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We analyze century-end projections for the tropical Pacific upper-ocean currents simulated within phase 6 of the Coupled Model Intercomparison Project (CMIP6) under global warming. We find that while the intensity of precipitation within the intertropical convergence zone (ITCZ) increases, the ITCZ also shifts toward the equator and broadens, which reduces wind stress curl north of the equator. Consequently, the North Equatorial Countercurrent (NECC) shifts equatorward, following the ITCZ, and weakens, despite the more intense ITCZ. The strength of the North Equatorial Current (NEC) and the South Equatorial Current (SEC) also decreases due to the weakening of the Walker circulation and the corresponding wind stress. However, despite the weaker winds, the Equatorial Undercurrent (EUC) intensifies as it shoals due to stronger vertical stratification induced by surface warming. Furthermore, we find a slightly stronger zonal pressure gradient along the core of the EUC, instead of a weaker one expected from weaker wind stress and sea surface height (SSH) gradient along the equator. Ultimately, we suggest that the reduced vertical friction, driven by enhanced ocean stratification and a higher Richardson number, is essential for the accelerated EUC. These intricate balances control future changes in equatorial currents, and the uncertainties of projected changes need to be further examined. [ABSTRACT FROM AUTHOR]

The likely changes to precipitation seasonality with warming are both impactful and not well understood. This work aims to describe areas that experience similar changes to seasonal precipitation irrespective of the original underlying precipitation seasonality. We train a self-organizing map on the difference between the seasonal cycle of precipitation in the past and in a high-warming future climate as represented by the Community Earth System Model, version 2, to create regions with similar changes in precipitation seasonality. This method is applied separately over land and ocean surfaces because of the differing processes leading to precipitation over each. This method indicates that future changes in seasonal precipitation are most varied in the tropics because of a southward shift in the intertropical convergence zone. The seasonal shifts found over midlatitude oceans indicate a poleward shift in atmospheric river activity. We find a correspondence between certain land-based precipitation changes and Köppen climate classification. The seasonality of large-scale and convective precipitation is examined for each region. The relationship between the seasonal changes to precipitation and associated atmospheric processes is discussed. These processes include atmospheric rivers, the intertropical convergence zone, tropical cyclones, and monsoons. [ABSTRACT FROM AUTHOR]

The intertropical convergence zone (ITCZ) is associated with a zonal band of strong precipitation that migrates meridionally over the seasonal cycle. Tropical precipitation also migrates zonally, such as from the South Asian monsoon in Northern Hemisphere summer (JJA) to the precipitation maximum of the west Pacific in Northern Hemisphere winter (DJF). To explore this zonal movement in the Indo-Pacific sector, we analyze the seasonal cycle of tropical precipitation using a 2D energetic framework and study idealized atmosphere–ocean simulations with and without ocean dynamics. In the observed seasonal cycle, an atmospheric energy and precipitation anomaly forms over South Asia in northern spring and summer due to heating over land. It is then advected eastward into the west Pacific in northern autumn and remains there due to interactions with the Pacific cold tongue and equatorial easterlies. We interpret this phenomenon as a "monsoonal mode," a zonally propagating moist energy anomaly of continental and seasonal scale. To understand the behavior of the monsoonal mode, we develop and explore an analytical model in which the monsoonal mode is advected by low-level winds, is sustained by interaction with the ocean, and decays due to the free tropospheric mixing of energy. Significance Statement: Regional concentrations of tropical precipitation, such as the South Asian monsoon, provide water to billions of people. These features have strong seasonal cycles that have typically been framed in terms of meridional shifts of precipitation following the sun's movement. Here, we study zonal shifts of tropical precipitation over the seasonal cycle in observations and idealized simulations. We find that land–ocean contrasts trigger a monsoon with concentrated precipitation over Asia in northern summer and near-surface eastward winds carry this precipitation into the west Pacific during northern autumn in what we call a "monsoonal mode." This concentrated precipitation remains over the west Pacific during northern winter, as further migration is impeded by the cold sea surface temperatures (SSTs) and easterly winds of the east Pacific. [ABSTRACT FROM AUTHOR]

Longstanding climate model biases in tropical precipitation exist over the east Pacific (EP) Ocean, especially during boreal winter and spring when models have excessive Southern Hemisphere (SH) precipitation near the intertropical convergence zone (ITCZ). In this study, we document the impact of convectively coupled waves (CCWs) on EP precipitation and the ITCZ using observations and reanalyses. We focus on the months when SH precipitation peaks in observations: February–April (FMA). CCWs explain 93% of total precipitation variance in the SH, nearly double the percent (48%) of the NH during FMA. However, we note that these percentages are inflated as they inevitably include the background variance. We further investigate the three leading high-frequency wave bands: mixed Rossby–gravity waves and tropical depression–type disturbances (MRG–TD type), Kelvin waves, and n = 0 eastward inertia–gravity waves (IG0). Compared to their warm pool counterparts, these three CCWs have a more zonally elongated and meridionally narrower precipitation structure with circulations that resemble past observational studies and/or shallow water theory. We quantify the contribution of all CCWs to four different daily ITCZ "states": Northern Hemisphere (NH) (nITCZ), SH (sITCZ), double (dITCZ), and equatorial (eITCZ) using a new precipitation-based ITCZ-state algorithm. We find that the percent of total precipitation variance explained by each of the CCWs is heightened for sITCZs and eITCZs and diminished for nITCZs. Last, we find that nITCZs are most prevalent weeks after strong CCW activity happens in the NH, whereas CCWs and sITCZs peak simultaneously in the SH. Significance Statement: Convectively coupled atmospheric waves (CCWs) are a critical feature of tropical weather and are an important source of precipitation near the region of highest precipitation on Earth called the intertropical convergence zone (ITCZ). Given three decades of climate model biases in CCWs and ITCZ precipitation over the east Pacific (EP) Ocean during spring, few studies have examined the relationship between CCWs and the springtime EP ITCZ. We explored the CCWs and EP ITCZ relationship through calculations of the percent of precipitation that comes from CCWs. A significant portion of the tropical precipitation is associated with CCWs during spring. CCWs are even more impactful when the ITCZ is in the SH or on the equator, which are both problematic in climate models. [ABSTRACT FROM AUTHOR]

The intertropical convergence zone (ITCZ), with its twice-annual passage over central Africa, is considered as the main driver of the rainfall seasonality. In this ITCZ paradigm, high rainfall occurs over regions of large low-level convergence. But recently, this paradigm was challenged over central Africa. Here, we show that a shallow meridional overturning circulation—driven by surface conditions—plays a thermodynamical control on the rainfall seasonality over central Africa. Indeed, due to the local evaporative cooling effect, the foot of the ascending branch of Hadley cells occurs where the temperature is the warmest, indicating a thermal low. This distorts the southern Hadley cell by developing its bottom-heavy structure. As result, both shallow and deep Hadley cells coexist over central Africa year-round. The deep mode is associated with the poleward transport of atmospheric energy at upper levels. The shallow mode is characterized by a shallow meridional circulation, with its moisture transport vanishing and converging in the midtroposphere rather than at lower troposphere. This midtropospheric moisture convergence is also the dominant component that shapes the vertically integrated moisture flux convergence, with little contribution of African easterly jets. This convergence zone thus controls the precipitating convection. Its meridional migration highlights the interhemispheric rainfall contrast over central Africa and outlines the unimodal seasonality. On the other hand, forced by the Congo basin cell, the precipitable water regulates the deep convection from the vegetated surface of Congo basin, acting as a continental sea. This nonlinear mechanism separates the rainfall into three distinct regimes: the moisture-convergence-controlled regime, with convective rainfall exclusively occurring in the rainy season; the local evaporation-controlled regime with drizzle in the dry season; and the precipitable-water-controlled regime, with exponential rainfall increase in the dry season. [ABSTRACT FROM AUTHOR]

The mid-Pliocene (approximately 3.3–3.0 Ma) was one of the past geological warm periods. Since this past warmer climate was in many respects comparable to near future warming projections, how the regional monsoonal rainfall changed during the mid-Pliocene is an important scientific and socioeconomic concern. Based on phase 2 of the Pliocene Model Intercomparison Project (PlioMIP2), this work examines the simulated changes of Sahel summer rainfall during the mid-Pliocene. The results show the considerable intermodel uncertainty of the simulated mid-Pliocene Sahel rainfall changes in the PlioMIP2 multimodel ensemble, which is due to the uncertainties of both the simulated dynamic and thermodynamic responses to this past warmer climate. In particular, we find that the intermodel spread in the simulated northern North American warming is a major source of the uncertainty of the mid-Pliocene Sahel summer rainfall changes through two processes. One is a direct dynamic process: a stronger warming over northern North America could enhance the meridional temperature gradient between the extratropical and tropical regions, inducing an interhemisphere energy imbalance of the atmosphere. This could lead to a northward shift of the intertropical convergence zone, strengthening the Western African summer monsoon (WASM) circulation and Sahel summer rainfall. Another is an indirect thermodynamic process: the strengthened WASM circulation could further induce anomalous moisture convergence over the Sahel region, increasing local atmospheric moisture at the low-level troposphere, in favor of a wetter Sahel. Our results suggest that an improved warming simulation over northern North America is essential for the hydrological cycle simulation around the Sahel in the mid-Pliocene warmer climate. Significance Statement: The Sahel summer rainfall change in a global warmer climate is a widespread scientific and socioeconomic issue because of its important impacts on regional agriculture, ecosystems, food security, water resources, and even cultural environment. However, the present study identifies a nonnegligible intermodel uncertainty of the simulated Sahel rainfall changes during the mid-Pliocene (one of the most recent geological warm periods in Earth's history) in the Pliocene Model Intercomparison Project phase 2 (PlioMIP2) multimodel ensemble. In particular, such an intermodel uncertainty of the simulated Sahel rainfall changes during the mid-Pliocene is attributed to that of the simulated northern North America warming via both the direct dynamic process and the indirect thermodynamic process. This is quite different from the uncertainty source of near-future projections of Sahel summer rainfall changes. The present results improve our understanding of the underlying physics of the hydrological cycle change around the Sahel region in the mid-Pliocene warmer climate, with implications for the future projections of regional monsoonal rainfall. [ABSTRACT FROM AUTHOR]

