Search

This contribution discusses the link between migrations in the intertropical convergence zone (ITCZ) and changes in the Atlantic meridional overturning circulation (AMOC), Atlantic multidecadal variability (AMV), and Pacific decadal oscillation (PDO). We use a coupled climate model that allows us to integrate over climate noise and assess underlying mechanisms. We use an ensemble of ten 300-yr-long simulations forced by a 50-yr oscillatory North Atlantic Oscillation (NAO)-derived surface heat flux anomaly in the North Atlantic, and a 4000-yr-long preindustrial control simulation performed with GFDL CM2.1. In both setups, an AMV phase change induced by a change in the AMOC’s cross-equatorial heat transport forces an atmospheric interhemispheric energy imbalance that is compensated by a change in the cross-equatorial atmospheric heat transport due to a meridional ITCZ shift. Such linkages occur on decadal time scales in the ensemble driven by the imposed forcing, and internally on multicentennial time scales in the control. Regional precipitation anomalies differ between the ensemble and the control for a zonally averaged ITCZ shift of similar magnitude, which suggests a dependence on timescale. Our study supports observational evidence of an AMV–ITCZ link in the twentieth century and further links it to the AMOC, whose long-time-scale variability can influence the phasing of ITCZ migrations. In contrast to the AMV, our calculations suggest that the PDO does not drive ITCZ migrations, because the PDO does not modulate the interhemispheric energy balance.

Using the Max Planck Institute Grand Ensemble (MPI-GE) with 200 members for the historical simulation (1850–2005), we investigate the impact of the spatial distribution of volcanic aerosols on the El Niño–Southern Oscillation (ENSO) response. In particular, we select three eruptions (El Chichón, Agung and Pinatubo) in which the aerosol is respectively confined to the Northern Hemisphere, the Southern Hemisphere or equally distributed across the Equator. Our results show that relative ENSO anomalies start at the end of the year of the eruption and peak in the following one. We especially found that when the aerosol is located in the Northern Hemisphere or is symmetrically distributed, relative El Niño-like anomalies develop, while aerosol distribution confined to the Southern Hemisphere leads to a relative La Niña-like anomaly. Our results point to the volcanically induced displacement of the Intertropical Convergence Zone (ITCZ) as a key mechanism that drives the ENSO response, while suggesting that the other mechanisms (the ocean dynamical thermostat and the cooling of tropical northern Africa or the Maritime Continent) commonly invoked to explain the post-eruption ENSO response may be less important in our model.

It has been well known that the uplift of the Tibetan Plateau (TP) can significantly enhance the Asian monsoon. Here, by comparing the sensitivity experiments with and without the TP, we find that the TP uplift can also increase the precipitation of the North American Summer Monsoon (NASM), with atmosphere teleconnection accounting for 6% and oceanic dynamical process accounting for another 6%. Physically, the TP uplift generates a stationary Rossby wave train traveling from the Asian continent to the North Atlantic region, resulting in an high-pressure anomaly over the tropical-subtropical North Atlantic. This high pressure system enhances the low-level easterly winds, forcing an enhanced upward motion over the North American monsoon (NAM) region and then an increase in summer precipitation there. In addition, the TP uplift enhances the Atlantic meridional overturning circulation, which reduces the meridional temperature gradient and leads to a northward shift of Hadley Cell over eastern Pacific-Atlantic section. The latter shifts the convection center northward to 10°N and further increases the NASM precipitation. The enhanced NASM precipitation can also be understood by the northward shift of Intertropical Convergence Zone. Our study implies that the changes of NAM climate can be affected by not only local process but also remote forcing, including those from Asian highland region.

Organised cloud bands are important features of tropical and subtropical rainfall. These structures are often regarded as convergence zones, alluding to an association with coherent atmospheric flow. However, the flow kinematics is not usually taken into account in classification methods for this type of event, as large-scale lines are rarely evident in instantaneous diagnostics such as Eulerian convergence. Instead, existing convergence zone definitions rely on heuristic rules of shape, duration and size of cloudiness fields. Here we investigate the role of large-scale turbulence in shaping atmospheric moisture in South America. We employ the Finite-Time Lyapunov Exponent (FTLE), a metric of deformation among neighboring trajectories, to define convergence zones as attracting Lagrangian Coherent Structures (LCSs). Attracting LCSs frequent tropical and subtropical South America, with climatologies consistent with the South Atlantic Convergence Zone (SACZ), the South American Low-level Jet (SALLJ) and the Intertropical Convergence Zone (ITCZ). In regions under the direct influence of the ITCZ and the SACZ, rainfall is significantly positively correlated with large-scale mixing measured by the FTLE. Attracting LCSs in South and Southeast Brazil are associated with significant positive rainfall and moisture flux anomalies. Geopotential height composites suggest that the occurrence of attracting LCSs in these regions is related with teleconnection mechanisms such as the Pacific-South Atlantic. We believe that this kinematical approach can be used as an alternative to region-specific convergence zone classification algorithms; it may help advance the understanding of underlying mechanisms of tropical and subtropical rain bands and their role in the hydrological cycle.

BCC-CSM2-HR is a high-resolution version of the Beijing Climate Center (BCC) Climate System Model (T266 in the atmosphere and 1/4∘ latitude × 1/4∘ longitude in the ocean). Its development is on the basis of the medium-resolution version BCC-CSM2-MR (T106 in the atmosphere and 1∘ latitude × 1∘ longitude in the ocean) which is the baseline for BCC participation in the Coupled Model Intercomparison Project Phase 6 (CMIP6). This study documents the high-resolution model, highlights major improvements in the representation of atmospheric dynamical core and physical processes. BCC-CSM2-HR is evaluated for historical climate simulations from 1950 to 2014, performed under CMIP6-prescribed historical forcing, in comparison with its previous medium-resolution version BCC-CSM2-MR. Observed global warming trends of surface air temperature from 1950 to 2014 are well captured by both BCC-CSM2-MR and BCC-CSM2-HR. Present-day basic atmospheric mean states during the period from 1995 to 2014 are then evaluated at global scale, followed by an assessment on climate variabilities in the tropics including the tropical cyclones (TCs), the El Niño–Southern Oscillation (ENSO), the Madden–Julian Oscillation (MJO), and the quasi-biennial oscillation (QBO) in the stratosphere. It is shown that BCC-CSM2-HR represents the global energy balance well and can realistically reproduce the main patterns of atmospheric temperature and wind, precipitation, land surface air temperature, and sea surface temperature (SST). It also improves the spatial patterns of sea ice and associated seasonal variations in both hemispheres. The bias of the double intertropical convergence zone (ITCZ), obvious in BCC-CSM2-MR, almost disappears in BCC-CSM2-HR. TC activity in the tropics is increased with resolution enhanced. The cycle of ENSO, the eastward propagative feature and convection intensity of MJO, and the downward propagation of QBO in BCC-CSM2-HR are all in a better agreement with observations than their counterparts in BCC-CSM2-MR. Some imperfections are, however, noted in BCC-CSM2-HR, such as the excessive cloudiness in the eastern basin of the tropical Pacific with cold SST biases and the insufficient number of tropical cyclones in the North Atlantic.

