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Abstract The climate response of tropical Southeast Asia to abrupt climate change originating from high northern latitudes remains poorly understood. Here, marine sediments recovered from the Northwest Borneo Trough are analyzed to trace variations in terrigenous material composition associated with sea level and palaeoflood activities over the past 40 ka. We describe for the first time the influence of sea level change on terrigenous deposition from a tropical island with small mountainous rivers surrounded by narrow continental shelves and the coupling between North Atlantic dynamics and climate feedbacks in tropical Southeast Asia during the Dansgaard‐Oeschger stadials. This study demonstrates that climate oscillations occurring at high northern latitudes can be transmitted to tropical Southeast Asia through the functions of sea level, the East Asian winter monsoon and the Intertropical Convergence Zone. Such information is fundamental to correctly assessing future sea level rise and flood risks in tropical Southeast Asia.

Abstract The Hadley Circulation and associated westerlies strengthened and moved equatorward across the mid‐Pleistocene transition (MPT). However, the evolution of the intertropical convergence zone (ITCZ) is still elusive due to the scarcity of long‐term hydrological records from regions sensitive to the ITCZ change. Here, high‐resolution sea surface salinity estimates derived from surface‐dwelling planktic foraminiferal δ18O and Mg/Ca in Ocean Drilling Program Site 871 reveal a long‐term freshening trend in the central equatorial Pacific across the MPT. We attribute this secular reorganization of the precipitation‐evaporation balance to the gradual southward migration and intensification of ITCZ in the North Pacific. It is inferred that the long‐term evolution of the ITCZ was modulated by the increased meridional sea surface temperature gradients and the enhancements of trade winds across the MPT.

Abstract An idealized aquaplanet moist global atmospheric model with realistic radiative transfer but no clouds and no convective parameterization is found to possess multiple climate equilibria. When forced symmetrically about the equator, in some cases the Inter Tropical Convergence Zone (ITCZ) migrates to an off‐equatorial equilibrium position. Mechanism denial experiments prescribing relative humidity imply that radiation‐circulation coupling is essential to this instability. The cross‐equatorial asymmetry occurs only when the underlying slab ocean is sufficiently deep and the atmosphere's spectral dynamical core is sufficiently coarse (∼T170 or less with our control parameters). At higher resolutions, initializing with an asymmetric state indicates metastability with very slow (thousands of days) return to hemispheric symmetry. There is some sensitivity to the model timestep, which affects the time required to transition to the asymmetric state, with little effect on the equilibrium climate. The instability is enhanced when the planetary boundary layer scheme favors deeper layers or by a prescribed meridional heat transport away from the equator within the slab. The instability is not present when the model is run with a convective parameterization scheme commonly utilized in idealized moist models. We argue that the instability occurs when the asymmetric heating associated with a spontaneous ITCZ shift drives a circulation that rises poleward of the perturbed ITCZ. These results serve as a warning of the potential for instability and non‐uniqueness of climate that may complicate studies with idealized models of the tropical response to perturbations in forcing.

Whether the South Asian summer monsoon (SASM) is controlled by local or remote insolation at the orbital band remains uncertain. Here, we perform a transient simulation forced by Earth's orbital parameters between 400 and 350 ka BP, a period characterized by significant contrast between local and remote insolation, to identify the SASM's response to insolation forcing. Simulation results suggest that the primary driver of orbital‐scale SASM variability is the Northern Hemisphere high‐latitude June insolation, as opposed to local insolation. High June insolation in the Southern Hemisphere might reduce the SASM intensity. Remote insolation influences the SASM by altering the latitudinal thermal gradient and, consequently, the meridional position of the South Asian high (SAH). The SAH is associated with intense convection and hence drives the meridional shift of the intertropical convergence zone and the SASM rain belt. Thus, orbital‐scale SASM variability is strongly related to remote insolation forcing. Plain Language Summary: Because the monsoon is directly related to the movement of the subsolar point, insolation changes are important in regulating the South Asian summer monsoon (SASM) intensity at an orbital timescale. However, it remains unclear whether the SASM is driven by local insolation that directly strikes southern Asia, or remote insolation in the high‐latitude Northern Hemisphere or the Southern Hemisphere at the precession band. We find a time window between 400 and 350 ka BP (ka = thousand years, BP = before present) when local and remote insolations differ significantly, allowing us to determine which insolation dominates the SASM at orbital timescales. We use an Earth system model with orbital forcing to run a transient simulation between 400 and 350 ka BP. Simulations suggest that the SASM is primarily influenced by Northern Hemisphere high‐latitude summer insolation rather than local insolation. Additionally, Southern Hemisphere insolation has a negative impact on orbital‐scale SASM variability. Remote insolation influences the SASM by inducing an anomalous meridional thermal gradient, which moves the South Asian high and thus the Intertropical Convergence Zone. Our findings are significant for future SASM projections because the obvious difference between remote and local insolation will reappear in the next 50 ka. Key Points: Local insolation is not the direct driver for orbital‐scale variability of the South Asian summer monsoon (SASM)The meridional shift of the South Asian high (SAH) affects orbital‐scale SASM variabilityRemote insolation influences the meridional shift in the SAH through altering the latitudinal thermal contrast [ABSTRACT FROM AUTHOR]

The El Niño Southern Oscillation (ENSO) in the tropical Pacific Ocean is an important driver of winter precipitation variability over western North America as a whole, but ENSO exhibits a weak and inconsistent relationship with precipitation in several critically important headwaters including the upper Colorado River Basin. We present interactions between North Atlantic sea surface temperatures (SSTs) and ENSO that influence western U.S. precipitation, accounting for substantial variability in areas where ENSO alone yields limited guidance. Specifically, we performed a statistical analysis on hemispheric SSTs and western U.S. winter precipitation in a century of observations and a 10,000-year perpetual current-climate simulation. In both frameworks, the leading coupled pattern is ENSO, and the second pattern links an Atlantic Quadpole Mode (AQM) of SST variability to precipitation anomalies over most of the western U.S., including the transition zone where ENSO provides little predictability. The AQM SST anomalies are expansive in latitude, but its primary mechanism appears to involve a strengthening/shifting of the intertropical convergence zone (ITCZ) over northern South America and the tropical Atlantic. The ENSO pattern accounts for a larger fraction of the total covariance between SSTs and precipitation (65% versus 12% for the AQM pattern), but the percent anomalies of precipitation associated with ENSO and the AQM are comparable in magnitude, meaning 20% or larger over much of the western U.S. The interaction between ENSO and AQM influences precipitation across the western U.S., with cold AQM generally reducing precipitation irrespective of ENSO whereas warm AQM increases the amount of precipitation and the area of influence of ENSO; knowledge of these interactions can increase predictability of western U.S. precipitation. [ABSTRACT FROM AUTHOR]

The mesoscale orography over one of the wettest regions of the world makes the Northeast India (NEI) vulnerable to hydrological disasters while sustaining a biodiversity "hotspot." The monsoon rainy season over the NEI is known to be longer than June–September (JJAS), but an objective definition has been lacking. Understanding the drivers and predictability of rainfall variability over the region is key for sustainable development planning and adaptation to increasing disasters in the backdrop of a warming climate but remained poor due to lack of a working definition of "summer monsoon season" over NEI. Here, using the relationship between rainfall over the region and winds at 850 hPa over North Bay of Bengal (BoB), we provide an objective definition of the monsoon season over NEI and argue that the "summer monsoon rainy season" over the NEI (NEIR) is from May to September (MJJAS). In contrast to a significant negative relationship between JJAS Central India (CI) rainfall and a JJAS El Niño–Southern Oscillation (ENSO) index, the MJJAS rainfall over the NEI has no relationship with MJJAS or JJAS ENSO. Instead, we unravel that (a) the tropical Northwest‐Pacific (TNWP) sea surface temperature (SST), (b) the South Equatorial Indian Ocean Dipole (SEIOD) and (c) the Atlantic Zonal mode (AZM) are potential drivers of the NEIR variability. Using a causal inference algorithm, namely the Peter and Clark Momentary Conditional Independence (PCMCI) method, we show that SEIOD directly influences NEIR while TNWP SST has a two‐way connection with the NEIR. While "internal variability" may be higher over the NEI, significant modulation of the variances of the subseasonal fluctuations by predictable drivers like the Atlantic Niño and North Atlantic water temperature provides optimism for seasonal prediction of the NEIR. [ABSTRACT FROM AUTHOR]