The Sahel is one of the most vulnerable regions to climate change. Robust estimation of future changes in the Sahel monsoon is therefore essential for effective climate change adaptation. Unfortunately, state-of-the-art climate models show large uncertainties in their projections of Sahel rainfall. In this study, we use 32 models from CMIP6 to identify the sources of this large intermodel spread of Sahel rainfall. By using maximum covariance analysis, we first highlight two new key drivers of this spread during boreal summer: the interhemispheric temperature gradient and equatorial Pacific sea surface temperature (SST) changes. This contrasts with previous studies, which have focused mainly on the Northern Hemisphere rather than the global scale, and in which the Pacific Ocean has been neglected in favor of the Atlantic. Next, we unravel the physical mechanisms behind these statistical relationships. First, the modulation of the interhemispheric temperature gradient across the models leads to varying latitudinal positions of the intertropical convergence zone and, consequently, varying Sahel rainfall intensity. Second, models that exhibit less warming than the multimodel mean in the equatorial Pacific, thereby projecting a less "El Niño–like" mean state, simulate enhanced precipitation over the central Sahel in the future through modulations of the Walker circulation, the tropical easterly jet, the meridional tropospheric temperature gradient, and hence regional zonal wind shear. Finally, we show that these two indices collectively explain 62% of Sahel rainfall change uncertainty: 40% due to the interhemispheric temperature gradient and 22% through equatorial Pacific SST. [ABSTRACT FROM AUTHOR]

The intertropical convergence zone (ITCZ) is a zonally elongated band of near-surface convergence and precipitation near the equator. During boreal spring, the eastern Pacific ITCZ migrates latitudinally on daily to subseasonal time scales, and climate models exhibit the greatest ITCZ biases during this time of the year. In this work, we investigate the air–sea interactions associated with the variability in the eastern Pacific ITCZ's latitudinal location for consecutive days when the ITCZ is only located north of the equator (nITCZ events) compared to when the ITCZ is on both sides of the equator or south of the equator (dsITCZ events) during February–April. The distribution of sea surface temperature (SST) anomalies and surface latent heat flux (SLHF) anomalies during the nITCZ and dsITCZ events follow the classic wind–evaporation–SST (WES) positive feedback mechanism. However, an alternative mechanism, embracing the effect of SST anomalies on vertical stratification and momentum mixing, gives rise to a negative WES feedback. Our results show that in the surface layer, there is a general progression of positive WES feedbacks happening in the weeks leading to the events followed by negative WES feedbacks occurring after the ITCZ events, with an alternate mechanism involving air–sea humidity differences limiting evaporation occurring in between. Additionally, the spatial structures of the components of the feedbacks are nearly mirror images for these opposite ITCZ events over the east Pacific during boreal spring. In closing, we find that understanding the air–sea interactions during daily to weekly varying ITCZ events (nITCZ and dsITCZ) helps to pinpoint how fundamental processes differ for ITCZs in different hemispheres. [ABSTRACT FROM AUTHOR]

There are no well-accepted mechanisms that can explain the annual frequency of tropical cyclones (TCs) both globally and in individual ocean basins. Recent studies using idealized models showed that the climatological frequency of TC genesis (TCG) is proportional to the Coriolis parameter associated with the intertropical convergence zone (ITCZ) position. In this study, we investigate the effect of the ITCZ position on TCG on the interannual time scale using observations over 1979–2020. Our results show that the TCG frequency is significantly correlated with the ITCZ position in the North Atlantic (NA) and western North Pacific (WNP), with more TCG events in years when the ITCZ is farther poleward. The ITCZ–TCG relationship in NA is dominated by TCG events in the tropics (0°–20°N), while the relationship in WNP is due to TCs formed in the east sector (140°E–180°). We further confirmed that ENSO has little effect on the ITCZ–TCG relationship despite the fact that it can affect the ITCZ position and TCG frequency separately. In NA and WNP, a poleward shift of ITCZ is significantly associated with large-scale environment changes favoring TCG in the main development region (MDR). However, the basinwide TCG frequency has a weak relationship with the ITCZ in other ocean basins. We showed that a poleward ITCZ in the eastern North Pacific and South Pacific favors TCG on the poleward flank of the MDR, while it suppresses TCG on the equatorward flank, leading to insignificant change in the basinwide TCG frequency. In the south Indian Ocean, the ITCZ position has weak effect on TCG frequency due to the mixed influences of environmental conditions. [ABSTRACT FROM AUTHOR]

This study investigates the transient evolution of tropical Pacific sea surface temperature (SST) responses to a constant northern high-latitude solar heating in fully coupled CESM 1.2. The study identifies two stages through multiple ensemble runs. 1) In the first 3 years, a hemispherically asymmetric pattern emerges, caused by air–sea interactions associated with the anomalous cross-equatorial Hadley cell. The northern tropics experience warming that is blocked north of the equator by the intertropical convergence zone. The southeast Pacific cooling reaches the equatorial region and is amplified by the equatorial Ekman divergence. 2) Within a decade, the equatorial cooling is replaced by warming in the eastern equatorial basin. The anomalous warming that appears faster than the time scales of the oceanic ventilation is attributed to anomalous meridional heat convergence and weakening of the northern subtropical cell. Our findings highlight the influence of ocean dynamics on the temporal and spatial evolution of tropical SST response to hemispherically asymmetric heating. The initial cooling caused by Ekman divergence delays the arrival of slow warming, while initial wind and temperature anomalies set the stage for the weakening of the subtropical cell. The results have important implications for understanding the evolution of tropical SST patterns in observational records and future climate change simulations, as they show strong interhemispheric temperature asymmetry in the extratropics. [ABSTRACT FROM AUTHOR]

The climate responses to Atlantic meridional overturning circulation (AMOC) fluctuations are investigated in a hierarchy of sensitivity experiments. We modify the baroclinic component of the North Atlantic Ocean currents online in an atmosphere–ocean general circulation model to reproduce typical AMOC multidecadal variability found in a preindustrial control simulation in the same model. An analogous experiment is also conducted using a slab-ocean experiment. The responses to a strong AMOC include a widespread warming in the Northern Hemisphere and a northward shift of the intertropical convergence zone over the Atlantic Ocean. The driving mechanism of climate responses is then investigated with the changes in the energy flows in the ocean and atmosphere. The large-scale atmospheric changes in the tropics are organized by an anomalous cross-equatorial Hadley circulation transporting energy southward and moisture and heat northward. Changes in the Indo-Pacific Ocean circulation and heat transport, driven by the wind stress associated with the abnormal Hadley cell, damp the atmospheric responses. The lack of Indo-Pacific transport and ocean heat storage leads to amplified atmospheric changes in the slab-ocean experiments, which are further amplified by a positive feedback due to the interhemispheric antisymmetric changes in low cloud cover. [ABSTRACT FROM AUTHOR]

El Niño–Southern Oscillation (ENSO) is the most influential interannual climate variability on Earth. The tendency of the mature phase of ENSO, characterized by the strongest sea surface temperature (SST) anomalies, to appear during the boreal winter is known as seasonal phase locking. Climate models are challenged by biases in simulating ENSO seasonal phase locking. Here, we evaluated the ENSO phase-locking simulation performance in 50 models of phase 6 of the Coupled Model Intercomparison Project (CMIP6) and found that the models with the intertropical convergence zone (ITCZ) poleward bias tended to simulate more ENSO events that peaked out of the boreal winter season. The contributions of the ITCZ poleward bias to the ENSO phase-locking bias were also evaluated, yielding a correlation coefficient of 0.55, which can explain approximately 30% of the ENSO seasonal phase-locking bias. The mechanism that influences the simulation of ENSO seasonal phase locking was also assessed. The ITCZ poleward bias induces a dry bias over the equatorial Pacific, especially during the boreal summer. During ENSO events, the meridional movement of the ITCZ is prevented, and the equatorial precipitation and convection anomalies that respond to ENSO events are also restrained. The restrained convection anomaly weakens the ENSO-related zonal wind anomaly, triggering a weaker eastern tropical Pacific thermocline anomaly during the following autumn. The weakened thermocline anomaly cannot sustain further development of ENSO-related SST anomalies. Therefore, ENSO events in models containing the ITCZ poleward bias are restrained during the boreal summer and autumn and, thus, tend to peak out of the winter season. [ABSTRACT FROM AUTHOR]

Tropical areas with mean upward motion}and as such the zonal-mean intertropical convergence zone (ITCZ)}are projected to contract under global warming. To understand this process, a simple model based on dry static energy and moisture equations is introduced for zonally symmetric overturning driven by sea surface temperature (SST). Processes governing ascent area fraction and zonal mean precipitation are examined for insight into Atmospheric Model Intercomparison Project (AMIP) simulations. Bulk parameters governing radiative feedbacks and moist static energy transport in the simple model are estimated from the AMIP ensemble. Uniform warming in the simple model produces ascent area contraction and precipitation intensification}similar to observations and climate models. Contributing effects include stronger water vapor radiative feedbacks, weaker cloud-radiative feedbacks, stronger convection-circulation feedbacks, and greater poleward moisture export. The simple model identifies parameters consequential for the inter-AMIPmodel spread; an ensemble generated by perturbing parameters governing shortwave water vapor feedbacks and gross moist stability changes under warming tracks inter-AMIP-model variations with a correlation coefficient ;0.46. The simple model also predicts the multimodel mean changes in tropical ascent area and precipitation with reasonable accuracy. Furthermore, the simple model reproduces relationships among ascent area precipitation, ascent strength, and ascent area fraction observed in AMIP models. A substantial portion of the inter-AMIP-model spread is traced to the spread in how moist static energy and vertical velocity profiles change under warming, which in turn impact the gross moist stability in deep convective regions}highlighting the need for observational constraints on these quantities. [ABSTRACT FROM AUTHOR]