Orography has considerable impacts on the large-scale atmospheric circulation, emphasizing the necessity of adequate representation of the impacts of orography in numerical models. The regional climate model version 4 (RegCM4) is used to investigate the impacts of orography on the large-scale atmospheric circulation. Three numerical experiments in four different years for winter and summer have been conducted over a large geographical area, covering Eurasia, Africa and Oceania. These experiments include control simulations using the real orography, simulations with the removed orography of the whole domain and simulations with the removed orography of the whole domain except the Tibetan Plateau. In winter, the Tibetan Plateau prevents the development of the sea-level high pressure in South Asia and contributes to the intensification of the Siberian high through blocking cold air advection from Siberia toward India. The Tibetan Plateau is also responsible for the southward displacement of low-level easterly flows in the North Indian Ocean, such that elimination of this Plateau is associated with more zonal orientation and intensification of easterly winds, as well as an increase of moisture flux over India and the Arabian Sea. Descending motions associated with lee waves of the Western Ghat Mountains contribute to a decrease of precipitation over the Arabian Sea. In summer, the Tibetan Plateau reinforces the South Asian low-pressure system and pushes the South Asian monsoon to South Asia. Both the tropical easterly jet stream over the southern Tibetan Plateau and the subtropical westerly jet stream over the Tibetan Plateau are weakened when the whole orography is removed. Removal of the whole orography is associated with a considerable equatorward displacement of the intertropical convergence zone over South Asia. In austral winter, low-level subtropical anticyclones in Southern Africa and Australia are intensified when the whole orography is removed.

After India separated from Gondwana, its 9000 km northward voyage from the Southern Hemisphere to its modern position joined with Eurasia took 160 million years. During that journey, India experienced a range of climatic conditions due to secular climate variations and its changing latitudinal position. Documenting India's climate during its trans-equatorial journey allows us to sample climate across the low latitudes, at a time when global temperatures were exceptionally high, such as we might experience in the future. Here, we reconstruct, quantitatively, the palaeoclimate of India during its voyage, focussing on the latest Maastrichtian–earliest Danian, late Paleocene, early Eocene and late Oligocene timeslices. We exploit the relationship between fossil leaf form and climate and use the Climate-Leaf Analysis Multivariate Program to reconstruct 24 individual palaeoclimate parameters, while fossil leaf traits also allow us to identify different monsoon types. We find that throughout the Paleogene India persistently experienced a monsoon climate similar to that of modern Indonesia-Australia; a monsoon system that exists today primarily as a function of seasonal trans-equatorial migrations of the Inter-tropical Convergence Zone (ITCZ). The reconstructed near-equatorial thermal regime of India was cooler, and showed a greater seasonal temperature variation than at similar latitudes today, suggesting a wider seasonal latitudinal migration of the ITCZ throughout the Paleogene. Leaf traits showing adaptation to the South Asia Monsoon only developed after the end of the Paleogene.

Due to different orbital configurations, high northern latitude summer insolation was higher during the Last Interglacial period (LIG; 129–116 thousand years before present, ka) than during the pre-industrial period (PI), while high southern latitude summer insolation was lower. The climatic response to these changes is studied here with focus on the Southern Hemisphere monsoons, by performing an equilibrium experiment of the LIG at 127 ka with the Australian Earth System Model, ACCESS-ESM1.5, as part of the Paleoclimate Model Intercomparison Project 4 (PMIP4). Simulated mean surface air temperature between 40 and 60∘ N over land during boreal summer is 6.5 ∘C higher at the LIG compared to PI, which leads to a northward shift of the Intertropical Convergence Zone (ITCZ) and a strengthening of the North African and Indian monsoons. Despite 0.4 ∘C cooler conditions in austral summer in the Southern Hemisphere (0–90∘ S), annual mean air temperatures are 1.2 ∘C higher at southern mid-latitudes to high latitudes (40–80∘ S). These differences in temperature are coincident with a large-scale reorganisation of the atmospheric circulation. The ITCZ shifts southward in the Atlantic and Indian sectors during the LIG austral summer compared to PI, leading to increased precipitation over the southern tropical oceans. However, weaker Southern Hemisphere insolation during LIG austral summer induces a significant cooling over land, which in turn weakens the land–sea temperature contrast, leading to an overall reduction (−20 %) in monsoonal precipitation over the Southern Hemisphere's continental regions compared to PI. The intensity and areal extent of the Australian, South American and South African monsoons are consistently reduced in LIG. This is associated with greater pressure and subsidence over land due to a strengthening of the Southern Hemisphere Hadley cell during austral summer.

Preliminary studies on ITCZ show that the displacement of this system can play an undeniable role in the climate of tropical regions as well as in surrounding areas. The purpose of this study is to investigate the annual ITCZ movement in the Indian Ocean and East Africa region and to select the appropriate and reliable index for ITCZ positioning. To conduct this research, the ERA-interim reanalysis gridded data from the European Centre for Medium-Range Weather Forecasts (ECMWF) was used to determine the location and spatial displacement of the ITCZ. The results showed that the HCC index is the appropriate index to detect the annual ITCZ displacement in all months of the year. The SH and P indices were in second place in terms of importance and accuracy in identifying the behavior and spatial displacement of the ITCZ system. The TCWV index has been a weak index to detect the ITCZ annual location and displacement. During the three months of winter, the ITCZ bends off the east coast of Africa to the southern latitudes and enters the waters of the Indian Ocean along the northwest-southeast. In the other words, the average ITCZ position on land is about 5˚–6˚ north latitude over the waters of the Indian Ocean. On the contrary, from April to the end of September, in the range of 30˚–50˚ E (Sudan), the ITCZ bends and extends to the northeast. During the summer months, this northward movement sometimes reaches up to 20˚.

To imply the gravity of their impact on Christmas celebration, the term Christmas typhoon recently became more popular to refer to tropical cyclones (TC) in the Western North Pacific (WNP) during its less active season. The past 9 years from 2012 to 2020 saw more than 70% (210%) increases in Christmas typhoon occurrences in the WNP (Philippines). Furthermore, Mindanao Island, which is located in southern Philippines, has experienced an unprecedented 480% increase in TC passage in the same period. Here we show that the detected recent increase in Christmas typhoons are mainly associated with the shift of the Pacific Decadal Oscillation to its positive phase in early 2010s, which led to favorable changes in the large-scale environment for TC development such as higher relative vorticity, anomalous low-level westerlies, warmer sea surface temperatures in the central Pacific, and extended WNP subtropical high. We also found that the poleward shift of the Intertropical Convergence Zone and possibly, the recent recovery of the Siberian High contributed to such increased occurrences. As opposed to the more active TC season, there is a wide research gap during the less active season. We aim to fill in this knowledge gap to gain better insights on TC risk reduction.