Climate variability in the Tropical Atlantic is complex with strong ocean–atmosphere coupling, where the sea surface temperature variability impacts the hydroclimate of the surrounding continents. Most of the Tropical Atlantic interannual variability is explained by its equatorial (Atlantic Zonal Mode, AZM) and meridional (Atlantic Meridional Mode, AMM) modes of variability. Our results using an ensemble of 37 models from the Coupled Model Intercomparison Project Phase 6 historical simulations show multidecadal changes in the Tropical Atlantic variability from 1900 to 2014. Within it, most models (at least 60%) show a decrease in variability after the 1970 s, in accordance with observational data sets. The agreement among simulations points out the role of the external forcing. After 1970, an anthropogenically induced warming trend is observed in the equatorial Atlantic accompanied by a weakening of the winds. This drives a weakening in the Bjerknes Feedback by deepening the thermocline in the eastern equatorial Atlantic, reduced SST sensitivity to thermocline anomalies, thus decreasing AZM variability. The interplay of the meridional asymmetric warming in the Tropical Atlantic related to the anthropogenic forcing, the relaxed northeast trade winds, and the background state of the negative Atlantic Multidecadal Variability, decreases AMM variability despite the individual increase in variability of the North and South Tropical Atlantic. Associated with these factors the African Sahel shows a positive precipitation trend and the Intertropical Convergence Zone tends to shift northward, which reinforces the increased precipitation. [ABSTRACT FROM AUTHOR]

Numerical simulations employing prognostic stable water isotopes can not only facilitate our understanding of hydrological processes and climate change but also allow for a direct comparison between isotope signals obtained from models and various archives. In the current work, we describe the performance and explore the potential of a new version of the Earth system model AWI-ESM (Alfred Wegener Institute Earth System Model), labeled AWI-ESM-2.1-wiso, in which we incorporated three isotope tracers into all relevant components of the water cycle. We present here the results of pre-industrial (PI) and mid-Holocene (MH) simulations. The model reproduces the observed PI isotope compositions in both precipitation and seawater well and captures their major differences from the MH conditions. The simulated relationship between the isotope composition in precipitation (δ18 O p) and surface air temperature is very similar between the PI and MH conditions, and it is largely consistent with modern observations despite some regional model biases. The ratio of the MH–PI difference in δ18 O p to the MH–PI difference in surface air temperature is comparable to proxy records over Greenland and Antarctica only when summertime air temperature is considered. An amount effect is evident over the North African monsoon domain, where a negative correlation between δ18 O p and the amount of precipitation is simulated. As an example of model applications, we studied the onset and withdrawal date of the MH West African summer monsoon (WASM) using daily variables. We find that defining the WASM onset based on precipitation alone may yield erroneous results due to the substantial daily variations in precipitation, which may obscure the distinction between pre-monsoon and monsoon seasons. Combining precipitation and isotope indicators, we suggest in this work a novel method for identifying the commencement of the WASM. Moreover, we do not find an obvious difference between the MH and PI periods in terms of the mean onset of the WASM. However, an advancement in the WASM withdrawal is found in the MH compared to the PI period due to an earlier decline in insolation over the northern location of Intertropical Convergence Zone (ITCZ). [ABSTRACT FROM AUTHOR]

The North Tropical Atlantic (NTA) mode is a dominant component of sea surface temperature (SST) variability over the northern tropical Atlantic, which is crucial to the occurrence of El Niño‐Southern Oscillation (ENSO). This study utilizes the historical simulation of the Coupled Model Intercomparison Project Phase 5 and 6 (CMIP5 and CMIP6) to evaluate the simulations of the NTA mode and NTA–ENSO connection. We found that the CMIP5 and CMIP6 could reproduce the spatial pattern of the NTA mode similarly. Most of them could reasonably reproduce the spatial pattern of the NTA mode similar to the observation. Still, there are some differences in the central values and position of the NTA mode. In addition, the analysis further suggests that the CMIP5/CMIP6 models have a large diversity in the NTA–ENSO connection, which may be due to their differences in simulating the spring climatological mean Atlantic Intertropical Convergence Zone (ITCZ) precipitation and spring‐to‐summer NTA SST persistence. Models with stronger spring climatological mean Atlantic ITCZ precipitation in favor of the spring NTA‐related SST anomalies stimulate local precipitation anomalies, resulting in a more powerful atmospheric circulation reaction over the tropical Pacific. Additionally, the stronger spring‐to‐summer NTA SST persistence for sustaining the teleconnection, which constantly transmits impacts from the tropical Atlantic into the tropical Pacific. The CMIP6 models show improvements to the CMIP5 models regarding simulations of the spring climatological mean Atlantic ITCZ precipitation and spring‐to‐summer NTA SST persistence. Therefore, the CMIP6 models simulate the NTA–ENSO connection better than the CMIP5 models. Plain Language Summary: El Niño‐Southern Oscillation (ENSO) significantly affects global climate variability through teleconnections. In particular, previous studies have suggested that ENSO impacts sea surface temperature (SST) variability over the North Tropical Atlantic (NTA). In addition, previous studies also showed that the NTA SST might feed back to the development of ENSO events and as a predictor for improving ENSO prediction. Therefore, evaluating the capability to capture the NTA mode and NTA–ENSO connection in the state‐of‐the‐art CMIP climate models gives some implications to improve the understanding of inter‐basin interactions and the ability of climate models to predict El Niño. The results show that the CMIP5 and CMIP6 could reproduce the spatial pattern of the NTA mode similarly. In addition, the CMIP5/CMIP6 models have a large diversity in the NTA–ENSO connection, which may be due to their differences in simulating the spring climatological mean Atlantic ITCZ precipitation and spring‐to‐summer NTA SST persistence. The CMIP6 models simulate the NTA–ENSO connection better than the CMIP5 models because the CMIP6 models show improvements to the CMIP5 models regarding simulations of the above two factors. Key Points: CMIP5 and CMIP6 could reproduce the spatial pattern of the North Tropical Atlantic (NTA) mode similarlyCMIP5/CMIP6 have large diversity in the NTA‐El Niño‐Southern Oscillation (ENSO) connection, CMIP6 simulate the connection better than CMIP5Differences in simulating the Atlantic Intertropical Convergence Zone (ITCZ) precipitation and NTA sea surface temperature persistence cause the bias in the NTA–ENSO connection [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) sea surface temperature (SST) anomaly skewness encapsulates the nonlinear processes of strong ENSO events and affects future climate projections. Yet, its response to CO2 forcing remains not well understood. Here, we find ENSO skewness hysteresis in a large ensemble CO2 removal simulation. The positive SST skewness in the central-to-eastern tropical Pacific gradually weakens (most pronounced near the dateline) in response to increasing CO2, but weakens even further once CO2 is ramped down. Further analyses reveal that hysteresis of the Intertropical Convergence Zone migration leads to more active and farther eastward-located strong eastern Pacific El Niño events, thus decreasing central Pacific ENSO skewness by reducing the amplitude of the central Pacific positive SST anomalies and increasing the scaling effect of the eastern Pacific skewness denominator, i.e., ENSO intensity, respectively. The reduction of eastern Pacific El Niño maximum intensity, which is constrained by the SST zonal gradient of the projected background El Niño-like warming pattern, also contributes to a reduction of eastern Pacific SST skewness around the CO2 peak phase. This study highlights the divergent responses of different strong El Niño regimes in response to climate change. [ABSTRACT FROM AUTHOR]

The article examines the El Niño–Southern Oscillation (ENSO) and its response to carbon dioxide reduction, revealing strong hysteresis effects in sea surface temperature variability and global teleconnections. Topics include the significant increase in Eastern Pacific SST anomaly variance during CO2 ramp-down, attributed to the tropical Pacific Intertropical Convergence Zone's meridional position response, impacting ENSO's prolonged influence under varied future mitigation pathways.