Oxygen isotope speleothems have been widely used to infer past climate changes over tropical South America (TSA). However, the spatial patterns of the millennial precipitation and precipitation δ18O (δ18Op) response have remained controversial, and their response mechanisms are unclear. In particular, it is not clear whether the regional precipitation represents the intensity of the millennial South American summer monsoon (SASM). Here, we study the TSA hydroclimate variability during the last deglaciation (20–11 ka ago) by combining transient simulations of an isotope-enabled Community Earth System Model (iCESM) and the speleothem records over the lowland TSA. Our model reasonably simulates the deglacial evolution of hydroclimate variables and water isotopes over the TSA, albeit underestimating the amplitude of variability. North Atlantic meltwater discharge is the leading factor driving the TSA’s millennial hydroclimate variability. The spatial pattern of both precipitation and δ18Op show a northwest–southeast dipole associated with the meridional migration of the intertropical convergence zone, instead of a continental-wide coherent change as inferred in many previous works on speleothem records. The dipole response is supported by multisource paleoclimate proxies. In response to increased meltwater forcing, the SASM weakened (characterized by a decreased low-level easterly wind) and consequently reduced rainfall in the western Amazon and increased rainfall in eastern Brazil. A similar dipole response is also generated by insolation, ice sheets, and greenhouse gases, suggesting an inherent stability of the spatial characteristics of the SASM regardless of the external forcing and time scales. Finally, we discuss the potential reasons for the model–proxy discrepancy and pose the necessity to build more paleoclimate proxy data in central-western Amazon. Significance Statement We want to reconcile the controversy on whether there is a coherent or heterogeneous response in millennial hydroclimate over tropical South America and to clearly understand the forcing mechanisms behind it. Our isotope-enabled transient simulations fill the gap in speleothem reconstructions to capture a complete picture of millennial precipitation/δ18Op and monsoon intensity change. We highlight a heterogeneous dipole response in precipitation and δ18Op on millennial and orbital time scales. Increased meltwater discharge shifts ITCZ southward and favors a wet condition in coastal Brazil. Meanwhile, the low-level easterly and the summer monsoon intensity reduced, causing a dry condition in the central-western Amazon. However, the millennial variability of hydroclimate response is underestimated in our model, together with the lack of direct paleoclimate proxies in the central-west Amazon, complicating the interpretation of changes in specific paleoclimate events and posing a challenge to constraining the spatial range of the dipole. Therefore, we emphasize the necessity to increase the source of proxies, enhance proxy interpretations, and improve climate model performance in the future. [ABSTRACT FROM AUTHOR]

Understanding the hydroclimate representations of precipitation δ18O (δ18Op) in tropical South America (TSA) is crucial for climate reconstruction from available speleothem caves. Our preceding study (Part I) highlights a heterogeneous response in millennial hydroclimate over the TSA during the last deglaciation (20–11 ka before present), characterized by a northwest–southeast (NW–SE) dipole in both rainfall and δ18Op, with opposite signs between central-western Amazon and eastern Brazil. Mechanisms of such δ18Op dipole response are further investigated in this study with the aid of moisture tagging simulations. In response to increased meltwater discharge, the intertropical convergence zone (ITCZ) migrates southward, causing a moisture source location shift and depleting the isotopic value of the vapor transported into eastern Brazil, which almost entirely contributes to the δ18Op depletion in eastern Brazil (SE pole). In contrast, the moisture source location change and local condensation change (due to the lowering convergence level and increased rain reevaporation in unsaturated subcloud layers) contribute nearly equally to the δ18Op enrichment in the central-western Amazon (NW pole). Therefore, although a clear inverse relationship between δ18Op and rainfall in both dipole regions seems to support the “amount effect,” we argue that the local rainfall amount only partially interprets the millennial δ18Op change in the central-western Amazon, while δ18Op does not document local rainfall change in eastern Brazil. Thus, the paleoclimate community should be cautious when using δ18Op as a proxy for past local precipitation in the TSA region. Finally, we discuss the discrepancy between the model and speleothem proxies on capturing the millennial δ18Op dipole response and pose a challenge in reconciling the discrepancy. Significance Statement We want to comprehensively understand the hydroclimate footprints of δ18Op and the mechanisms of the millennial variability of δ18Op over tropical South America with the help of water tagging experiments performed by the isotope-enabled Community Earth System Model (iCESM). We argue that the millennial δ18Op change in eastern Brazil mainly documents the moisture source location change associated with ITCZ migration and the change of the isotopic value of the incoming water vapor, instead of the local rainfall amount. In contrast, the central-western Amazon partially documents the moisture source location shift and local precipitation change. Our study cautions that one should not simply resort to the isotopic “amount effect” to reconstruct past precipitation in tropical regions without studying the mechanisms behind it. [ABSTRACT FROM AUTHOR]

The western-central equatorial Pacific (WCEP) zonal wind affects El Niño-Southern Oscillation (ENSO) by involving a series of multiscale air-sea interactions. Its interannual variation contributes the most to ENSO amplitude. Thus, understanding the predictability of the WCEP interannual wind is of great importance for better predictions of ENSO. Here, we show that the North Pacific Oscillation (NPO) and the South Pacific Oscillation (SPO) alternate in fueling this interannual wind from late boreal winter to austral winter in the presence of background trade winds in different hemispheres. During the boreal winter-spring, the NPO registers footprints in the tropics by benefiting from the Pacific meridional mode and modulating the northwestern Pacific intertropical convergence zone (NITCZ). However, as austral winter approaches, the SPO takes over the role of the NPO in maintaining the anomalous NITCZ. Moreover, the interannual wind is further driven to the east in the positive phase of the SPO, by intensified central-eastern equatorial Pacific convection resulting from tropical-extratropical heat flux adjustments. A reconstructed WCEP interannual wind index involving only the NPO and the SPO possesses a long lead time for ENSO prediction of nearly one year. These two extratropical boosters enhance the viability of equatorial Pacific zonal wind anomalies associated with the large growth rate of ENSO, and the one in the winter hemisphere seems to be more efficient in forcing the tropics. Our result further indicates that the NPO benefits a long-lead prediction of the WCEP interannual wind and ENSO, while the SPO is the dominant extratropical predictor of ENSO amplitude. [ABSTRACT FROM AUTHOR]

The response of zonal-mean precipitation minus evaporation (P 2 E) to global warming is investigated using a moist energy balance model (MEBM) with a simple Hadley cell parameterization. The MEBM accurately emulates zonal-mean P 2 E change simulated by a suite of global climate models (GCMs) under greenhouse gas forcing. The MEBM also accounts for most of the intermodel differences in GCM P 2 E change and better emulates GCM P 2 E change when compared to the "wet-gets-wetter, dry-gets-drier" thermodynamic mechanism. The intermodel spread in P 2 E change is attributed to intermodel differences in radiative feedbacks, which account for 60%-70%of the intermodel variance, with smaller contributions from radiative forcing and ocean heat uptake. Isolating the intermodel spread of feedbacks to specific regions shows that tropical feedbacks are the primary source of intermodel spread in zonal-mean P 2 E change. The ability of the MEBM to emulate GCM P 2 E change is further investigated using idealized feedback patterns. A less negative and narrowly peaked feedback pattern near the equator results in more atmospheric heating, which strengthens the Hadley cell circulation in the deep tropics through an enhanced poleward heat flux. This pattern also increases gross moist stability, which weakens the subtropical Hadley cell circulation. These two processes in unison increase P 2 E in the deep tropics, decrease P 2 E in the subtropics, and narrow the intertropical convergence zone. Additionally, a feedback pattern that produces polar-amplified warming partially reduces the poleward moisture flux by weakening the meridional temperature gradient. It is shown that changes to the Hadley cell circulation and the poleward moisture flux are crucial for understanding the pattern of GCM P 2 E change under warming. [ABSTRACT FROM AUTHOR]

Tropical precipitation is climatologically most intense at the heart of the intertropical convergence zone (ITCZ), but this is not always true in instantaneous snapshots. Precipitation is amplified along the ITCZ edge rather than at its center from time to time. In this study, satellite observations of column water vapor, precipitation, and radiation as well as the thermodynamic field from reanalysis data are analyzed to investigate the behavior of ITCZ convection in light of the local atmospheric energy imbalance. The analysis is focused on the eastern Pacific ITCZ, defined as the areas where column water vapor exceeds 50 mm over a specified width (typically 400-600 km) in the domain of 208S-208N, 1808-908W. The events with a precipitation maximum at the southern and northern edges of the ITCZ are each averaged into composite statistics and are contrasted against the reference case with peak precipitation at the ITCZ center. The key findings are as follows. When precipitation peaks at the ITCZ center, suppressed radiative cooling forms a prominent positive peak in the diabatic forcing to the atmosphere, counteracted by an export of moist static energy (MSE) owing to a deep vertical advection and a large horizontal export of MSE. When convection develops at the ITCZ edges, to the contrary, a positive peak of the diabatic forcing is only barely present. An import of MSE owing to a shallow ascent on the ITCZ edges presumably allows an edge intensification to occur despite the weak diabatic forcing. [ABSTRACT FROM AUTHOR]

The position and strength of the Hadley cell circulation determine the habitable zones in the tropics, yet our understanding of and ability to predict changes in the circulation is limited. One potentially important source of uncertainty is the dependence of the Hadley cell on turbulent drag. Here, the sensitivity of the Hadley cell and associated features such as the intertropical convergence zone to variations in the magnitude of the turbulent drag is explored with an atmospheric general circulation model in aquaplanet configuration. The tropical circulation and precipitation, and extratropical features such as the polar jet stream, displayed a strong sensitivity to the strength of the parameterized turbulent drag, with distinct low- or high-drag regimes. However, the response of the meridional heat transport produced a surprising departure from previous expectations: with greater drag, simulations exhibited less heat transport than low-drag simulations, which is in the opposite sense to that from Held and Hou. This may be due to the energetic constraints in the present model framework. When exposed to a uniform global warming, the response of the ITCZ precipitation depends strongly on the choice of drag, whereas most simulations exhibit a poleward expansion of the subtropics. [ABSTRACT FROM AUTHOR]