Anomalous seasonal patterns of precipitation variability over South America (SA) associated with El Nino-Southern Oscillation (ENSO) and non-ENSO (residual) conditions were assessed during 1951–2016. Patterns were obtained from empirical orthogonal functions analysis of total and residual precipitation seasonal anomalies. In austral spring and summer, precipitation variability is dominated by a dipolar anomaly mode with a centre extending from northwestern to northeastern SA and another in central-eastern Brazil and part of southeastern SA (SESA) during spring, and a centre in northwestern SA and another extending from northeastern SA to central and eastern Brazil and central SESA, during summer. These modes are associated with ENSO to a greater extent during spring than summer. In summer, there is a strong association of the dipolar precipitation pattern with sea surface temperature (SST) anomalies on the east coast of Brazil, which indicates local influence. In austral fall, SST anomalies in the tropical South Atlantic relate to precipitation anomalies in northeast SA, and those in the tropical north Atlantic (TNA) to precipitation anomalies in northwestern SA, through the intertropical convergence zone anomalous position modulated by SST anomalies. In this same condition, the ENSO acts only to intensify or weaken the dominant precipitation pattern, depending on its phase, mainly over SESA. In contrast, the second variability mode in fall is characterized by positive SST anomalies in the Indian Ocean and equatorial and southern Atlantic Ocean and negative in the TNA. The importance of ENSO and the Indian Ocean in the characterization of the SST dipole in the tropical Atlantic explains the main changes in precipitation patterns over northeastern Brazil not been discussed in previous studies.

The fidelity of the eight Asia-Pacific Economic Cooperation (APEC) Climate Center (APCC) models in representing the inter-annual variability and decadal shift in the northeast monsoon (NEM; October–December) rainfall over Southern Peninsular India (SPI) is evaluated. The hindcast data is used for the period of 28 years from 1983 to 2010 based on September initial conditions. The observations showed a clear inter-annual and inter-decadal variability of NEM rainfall during the study period. The analysis suggests that most of the models exhibited poor skill in representing the inter-annual variability. Only APCC model rainfall is in phase with observed SPI rainfall variations on the inter-annual time scale. It is noticed from the observed NEM rainfall time series that the period 1990–1999 (first decade) displays an above-normal rainfall and the period 2000–2010 (second decade) displays a below normal rainfall over the SPI region. It is also evident from the observations that NEM rainfall for most of the years displayed negative anomalies in the second decade including El Nino and La Nina years, while in the first decade positive anomalies are noted, suggesting the presence of decadal variability in the NEM. Rainfall variations in most of the coupled models are in phase with the observations during the second decade but are out of phase during the first decade. As evidenced from the observations that the intensified deep convection over the Indo-western Pacific region results in too far southward movement of the intertropical convergence zone (ITCZ) during the second decade. The southward shift in the strong upper-level divergence associated with lower-level convergence over the south Indian Ocean caused negative rainfall anomalies over the SPI in this decade. Further, the difference between the second and first decade demonstrates that an anomalous anticyclonic circulation over the Indian subcontinent is accountable for the vigorous dry northerly flow towards the SPI region and the resultant decadal shift in the rainfall pattern. Though the southward shift in the rainfall and large-scale circulation patterns are mildly captured by some models from decade to decade, most of the models completely misrepresented it. This study suggests that the coupled models displayed a very limited skill not only in capturing the inter-annual variability but also in representing the decadal variability of NEM rainfall.

From a climate perspective, land differs from the ocean in several fundamental physical ways, including albedo, heat capacity, amount of water storage, and differences in resistance to evaporation. These differences alter the surface energy and water budgets over land compared to ocean, with implications for both surface climate and atmospheric circulation. In this study, we use an idealized general circulation model (Isca) to explore the climate state of Northland, a planet with a northern land hemisphere and a southern ocean hemisphere. These idealized simulations are motivated by the asymmetry of continental distribution on the globe, with a greater concentration of landmasses in the northern hemisphere and a larger area of ocean in the southern hemisphere, and further illuminate the basic role that land-sea contrasts play in global atmospheric dynamics. We find a much larger seasonal cycle of temperature over land compared to ocean, as expected. The continent is seasonally wet in the tropics, has a subtropical desert, and a moist high-latitude ``swamp, where moisture transported from the tropics accumulates. Decreasing the land albedo leads to warming. In contrast to past studies, suppressing evaporation from the land surface cools the climate, resulting from decreased atmospheric water vapor and reduced trapping of longwave radiation, which dominates over the warming associated with reduced evaporative cooling at the surface. The ITCZ in the Northland simulations extends farther polewards over both the land and ocean hemispheres than the ITCZ in an aquaplanet. Our results demonstrate the potential for land and hemispheric asymmetries in controlling the large-scale axisymmetric atmospheric circulation.

Most precipitation from deep clouds over the continents and in the intertropical convergence zone is strongly influenced by the presence of ice crystals whose formation requires the presence of ice nucleating particles (INPs). Although there are a large number of INP sources, the ice nucleating abilities of aerosol particles originating from oceans, deserts, and wildfires are poorly described at tropical latitudes. To fill this gap in knowledge, the National Autonomous University of Mexico micro-orifice uniform deposit impactor droplet freezing technique (UNAM-MOUDI-DFT) was constructed to measure the ice nucleating activity of aerosol samples that were collected in Sisal and Mérida, Yucatán (Mexico) under the influence of cold fronts, biomass burning (BB), and African dust (AD) intrusions during five short-term field campaigns between January 2017 and July 2018. The three different aerosol types were distinguished by their physicochemical properties. Marine aerosol (MA), BB, and AD air masses were found to contain INPs; the highest concentrations were in AD (from 0.071 to 36.07 L−1 at temperatures between −18 and −27 ∘C), followed by MA (from 0.068 to 18.90 L−1 at temperatures between −15 and −28 ∘C) and BB (from 0.063 to 10.21 L−1 at temperatures between −20 and −27 ∘C). However, MA had the highest surface active site densities (ns) between −15 and −30 ∘C. Additionally, supermicron particles contributed more than 72 % of the total INP concentration independent of aerosol type.

Recently, El Niño diversity has been paid much attention because of its different global impacts. However, most studies have focused on a single warm peak in sea surface temperature anomalies (SSTAs), either in the central Pacific or the eastern Pacific Ocean. Here, we demonstrate from observational analyses that several recent El Niño events show double warm peaks in SSTA—called “double-peaked (DP) El Niño”—that have only been observed since 2000. The DP El Niño has two warm centers, which grow concurrently but separately, in both the central and eastern Pacific. In general, the atmospheric and oceanic patterns of the DP El Niño are similar to those of the warm-pool (WP) El Niño from the development phase, such that the central Pacific peak is developed by the zonal advective feedback and reduced wind speed anomalies. However, a distinctive difference exists in the eastern Pacific where the DP El Niño has a second SSTA peak. In addition, the DP El Niño shows more distinctive anomalous precipitation along the Pacific intertropical convergence zone (ITCZ) when compared with the WP El Niño. We demonstrate that the peculiar precipitation anomalies along the Pacific ITCZ play a critical role in enhancing the equatorial westerly wind stress anomalies, which help to develop the eastern SSTA peak by deepening the thermocline in the eastern Pacific.

Future changes in the position of the intertropical convergence zone (ITCZ; a narrow band of heavy precipitation in the tropics) with climate change could affect the livelihood and food security of billions of people. Although models predict a future narrowing of the ITCZ, uncertainties remain large regarding its future position, with most past work focusing on zonal-mean shifts. Here we use projections from 27 state-of-the-art (CMIP6) climate models and document a robust zonally-varying ITCZ response to the SSP3-7.0 scenario by 2100, with a northward shift over eastern Africa and the Indian Ocean, and a southward shift in the eastern Pacific and Atlantic Oceans. The zonally-varying response is consistent with changes in the divergent atmospheric energy transport, and sector-mean shifts of the energy flux equator. Our analysis provides insight about mechanisms influencing the future position of the tropical rainbelt, and may allow for more robust projections of climate change impacts.