Despite the fact that the response of tropical hydroclimate to North Atlantic cooling events during the Heinrich Stadial 1 (HS1) has been extensively studied in African, South American and Indonesia, the nature of such responses remains debated. Here we investigate the tropical hydroclimate pattern over the Indo-Asian-Australian monsoon region during the HS1 by integrating hydroclimatic records, and examining a δ18Oseawater record from Globigerinoides ruber (white) in the tropical Indian Ocean. Our findings indicate that tropical hydrological conditions were synchronously arid in both hemispheres during the early HS1 (~18.3-16.3 ka) in the Indo-Asian-Australian monsoon region, except for a narrow, wet hydrological belt in northern low latitudes, suggesting the existence of a contracted tropical precipitation belt at that time. This study reveals that the meltwater discharge and resulting changes in global temperatures and El Niño exerted a profound influence on the tropical hydroclimate in the Indo-Asian-Australian monsoon region during the early HS1. New and existing paleoclimatic records show the rain-belt range of the Intertropical Convergence Zone (ITCZ) in the Indo-Asian-Australian monsoon region contracted to the northern low latitudes during the early Heinrich Stadial 1 [ABSTRACT FROM AUTHOR]

This article presents results of the structural and morphological analysis of the fracture zones that are part of Doldrums Megatransform System (DMS), located in the northern part of the Equatorial Atlantic (6.5°–9° N) that include Vernadskiy and Bogdanov transform faults and the Doldrums and Pushcharovskiy megatransforms. Bathymetric map, based on the multibeam echo sounding data, collected during the 45th cruise of the R/V Akademik Nikolaj Strakhov was used for this analysis. It was established that large-scale variations in the width of fracture zone valleys are determined by the distribution of stresses perpendicular to the fracture zone. In the areas with compressive stresses, the fracture zone valleys are narrower and the extension areas are wider. The difference in geodynamic settings within the DMS is due to the difference in spreading directions, which change from 89° to 93° when moving from south to north. The depth of fracture zone valleys consistently increases from the periphery of the DMS (Bogdanov and Doldrums faults) to the center (Pushcharovskiy fracture zone) in accordance with a decrease in the upper mantle temperature. In each fracture zone, the valley depth decreases from the rift- fracture zone intersections towards the center of the active part to a certain background depth. It is assumed that this phenomenon is the result of the uplift of the valley bottom, which occurred due to the decompaction of the lithosphere, caused by the serpentinization of ultramafic rocks. The violation of the revealed variations in the width and depth of fracture zone valley patterns occurs as a result of various ridges and uplifts formation in the fracture zone. In the axial zones of the active parts of the fracture zone valleys median ridges are widespread, extending parallel to the fracture zone and representing serpentinite diapirs squeezed out above the bottom surface. Transverse ridges that were formed 10‒11 million years ago as a result of the lithospheric plate edge flexural bending under extensional conditions are now located in the western passive parts on the southern sides of the of Doldrums and Pushcharovskiy fracture zone valleys. The transverse ridge on the northern side of the Vernadskiy fracture zone, which includes Mount Peyve, was formed between 3.65‒2.4 Ma. Due to the frequent jumps of the spreading axis in this region, it was divided into three segments. There are interfracture zone ridges in megatransforms, which in the active part consist of two fracture zone valleys. The times of their formation were in the Pushcharovskiy megatransform, 30‒32 million years ago and in the Doldrums megatransform, about 4 million years ago. Due to the curvilinearity of the outlines and under the pressure of moving lithospheric plates, the interfracture zone ridges experience longitudinal (along the fault) compressive and tensile stresses, which are compensated by vertical uplifts of their separate blocks and the formation of depressions, pull apart depressions, and spreading centers (the latter are only in Pushcharovskiy megatransform). The structure-forming processes that determine the patterns and morphology of the fracture zones as a part of the DMS are related in their origin to the spreading and transform geodynamic systems. [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]

The current study investigates changes in Austral Summer Monsoon based on numerical experiments conducted with the coupled ICTP-CGCM model. The interannual variability and intensity of the monsoonal system have been analyzed from vorticity indices and air-sea interaction in Africa, Australia and South America. We focus on interglacial stages MIS5e (127 ka), MIS11c (409 ka) and MIS31 (1072 ka). Results show orbitally-driven decreased summer precipitation and slightly shifted monsoon onset and demise with respect to present day conditions. Sensitivity experiments indicate that monsoons are forced not only by the dominant effect of insolation, but also by remote teleconnections, such as the equatorial Atlantic and Pacific ocean basins. During those interglacial stages, cooling occurs in the Southern Hemisphere whereas Northern Hemisphere substantially warms. This induces meridional displacement of oceanic subtropical high pressure systems and the equatorial convergence zone. Regionally, these mechanisms contribute to droughts over the Amazon and northeastern Brazil, northern Australia and southern Africa. Monsoonal rainfall shows different responses to precessional forcing, as well as the relationship between the monsoon and Niño 3.4 differs among the interglacial stages. Results also indicate a weaker influence of the equatorial Pacific Ocean on the Austral summer monsoon for the MIS31 interglacial stage as compared to current climate conditions across Africa and Australia. On the other hand, South America monsoon is strongly influenced by Niño 3.4 and tropical Atlantic. [ABSTRACT FROM AUTHOR]

The evolution of Sahel summer rainfall in the context of global warming is a severe socio-economic concern because of its widespread influences on local agriculture, water resource management, food security, infrastructure planning, and ecosystems. Based on the mid-Pliocene simulations from the Pliocene Model Intercomparison Project Phase 2 and the historical simulations and shared socio-economic pathway 5–8.5 experiments from the Coupled Model Intercomparison Project phase 6, the present study contrasts the Sahel summer rainfall changes between the past mid-Pliocene and near future global warming climates. The results show that the Western African summer monsoon (WASM) circulation, closely linked with the Sahel summer rainfall change, tends to strengthen in both the past and future global warming climates, but the monsoonal circulation strengthening is much more intense in the past warm period than in the projected warm future. This causes that the multi-model ensemble (MME) mean increase ratio of Sahel summer rainfall in the past warming climate is about twice to three times larger than that in the future warming climate for the same increase of global mean surface temperature (the regional rainfall increase ratio in the MME mean: about 19.6% per one degree Celsius of global warming in the mid-Pliocene simulations versus about 7.7% per one degree Celsius of global warming in the SSP5-8.5 future projections). Such a striking discrepancy in the regional circulation and hydrological cycle changes is mainly attributed to a dramatically stronger warming over the Canadian Archipelago and Greenland during the mid-Pliocene warm period relative to the projected near future. The more significant northern high-latitude warming during the mid-Pliocene enhances the meridional temperature gradient between the extratropical and tropical regions, which could induce an excessive northward shift of the Intertropical Convergence Zone and a stronger WASM, and thus result in a more intense hydrological cycle around the Sahel region. Our results highlight that besides the global mean temperature increase, meridional warming patterns are also essential for the changes of WASM and regional hydrological cycle in a warmer world. Implications for projecting the regional monsoon and hydrological cycle changes at longer time scales than in the near future are discussed. [ABSTRACT FROM AUTHOR]

Northern Hemispheric summer monsoons were more intense during the mid-Holocene (MH ~ 6000 years ago) and coincided with a northward shift of the Intertropical Convergence Zone (ITCZ) compared to the pre-industrial (PI) climate. Ancient civilizations in the Indus valley, Mesopotamia, and Egypt appear to have flourished during this period, thanks to abundant water availability. This study exploits a high-resolution variable grid global atmosphere model to understand the role of orbital forcing and ocean surface conditions in strengthening the monsoons and shifting the ITCZ northward over Africa, India, and East Asia during the MH. The combined impact of orbital forcing and sea surface temperature (SST) boundary conditions led to a change in monsoon rainfall of around 42, 30, 21, and 41% over Africa, East Asia, India, and northwest India (NWI) relative to the PI conditions. Changes in orbital parameters alone account for more than 36 and 26% of total rainfall increases in Africa and East Asia. Over the Indian subcontinent, the strengthening of monsoon was primarily a combined effect of SST and orbital forcing. In contrast, the SST boundary condition alone could explain the 39% of rainfall increase over NWI, where the Indus valley civilization once existed. Through moisture budget analysis, the study further illustrates the role of dynamic and thermodynamic factors responsible for the changes in monsoon precipitation. The enhanced monsoon resulted in a northward shift of ITCZ by around 3°N, 1.9°N, and 2.5°N over Africa, East Asia, and India, respectively, compared to its PI position. Analogous to the precipitation changes, orbital forcing mostly mediated ITCZ changes across Africa and East Asia, but the combined impact of orbital forcing and SST was responsible for the changes over India. [ABSTRACT FROM AUTHOR]