Thermohaline structure and time evolution in the subsurface ocean play a critical role in climate variability and predictability. They are still poorly represented in ocean and climate models. Here, the characteristics of subsurface thermohaline biases in the southern tropical Pacific and their causes are investigated through CMIP-based analyses and model-based experiments. There exists a pronounced subsurface cold bias at 200-m depth over the southern tropical Pacific in CMIP6 simulations with an ensemble mean of about −4°C and an extreme close to −10°C. This cold bias is accompanied by a fresh subsurface bias of about −0.9 psu in the ensemble mean (−1.9 psu minimum). Similar subsurface thermohaline biases also exist in CMIP5 outputs, indicating that reduction of these biases remains a long-standing challenge for model developments. To understand the causes of these biases, attribution analyses and POP2-based sensitivity experiments are performed. It is found that the subsurface thermohaline biases are attributed to the model deficiencies in simulating wind stress curl and precipitation in the southern tropical Pacific. By conducting CESM2-based coupled experiments, a warm SST bias in the southeastern tropical Pacific is found to be responsible for the poor simulations in wind stress curl and precipitation. The consequences of these biases are also analyzed. The subsurface thermohaline biases cause the density field to increase substantially along 10°S, flattening the zonal isopycnal surface and reducing equatorward interior transport. In addition, the anomalously cold and fresh subsurface signals in the southern tropical Pacific are seen to propagate to the equator, leading to an overall spurious cooling in the equatorial subsurface. Significance Statement: Subsurface biases severely degrade the credibility of climate models in their predictions and projections; hence, it is important to understand the causes of these subsurface biases. Our study analyzes the characteristics of subsurface thermohaline biases in the southern tropical Pacific and investigates their causes. A pronounced subsurface cold bias is found over the southern tropical Pacific, accompanied by an obvious subsurface fresh bias. By performing attribution analyses and numerical experiments, it is found that the subsurface thermohaline biases are attributed to the model deficiencies in simulating wind stress and precipitation, which are further attributed to the warm SST bias in the southeastern tropical Pacific. These results provide a guide for improving climate model performances. [ABSTRACT FROM AUTHOR]

High-frequency observations of surface winds over the open ocean are available only at limited locations. However, these observations are essential for assessing atmospheric influences on the ocean, validating reanalysis products, and building parameterization schemes. By analyzing high-frequency measurements from the Global Tropical Moored Buoy Array, the effects of subdaily winds on the mean surface wind stress magnitude are systematically examined. Subdaily winds account for 12.4% of the total stress magnitude on average. The contribution is enhanced over the intertropical convergence zone and reaches a maximum (28.5%) in the equatorial western Pacific. The magnitude of the contribution is primarily determined by the kinetic energy of subdaily winds. Compared to the buoy observations, the ERA5 and MERRA2 subdaily winds underestimate this contribution by 51% and 63% due to underestimations of subdaily kinetic energy, leading to 7% and 8% underestimations in the total stress magnitude, respectively. Two new gustiness parameterization schemes related to precipitation are developed to account for the effect of subdaily winds, explaining ∼80% of the contribution from subdaily winds. Considering the importance of wind stress for ocean–atmosphere interactions, the inclusion of these parameterization schemes in climate models is expected to substantially improve simulations of large-scale climate variability. Significance Statement: Surface wind stress drives upper-ocean circulation, which is critical for the redistribution of mass, momentum, and energy in the ocean. Moreover, it is one of the key factors controlling oceanic turbulent mixing and therefore has significant impacts on the distribution of temperature, salinity, and associated ocean variability. Using high-resolution buoy observations, this study highlights the importance of subdaily winds for integrated wind stress estimates. In addition, it finds that current state-of-the-art and widely used reanalysis products largely underestimate the effect of subdaily winds. Two new parameterization schemes are developed, leading to a better representation of the effect of subdaily winds. Including the proposed parameterization schemes in climate models is expected to substantially improve their simulations of large-scale climate variability. [ABSTRACT FROM AUTHOR]

The tropical rain belt varies between unimodal and bimodal meridional precipitation distributions, both regionally and on seasonal to geological time scales. Here we show that this variation is largely driven by equatorial precipitation inhibition, and quantify it using an equatorial modality index (EMI) that varies continuously between 1 and 2 for purely unimodal and bimodal distributions. We show that tropical modality is a fundamental characteristic of tropical climate, which we define as annual-mean EMI. We examine large-scale aspects of tropical modality across 73 climate models from phases 5 and 6 of the Coupled Model Intercomparison Project, 45 paleo simulations (;300 million years ago to present), and observations. We find increased tropical modality to be strongly related to increased width of the tropical rain belt, wider and weaker meridional overturning circulation, colder equatorial cold tongues, and more severe double intertropical convergence zone bias in modern climate models. Tropical sectors (or global zonal means) with low tropical modality are characterized by monsoonal seasonal variations (i.e., seasonal migrations of rainbands following the sun). In sectors with high tropical modality we identify three important seasonal modes: (i) migration of the precipitation distribution toward the warmer hemisphere, (ii) variation in the latitudinal separation between hemispheric rainbands, and (iii) seesaw variation in the intensity of the hemispheric rainbands. In high tropical modality sectors, due to contrasting shifts of the migration and separation modes, counter to general wisdom, seasonal migrations of tropical rainbands cannot be generally assumed to follow the sun. [ABSTRACT FROM AUTHOR]

This study reveals the role of the tropical Atlantic variability in modulating barrier layer thickness (BLT) in peak seasons. Based on reanalysis data during 1980–2016, statistical and dynamical analyses are performed to investigate the mechanism of BLT variability associated with the tropical Atlantic modes. The regions with significant correlation between BLT and tropical Atlantic modes are located on the northwest and southeast coasts of the tropical Atlantic, which are consistent with BLT maximum variability regions. In boreal spring, BLT decreases in the northwest because less latent heat release affected by weak trade wind related to the Atlantic meridional mode (AMM) shoals the isothermal layer depth (ITLD). In the south equatorial Atlantic, deepened mixed layer depth (MLD) is controlled by the decreasing freshwater input brought by a northward shift of the intertropical convergence zone (ITCZ) and further leads to a thinner barrier layer (BL). However, a shoaling MLD appears in the north equatorial Atlantic, which results from excessive freshwater input, causing a thick BL there. In boreal summer, positive runoff anomaly caused by the Atlantic equatorial mode (AEM) leads to upper warming of the tropical northwest Atlantic and a shallowing ITLD, favoring a thinner BL there. However, a southward shift of ITCZ brings more freshwater into the south equatorial Atlantic, inducing a shallowing MLD as well as a thicker BL. AEM-driven horizontal heat advection of the south equatorial current contributes to a thick ITLD in the central southern tropical Atlantic and thus increases BLT. Significance Statement: This research aims to reveal how the tropical Atlantic meridional and equatorial interannual climatic modes affect barrier layer thickness (BLT). These two climate modes can affect the wind field, ocean current, and precipitation through air–sea interaction processes, and further affect mixing, heat–salt transport, and stratification in the upper ocean and thus BLT. This finding is important because the barrier layer restricts the exchange of heat, momentum, mass, and nutrients between the mixed layer and the thermocline, thereby impacting local and remote weather events, the ecological environment, and the climate. Our results provide guidance for interpreting the interannual variability of BLT in the tropical Atlantic. [ABSTRACT FROM AUTHOR]

This study reveals that the impact of the spring North Pacific meridional mode (PMM) on the following-winter El Niño–Southern Oscillation (ENSO) shows a continuing increase in the past. A comparative analysis is conducted for the high- and low-correlation periods to understand the factors for the strengthened impact of the PMM. The spring PMM-related sea surface temperature (SST) and atmospheric anomalies over the subtropical northeastern Pacific propagate southwestward to the tropical central Pacific via wind–evaporation–SST feedback in the high-correlation period. The tropical SST and atmospheric anomalies further develop to an ENSO-like pattern via positive air–sea interaction. In the low-correlation period, SST and atmospheric anomalies over the subtropical northeastern Pacific related to the PMM cannot extend to the deep tropics. Therefore, the spring PMM has a weak impact on ENSO. The extent to which the PMM-related SST and atmospheric anomalies extend toward the tropics is related to the background flow. The stronger mean trade winds in the high-correlation period lead to an increase in the air–sea coupling strength over the subtropical northeastern Pacific. As such, the spring PMM-related SST and atmospheric anomalies can more efficiently propagate southwestward to the tropical Pacific and exert stronger impacts on the succeeding ENSO. In addition, the southward shifted intertropical convergence zone in the high-correlation period also favors the southward extension of the PMM-related SST anomalies to the tropics and contributes to a stronger PMM–ENSO relation. The variation and its formation mechanism of the spring PMM–winter ENSO relationship appear in both the observations and the long historical simulation of Earth system models. Significance Statement: The North Pacific meridional mode (PMM) is the leading atmosphere–ocean coupling pattern over the subtropical northeastern Pacific after removing the ENSO variability, with maximum variance during boreal spring. Previous studies indicated that the PMM plays an important role in relaying the impact of the atmosphere–ocean forcings over the extratropics on the tropical ENSO. This study reveals that the impact of the spring PMM on the following winter ENSO shows a continuing increase in the past 70 years. The physical mechanisms for this strengthened impact are further examined. Results obtained in this study have important implications for improving the prediction of the tropical ENSO variability. [ABSTRACT FROM AUTHOR]