Each movement, displacement, and change in the intertropical convergence zone (ITCZ) position play an important role in atmospheric circulation and moisture provided for precipitation systems in the North East African and Middle East regions. Accordingly, the present study aimed to evaluate the role of the Arabian subtropical high pressure (ASHP) position as part of Hadley circulation in ITCZ changes in the Indian Ocean and East African region. To this end, the ASHP displacement pattern was modeled for each month with regard to the position of the central core, expansion of the first closed curve around the central core, and the last closed curve of ASHP. The evaluation of ASHP spatial displacement demonstrated that refractions along ITCZ begin early in the winter and move steeply in the range of 40° east (the south of Sudan and Ethiopia) toward the southern latitudes as the eastward and southward displacement, as well as the expansion of ASHP and its troughs. During the summer, ITCZ is placed on the north of the equator in addition to Sudan and Ethiopia, with the deployment of the ASHP central core on the eastern shore of the Red Sea as well as the orbiting of its troughs. In conclusion, the spatial displacement of ASHP and its troughs, especially at 700 hPa and above, is assumed to be a highly important factor in the northward and southward displacement of ITCZ and its significant refraction in both winter and summer in the Horn of Africa.

Warm SST bias underlying the spurious southern ITCZ has long been recognized as one of the main causes for double-ITCZ bias in coupled GCMs in the central Pacific. This study demonstrates that the NCAR CESM1.2 can still simulate significant double-ITCZ bias even with cold SST bias in the southern ITCZ region, indicating that warm SST bias is not a necessary condition for double-ITCZ bias in the central Pacific. Further analyses suggest that the equatorial cold tongue (ECT) biases play important roles in the formation of double-ITCZ bias in the central Pacific. The severe cold SST biases in the ECT region in the central Pacific may enhance the SST gradient between the ECT and southern ITCZ region, strengthening the lower-troposphere dynamical convergence and hence convection in the southern ITCZ region. The formation mechanism of excessive ECT bias is further investigated. It is shown that the cold SST biases in the ECT region can be largely attributed to the anomalous cooling tendency produced by the upper-ocean zonal advection due to overly strong zonal currents. In the ECT region, the westward ocean surface zonal current is driven by the equatorial easterly surface winds. It is shown that convection bias simulated by the atmospheric model in the equatorial Amazon region may lead to easterly wind bias in the downwind side (west) of convection region. The mean Walker circulation transports these easterly wind momentum anomalies downward and westward to the surface, resulting in the overly strong surface easterly wind in the central equatorial Pacific.

The cause of seasonal hydrologic changes in tropical East Asia during interstadial/stadial oscillations of the last glaciation remains controversial. Here, we show seven seasonal drought events that occurred during the relatively warm interstadials by phytolith and pollen records. These events are significantly manifested as high percentages of bilobate phytoliths and are consistent with the large zonal sea-surface temperature (SST) gradient from the western to eastern tropical Pacific, suggesting that the reduction in seasonal precipitation could be interpreted by westward shifts of the western Pacific subtropical high triggered by changes of zonal SST gradient over the tropical Pacific and Hadley circulation in the Northern Hemisphere. Our findings highlight that both zonal and meridional ocean-atmosphere circulations, rather than solely the Intertropical Convergence Zone or El Niño-Southern Oscillation, controlled the hydrologic changes in tropical East Asia during the last glaciation.

Cirrus clouds at heights of 10–18 km are investigated for the seasonal migrations based on 3 years observations of CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) in 2008–2012. Past studies have shown cirrus clouds migrate from their winter peaks in the equatorial regions to the maximum latitudes of ± 30° in the boreal summer as the migration of ITCZ (Intertropical Convergence Zone). However, this general picture does not show detailed distributions by including height, seasons and latitudinal dependences considering cirrus extended height and latitudinal coverages. We are also interested in understanding the occurrences of thin cirrus in the tropical tropopause and formation of all cirrus in the extratropics where little studies about their migration have been made. It is not well know if the summer enhancement in the extratropics is only related to the migration or by other factors. Cirrus distributions are studied in two height groups of low ( 15 km), and in three regions of EQ (− 15° to 15° latitudes), NH (15°–30°), and SH (− 15° to − 30°) for different seasons. Our results show a major part of cirrus, the low cirrus, migrate to the extratropics in the summer. Like ITCZ, they stay mostly in the north (64%) less in the south (36%) following the same migration pattern. High cirrus cloud is decoupled from ITCZ with the major part showing a boreal winter peak spread to higher latitudes with seasons. In the extratropics, enhanced production of high cirrus can be attributed to the convective production in the summer monsoons. The summer convective production is used to estimate the formation of equatorial high cirrus in terms of rapid convection or slow ascending with a ratio determined.

Climate models in phase 5 of the Coupled Model Intercomparison Project (CMIP5) vary significantly in their ability to simulate the phase and amplitude of atmospheric stationary waves in the midlatitude Southern Hemisphere. These models also suffer from a double intertropical convergence zone (ITCZ), with excessive precipitation in the tropical eastern South Pacific, and many also suffer from a biased simulation of the dynamics of the Agulhas Current around the tip of South Africa. The intermodel spread in the strength and phasing of SH midlatitude stationary waves in the CMIP archive is shown to be significantly correlated with the double-ITCZ bias and biases in the Agulhas Return Current. An idealized general circulation model (GCM) is used to demonstrate the causality of these links by prescribing an oceanic heat flux out of the tropical east Pacific and near the Agulhas Current. A warm bias in tropical east Pacific SSTs associated with an erroneous double ITCZ leads to a biased representation of midlatitude stationary waves in the austral hemisphere, capturing the response evident in CMIP models. Similarly, an overly diffuse sea surface temperature gradient associated with a weak Agulhas Return Current leads to an equatorward shift of the Southern Hemisphere jet by more than 3° and weak stationary wave activity in the austral hemisphere. Hence, rectification of the double-ITCZ bias and a better representation of the Agulhas Current should be expected to lead to an improved model representation of the austral hemisphere.

Author(s): Huang, Huilin; Xue, Yongkang; Chilukoti, Nagaraju; Liu, Ye; Chen, Gang; Diallo, Ismaila | Abstract: AbstractLand-use and land-cover change (LULCC) is one of the most important forcings affecting climate in the past century. This study evaluates the global and regional LULCC impacts in 1950–2015 by employing an annually updated LULCC map in a coupled land–atmosphere–ocean model. The difference between LULCC and control experiments shows an overall land surface temperature (LST) increase by 0.48 K in the LULCC regions and a widespread LST decrease by 0.18 K outside the LULCC regions. A decomposed temperature metric (DTM) is applied to quantify the relative contribution of surface processes to temperature changes. Furthermore, while precipitation in the LULCC areas is reduced in agreement with declined evaporation, LULCC causes a southward displacement of the intertropical convergence zone (ITCZ) with a narrowing by 0.5°, leading to a tripole anomalous precipitation pattern over the warm pool. The DTM shows that the temperature response in LULCC regions results from the competing effect between increased albedo (cooling) and reduced evaporation (warming). The reduced evaporation indicates less atmospheric latent heat release in convective processes and thus a drier and cooler troposphere, resulting in a reduction in surface cooling outside the LULCC regions. The southward shift of the ITCZ implies a northward cross-equatorial energy transport anomaly in response to reduced latent/sensible heat of the atmosphere in the Northern Hemisphere, where LULCC is more intensive. Tropospheric cooling results in the equatorward shift of the upper-tropospheric westerly jet in both hemispheres, which, in turn, leads to an equatorward narrowing of the Hadley circulation and ITCZ.