Relatively high opal concentrations are measured in equatorial Atlantic sediments from the most recent deglaciation. To shed light on their causes, seven cores were analyzed for their content of siliceous (diatom, silicoflagellates, radiolarians, phytoliths, and sponge spicules) and calcareous (coccolithophores) microfossils. An early deglacial signal is detected at the time of rising boreal summer insolation ca. 18 ka by the coccolithophores. The surface freshening is likely due to the rain belt associated with the intertropical convergence zone (ITCZ), implying its southward shift relatively to its present‐day average positioning. The diatom assemblages corresponding to the following increase in diatom abundances ca. 15.5 ka suggest the formation of a cold tongue of upwelled water associated with tropical instability waves propagating westward. Such conditions occur at present during boreal summer, when southerly trade winds are intensified, and the ITCZ shifts northward. The presence of the diatom Ethmodiscus rex (Wallich) Hendey and the coccolithophore Florisphera profunda indicates a deep thermocline and nutrient enrichment of the lower photic zone, revealing that Si‐rich southern sourced water (SSW) likely contributed to enhanced primary productivity during this time interval. The discrepancies between the maximum opal concentrations and siliceous marine microfossils records evidence the contribution of freshwater diatoms and phytoliths, indicative of other processes. The definition of the nature of the opal record suggests successive productivity conditions associated with specific atmospheric settings determining the latitudinal ITCZ positioning and the development of oceanic processes; and major oceanic circulation changes permitting the contribution of SSW to marine productivity at this latitude. Key Points: Microfossil evidence for the contribution by Si‐rich southern‐sourced water to enhanced primary productivity in the equatorial AtlanticThe position of the intertropical convergence zone plays a role in determining the different productivity conditionsOpal accumulation does not necessarily reflect solely diatom burial [ABSTRACT FROM AUTHOR]

This study conducted high-resolution paleoclimatic analyses based on the identification of palynological groups of late Aptian age (biozone Sergipea veriverrucata) in the Bragança and Codó formations within the Bragança–Viseu, São Luís, and Parnaíba basins. The analysis comprised 40 palynological samples, with 200 palynomorphs per slide counted when possible. Bioclimatic analysis was mainly supported by the identification of botanical affinities, and ecological and climatic parameters such as the wet / arid trend (Fs/X), Shannon–Wiener diversity, and indicator species analysis (IndVal) were used. Statistical analyses such as principal component and cluster analyses were employed to support the paleoclimatic interpretations. The study recognized 69 genera distributed among the main groups of living plants, including bryophytes, ferns, lycophytes, gymnosperms, and angiosperms. It was possible to attribute botanical affinity in 94.2 % of the taxa, and nine genera occurred in all sections studied: Afropollis, Araucariacites, Callialasporites, Cicatricosisporites, Classopollis, Cyathidites, Deltoidospora, Equisetosporites, and Verrucosisporites, with Classopollis being the most abundant. The stratigraphic distribution of the bioclimatic groups (hydrophytes, hygrophytes, lowland tropical flora, upland flora, and xerophytes) allowed for the identification of climatic phases: pre-evaporitic, evaporitic, and post-evaporitic. In the pre-evaporitic phase, the most significant abundances were between the hygrophytes and upland flora, indicating a certain level of humidity. Xerophytes were the most abundant in all phases, with a conspicuous increase in the evaporitic phase, reflecting an increase in aridity. In the post-evaporitic phase, there was a significant increase in the upland flora with the return of wetter conditions. This study confirmed an increasing humidity trend in the analyzed sections, probably owing to the influence of the Intertropical Convergence Zone that was already operating during the late Aptian. [ABSTRACT FROM AUTHOR]

The article presents a study conducted to analyze whether the Hadley cell (HC) edge's shift towards poles could be reversed through removal of carbon dioxide (CO2). It states that reducing CO2 concentrations does not return the poleward-shifted HC edge to its original state. It mentions that hemispherically asymmetric changes are linked to variations in vertical wind shear due to the prolonged ocean response to CO2 removal.

We investigated long‐term trends in convective and large‐scale precipitation within the Intertropical Convergence Zone during the boreal summer (June–August) for the period 1980–2020 using five different reanalysis datasets. We found that there exists a large diversity in the trends. In particular, we focused on the European Center for Medium‐Range Weather Forecasts reanalysis version 5 (ERA5) and the Japanese 55‐year reanalysis (JRA55) datasets; because the convective precipitation amount statistically significantly decreases in the former, but increases in the later. In contrast, the large‐scale precipitation amount increases in both of these reanalysis datasets. We examine the physical factors leading to contrasting features in the respective trends in convective and large‐scale precipitation amount between two reanalysis datasets. A significant decrease of convective available potential energy, which is caused by enhanced atmospheric stability, leads to a decreasing trend of convective precipitation amount in the ERA5 dataset. In contrast, a significant increase of precipitable water is noted in JRA55 dataset. Additionally, the Clausius–Clapeyron relation is larger in JRA55 than ERA5, implying that the sensitivity of humidity to the temperature change is larger in JRA55 than that in ERA5 dataset. On the other hand, the increase of the large‐scale precipitation is associated with the increase of medium cloud amount in both ERA5 and JRA55 datasets. Plain Language Summary: Typically, the total amount of precipitation is divided into convective precipitation and large‐scale precipitation in which the associated mechanisms are quite different. We investigated long‐term trends in convective and large‐scale precipitation within the Intertropical Convergence Zone, where tropical rainfall is concentrated in a narrow band that accounts for 32% of global precipitation, during the boreal summer for 1980–2020 using five different reanalysis datasets. We focused on the European Center for Medium‐Range Weather Forecasts reanalysis version 5 and the Japanese 55‐year reanalysis datasets; because the convective precipitation amount decreases significantly in the former, but increases in the later. In contrast, the large‐scale precipitation amount increases in both of these reanalysis datasets. The interesting challenge in these results is that there is little agreement in the mechanisms for those trends and this disagreement is most notable in terms of convective precipitation. If our reanalysis products disagree in terms of mechanisms for the trends, understanding future projections will be even more difficult. Key Points: We investigated long‐term trends in convective and large‐scale precipitation within the Intertropical Convergence Zone (ITCZ), using five different reanalysis datasetsA significant decrease of convective available potential energy leads to a decreasing trend of convective precipitation amount in the European Center for Medium‐Range Weather Forecasts reanalysis version 5 (ERA5), which is in contrast to Japanese 55‐year reanalysis (JRA55)The increasing of the medium cloud may contribute to the increase of large‐scale precipitation amount within the ITCZ in the ERA5 and JRA55 [ABSTRACT FROM AUTHOR]