This study revises the collisional growth, heterogeneous ice nucleation, and homogeneous ice nucleation processes in a double-moment bulk cloud microphysics scheme implemented in the Nonhydrostatic Icosahedral Atmospheric Model (NICAM). The revised cloud microphysical processes are tested by 10-day global simulations with a horizontal resolution of 14 km. It is found that both the aggregation of cloud ice with smaller diameters and the graupel production by riming are overestimated in the current schemes. A new method that numerically integrates the collection kernel solves this issue, and consequently, the lifetime of cloud ice is reasonably extended in reference to satellite observations. In addition, the results indicate that a reduction in graupel modulates the convective intensity, particularly in intense rainfall systems. The revision of both heterogeneous and homogeneous ice nucleation significantly increases the production rate of cloud ice number concentration. With these revisions, the new version of the cloud microphysics scheme successfully improves outgoing longwave radiation, particularly over the intertropical convergence zone, in reference to satellite observations. Therefore, the revisions are beneficial for both long-term climate simulations and representing the structure of severe storms. Significance Statement: Very high-resolution global atmospheric models have been developed to simultaneously address global climate and regional weather. In general, cloud microphysics schemes used in such global models are introduced from regional weather forecasting models to realistically represent mesoscale cloud systems. However, a cloud microphysics scheme that was originally developed with the aim of weather forecasting can cause unexpected errors in global climate simulations because such a cloud microphysics scheme is not designed for interdisciplinary usage across spatiotemporal scales. This study focuses on systematic model biases in evaluating the terminal velocity of ice cloud particles and proposes a method to accurately calculate the growth rate of ice cloud particles. Improvements in ice cloud modeling successfully reduce model biases in the global energy budget. In addition, the internal structure of intense rainfall systems is modified using the new cloud model. Therefore, improvements in ice cloud modeling could further increase the reliability of weather forecasting, seasonal prediction, and climate projection. [ABSTRACT FROM AUTHOR]

Tropical precipitation in climate models presents significant biases in both the large-scale pattern (i.e., double intertropical convergence zone bias) and local-scale characteristics (i.e., drizzling bias with too frequent drizzle/convection and reduced occurrences of no and heavy precipitation). By untangling the coupled system and analyzing the biases in precipitation, cloud, and radiation, this study shows that local-scale drizzling bias in atmospheric models can lead to large-scale double-ITCZ bias in coupled models by inducing convective-regime-dependent biases in precipitation and cloud radiative effects (CRE). The double-ITCZ bias consists of a hemispherically asymmetric component that arises from the asymmetric SST bias and a nearly symmetric component that exists in atmospheric models without the SST bias. By increasing light rain but reducing heavy rain, local-scale drizzling bias induces positive (negative) precipitation bias in the moderate (strong) convective regime, leading to the nearly symmetric wet bias in atmospheric models. By affecting the cloud profile, local-scale drizzling bias induces positive (negative) CRE bias in the stratocumulus (convective) regime in atmospheric models. Because the stratocumulus (convective) region is climatologically more pronounced in the southern (northern) tropics, the CRE bias is deemed to be hemispherically asymmetric and drives warm and wet (cold and dry) biases in the southern (northern) tropics when coupled to ocean. Our results suggest that correcting local-scale drizzling bias is critical for fixing large-scale double-ITCZ bias. The drizzling and double-ITCZ biases are not alleviated in models with mesoscale (0.25°–0.5°) or even storm-resolving (∼3 km) resolution, implying that either large-eddy simulation or fundamental improvement in small-scale subgrid parameterizations is needed. [ABSTRACT FROM AUTHOR]

Precipitation often happens along organized filaments or bands of moisture such as convergence zones. Recent regional studies have shown that these moisture filaments arise from synoptic-scale mixing features known as attracting Lagrangian coherent structures (LCSs). In this study, we present a climatology of synoptic-scale mixing and investigate its covariability with precipitation on temporal scales ranging from weekly to interannual. We characterize mixing with the finite-time Lyapunov exponent (FTLE), a measure of parcel deformation, in ERA5 data between 1980 and 2009. Attracting LCSs are identified as ridges of the FTLE. At the interannual time scale, we compare El Niño and La Niña events and find that composites of precipitation and mixing anomalies share similar spatial patterns. We also compare summer and winter seasons and find that composites of seasonal-mean precipitation and mixing anomalies present similar characteristics, with precipitation being particularly intense (weak) where mixing is strong (weak). In particular, these patterns closely match the typical signatures of the intertropical convergence zone (ITCZ) and monsoon systems and the migrations of extratropical cyclone tracks. At the subseasonal scale, we employ daily composites to investigate the influence of the Madden–Julian oscillation and the North Atlantic Oscillation on the mixing regimes of the Atlantic and east Pacific; our results indicate that these oscillations control the synoptic-scale horizontal mixing and the occurrence of LCSs as to suppress or enhance precipitating systems like the ITCZ and the South Atlantic convergence zone. The results presented in this first climatology of synoptic-scale mixing and LCSs indicate that these are powerful diagnostics to identify circulation mechanisms underlying precipitation variability. [ABSTRACT FROM AUTHOR]

Numerical simulations allow us to gain a comprehensive understanding of the underlying mechanisms of past, present, and future climate changes. The mid-Holocene (MH) and the last interglacial (LIG) were the two most recent warm episodes of Earth's climate history and are the focus of paleoclimate research. Here, we present results of MH and LIG simulations with two versions of the state-of-the-art Earth system model AWI-ESM. Most of the climate changes in MH and LIG compared to the preindustrial era are agreed upon by the two model versions, including 1) enhanced seasonality in surface temperature that is driven by the redistribution of seasonal insolation; 2) a northward shift of the intertropical convergence zone (ITCZ) and tropical rain belt; 3) a reduction in annual mean Arctic sea ice concentration; 4) weakening and northward displacement of the Northern Hemisphere Hadley circulation, which is related to the decrease and poleward shift of the temperature gradient from the subtropical to the equator in the Northern Hemisphere; 5) a westward shift of the Indo-Pacific Walker circulation due to anomalous warming over the Eurasia and North Africa during boreal summer; and 6) an expansion and intensification of Northern Hemisphere summer monsoon rainfall, with the latter being dominated by the dynamic component of moisture budget (i.e., the strengthening of wind circulation). However, the simulated responses of the Atlantic meridional overturning circulation (AMOC) in the two models yield different results for both the LIG and the MH. AMOC anomalies between the warm interglacial and preindustrial periods are associated with changes in North Atlantic westerly winds and stratification of the water column at the North Atlantic due to changes in ocean temperature, salinity, and density. [ABSTRACT FROM AUTHOR]

Although Earth's troposphere does not superrotate in the annual mean, for most of the year—from October to May—the winds of the tropical upper troposphere are westerly. We investigate this seasonal superrotation using reanalysis data and a single-layer model for the winds of the tropical upper troposphere. We characterize the temporal and spatial structures of the tropospheric superrotation, and quantify the relationships between the superrotation and the leading modes of tropical interannual variability. We also find that the strength of the superrotation has remained roughly constant over the past few decades, despite the winds of the tropical upper troposphere decelerating (becoming more easterly) in other months. We analyze the monthly zonal-mean zonal momentum budget and use numerical simulations with an axisymmetric, single-layer model of the tropical upper troposphere to study the underlying dynamics of the seasonal superrotation. Momentum flux convergence by stationary eddies accelerates the superrotation, while cross-equatorial easterly momentum transport associated with the Hadley circulation decelerates the superrotation. The seasonal modulations of these two competing factors shape the superrotation. The single-layer model is able to qualitatively reproduce the seasonal progression of the winds in the tropical upper troposphere, and highlights the northward displacement of the intertropical convergence zone in the annual mean as a key factor responsible for the annual cycle of the tropical winds. [ABSTRACT FROM AUTHOR]

Cross-equatorial ocean heat transport (OHT) changes have been found to damp meridional shifts of the intertropical convergence zone (ITCZ) induced by hemispheric asymmetries in radiative forcing. Zonal-mean energy transport theories and idealized model simulations have suggested that these OHT changes occur primarily due to wind-driven changes in the Indo-Pacific's shallow subtropical cells (STCs) and buoyancy-driven changes in the deep Atlantic meridional overturning circulation (AMOC). In this study we explore the partitioning between buoyancy and momentum forcing in the ocean's response. We adjust the top-of-atmosphere solar forcing to cool the Northern Hemisphere (NH) extratropics in a novel set of comprehensive climate model simulations designed to isolate buoyancy-forced and momentum-forced changes. In this case of NH high-latitude forcing, we confirm that buoyancy-driven changes in the AMOC dominate in the Atlantic. However, in contrast with prior expectations, buoyancy-driven changes in the STCs are the primary driver of the heat transport changes in the Indo-Pacific. We find that buoyancy-forced Indo-Pacific STC changes transport nearly 4 times the amount of heat across the equator as the shallower wind-driven STC changes. This buoyancy-forced STC response arises from extratropical density perturbations that are amplified by the low cloud feedback and communicated to the tropics by the ventilated thermocline. While the ocean's specific response is dependent on the forcing scheme, our results suggest that partitioning the ocean's total response to energy perturbations into buoyancy and momentum forcing provides basin-specific insight into key aspects of how the ocean damps ITCZ migrations that previous zonal-mean frameworks omit. [ABSTRACT FROM AUTHOR]

Observational and climate model analysis showed that the anomalous sea surface temperature in the north tropical Atlantic (NTA) in boreal spring can trigger El Niño–Southern Oscillation (ENSO) in the subsequent winter. Similarly, the climate models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) are known to reasonably simulate the NTA effect. Nevertheless, the strengths of the NTA effect on ENSO among the climate models are also diverse. In this light, we revisited the possible causes that contributed to the different NTA effects on ENSO in the CMIP5 climate models. We found that the strength of the NTA triggering ENSO in the climate model tended to be proportional to the intensity of the climatological subtropical North Pacific high system in boreal spring. The stronger climatological subtropical North Pacific high accompanied enhanced trade wind, precipitation reduction, and cold sea surface temperature over the subtropics. Under these conditions, the moist static energy feedback process, also known as the moist enthalpy advection mechanism, effectively operated around the Pacific intertropical convergence zone. That is, the NTA-induced signals in the subtropical North Pacific readily intruded into the deep tropical Pacific with the aid of the feedback processes, leading to an ENSO event. Consistent with the CMIP5 analysis results, the observed NTA effect on ENSO became stronger during the decades when the climatological North Pacific subtropical high intensified, underpinning the importance of climatology in the subtropical North Pacific in the NTA–ENSO connection. [ABSTRACT FROM AUTHOR]