In this study, long-term structural changes in the intertropical convergence zone (ITCZ) and associated precipitation–radiation–circulation feedback processes are examined using multiple sources of reanalysis data for temperature, winds, moisture, and observed precipitation and outgoing longwave radiation (OLR) during 1980–2014. Consistent with CMIP5 climate model projections of the “deep tropical squeeze” under greenhouse warming, this period witnessed a warming and wetting (increased specific humidity) global trend, characterized by a narrowing of the ITCZ core with increased precipitation, coupled to widespread tropospheric drying (deficient relative humidity), increased OLR in the subtropics and midlatitudes, a widening of the descending branches of the Hadley circulation, and a poleward shift of the jet streams in both hemispheres. The widespread tropospheric drying stems from 1) a faster rate of increased saturated water vapor with warming, relative to the increase in ambient moisture due to convective and large-scale transport, and 2) enhanced anomalous subsidence, and low-level moisture divergence in the subtropics and midlatitudes. The long-term trend in enhanced precipitation (latent heating) in the ITCZ core region is strongly coupled to increasing OLR (radiative cooling to space) in the expanding dry zones, particularly over land regions in the subtropics and midlatitudes, arguably as a necessary condition for global thermodynamic energy balance. Analyses of the trend patterns in vertical profiles of p velocity, temperature, and relative humidity with respect to ITCZ precipitation rate and OLR reveal that the contrast between the wet and dry regions in the troposphere has been increasing globally, with the ITCZ core getting wetter and contracting, while the marginal convective and dry zones are getting drier and expanding.

After each of the 1963 Agung, 1982 El Chichón, and 1991 Pinatubo eruptions, an El Niño was observed. The increased likelihood of an El Niño after a tropical eruption has also been found in long-term reconstructed proxy data. Through examining simulations over the last millennium by 11 different models, we show that a tropical volcano eruption can robustly excite a western-to-central equatorial Pacific (WCEP) westerly anomaly at 850 hPa in eight out of the 11 models; such a westerly anomaly is favorable for El Niño development. Under the volcanic forcing, there are significant extratropical continent surface cooling and tropical drying with negative precipitation anomalies over the South–South East Asia (SSEA), West African monsoon, and Intertropical Convergence Zone (ITCZ) regions. This common precipitation suppression response occurs in most of the models. Sensitivity experiments show that a WCEP westerly anomaly can be excited by the tropical land cooling, especially the SSEA cooling induced precipitation suppression rather than by the extratropical land surface cooling. Theoretical results show that a WCEP westerly anomaly is excited due to a Gill response to reduced precipitation over the SSEA and West African monsoon regions; and the SSEA contributes more than the West African monsoon does. The ITCZ weakening, however, excites an easterly wind anomaly. The models with more sensitive convective feedback tend to simulate an El Niño more easily, while a failed simulation of an El Niño after a robust westerly anomaly in some models calls for further studies on these models’ delayed responses to radiative forcing induced by volcano eruptions.

To better understand the climate response under stabilized, overshoot, and transient global warming, four types of ensemble experiments on 1.5 °C/2 °C global warming scenarios (i.e., stabilized 1.5 °C, 1.5 °C overshoot, stabilized 2 °C, and transient 2 °C) are elaborately designed using the Nanjing University Information Science and Technology Earth System Model (NESM). Compared with the modern climate (1985–2014), the projected surface air temperature (SAT) change is characterized by a robust ‘Northern Hemisphere (NH)-warmer than-Southern Hemisphere (SH)’ and ‘land-warmer than-ocean’ patterns. The projected precipitation change exhibits ‘NH-wetter than-SH’ pattern in the tropics. Although the response of SAT and precipitation climatology show similar pattern between stabilized and overshoot scenarios, some significant differences are still found. The projected change in the Northern Hemisphere land monsoon precipitation (NHLMP) is 30% larger in the transient 2 °C experiment compared with that in the stabilized 2 °C experiment. The more vigorous NHLMP in the transient global warming scenario is mainly due to the enhanced land–sea thermal contrast and interhemispheric temperature difference. The enlarged land–sea thermal contrast increases the surface pressure gradient between the NH continents and its adjacent oceans, thus enhancing the NH monsoon circulation and moisture convergence. The enhanced interhemispheric temperature difference shifts the Hadley circulation and intertropical convergence zone northward, leading to the enhanced moisture convergence and the shifts of tropical rain band over the NH monsoon region. This result highlights that climate responses may depend on different warming trajectories and, which could facilitate the strategic planning of governments.

Bursts of carbon dioxide, released into the atmosphere and occurring on centennial time scales, were seen during the cold periods of the last glacial cycle but not in older or warmer conditions. Nehrbass-Ahles et al. present a record of atmospheric carbon dioxide concentrations retrieved from the European Project for Ice Coring in Antarctica Dome C ice core showing that these carbon dioxide jumps occurred during both cold and warm periods between 330,000 and 450,000 years ago. They relate these pulses to disruptions of the Atlantic meridional overturning circulation caused by freshwater discharge from ice sheets. Such rapid carbon dioxide increases could occur in the future if global warming also disrupts this ocean circulation pattern.Science, this issue p. 1000Pulse-like carbon dioxide release to the atmosphere on centennial time scales has only been identified for the most recent glacial and deglacial periods and is thought to be absent during warmer climate conditions. Here, we present a high-resolution carbon dioxide record from 330,000 to 450,000 years before present, revealing pronounced carbon dioxide jumps (CDJ) under cold and warm climate conditions. CDJ come in two varieties that we attribute to invigoration or weakening of the Atlantic meridional overturning circulation (AMOC) and associated northward and southward shifts of the intertropical convergence zone, respectively. We find that CDJ are pervasive features of the carbon cycle that can occur during interglacial climate conditions if land ice masses are sufficiently extended to be able to disturb the AMOC by freshwater input.

In the present paper, we study the climatological annual mean Hadley circulation simulated by 47 coupled atmospheric and ocean general circulation models (AOGCMs) of the phase 5 of the Coupled Model Intercomparison Project (CMIP5). Our results show that the climatological annual mean states of the Hadley circulation, i.e., strength, central line, and poleward boundaries of the Hadley cells, have a large inter-model spread. The strength of the Hadley cells, i.e., the maximum value of mean meridional streamfunction (MMS), varies from 7.0 × 1010 to 11.1 × 1010 kg s− 1. The variation range is about half of the multi-model ensemble mean strength of about 8.7 × 1010 kg s− 1. The central line location between the two Hadley cells varies from 3.7° S to 7.8° N, and the poleward boundary latitudes have a variation range of 5.5° in latitude. In contrast, inter-model spreads of the Hadley circulation are much smaller in reanalyses. Using a method of inter-model empirical orthogonal function (EOF) analysis of annual-mean MMS, we show that the first principal component (PC1) is closely correlated with the central line latitudes of the Hadley cells, and that the second principal component (PC2) has a high correlation with the poleward boundaries. Both PC1 and PC2 are significantly correlated with the strength of the Hadley cells. Regressions of sea surface temperatures (SSTs) and precipitation on PC1 and PC2 show that EOF1 is associated with the well-known double intertropical convergence zone (ITCZ) problem in AOGCMs, and that EOF2 is related to the cold tongue bias. The results suggest that the large inter-model spread of the climatological mean Hadley circulation is largely due to the double ITCZ and cold tongue biases, and that the key for improving the simulation performance of the Hadley circulation is to reduce the double ITCZ and cold tongue bias in AOGCMs.