Due to the unique local air chemistry, the transport history of tropospheric air masses above the remote tropical West Pacific (TWP) is reflected by local ozone (O3) and relative humidity (RH) characteristics. In boreal winter, the TWP is the main global entry point for air masses into the stratosphere and therefore a key region of atmospheric chemistry and dynamics. However, a long-term in situ monitoring of tropospheric O3 to assess the variability of TWP air masses and the respective controlling processes has yet been missing. The aim of our study was to identify air masses with different origins and pathways to the TWP and their seasonality using the new Palau time series (2016–2019) of mostly fortnightly Electrochemical Concentration Cell ozone and radio soundings. Based on monthly statistics of O3 volume mixing ratios and RH we defined a free tropospheric locally-controlled background and analyzed anomalies for both tracers in the 5–10 km altitude range. We found that anomalously high O3 indicates a remote origin, while RH is controlled by a range of dynamical processes resulting in a bimodality in RH anomalies. The Palau time series confirms a year-round presence of low O3 background air masses and a seasonal mid-tropospheric cycle in O3 with a prominent anti-correlation between O3 volume mixing ratios and RH. We assumed five different types of air masses with differing tracer characteristics and origin which we validated by analyzing backward trajectories calculated with the transport module of the Lagrangian chemistry and transport model ATLAS. The main result is a clear separation of origin and pathways for the two most contrasting types of air masses, i.e. ozone-poor and humid versus ozone-rich and dry air. Both, potential vorticity and air mass origin analyses, reveal no indication for stratospheric influence for the ozone-rich dry air masses. Rather, we found indications for O3 production due to biomass burning or anthropogenic pollution at the origins of these air masses and drying due to clear sky subsidence during long-range transport. The seasonal occurrence is tied to the position of the Intertropical Convergence Zone which opens a pathway from potential source regions which are confirmed by the trajectory analysis. We conclude, that dominant ozone-poor and humid air masses are of local or Pacific convective origin and occur year-round, but dominate from August until October. Anomalously dry and ozone-rich air is generated in Tropical Asia and subsequently transported to the TWP via an anti-cyclonic route, mostly from February to April. The areas of origin suggest different sources of ground pollution as a cause for O3 production. We propose large-scale descent within the tropical troposphere and subsequent radiative cooling in connection with the Hadley circulation as responsible for the vertical displacement and dehydration. [ABSTRACT FROM AUTHOR]

The objective of this work is to assess the wind resources of the east coast of Maranhão, Brazil. Wind profilers were combined with micrometeorological towers and atmospheric reanalysis to investigate micro- and mesoscale aspects of wind variability. Field campaigns recorded winds in the dry and wet seasons, under the influence of the Intertropical Convergence Zone. The dry season was characterized by strong winds (8 to 12 m s − 1 ) from the northeast. Surface heat fluxes were generally positive (250 to 320 W m − 2 ) at midday and negative (−10 to −20 W m − 2 ) during the night. Convective profiles predominated near the beach, with strongly stable conditions rarely occurring before sunrise. Further inland, convective to strongly convective profiles occurred during the day, and neutral to strongly stable profiles at night. Wind speeds decreased during the rainy season (4 to 8 m s − 1 ), with increasingly easterly and southeasterly components. Cloud cover and precipitation reduced midday heat fluxes (77 W m − 2 ). Profiles were convective during midday and stable to strongly stable at night. Terrain roughness increased with distance from the ocean ranging from smooth surfaces ( z o = 0.95 mm) and rough pastures ( z o = 15.33 mm) to crops and bushes ( z o = 52.68 mm), and trees and small buildings ( z o = 246.46 mm) farther inland. Seasonal variations of the mean flow and sea and land breezes produced distinct diurnal patterns of wind speeds. The strongest (weakest) breeze amplitudes were observed in the dry (rainy) period. Daily changes in heat fluxes and fetch over land controlled the characteristics of wind profiles. During sea breezes, winds approached the coast at right angles, resulting in shorter fetches over land that maintained or enhanced oceanic convective conditions. During land breezes, winds blew from the mainland or with acute angles against the coastline, resulting in large fetches with nighttime surface cooling, generating strongly stable profiles. Coastal observations demonstrated that with increasing monopiles from 100 to 130 m it is possible to obtain similar capacity factors of beachfront turbines. [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]

As global temperatures rise due to increasing concentrations of greenhouse gasses in the atmosphere, melting in Antarctica and Greenland is accelerating, initiating feedback systems that will result in record losses in sea and land ice by the end of this century if left unabated. The freshening of polar waters changes the production of deepwater masses such as the Antarctic Bottom Water and the North Atlantic Deep Water, which rely on salinity and temperature gradients to form. Thermohaline Circulation, dependent on these gradients, transports heat, nutrients, and carbon around the globe, but is disrupted as freshwater forcing from ice melt reduces the capacity for overturning of the Atlantic Meridional Overturning Circulation (ΑMOC). As the ΑMOC weakens, it changes the trajectory of the Gulf Stream, which produces a southerly shift to the Intertropical Convergence Zone (ITCZ), altering the temperature of the lower atmosphere and thus precipitation over most of the Northern Hemisphere. Multidecadal and multiannual atmospheric circulation patterns are also affected, increasing the intensity and prevalence of storms in some regions, and drought conditions in others. Such climatic changes can have implications for food security through variations in land and water availability for agriculture, and through changes to ocean circulation that alter upwelling and marine primary productivity. Many countries that would be affected are the least developed and are already, or most at risk of becoming, food insecure. [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]

When projecting future monsoon changes by carbon dioxide (CO2) pathway, most studies have analyzed precipitation responses without considering monsoon area (total area of monsoon domain, MA) variations. However, how MA responds to CO2 removal remains uncertain. This study evaluates MA variations and impacts in idealized CO2 ramp‐up (toward CO2 quadrupling), ramp‐down, and stabilized simulations using the Community Earth System Model version 1. Global MA negatively overshoots (i.e., recovery with decreasing tendency beyond the original MA) during the ramp‐down period due to reduced or rapidly recovered MA in several regional monsoons, including Northern and Southern Africa, South and East Asia, and South America, showing hysteresis when comparing CO2 ramp‐up and ramp‐down periods despite similar global warming levels. These non‐linear regional MA variations come from distinct regional summer and winter precipitation variations, which are found to be mostly associated with Intertropical Convergence Zone movements and El Niño‐like response. Further, regional monsoon precipitation characteristics also vary through CO2 ramp‐up and ramp‐down periods consistently with overall hysteresis. Changes in total monsoon precipitation resemble the distinct responses of MA. Our results suggest that regions characterized by a monsoonal climate may experience reduced seasonal rainfall variations under net‐negative CO2 emissions. Plain Language Summary: Many studies have researched how increasing carbon dioxide (CO2) affects monsoon. Most have focused on precipitation changes without considering how monsoon area (i.e., total area of monsoon domain, MA) will change. Furthermore, it is unknown how global and regional MA responds to CO2 removal. In this study, we first investigate how global to regional MA changes under CO2 concentration increase, decrease and then equilibration using climate model simulations. After CO2 removal, the global MA is largely reduced below the present‐day state, as MA decreases over several monsoon regions. Further, when we compare MA at the same global warming level in CO2 ramp‐up and ramp‐down periods, many individual monsoon regions show reduced MA during the ramp‐down period. As MA is defined based on local contrast of summer and winter precipitation, we investigate hemispheric summer and winter precipitation responses and how they affect MA. The regional changes in MA are found to be due to distinct variations of local summer or winter precipitation, which are commonly associated with large‐scale atmospheric and oceanic variations. Further impacts of changes in MA with CO2 removal on the hydrological cycle more generally, are identified. Key Points: Non‐linear responses in global and regional monsoon areas to CO2 pathway are investigated using a coupled climate modelDistinct hysteresis in monsoon area variation is identified over global to regional monsoons between CO2 ramp‐up and ramp‐down periodThe hysteresis emerges due to distinct precipitation changes with large‐scale atmospheric and oceanic responses [ABSTRACT FROM AUTHOR]

Studying the evolution history of the southwest summer monsoon (SSM) throughout geological time, particularly during its strongest period in the Holocene, can improve our understanding of its variation and driving mechanisms, and even help predict future climate changes, due to its significant social and economic implications. Here, we reconstructed the history of chemical weathering intensity since the Last Glacial Maximum (LGM) based on clay mineral proxies [(illite/smectite)/(illite + chlorite) and illite crystallinity] obtained from Qionghai Lake sediments and examined its response to paleoclimate and SSM. Our findings indicate that the intensity of chemical weathering generally aligned with changes in paleoclimate, exhibiting strong chemical weathering intensity during warm and humid climate conditions. In addition, the intensity of chemical weathering basically tracks the evolution of the SSM since the LGM. Our results support the view that the highest SSM intensity occurred during the early‐middle Holocene, followed by gradual weakening during the late Holocene, with Northern Hemisphere summer insolation being the primary driver of the SSM evolution. The variations of the SSM and the corresponding intensity of chemical weathering were also influenced by the cumulative effects of glacier boundary conditions, North Atlantic climate fluctuations, and Intertropical Convergence Zone migrations. Key Points: Clay minerals can effectively reflect regional chemical weathering intensityChemical weathering intensity was following the monsoon variabilityThe strongest southwest summer monsoon occurred in the early‐middle Holocene [ABSTRACT FROM AUTHOR]