Through a series of global convection-permitting simulations and geostationary satellite observations, this study investigates the role of deep moist convection in atmospheric kinetic energy (KE) and brightness temperature (BT) spectra in a realistic framework. The control simulation was produced on a quasi-uniform 3-km global mesh, which allowed the explicit representation of deep convection. To assess the impact of deep moist convection, a fake-dry simulation was performed with latent heating–cooling feedback in the microphysics removed for comparison. The impacts of deep moist convection on mesoscale KE spectrum are concentrated on energizing the mesoscale at the upper troposphere and the lower stratosphere through buoyancy production. BT spectra for the control simulation have a similar shallow slope in the mesoscale as that for the observations. The greater spectral power of BT for the control simulation compared to the observed is attributed to the dislocation and higher intensity of simulated convection. The observed BT spectra exhibit a large diurnal variability due to the diurnal variation of the intensity of convection. The simulated BT spectrum is dependent on convective systems at different scales. Deep convection in the intertropical convergence zone (ITCZ) and shallow convection in the North Pacific storm-track region play an important role in energizing the convective scale of the BT spectrum. In the mesoscale, the BT spectrum is mainly energized by mesoscale convective systems (MCSs) in the ITCZ. Tropical equatorial waves and baroclinic waves in the southern midlatitudes are critical in producing the shallow slope near −5/3 and providing energy in the BT spectrum at the synoptic scale. Significance Statement: We further explore the role of deep moist convection in kinetic energy and brightness temperature spectra through high-resolution radiance observations and convection-permitting simulations. Moist processes can energize the mesoscale of kinetic energy. Brightness temperature spectra show dependence on convective systems at different scales. These results point the way toward a new approach to evaluate the predictability of convective systems, and future development of model dynamics and parameterization. [ABSTRACT FROM AUTHOR]

Rainfall projections from the Coupled Model Intercomparison Project (CMIP) models are strongly tied to projected sea surface temperature (SST) spatial patterns through the "warmer-gets-wetter" mechanism. While these models consistently project an enhanced equatorial warming, they, however, indicate much more uncertain changes in zonal SST gradients. That translates into large uncertainties on rainfall projections. Here, we force an atmospheric model with synthetic SSTs whose zonal SST gradient changes span the range of CMIP5 uncertainties in the presence and in the absence of the robust equatorially enhanced warming. Our results confirm that projected rainfall changes are dominated by the effect of circulation changes, which are tied to SST through the "warmer-gets-wetter" mechanism. We show that SPCZ rainfall changes are entirely driven by the uncertain zonal SST gradient changes. The western equatorial Pacific rainfall increase is largely controlled by the robust enhanced equatorial warming for modest zonal SST gradient changes. However, for larger values, the effect of the zonal SST gradient change on rainfall projections becomes dominant due to nonlinear interactions with the enhanced equatorial warming. Overall, our study demonstrates that uncertainties in the zonal SST gradient changes strongly contribute to uncertainties in rainfall projections over both the South Pacific convergence zone and western equatorial Pacific. It is thus critical to reduce these uncertainties to produce more robust precipitation estimates. [ABSTRACT FROM AUTHOR]

Based on the abrupt-4XCO2 scenario in phase 6 of the Coupled Model Intercomparison Project (CMIP6), this study investigates the response of the rainfall changes to El Niño–like SST warming and the role of ocean dynamical processes in the salinity changes in the tropical Pacific. The results show that the Walker circulation weakening and eastward shift, related to El Niño–like SST warming, dominates the zonal precipitation change. Precipitation decreases (increases) in the Maritime Continent (the equatorial Pacific), partly offsetting the effect of specific humidity. At the same time, the El Niño–like warming triggers convergence of meridional winds, which leads to a precipitation increase in the equatorial Pacific and a decrease in the intertropical convergence zone and the South Pacific convergence zone, following the "warmer-get-wetter" mechanism. Unlike the spatial pattern of precipitation changes, the sea surface salinity changes become fresher in the tropical western Pacific, related to the precipitation and the mean horizontal advection. The precipitation increase leads to negative salinity anomalies in the equatorial central Pacific. The westward climatological zonal currents transport the negative salinity anomalies westward. The meridional currents advect the salinity anomalies to both sides of the equator, partly offsetting the contribution of the freshwater flux on the salinity change. In addition, shallower mixed layer depth and weakening upwelling bring less high-salinity water to the surface and impact salinity redistribution through the vertical process in the equatorial regions. [ABSTRACT FROM AUTHOR]

The subseasonal variation of the anomalous western North Pacific anticyclone (WNPAC) has important implications for East Asian summer monsoon variability. How the WNPAC evolves on the subseasonal time scale under the different configurations of tropical North Atlantic (TNA) Ocean and north Indian Ocean (NIO) SST warming is elucidated in this study. The WNPAC forced by individual TNA SST warming shows an obvious subseasonal variation with a stepwise northward movement. In contrast, the WNPAC forced by individual NIO SST warming shows a weak subseasonal variation, being nearly stabilized at around 208N from June to August and thereby causing long-lasting and intense positive mei-yu-baiu-changma rainfall anomalies. The physical mechanism for the different subseasonal variation of WNPAC is further investigated. The TNA SST warming generates a WNPAC via a Rossby wave-induced divergence/convergence chain response. In this process, the TNA SST warming-induced suppressed convection over the western Pacific moves northward with the northward movement of climatological intertropical convergence zone and summer monsoon region, which generates a northward shift of the WNPAC. However, the NIO SST warming produces a WNPAC via a Kelvin wave-induced suppressed convection over the western Pacific Ocean. This suppressed convection is stabilized at around 208N because of the Kelvin wave activity scope being limited within 208N, which finally produces a nearly stationary WNPAC from June to August. In addition, under the simultaneous occurrence of the TNA and NIO SST warming, the subseasonal variation of WNPAC bears a resemblance to that for the individual NIO SST warming condition, where the TNA SST warming fails to exert its impact. [ABSTRACT FROM AUTHOR]

Rainfall characteristics are crucial in monsoon regions, in particular for agriculture. Crop yields indeed depend on the rainfall seasonal amounts, but also on other rainfall characteristics such as the onset of the rainy season or the distribution of rainy days. In the Sahel region, while the average amount of seasonal rainfall has been shown to be marked by strong decadal variability, the modulation of rainfall characteristics has received less attention in the literature so far. In this study, we show that the frequency of light, heavy, and extreme rainfall events and the mean intensity of rainfall events in Senegal exhibit a marked decadal variability over the 1918–2000 period, strongly similar to that of the mean seasonal rainfall. The decadal modulations of these events show a strong and positive link with the Atlantic multidecadal variability (AMV). Indeed, positive sea surface temperature anomalies over the North Atlantic and Mediterranean related to a warm AMV phase are associated with negative sea level pressure anomalies over the northern Atlantic and a northward shift of the intertropical convergence zone. We also find that the onset and cessation dates as well as the length of the rainy season show relatively less decadal variability, which is more related to the interdecadal Pacific oscillation (IPO), a positive phase of the latter leading to a late onset, an early cessation, and an overall shorter rainy season in Senegal. [ABSTRACT FROM AUTHOR]

The Indian Ocean has an intriguing intertropical convergence zone (ITCZ) south of the equator year-round, which remains largely unexplored. Here we investigate this Indian Ocean ITCZ and the mechanisms for its origin. With a weak semiannual cycle, this ITCZ peaks in January–February with the strongest rainfall and southernmost location and a northeast–southwest orientation from the Maritime Continent to Madagascar, reaches a minimum around May with a zonal orientation, grows until its secondary maximum around September with a northwest–southeast orientation, weakens slightly until December, and then regains its mature phase in January. During austral summer, the Indian Ocean ITCZ exists over maximum surface moist static energy (MSE), consistent with convective quasi-equilibrium theory. This relationship breaks up during boreal summer when the surface MSE maximizes in the northern monsoon region. The position and orientation of the Indian Ocean ITCZ can be simulated well in both a linear dynamical model and the state-of-the-art Community Atmosphere Model version 6 (CAM6) when driven by observed sea surface temperature (SST). To quantify the contributions of the planetary boundary layer (PBL) and free-atmosphere processes to this ITCZ, we homogenize the free-atmosphere diabatic heating over the Indian Ocean in CAM6. In response, the ITCZ weakens significantly, owing to a weakened circulation and deep convection. Therefore, in CAM6, the SST drives the Indian Ocean ITCZ directly through PBL processes and indirectly via free-atmosphere diabatic heating. Their contributions are comparable during most seasons, except during the austral summer when the free-atmosphere diabatic heating dominates the mature-phase ITCZ. [ABSTRACT FROM AUTHOR]

The western North Pacific anomalous anticyclone (WNPAC) is the most important interannual circulation anomaly over the western Pacific warm pool. It can persist from boreal winter to the following summer and has great impacts on the East Asian monsoon. The maintenance of the WNPAC in boreal summer was explained by Ekman divergence anomalies over the western North Pacific (WNP) induced by the equatorial easterly Kelvin waves from the tropical Indian Ocean and the Maritime Continent. In this study, we propose a new mechanism for the maintenance of the WNPAC in the season, which we refer to as the "wind-induced moist enthalpy advection" (WIMEA) mechanism. Warm anomalies over the tropical Indian Ocean enhance local deep convection and thus excite atmospheric easterly Kelvin waves to the east. Climatological moist enthalpy decreases eastward over the WNP due to the local northward extension of the intertropical convergence zone (ITCZ) with the establishment of the WNP summer monsoon. Hence, the easterly anomalies transport low moist enthalpy (dry) air westward to the WNP. The induced negative moist enthalpy advection anomalies drive downward motion anomalies and thus suppress deep convection over the WNP under the constraint of the moist static energy budget balance. This moist teleconnection mechanism does not rely on Ekman processes. Numerical experiments using a dry linear baroclinic model indicate that the WIMEA and the conventional Kelvin wave–induced Ekman divergence mechanism have nearly equal relative contributions to the maintenance of the summer WNPAC, with the former mainly driven by the enhanced convection over the tropical Indian Ocean, while the latter is driven by that over the Maritime Continent. [ABSTRACT FROM AUTHOR]