This study provides an updated dynamical analysis of the observed climate conditions during the longest, most severe and continuous drought (2012–2016) ever recorded in Northeast Brazil (NEB). This investigation was carried out based on temperature, precipitation, sea surface temperature, outgoing long-wave radiation, and ERA-Interim reanalysis data from 1981 to 2016. This was done by first looking for the area of the NEB where the drought was most severe, and then calculating the main rainy season anomalies and atmospheric parameters from 2012 to 2016 in relation to the climatological conditions (1981–2010). Results show this drought event was influenced mainly by two distinct oceanic conditions that induced anomalous atmospheric circulation patterns. The observed negative precipitation anomalies were firstly related to the northward displacement of the Intertropical Convergence Zone (ITCZ), followed by an anomalous upward motion over the Western Amazon, as well as the anomalous precipitation deficit along the South Atlantic Convergence Zone, that was slightly displaced southward of the climatological position. From 2015 to 2016 the subsidence over NEB Brazil was linked to the descending branch of the Walker circulation, as well as to the strong anomalous flow induced by convection in the ITCZ over Africa and Gulf of Mexico.

Despite the pervasive impact of drought on human and natural systems, the large-scale mechanisms conducive to regional drying remain poorly understood. Here we use a multivariate approach1,2 to identify two distinct externally forced fingerprints from multiple ensembles of Earth system model simulations. The leading fingerprint, FM1(x), is characterized by global warming, intensified wet–dry patterns3 and progressive large-scale continental aridification, largely driven by multidecadal increases in greenhouse gas (GHG) emissions. The second fingerprint, FM2(x), captures a pronounced interhemispheric temperature contrast4,5, associated meridional shifts in the intertropical convergence zone6–9 and correlated anomalies in precipitation and aridity over California10, the Sahel11,12 and India. FM2(x) exhibits nonlinear temporal behaviour: the intertropical convergence zone moves southwards before 1975 in response to increases in hemispherically asymmetric sulfate aerosol emissions, and it shifts northwards after 1975 due to reduced sulfur dioxide emissions and the GHG-induced warming of Northern Hemisphere landmasses. Both fingerprints are statistically identifiable in observations of joint changes in temperature, rainfall and aridity during 1950–2014. We show that the reliable simulation of these changes requires combined forcing by GHGs, direct and indirect effects of aerosols, and large volcanic eruptions. Our results suggest that GHG-induced aridification may be modulated regionally by future reductions in sulfate aerosol emissions. Large-scale mechanisms causing regional drying are not well understood. Models and observational data reveal that human-caused changes in GHGs and aerosols led to detectable global and hemispheric signals in the joint behaviour of precipitation, temperature and aridity since the 1950s.

The semi-arid African Sahel region is highly sensitive to changes in monsoon precipitation, as much of the region’s workforce is employed in the agricultural industry (Hamro-Drotz and Programme 2011). Thus, studying the response of rainfall and aridity in this region to radiative perturbations is a matter of pressing humanitarian relevance. In addition, there is evidence to suggest that spatially asymmetric volcanic aerosols produce different hydroclimatic responses based on their hemispheric symmetry, both globally and in the Sahel. We use two different climate models, GFDL’s FLOR model (Vecchi et al. in J Clim 27(21):7994–8016, 2014) and NCAR’s CESM 1.1 model (Otto-Bliesner et al. in Bull Am Meteorol Soc 97(5):735–754, 2016), to characterize the response of rainfall in the Sahel to large volcanic eruptions based on the meridional symmetry of the volcanic eruptions. We find that in both the FLOR experiments simulating three large twentieth century eruptions and in the CESM Last Millennium Ensemble simulations of 46 historic volcanic eruptions, asymmetric Northern Hemisphere cooling causes a subsequent drying response in the Sahel, and Southern Hemisphere cooling causes a wetting, or “greening” response. Symmetric volcanic eruptions have a relatively small effect on rainfall in the Sahel. We also find that the FLOR results show a consistent response in the annual rainfall cycle in the Sahel for all three of the eruptions analyzed, with a reduction in rainfall in early summer followed by an increased rainfall in late summer. The annual cycle response of rainfall in the Sahel from the CESM experiments is different, in that the SH eruptions cause a rainfall maximum in August, NH eruptions cause a rainfall minimum in September, and symmetric eruptions show a slight increase in August and a decrease in October. Our results highlight the need for accurate meridional structures in historic volcanic forcing data used for climate models as well as the need for further study on regional effects of hemispherically asymmetric radiative forcing, especially as they might pertain to aerosol geoengineering.

This study examines the climatology of cloud phase over Southeast Asia (SEA) based on A-Train satellite observations. Using the combined CloudSat–CALIPSO (CC) data, five main cloud groups are investigated: ice-only, ice-above-liquid, liquid-only, ice-above-mixed, and mixed-only clouds that have annual mean frequencies of 28.6 %, 20.1 %, 16.0 %, 9.3 %, and 6.7 %, respectively. Liquid-only clouds tend to occur in relatively cold, dry, and stable lower troposphere. The other four cloud groups appear more frequently in relatively warm, humid, and unstable conditions, and their seasonal distributions move with the Asian monsoon and the Intertropical Convergence Zone (ITCZ). Liquid clouds are found to be highly inhomogeneous based on the heterogeneity index (Hσ) from Aqua Moderate Resolution Imaging Spectroradiometer (MODIS), while ice-only and mixed-only clouds are often very smooth. Ice-above-liquid clouds are more heterogeneous than ice-only clouds owing to ice clouds being optically thin. We demonstrate that the distribution of clear-sky Hσ has a long tail towards heterogeneous values that are caused by undetected subpixel cloud within both CC and MODIS datasets. The reflectance at 0.645 µm (R0.645) and brightness temperature at 11 µm (BT11) of CC ice-only, liquid-only, and ice-above-liquid clouds show peak frequencies near that of clear sky (R0.645∼0.02; BT11∼294 K), which explains why up to 30 % of these CC cloud groups are classified as clear by MODIS. In contrast, mixed-only clouds are thick (average top ∼13 km), bright (average R0.645∼0.6), and cold (average BT11 ∼234 K). Cloud phase comparison between CC and MODIS reveals only modest agreement, with the best agreement (73 %) occurring between CC ice-above-mixed and MODIS ice clouds. The intraseasonal and interannual behaviors of the all-sky Hσ and spectral signatures follow that of cloud phase and vary with the Madden–Julian oscillation (MJO) and the El Niño–Southern Oscillation (ENSO) phases.