Lake Petén Itzá (Guatemala) possesses one of the longest lacustrine sediment records in the northern Neotropics, which enabled study of paleoclimate variability in the region during the last ∼400000 years. We used geochemical (Ti, Ca/(Ti+Fe) and Mn/Fe) and mineralogical (carbonates, gypsum, quartz, clay) data from sediment core PI-2 to infer past changes in runoff, lake evaporation, organic matter sources and redox conditions in the water column, caused by hydrological changes in the northern Neotropics during Marine Isotope Stages (MISs) 3–2. From 59 to 39 cal ka BP climate conditions were relatively wet, and the lake was marked by higher primary productivity and anoxic bottom waters. This wet environment was interrupted for two periods of possible low water level at 52 and 46 cal ka BP, when our data suggest higher evaporation, high terrestrial organic matter input and persistent oxic conditions. Between 39 and 23 cal ka BP, evaporation and input of terrestrial organic matter increased considerably, lake level declined, and lake bottom waters generally became oxic. These conditions reversed during the Last Glacial Maximum (23.5–18.0 cal ka BP), when runoff and lake productivity increased, and rising lake level caused bottom waters to again become anoxic. Comparison of our hydrologic proxy data with sea surface temperature anomalies between the eastern Pacific and the Caribbean suggests that changes in the intensity of the Caribbean Low-Level Jet (CLLJ) may have influenced long-term changes in runoff during MISs 3–2. Higher intensity of the CLLJ during the onset of MIS 3 and the LGM might have led to greater runoff into the lake, whereas the MIS 3–2 transition experienced a weaker CLLJ and consequently less runoff. A refined, high-resolution age–depth model for the PI-2 sediment core enabled us to identify millennial-scale Greenland interstadials (GIs) 14–2, Greenland stadials (GSs) 14–2 and Heinrich stadials (HSs) 5–1. In general, HSs and GSs were characterized by drier conditions. In contrast to GSs and HSs, GIs were characterized by greater runoff and overall wetter conditions, with the most pronounced GI peaks between 40 and 30 cal ka BP. Whereas GSs 9, 8, 7 and 6 began with abrupt increases in evaporation and ended with gradual increases in humidity, GSs 11 and 10 showed reversed patterns. The Lake Petén Itzá paleohydrology record, along with other regional paleoclimate records, led us to conclude that shifts in the position of the Intertropical Convergence Zone (ITCZ) altered moisture delivery to the lake on millennial timescales. During GSs and HSs, high evaporation from Petén Itzá (dry climate conditions) was associated with a more southerly position of the ITCZ, whereas wetter GIs prevailed during a more northerly ITCZ position. Although abrupt millennial-scale shifts in ITCZ and hydroclimate between GSs/HSs and GIs can be linked to instabilities in the Atlantic Meridional Overturning Circulation (AMOC), longer-term changes were additionally influenced by changes in atmospheric convection linked to modulations of the CLLJ in response to Δ SST between the equatorial Pacific and tropical Atlantic. [ABSTRACT FROM AUTHOR]

Over one hundred years of vigorous progress in tropical cyclone (TC) research, the genesis of the cyclone (hereafter, tropical cyclogenesis) is remarkable as a doubtful subject. Furthermore, predicting tropical cyclogenesis, particularly in the lesser latitude, remains a significant challenge. Therefore, understanding the complex interactions in developing tropical cyclogenesis over the region is vital to improving tropical cyclogenesis forecasting. Hence, the Indonesia Maritime Continent is a tropical cyclone-free region due to decreasing the Coriolis effect. However, Seroja TC hit Flores (8.6° S, 120° E), east Nusa Tenggara, Indonesia, on 4 April 2021, and was recorded as the first TC that occurred over the mainland, which brought a catastrophic disaster in the region. This study investigated the tropical cyclogenesis of Seroja by using observational and numerical studies. The results indicate that a marine heatwave and double vortices were favorable conditions that produced preconditions for developing tropical cyclogenesis over the Maluku Sea. Thus, tropical cyclogenesis is formed by the breakdown of the intertropical convergence zone (ITCZ) associated with synoptic-scale wave train driven under the interaction of the Madden Julian oscillation (MJO) and equatorial Rossby waves. Moreover, our finding suggested that an extensive background cyclonic vorticity under the cold pool mechanisms is responsible for maintaining tropical cyclogenesis into a persistent Seroja TC. [ABSTRACT FROM AUTHOR]

This study investigates the effect of mesoscale eddies on air–sea heat and fresh water exchange in the tropical Atlantic Ocean (TAO) using 8 years of satellite altimetry data, combined with sea surface temperature (SST), latent and sensible heat fluxes (LHF and SHF), infrared fluxes (IRF) and precipitation (PR) data. Results indicate that approximately ∼40% of cyclonic eddies contribute to warm SST anomalies, and ∼40% of anticyclonic eddies contribute to cold SST anomalies. Eddies were found to play a role in the variability in LHF, SHF and IRF, contributing 10–35% of their total variability, with the largest contributions observed beneath the intertropical convergence zone (ITCZ) and frontal SST areas. Composite analysis of SST and heat flux anomalies over eddies suggested that the anomalies created through horizontal advection processes may not significantly impact the overall LHF, SHF and IRF over eddies, contrary to vertical processes. Despite a lack of clear correlation between heat flux and PR anomalies over eddies in the TAO, significant correlations were found beneath the ITCZ, suggesting that eddies may impact both heat fluxes and PR in the ITCZ region. This study provides an original contribution to the understanding of the impact of ocean mesoscale eddies on the atmosphere in the TAO. [ABSTRACT FROM AUTHOR]

The Tropical West Pacific is recognized as an important region for stratosphere-troposphere exchange, but has been a measurement gap in the global ozone sounding network. The Palau Atmospheric Observatory (PAO) was established to study the atmospheric composition above the remote Tropical West Pacific with a comprehensive instrumental setup. Since 2016, two laboratory containers in Palau host an Fourier-transform infrared spectrometer, a lidar (micro lidar until 2016, cloud and aerosol lidar from 2018), a Pandora 2S photometer and laboratory space for weather balloon soundings with ozone-, water-vapor-, aerosol- and radiosondes. In this analysis, we focus on the continuous, fortnightly ozone sounding program with Electrochemical Concentration Cell (ECC) ozone sondes. The aim of this study is to introduce the PAO and its research potential, present the first observation of the typical seasonal cycle of tropospheric ozone in the Tropical West Pacific based on a multiannual record of in situ observations, and investigate major drivers of variability and seasonal variation from 01/2016 until 12/2021 related to the large scale atmospheric circulation. We present the PAO ozone (O3) volume mixing ratios (VMR) and relative humidity (RH) time series complemented by other observations. The site is exposed to year-round high convective activity reflected in dominating low O3 VMR and high RH. In 2016, the impact of the strong El Niño is evident as a particularly dry, ozone-rich episode. The main modulator of annual tropospheric O3 variability is identified as the movement of the Intertropical Convergence Zone (ITCZ), with lowest O3 VMR in the free troposphere during the ITCZ position north of Palau. An analysis of the relation of O3 and RH for the PAO and selected sites from the Southern Hemispheric ADditional OZonesondes (SHADOZ) network reveals three different regimes. Palau's O3/RH distribution resembles the one in Fiji, Java and American Samoa, but is unique in its seasonality and its comparably narrow Gaussian distribution around low O3 VMR and the evenly distributed RH. A previously found bimodal distribution of O3 VMR and RH could not be seen in the Palau record. Due to its unique remote location, Palau is an ideal atmospheric background site to detect changes in air dynamics imprinted on the chemical composition of the tropospheric column. The efforts to establish, run and maintain the PAO have succeeded to fill an observational gap in the remote Tropical West Pacific and give good prospects for ongoing operations. The ECC sonde record will be integrated into the SHADOZ database in the near future. [ABSTRACT FROM AUTHOR]