In this study, we identified the origin of the moisture associated with the tropical cyclones' (TCs) precipitation in the North Atlantic Ocean basin during their three well-differentiated life stages between 1980 and 2018. The HURDAT2 database was used to detect the location of 598 TCs during their genesis, maximum intensification peak, and dissipation phases. The global outputs of the Lagrangian FLEXPART model were then used to determine the moisture sources. Using a k-means cluster analysis technique, seven different regions were identified as the most common locations for the genesis and maximum intensity of the TC phases, while six regions were found for the dissipation points. Our results showed that the origin of moisture precipitating was not entirely local over the areas of TC occurrence. The North Atlantic Ocean to the north of the intertropical convergence zone at 10°N (NATL)—especially from tropical latitudes, the Caribbean Sea, and the Gulf of Mexico—provides most of the moisture for TCs (∼87%). The Atlantic Ocean basin southward of the ITCZ (SATL) played a nonnegligible role (∼11%), with its contribution being most pronounced during the TC genesis phase, while the eastern tropical Pacific Ocean made the smallest contribution (∼2%). The moisture supported by TCs varied depending on their category, being higher for hurricanes than for major hurricanes or tropical storms. Additionally, the approach permitted the estimation of the mean residence time of the water vapor uptake that produces the precipitation during TC activity, which ranged between 2.6 and 2.9 days. Significance Statement: Atmospheric moisture transport plays an important role in the genesis and intensification of tropical cyclones (TCs). In this study, we investigated the moisture source for the genesis, intensification, and dissipation of TCs in the North Atlantic Ocean basin using a Lagrangian approach. This model allowed us to track air masses backward in time from the target area to identify regions where air masses experienced an uptake of moisture prior to reaching the area of interest. The sources were identified individually for each TC, and the results were then combined to provide a broad general picture with some surprising outstanding results, such as the role of the North and South Atlantic and the eastern tropical Pacific as important moisture sources during the different TCs phases and intensities. [ABSTRACT FROM AUTHOR]

The latitudinal location of the east Pacific Ocean intertropical convergence zone (ITCZ) changes on time scales of days to weeks during boreal spring. This study focuses on tropical near-surface dynamics in the days leading up to the two most frequent types of ITCZ events, nITCZ (Northern Hemisphere) and dITCZ (double). There is a rapid daily evolution of dynamical features on top of a slower, weekly evolution that occurs leading up to and after nITCZ and dITCZ events. Zonally elongated bands of anomalous cross-equatorial flow and off-equatorial convergence rapidly intensify and peak 1 day before or the day of these ITCZ events, followed 1 or 2 days later by a peak in near-equatorial zonal wind anomalies. In addition, there is a wide region north of the southeast Pacific subtropical high where anomalous northwesterlies strengthen prior to nITCZ events and southeasterlies strengthen before dITCZ events. Anomalous zonal and meridional near-surface momentum budgets reveal that the terms associated with Ekman balance are of first-order importance preceding nITCZ events, but that the meridional momentum advective terms are just as important before dITCZ events. Variations in cross-equatorial flow are promoted by the meridional pressure gradient force (PGF) prior to nITCZ events and the meridional advection of meridional momentum in addition to the meridional PGF before dITCZ events. Meanwhile, variations in near-equatorial easterlies are driven by the zonal PGF and the Coriolis force preceding nITCZ events and the zonal PGF, the Coriolis force, and the meridional advection of zonal momentum before dITCZ events. [ABSTRACT FROM AUTHOR]

Long-term continuous monitoring of precipitation isotopes has great potential to advance our understanding of hydrometeorological processes that determine stable isotope variability in the monsoon regions. This study presents 4-yr daily precipitation isotopes from Yungui Plateau in southwestern China that are influenced by Indian summer monsoon and East Asian monsoon. The local meteoric water line (LMWL; δ2H = 8.12δ18O + 11.2) was first established at the Tengchong (TC) site, which was close to the global meteoric water line (GMWL; δ2H = 8δ18O + 10), indicating little secondary subcloud evaporation in the falling rain. Precipitation δ18O values exhibited significant inverse relationships with precipitation amount (r = −0.42), air temperature (r = −0.43), and relative humidity (r = −0.41) with lower correlation coefficients throughout the entire period, which indicated that precipitation isotopic variability in TC could not be well explained by the local meteorological factors but influenced by other combined factors of regional precipitation amount and upstream rainout. Precipitation δ18O values showed a clear V-shaped trend throughout the observation period, characterized by higher δ18O values during the premonsoon period whereas lower values during the postmonsoon period. This seasonal variation of precipitation δ18O values was associated with the seasonal movement of the intertropical convergence zone and seasonal changes in moisture transport. Combined with backward trajectory analysis, precipitation δ18O values were estimated by a Rayleigh distillation model showing that upstream rainout processes from the Bay of Bengal (BoB) toward land (Myanmar) and recycling moisture over land were key factors affecting the isotopic compositions of the TC precipitation. These findings could enhance our understanding of atmospheric dynamics and moisture source in the monsoon regions and will potentially facilitate the interpretation of numerous isotopic proxy records from this region. SIGNIFICANCE STATEMENT: The variability of the summer monsoon and its onset, duration, and failure directly determine the strong rainfall and drought in a given region and have great impacts on regional societies and agriculture. To better understand this variability, this study presented a 4-yr daily dataset of precipitation isotopes on the Yungui Plateau of southwestern China to explore atmospheric processes and moisture sources that drive isotopic variability in this region. Precipitation δ18O exhibited remarkably seasonal variability, with higher values in premonsoon period and lower values in the postmonsoon period. During the Indian summer monsoon period, moisture sources primarily originated from the BoB toward the TC site, experiencing rainout processes and local moisture recycling over land using a Rayleigh fractionation model. These findings shed new light on the temporal variations of precipitation stable isotopes and facilitate our understanding of hydrological cycle in the monsoon regions. [ABSTRACT FROM AUTHOR]

Energetic constraints on the time-dependent response of the intertropical convergence zone (ITCZ) to volcanic eruptions are analyzed using the Community Earth System Model Last Millennium Ensemble project. The energetic constraints are found to vary during the first few years, governed by conjoined variations of the energy budgets of the stratosphere, troposphere, and oceans. Specifically, following eruptions, sulfate aerosols heat the stratosphere by longwave absorption and cool the surface by shortwave reflection, leading to contrasting energy transport anomalies in the stratosphere and troposphere, which are of comparable strength during the first year. Similar contrasting responses are also seen by the mean and eddy components of atmospheric energy transport (AET). Consequently, ocean energy transport (OET) dominates the anomalous total interhemispheric energy transport during the first year. However, wind-driven OET, generally assumed to constrain shifts of the ITCZ, has a negligible role in the transient ocean response. Consistent with theory, anomalous cross-equatorial tropospheric energy transport, dominated by the anomalous Hadley circulation, is strongly negatively correlated with ITCZ shifts. However, due to the strong anomalous stratospheric energy fluxes, the commonly used energy flux equator (derived from net AET) is a poor predictor of transient ITCZ shifts following eruptions. El Niño–like conditions typically appear during the second year after eruptions, and La Niña–like conditions appear after the third year. These variations modulate ITCZ shifts in a complex manner, via changes in surface conditions and in associated energy transport variations in the atmosphere and oceans. Significance Statement: Volcanic eruptions can lead to strong climatic impacts, which include global cooling and shifts of the tropical rain belt, with major socioeconomic implications. The global impacts of volcanic events motivate potential future geo-engineering applications, based on mimicking the effect of volcanic eruptions by artificial aerosol loading of the stratosphere. Understanding the transient and long-term response to volcanic eruptions is therefore of prime importance. Here we analyze energetic constraints on time-dependent shifts of the tropical rain belt in response to volcanic eruptions. In contrast to previous findings, we show that due to contrasting stratospheric and tropospheric responses, net atmospheric energy transport is a poor predictor for ITCZ shifts during the first year following eruptions. Instead, we elucidate the roles of components of atmospheric and oceanic energy transport in setting the transient response. El Niño–like conditions typically appear during the second year after eruptions, and La Niña–like conditions appear after the third year. These variations affect the tropical rain belt in a complex manner, via changes in surface conditions and in associated energy transport variations in the atmosphere and oceans. [ABSTRACT FROM AUTHOR]

Easterly waves (EWs) are off-equatorial tropical synoptic disturbances with a westward phase speed between 11 and 14 m s−1. Over the east Pacific in boreal summer, the combination of EWs and other synoptic disturbances, plus local mechanisms associated with sea surface temperature (SST) gradients, define the climatological structure of the intertropical convergence zone (ITCZ). The east Pacific ITCZ has both deep and shallow convection that is linked to deep and shallow meridional circulations, respectively. The deep convection is located around 9°N over warm SSTs. The shallow convection is located around 6°N and is driven by the meridional SST gradient south of the ITCZ. This study aims to document the interaction between east Pacific EWs and the deep and shallow meridional circulations during the Organization of Tropical East Pacific Convection (OTREC) field campaign in 2019 using field campaign observations, ERA5, and satellite precipitation. We identified three EWs during the OTREC period using precipitation and dynamical fields. Composite analysis shows that the convectively active part of the EW enhances ITCZ deep convection and is associated with an export of column-integrated moist static energy (MSE) by vertical advection. The subsequent convectively suppressed, anticyclonic part of the EW produces an increase of moisture and column-integrated MSE by horizontal advection that likely enhances shallow convection and the shallow overturning flow at 850 hPa over the southern part of the ITCZ. Therefore, EWs appear to strongly modulate shallow and deep circulations in the east Pacific ITCZ. [ABSTRACT FROM AUTHOR]