The simulated climate of the American monsoon system (AMS) in the UK models HadGEM3 GC3.1 (GC3) and the Earth system model UKESM1 is assessed and compared to observations and reanalysis. We evaluate the pre-industrial control, AMIP and historical experiments of UKESM1 and two configurations of GC3: a low (1.875∘×1.25∘) and a medium (0.83∘×0.56∘) resolution. The simulations show a good representation of the seasonal cycle of temperature in monsoon regions, although the historical experiments overestimate the observed summer temperature in the Amazon, Mexico and Central America by more than 1.5 K. The seasonal cycle of rainfall and general characteristics of the North American monsoon of all the simulations agree well with observations and reanalysis, showing a notable improvement from previous versions of the HadGEM model. The models reasonably simulate the bimodal regime of precipitation in southern Mexico, Central America and the Caribbean known as the midsummer drought, although with a stronger-than-observed difference between the two peaks of precipitation and the dry period. Austral summer biases in the modelled Atlantic Intertropical Convergence Zone (ITCZ), cloud cover and regional temperature patterns are significant and influence the simulated regional rainfall in the South American monsoon. These biases lead to an overestimation of precipitation in southeastern Brazil and an underestimation of precipitation in the Amazon. The precipitation biases over the Amazon and southeastern Brazil are greatly reduced in the AMIP simulations, highlighting that the Atlantic sea surface temperatures are key for representing precipitation in the South American monsoon. El Niño–Southern Oscillation (ENSO) teleconnections, of precipitation and temperature, to the AMS are reasonably simulated by all the experiments. The precipitation responses to the positive and negative phase of ENSO in subtropical America are linear in both pre-industrial and historical experiments. Overall, the biases in UKESM1 and the low-resolution configuration of GC3 are very similar for precipitation, ITCZ and Walker circulation; i.e. the inclusion of Earth system processes appears to make no significant difference for the representation of the AMS rainfall. In contrast, the medium-resolution HadGEM3 N216 simulation outperforms the low-resolution simulations due to improved SSTs and circulation.

International audience This study presents the global climate model IPSL-CM6A-LR developed at Institut Pierre-Simon Laplace (IPSL) to study natural climate variability and climate response to natural and anthropogenic forcings as part of the sixth phase of the Coupled Model Intercomparison Project (CMIP6). This article describes the different model components, their coupling, and the simulated climate in comparison to previous model versions. We focus here on the representation of the physical climate along with the main characteristics of the global carbon cycle. The model's climatology, as assessed from a range of metrics (related in particular to radiation, temperature, precipitation, and wind), is strongly improved in comparison to previous model versions. Although they are reduced, a number of known biases and shortcomings (e.g., double Intertropical Convergence Zone [ITCZ], frequency of midlatitude wintertime blockings, and El Niño–Southern Oscillation [ENSO] dynamics) persist. The equilibrium climate sensitivity and transient climate response have both increased from the previous climate model IPSL-CM5A-LR used in CMIP5. A large ensemble of more than 30 members for the historical period (1850–2018) and a smaller ensemble for a range of emissions scenarios (until 2100 and 2300) are also presented and discussed.

The interannual variability of precipitation in southern Mexico, northwestern Central America, and part of the northeastern tropical Pacific and its relation with previous oceanic and atmospheric anomalies are studied using the National Centers for Environmental Prediction Climate Forecast System Reanalysis data for 1979–2015. The rainy season over the study region was divided into three intraseasonal phases: before, during, and after the mid-summer drought. The interannual precipitation anomalies between the three phases are significantly correlated with each other, although there are years in which the precipitation anomalies for different phases differ in their sign. The interannual variability of the precipitation in this region is closely related with previous oceanic and atmospheric anomalies that occurred in the 60–31 days before the beginning of each phase. Multilinear regression models, between precipitation anomalies and previous anomalies of oceanic and atmospheric variables, explain 49, 60, and 55% of the variance of the precipitation interannual variability for the respective phase. Within a phase, in more than 76% of the cases, the estimated precipitation anomalies had the correct sign but in most cases their amplitude, irrespective of sign, were underestimated, as is expected from a regression model which aims to estimate the conditional mean. The more relevant processes associated with the interannual variability of precipitation in the study area are the westward displacement and strengthening of the North Atlantic Subtropical High, the meridional displacement of the Intertropical Convergence Zone and the strength of the Caribbean Low-Level Jet. In general, the mechanisms that explain interannual variability of precipitation are the same through the phases. However, the magnitude of the anomalies and the size of the regions evolve in time in a particular way for each variable.

In phases 5 and 6 of the state-of-the-art Coupled Model Intercomparison Project (CMIP5 and CMIP6, respectively) models, there is an apparent excessive rainfall bias with a negative SST bias in the tropical Pacific intertropical convergence zone (ITCZ). The regime of the excessive ITCZ but negative SST bias is inconsistent with the common positive rainfall–SST correlation of climate anomalies over the tropics. Using a two-mode model, we decomposed the rainfall bias into two components and found that the surface convergence (SC) bias is the key factor forming the excessive ITCZ bias in the historical runs of 25 CMIP5 models and 23 CMIP6 models. A mixed layer model was further applied to connect the formation of the SC bias with the SST pattern bias. The results suggest that the meridional pattern of the SST bias plays a key role in forming the SC bias. In the CMIP5 and CMIP6 models, the overall negative SST bias has two apparent meridional troughs at around 10°S and 10°N, respectively. The two meridional troughs in the SST bias drive two convergence centers in the SC bias favoring the excessive ITCZ, even though the local SST bias is negative.

Regional-scale precipitation responses over Indonesia to major climate modes in the tropical Indo–Pacific Oceans, namely canonical El Niño, El Niño Modoki, and the Indian Ocean dipole (IOD), and how the responses are related to large-scale moisture convergences are investigated. The precipitation responses, analyzed using a high-spatial-resolution (0.5° × 0.5°) terrestrial precipitation dataset for the period 1960–2007, exhibit differences between the dry (July–September) and wet (November–April) seasons. Canonical El Niño strongly reduces precipitation in central to eastern Indonesia from the dry season to the early wet season and northern Indonesia in the wet season. El Niño Modoki also reduces precipitation in central to eastern Indonesia during the dry season, but conversely increases precipitation in western Indonesia in the wet season. Moisture flux analysis indicates that corresponding to the dry (wet) season precipitation reduction due to the canonical El Niño and El Niño Modoki anomalous divergence occurs around the southern (northern) edge of the convergence zone when one of the two edges is located near the equator (10°S–15°N) associated with their seasonal migration. This largely explains the seasonality and regionality of precipitation responses to canonical El Niño and El Niño Modoki. IOD reduces precipitation in southwestern Indonesia in the dry season, associated with anomalous moisture flux divergence. The seasonality of precipitation response to IOD is likely to be controlled by the seasonality of local sea surface temperature anomalies in the eastern pole of the IOD.

Many recent studies have aimed to better understand changes in the characteristics of the intertropical convergence zone (ITCZ), including ITCZ location, width, and precipitation intensity. However, very few studies have looked at the relationship between characteristics of convection within the ITCZ and ITCZ width. The present work uses information from an ITCZ identification database and Tropical Rainfall Measuring Mission (TRMM) precipitation feature (PF) database to quantify variations in convective characteristics across the ITCZ in the Pacific Ocean. Data are partitioned into wide and narrow ITCZ regimes to quantify differences in convection between different ITCZ regimes. Under the wide regime, convection deeper than 5 km, with areas greater than 100 km2, or stratiform rain fractions greater than 0.5 is, on average, 24%, 23%, and 12% more frequent, respectively. In the narrow regime, the signal is reversed, with average increases in the frequency of convection with heights below 5 km, areas less than 100 km2, or stratiform rain fractions less than 0.5 of 15%, 4%, and 6%, respectively. Positive and negative anomalies in columnar water vapor (CWV) and sea surface temperature (SST) across the ITCZ are observed in the wide and narrow regimes, respectively. There is also a strong positive correlation between an El Niño–Southern Oscillation (ENSO) index and ITCZ width anomalies, with wide (narrow) ITCZs occurring during warm (cold) phases of ENSO. This implies that the strengthening and weakening of the Walker circulation associated with ENSO may play a role in modulating the convective populations that contribute to the Pacific ITCZ width variations.