This study examines the role of tropical dynamics in the formation of global tropical cyclone (TC) clusters. Using theoretical analyses and idealized simulations, it is found that global TC clusters can be produced by the internal dynamics of the tropical atmosphere, even in the absence of landmass surface and zonal sea surface temperature (SST) anomalies. Our analyses of a two-dimensional InterTropical Convergence Zone (ITCZ) model capture indeed some planetary-scale stationary modes whose zonal and meridional structures can support the formation of TC clusters at the global scale. Additional idealized simulations using the Weather Research and Forecasting (WRF) model confirm these results in a range of aqua-planet experiments. Specifically, the examination of two common tropical waves including the equatorial Rossby (ER) wave and the equatorial Kelvin (EK) wave shows that ER waves could develop and maintain a planetary-scale stationary structure for a range of zonal wavenumbers [5–11], while EK waves do not. This numerical result is consistent with the ITCZ breakdown model and reveals some forcing structures that can support stationary 'hot spots' for global TC formation. The findings in this study offer different insights into the importance of tropical waves in producing global TC clusters beyond the traditional explanation based on zonal SST anomalies. [ABSTRACT FROM AUTHOR]

We investigate global and regional changes in the intertropical convergence zone (ITCZ) position, width, and intensity during the last glacial maximum (LGM) relative to the preindustrial period using multiple simulations from Phases 3 and 4 of the Paleoclimate Modelling Intercomparison Project (PMIP3/4). On annual scale, most models show that LGM tropical precipitation decreases, and the deficit in the Northern Hemisphere is larger than that in the Southern Hemisphere, resulting in the southward shift, narrowing, and weakening of the ITCZ at the global scale. The arithmetic mean of 13 models shows that the global zonal mean ITCZ shifts southward by 0.85° (1σ = 0.86°), narrows by 1.05° (1σ = 1.33°), and weakens by 7% (1σ = 4%) during the LGM. Regionally, position and intensity changes are larger in the central and eastern Pacific, while width changes are most obvious in the Indian Ocean–western Pacific. Precipitation changes in the central and eastern Pacific and Atlantic oceans are dominated by the dynamic term. In the Indian Ocean–western Pacific, the thermodynamic term is the main cause for precipitation changes within 10°S–10°N, while the dynamic term plays a leading role at other tropical latitudes. Seasonally, the September–October–November and June–July–August mainly contribute to the annual ITCZ position, width, and intensity changes globally and in most regions. The convergence factor dominates both the dynamic and thermodynamic terms annually and seasonally. The model results are compatible with the existing site reconstructions on the southward shift of the LGM ITCZ. Plain Language Summary: The intertropical convergence zone (ITCZ) is a heavy precipitation belt near the equator, which has an important impact on the Earth's climate. The changes in ITCZ under past and future climate conditions remain inconclusive. Here we use state of the art computational models of the global climate system to investigate how it changes during the recent glacial period, namely the last glacial maximum (LGM), when the climate conditions differed dramatically from the present day and are opposite to the warming future. We find that, relative to the preindustrial period, the LGM ITCZ shifts southward, narrows and weakens both globally and regionally, which are caused by the simulated cooling and associated circulation changes at the LGM. These model simulated changes are consistent with the available geological evidence. Key Points: Changes of the intertropical convergence zone (ITCZ) during the last glacial maximum (LGM) are investigated in this studyThe LGM ITCZ significantly shifted southward, narrowed, and weakened both at global and regional scales relative to the preindustrial periodLGM ITCZ changes originated from an uneven decrease of precipitation between the two hemispheres due to dynamic and thermodynamic effects [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 Southern Ocean has warmed substantially, and up to early 21st century, Antarctic stratospheric ozone depletion and increasing atmospheric CO 2 have conspired to intensify Southern Ocean warming. Despite a projected ozone recovery, fluxes to the Southern Ocean of radiative heat and freshwater from enhanced precipitation and melting sea ice, ice shelves, and ice sheets are expected to increase, as is a Southern Ocean westerly poleward intensification. The warming has far-reaching climatic implications for melt of Antarctic ice shelf and ice sheet, sea level rise, and remote circulations such as the intertropical convergence zone and tropical ocean-atmosphere circulations, which affect extreme weathers, agriculture, and ecosystems. The surface warm and freshwater anomalies are advected northward by the mean circulation and deposited into the ocean interior with a zonal-mean maximum at ∼45°S. The increased momentum and buoyancy fluxes enhance the Southern Ocean circulation and water mass transformation, further increasing the heat uptake. Complex processes that operate but poorly understood include interactive ice shelves and ice sheets, oceanic eddies, tropical-polar interactions, and impact of the Southern Ocean response on the climate change forcing itself; in particular, limited observations and low resolution of climate models hinder rapid progress. Thus, projection of Southern Ocean warming will likely remain uncertain, but recent community effort has laid a solid foundation for substantial progress. [ABSTRACT FROM AUTHOR]

This study focuses on the distribution of hydrogen peroxide (H 2 O 2) in the upper tropical troposphere at altitudes between 8 and 15 km based on in situ observations during the Chemistry of the Atmosphere: Field Experiment in Africa (CAFE-Africa) campaign conducted in August–September 2018 over the tropical Atlantic Ocean and western Africa. The measured hydrogen peroxide mixing ratios in the upper troposphere show no clear trend in the latitudinal distribution with locally increased levels (up to 1 ppbv​​​​​​​) within the Intertropical Convergence Zone (ITCZ), over the African coastal area, as well as during measurements performed in proximity to the tropical storm Florence (later developing into a hurricane). The observed H 2 O 2 distribution suggests that mixing ratios in the upper troposphere seem to be far less dependent on latitude than assumed previously and the corresponding factors influencing the photochemical production and loss of H 2 O 2. The observed levels of H 2 O 2 in the upper troposphere indicate the influence of convective transport processes on the distribution of the species not only in the tropical but also in the subtropical regions. The measurements are compared to observation-based photostationary steady-state (PSS) calculations and numerical simulations by the global ECHAM/MESSy Atmospheric Chemistry (EMAC) model. North of the ITCZ, PSS calculations produce mostly lower H 2 O 2 mixing ratios relative to the observations. The observed mixing ratios tend to exceed the PSS calculations by up to a factor of 2. With the exception of local events, the comparison between the calculated PSS values and the observations indicates enhanced H 2 O 2 mixing ratios relative to the expectations based on PSS calculations in the north of the ITCZ. On the other hand, PSS calculations tend to overestimate the H 2 O 2 mixing ratios in most of the sampled area in the south of the ITCZ by a factor of up to 3. The significant influence of convection in the ITCZ and the enhanced presence of clouds towards the Southern Hemisphere indicate contributions of atmospheric transport and cloud scavenging in the sampled region. Simulations performed by the EMAC model also overestimate hydrogen peroxide levels particularly in the Southern Hemisphere, most likely due to underestimated cloud scavenging. EMAC simulations and PSS calculations both indicate a latitudinal gradient from the Equator towards the subtropics. In contrast, the measurements show no clear gradient with latitude in the mixing ratios of H 2 O 2 in the upper troposphere with a slight decrease from the ITCZ towards the subtropics, indicating a relatively low dependency on the solar radiation intensity and the corresponding photolytic activity. The largest model deviations relative to the observations correspond with the underestimated hydrogen peroxide loss due to enhanced cloud presence, scavenging, and rainout in the ITCZ and towards the south. [ABSTRACT FROM AUTHOR]

The article presents a study which explored the influence of Australian wildfires on sea surface temperature (SST) in the tropical Pacific Ocean and analyzed the mechanisms connecting the two. Topics discussed include responses in the tropical Pacific; responses of clouds, albedo, and radiation in the southeastern subtropical Pacific; and simulation of a subsequent multiyear ensemble mean cooling of the tropical Pacific.