Diurnal variation in surface latent heat flux (LHF) and the effects of diurnal variations in LHF-related variables on the climatological LHF are examined using observations from the Global Tropical Moored Buoy Array. The estimated amplitude of the climatological diurnal LHF over the Indo-Pacific warm pool and the equatorial Pacific and Atlantic cold tongues is remarkable, with maximum values exceeding 20.0 W m−2. Diurnal variability of sea surface skin temperature (SSTskin) is the primary contributor to the diurnal LHF amplitude. Because the diurnal SSTskin amplitude has an inverse relationship with surface wind speed over the tropical oceans, an inverse spatial pattern between the diurnal LHF amplitude and surface wind speed results. Resolving diurnal variations in the SSTskin and wind improves the estimate of the climatological LHF by properly capturing the daytime SSTskin and daily mean wind speed, respectively. The diurnal SSTskin-associated contribution is large over the warm pool and equatorial cold tongues where low wind speeds tend to cause strong diurnal SSTskin warming, while the magnitude associated with the diurnal winds is large over the highly dynamic environment of the intertropical convergence zone. The total diurnal contribution is about 9.0 W m−2 on average over the buoy sites. There appears to be a power function (linear) relationship between the diurnal SSTskin-associated (wind-associated) contribution and surface mean wind speed (wind speed enhancement from diurnal variability). The total contribution from diurnal variability can be estimated accurately from high-frequency surface wind measurements using these relationships. [ABSTRACT FROM AUTHOR]

A large fraction of tropical cyclones (TCs) are generated near the intertropical convergence zone (ITCZ), and barotropic instability of the related wind shear has been shown to be an important generation mechanism. The latitudinal position of the ITCZ shifts seasonally and may shift poleward in response to global warming. Aquaplanet GCM simulations have shown TC-generation frequency to vary with position of the ITCZ. These results, and that moisture plays an essential role in the dynamics, motivate the present study on the growth rates of barotropic instability in ITCZ-like zonal wind profiles. Base-state zonal wind profiles are generated by applying a prescribed forcing (representing zonally averaged latent heat release in the ITCZ) to a shallow-water model. Shifting the latitudinal position of the forcing alters these profiles, with a poleward shift leading to enhanced barotropic instability. Next, an examination of how latent release impacts the barotropic breakdown of these profiles is considered. To do this, moisture is explicitly represented using a tracer variable. Upon supersaturation, precipitation occurs and the related latent heat release is parameterized as a mass transfer out of the dynamically active layer. Whether moisture serves to enhance or reduce barotropic growth rates is found to depend on how saturation humidity is represented. In particular, taking it to be constant or a function of the layer thickness (related to temperature) leads to a reduction, whereas taking it to be a specified function of latitude leads to an enhancement. Simple arguments are given to support the idea that moisture effects should lead to a reduction in the moist shallow-water model and that a poleward shift of the ITCZ should lead to an enhancement of barotropic instability. [ABSTRACT FROM AUTHOR]

Changes in land surface albedo and land surface evaporation modulate the atmospheric energy budget by changing temperatures, water vapor, clouds, snow and ice cover, and the partitioning of surface energy fluxes. Here idealized perturbations to land surface properties are imposed in a global model to understand how such forcings drive shifts in zonal mean atmospheric energy transport and zonal mean tropical precipitation. For a uniform decrease in global land albedo, the albedo forcing and a positive water vapor feedback contribute roughly equally to increased energy absorption at the top of the atmosphere (TOA), while radiative changes due to the temperature and cloud cover response provide a negative feedback and energy loss at TOA. Decreasing land albedo causes a northward shift in the zonal mean intertropical convergence zone (ITCZ). The combined effects on ITCZ location of all atmospheric feedbacks roughly cancel for the albedo forcing; the total ITCZ shift is comparable to that predicted for the albedo forcing alone. For an imposed increase in evaporative resistance that reduces land evaporation, low cloud cover decreases in the northern midlatitudes and more energy is absorbed at TOA there; longwave loss due to warming provides a negative feedback on the TOA energy balance and ITCZ shift. Imposed changes in land albedo and evaporative resistance modulate fundamentally different aspects of the surface energy budget. However, the patterns of TOA radiation changes due to the water vapor and air temperature responses are highly correlated for these two forcings because both forcings lead to near-surface warming. [ABSTRACT FROM AUTHOR]

Rainfall over tropical South America is known to be modulated by convectively coupled Kelvin waves (CCKWs). In this work, the origin and dynamical features of South American Kelvin waves are revisited using satellite-observed brightness temperature, radiosonde, and reanalysis datasets. Two main types of CCKWs over the Amazon are considered: Kelvin waves with a Pacific precursor, and Kelvin waves with a precursor originating over South America. Amazonian CCKWs associated with a preexisting Kelvin convection in the eastern Pacific account for about 35% of the total events. The cases with South American precursors are associated with either pressure surges in the western Amazon from extratropical wave train activity, responsible for 40% of the total events, or "in situ" convection that locally excites CCKWs, accounting for the remaining 25%. The analysis also suggests that CCKWs associated with different precursors are sensitive to Pacific sea surface temperature. Kelvin wave events with a Pacific precursor are more common during ENSO warm events, while Kelvin waves with extratropical South American precursors show stronger activity during La Niña events. This study also explores other triggering mechanisms of CCKWs over the Amazon. These mechanisms are associated with 1) extratropical Rossby wave trains not necessarily of extratropical South American origin; 2) CCKWs initiated in response to the presence of the southern and/or double intertropical convergence zone (ITCZ) in the eastern Pacific Ocean; and 3) possible interaction between CCKWs and other equatorial waves in the Amazon. [ABSTRACT FROM AUTHOR]

This study examines the temporal evolution of the extratropically forced tropical response in an idealized aquaplanet model under equinox condition. We apply a surface thermal forcing in the northern extratropics that oscillates periodically in time. It is shown that tropical precipitation is unaltered by sufficiently high-frequency extratropical forcing. This sensitivity to the extratropical forcing periodicity arises from the critical time required for sea surface temperature (SST) adjustment. Low-frequency extratropical forcing grants sufficient time for atmospheric transient eddies to diffuse moist static energy to perturb the midlatitude SSTs outside the forcing region, as demonstrated by a one-dimensional energy balance model with a fixed diffusivity. As the transient eddies weaken in the subtropics, a further equatorward advection is accomplished by the Hadley circulation. The essential role of Hadley cell advection in connecting the subtropical signal to the equatorial region is supported by an idealized thermodynamical-advective model. Associated with the SST changes in the tropics is a meridional shift of the intertropical convergence zone. Since the time needed for SST adjustment increases with increasing mixed layer depth, the critical forcing period at which the extratropical forcing can affect the tropics scales linearly with the mixed layer depth. Our results highlight the important role of decadal-and-longer extratropical climate variability in shaping the tropical climate system. We also raise the possibility that the transient behavior of a tropical response forced by extratropical variability may be strongly dependent on cloud radiative effects. [ABSTRACT FROM AUTHOR]

The TRACMIP (Tropical Rain Belts with an Annual Cycle and Continent Model Intercomparison Project) ensemble includes slab-ocean aquaplanet controls and experiments with a highly idealized tropical continent, characterized by modified aquaplanet grid cells with increased evaporative resistance, increased albedo, reduced heat capacity, and no ocean heat transport (zero Q-flux). In the annual mean, an equatorial cold tongue develops west of the continent and induces dry anomalies and a split in the oceanic intertropical convergence zone (ITCZ). Ocean cooling is initiated by advection of cold, dry air from the winter portion of the continent; warm, humid anomalies in the summer portion are restricted to the continent by anomalous surface convergence. The surface energy budget suggests that ocean cooling persists and intensifies because of a positive feedback between a colder surface, drier and colder air, reduced downwelling longwave (LW) flux, and enhanced net surface LW cooling (LW feedback). A feedback between wind, evaporation, and SST (so-called WES feedback) also contributes to the establishment and maintenance of the cold tongue. Simulations with a gray-radiation model and simulations that diverge from protocol (with negligible winter cooling) confirm the importance of moist-radiative feedbacks and of rectification effects on the seasonal cycle. This mechanism coupling the continental and oceanic climate might be relevant to the double ITCZ bias. The key role of the LW feedback suggests that the study of interactions between monsoons and oceanic ITCZs requires full-physics models and a hierarchy of land models that considers evaporative processes alongside heat capacity as a defining characteristic of land. [ABSTRACT FROM AUTHOR]

Investigating Pacific meridional modes (PMMs) without the influence of tropical Pacific variability is technically difficult if based on observations or fully coupled model simulations due to their overlapping spatial structures. To confront this issue, the present study investigates both the North PMM (NPMM) and South PMM (SPMM) in terms of their associated atmospheric forcing and response processes based on a mechanically decoupled climate model simulation. In this experiment, the climatological wind stress is prescribed over the tropical Pacific, which effectively removes dynamically coupled tropical Pacific variability (e.g., El Niño–Southern Oscillation). Interannual NPMM in this experiment is forced not only by the North Pacific Oscillation but also by a North Pacific tripole (NPT) pattern of atmospheric internal variability, which primarily forces decadal NPMM variability. Interannual and decadal variability of the SPMM is partly forced by the South Pacific Oscillation. In turn, both interannual and decadal NPMM variability can excite atmospheric teleconnections over the Northern Hemisphere extratropics by influencing the meridional displacement of the climatological intertropical convergence zone throughout the whole year. Similarly, both interannual and decadal SPMM variability can also excite atmospheric teleconnections over the Southern Hemisphere extratropics by extending or shrinking the climatological South Pacific convergence zone in all seasons. Our results highlight a new poleward pathway by which both the NPMM and SPMM feed back to the extratropical climate, in addition to the equatorward influence on tropical Pacific variability. [ABSTRACT FROM AUTHOR]