The Marine Isotope Stage (MIS5e) is characterized by a warmer climate than that of the pre-industrial period, and serves as an analog for the Current Warm Period (CWP). However, uncertainties persist regarding its climatic stability. Here, we retrieved a stalagmite (WXB075) from Wanxiang Cave in the eastern Qinghai-Tibetan Plateau, and employed absolute 230Th dating and relative annual layer data to establish a high-precision chronological framework for reconstructing the history of the Asian summer monsoon (ASM) and environmental evolution during early MIS5e with multiple proxies. The findings indicate that the annually laminated stalagmite was formed during Cooling Event 27 (C27). The deposition of WXB075 experienced a hiatus (∼125.58 ka BP) due to a significant cooling event in the North Atlantic, which may be linked to the unstable climate in the Northern Hemisphere. Additionally, the impact of meltwater discharge in high northern latitudes results in a two-phase evolution of the ASM, i.e., an initial weaker stage followed by a gradual increase (with the exception of deposition hiatus). The climatic instability of ASM is generally characterized by a quasi-60 year cycle that affects vegetation conditions, biological productivity, and karst hydroclimate dynamics. However, the increase in meltwater and decrease in temperature in the Northern Hemisphere have led to a weakened ASM and subsequent reduction in precipitation. Consequently, vegetation degradation above the cave has occurred along with a slowdown of karst hydroclimate. The vegetation conditions, organic matter content, and wet/drought of the karst hydroclimate were affected by both the large-scale monsoon circulation and local environment during extreme weakening (strengthening) of the monsoon when high-frequency climatic events of ASM occurred. A comparison of δ18O records between early MIS5e and the past 2000 years reveals that the climate during early MIS5e differed significantly from that of CWP, Medieval Warm Period (MWP), and Dark Age Cold Period (DACP) but was similar to Little Ice Age (LIA). Comparison with other geological records from the Northern Hemisphere indicates that climate instability was a widespread phenomenon during MIS5e. The power spectrum analysis of WXB075 δ18O reveals significant quasi-60 and 35 a cycles during the early MIS5e, which is consistent with the Atlantic Multidecadal Oscillation (AMO). The comprehensive results demonstrate that the ASM in the early MIS5e was closely linked to solar activity, Intertropical Convergence Zone (ITCZ) position, and Atlantic Meridional Overturning Circulation (AMOC). [ABSTRACT FROM AUTHOR]

Understanding long-term climate change is particularly relevant in Australasia, where precipitation exhibits marked variability on annual and decadal timescales. Although numerous studies have investigated hydroclimate change during the Common Era in Australasia, high-quality records covering the late Holocene remain scarce in northern Australia. Here we present an isoprenoid glycerol dialkyl glycerol tetraethers (GDGT) record from Lake Barrine, to evaluate the environmental significance of archaeal lipid-based proxies in the lake system and possible drivers of changes during the late Holocene. The fractional composition of isoprenoid GDGTs in the sediments from Lake Barrine is dominated by GDGT-0 and crenarchaeol. The ratio of crenarchaeol and its regio-isomer (cren/cren'), GDGT-0/crenarchaeol ratios and Methane Index suggest that a dominant contribution of archaeal lipids in Lake Barrine derived from Group Ⅰ.1a Thaumarchaeota, with a minor contribution from Group Ⅰ.1b Thaumarchaeota and methanogenic archaea. Centennial-scale variation in cren/cren' ratios correlates well with other hydroclimate records in the tropical Australia, due to a preference of Group Ⅰ.1a Thaumarchaeota for deeper lakes, and humid climates typically do not offer a large niche for soil Group Ⅰ.1b Thaumarchaeota. Variability of hydroclimate in northern Australia is linked to the mean position and strength of the intertropical convergence zone. Furthermore, the TEX 86 proxy can effectively record lake surface temperature during the past 0.8 kyr, however, a slight increase in the contribution of non-Thaumarchaeota in deeper sediments lead to underestimate of the paleotemperature. [ABSTRACT FROM AUTHOR]

The cloud thermodynamic phase is a crucial parameter to understand the Earth's radiation budget, the hydrological cycle, and atmospheric thermodynamic processes. Spaceborne active remote sensing such as the synergistic radar-lidar DARDAR product is considered the most reliable method to determine cloud phase; however, it lacks large-scale observations and high repetition rates. These can be provided by passive instruments such as SEVIRI aboard the geostationary Meteosat Second Generation (MSG) satellite, but passive remote sensing of the thermodynamic phase is challenging and confined to cloud top. Thus, it is necessary to understand to what extent passive sensors with the characteristics of SEVIRI are expected to provide a relevant contribution to cloud phase investigation. To reach this goal, we collect five years of DARDAR data to model the cloud top phase (CTP) for MSG/SEVIRI and create a SEVIRI-like CTP through an elaborate aggregation procedure. Thereby, we distinguish between ice (IC), mixed-phase (MP), supercooled (SC), and warm liquid (LQ). Overall, 65% of the resulting SEVIRI pixels are cloudy, consisting of 49% IC, 14% MP, 13% SC, and 24% LQ cloud tops. The spatial resolution has a significant effect on the occurrence of CTP, especially for MP cloud tops, which occur significantly more often at the lower SEVIRI resolution than at the higher DARDAR resolution (9%). We find that SC occurs most frequently at high southern latitudes, while MP is found mainly in both high southern and high northern latitudes. LQ dominates in the subsidence zones over the ocean, while IC occurrence dominates everywhere else. MP and SC show little seasonal variability apart from high latitudes, especially in the south. IC and LQ are affected by the shift of the Intertropical Convergence Zone. The peak of occurrence of SC is at −3 ∘ C, followed by that for MP at −13 ∘ C. Between 0 and −27 ∘ C, the occurrence of SC and MP dominates IC, while below −27 ∘ C, IC is the most frequent CTP. Finally, the occurrence of cloud top height (CTH) peaks lower over the ocean than over land, with MP, SC, and IC being undistinguishable in the tropics but with separated CTH peaks in the rest of the MSG disk. Finally, we test the ability of a state-of-the-art AI-based ice cloud detection algorithm for SEVIRI named CiPS (Cirrus Properties for SEVIRI) to detect cloud ice. We confirm previous evaluations with an ice detection probability of 77.1% and find a false alarm rate of 11.6%, of which 68% are due to misclassified cloud phases. CiPS is not sensitive to ice crystals in MP clouds and therefore not suitable for the detection of MP clouds but only for fully glaciated (i.e., IC) clouds. Our study demonstrates the need for the development of dedicated cloud phase distinction algorithms for all cloud phases (IC, LQ, MP, SC) from geostationary satellites. [ABSTRACT FROM AUTHOR]

Aeolus is the first satellite mission to acquire vertical profiles of horizontal line‐of‐sight winds globally and thus fills an important gap in the Global Observing System, most notably in the Tropics. This study explores the impact of this dataset on analyses and forecasts from the European Centre for Medium‐Range Weather Forecasts (ECMWF) and Deutscher Wetterdienst (DWD), focusing specifically on the West African Monsoon (WAM) circulation during the boreal summers of 2019 and 2020. The WAM is notoriously challenging to forecast and is characterized by prominent and robust large‐scale circulation features such as the African Easterly Jet North (AEJ‐North) and Tropical Easterly Jet (TEJ). Assimilating Aeolus generally improves the prediction of zonal winds in both forecasting systems, especially for lead times above 24 h. These improvements are related to systematic differences in the representation of the two jets, with the AEJ‐North weakened at its southern flank in the western Sahel in the ECMWF analysis, while no obvious systematic differences are seen in the DWD analysis. In addition, the TEJ core is weakened in the ECMWF analysis and strengthened on its southern edge in the DWD analysis. The regions where the influence of Aeolus on the analysis is greatest correspond to the Intertropical Convergence Zone (ITCZ) region for ECMWF and generally the upper troposphere for DWD. In addition, we show the presence of an altitude‐ and orbit‐dependent bias in the Rayleigh‐clear channel, which causes the zonal winds to speed up and slow down diurnally. Applying a temperature‐dependent bias correction to this channel contributes to a more accurate representation of the diurnal cycle and improved prediction of the WAM winds. These improvements are encouraging for future investigations of the influence of Aeolus data on African Easterly Waves and associated Mesoscale Convective Systems. [ABSTRACT FROM AUTHOR]