The sea surface temperature (SST) contrast between the Northern Hemisphere (NH) and Southern Hemisphere (SH) influences the location of the intertropical convergence zone (ITCZ) and the intensity of the monsoon systems. This study examines the contributions of external forcing and unforced internal variability to the interhemispheric SST contrast in HadSST3 and ERSSTv5 observations, and 10 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) from 1881 to 2012. Using multimodel mean fingerprints, a significant influence of anthropogenic, but not natural, forcing is detected in the interhemispheric SST contrast, with the observed response larger than that of the model mean in ERSSTv5. The forced response consists of asymmetric NH–SH SST cooling from the mid-twentieth century to around 1980, followed by opposite NH–SH SST warming. The remaining best-estimate residual or unforced component is marked by NH–SH SST maxima in the 1930s and mid-1960s, and a rapid NH–SH SST decrease around 1970. Examination of decadal shifts in the observed interhemispheric SST contrast highlights the shift around 1970 as the most prominent from 1881 to 2012. Both NH and SH SST variability contributed to the shift, which appears not to be attributable to external forcings. Most models examined fail to capture such large-magnitude shifts in their control simulations, although some models with high interhemispheric SST variability are able to produce them. Large-magnitude shifts produced by the control simulations feature disparate spatial SST patterns, some of which are consistent with changes typically associated with the Atlantic meridional overturning circulation (AMOC).

Sea salt chloride is a major component of atmospheric aerosol and its behavior is an essential element in determining the climate. Two atmospheric chlorine deposition measurement procedures were performed between 2004 and 2008 by the French Guiana Regional Air Observatory (ORA), in the coastal plain of Sinnamary. The main goal was to determine the background spatial distribution of marine chlorine in order to evaluate the impact of Ariane rocket hydrogen chloride emission. To determine the chlorine concentration level, weekly samples from 10 sites were analyzed. A seasonal pattern was identified. For every site, a high sea salt chlorine deposition level was observed from December to April and a low level from July to October. The ratio of high to low mean chloride concentration RHC/LC shows that just under half of the variation in chlorine deposition can be linked to the variation of sea salt production with low-level wind speed. Equations relating mean sea salt chlorine concentration and the distance to shore were studied, taking into account parameters found in other tropical regions, with a focus on neighboring Brazil. The wind rotation between the two seasons, and the sea salt chlorine loss per kilometer equation found for dry deposition, were used to calculate RHC/LC. It appears that the observed rotation (60°) explains 88% of RHC/LC. Finally, inland sea salt chlorine deposition behavior in this region was linked to intertropical convergence zone oscillation through variations in wind speed and direction.

This study investigates the joint impact of the boreal late winter–early spring North Pacific Victoria mode (VM) and South Pacific Quadrapole (SPQ) mode on the following summer Pacific Intertropical Convergence Zone (ITCZ) precipitation. Here, both observational data and historical simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) models show that the precipitation anomalies are enhanced when the VM and the SPQ are in phase but are weakened when they are out of phase. The strength of the anomalous vertical velocity (ω) and outgoing longwave radiation also tend to be stronger (weaker) in same-sign (opposite-sign) events. One possible reason for this is that when the VM and the SPQ are in (out of) phase, westerly anomalies over the western–central tropical Pacific are greatly increased (reduced) due to the interactions between preceding North and South Pacific sea surface temperature anomalies, thereby enhancing (reducing) convective upward motion over the western and central Pacific. In addition, an empirical linear regression model is also developed to forecast Pacific ITCZ summer precipitation based on both the late winter–early spring VM and SPQ indices. The combination of the two precursors in North and South Pacific show higher prediction skills than the single VM or SPQ model, which means it is necessary to consider their joint impact when we focus on the Pacific ITCZ precipitation.

The annual cycles of tropical and subtropical precipitation were estimated by parameterizing them using the inferential circular statistics approach. The Tropical Rainfall Measuring Mission (TRMM) Multi-satellite Precipitation Analysis (TMPA) version 7 dataset which has spatial coverage from 50°S to 50°N and temporal coverage from 1998 to 2018 (21 years) was used to compute the monthly precipitation climatology data. Their annual cycles can be estimated using the bimodal von Mises distribution. The estimated parameters were found to sufficiently capture some interesting characteristics such as the non-uniformity and bimodality of precipitation rate associated with regions of convergence zones and subtropical highs. The difference in timings of maximum and minimum precipitation suggest that precipitation maxima over the continent were found to follow the position of the Sun at an average time lag of 1.2 months. Three examples of applying these parameters are given including the regional classification of precipitation which emphasizes on the bimodality the non-uniformity and the bimodality of their annual cycles, the evolution of the southern Intertropical Convergence Zone (ITCZ) in the Pacific Ocean which was found to propagate westward from 90°W in mid-March to 150°W in early-April at an estimated speed of 21.6 km/h, and the precipitation variability in the case of Paris in France in which the precipitation anomaly tend to decrease at a highly significant rate of −9.716 mm/month per decade (p value ≤0.01).

On 8 April 2005, strong gravity wave (GW) activity (over a period of more than 3 h) was observed in São João do Cariri (7.4∘ S, 36.5∘ W). These waves propagated to the southeast and presented different spectral characteristics (wavelength, period and phase speed). Using hydroxyl (OH) airglow images, the characteristics of the observed GWs were calculated; the wavelengths ranged between 90 and 150 km, the periods ranged from ∼26 to 67 min and the phase speeds ranged from 32 to 71 m s−1. A reverse ray-tracing analysis was performed to search for the possible sources of the waves that were detected. The ray-tracing database was composed of temperature profiles from the Naval Research Laboratory Mass Spectrometer Incoherent Scatter (NRLMSISE-00) model and SABER measurements as well as wind profiles from the Horizontal Wind Model (HWM) and meteor radar data. According to the ray tracing result, the likely source of these observed gravity waves was the Intertropical Convergence Zone, which caused intense convective processes to take place in the northern part of the observatory. Also, the observed preferential propagation direction of the waves to the southeast could be explained using blocking diagrams, i.e. due to the wind filtering process.

The Tropical Rain Belts with an Annual Cycle and Continent Model Intercomparison Project (TRACMIP) ensemble—a multimodel ensemble of slab-ocean simulations in idealized configurations—provides a test of the relationship between the zonal mean ITCZ and the cross-equatorial atmospheric energy transports (AHTeq). In a gross sense, the ITCZ position is linearly related to AHTeq, as expected from the energetic framework. Yet, in many aspects, the TRACMIP model simulations do not conform to the framework. Throughout the annual cycle there are large excursions in the ITCZ position unrelated to changes in the AHTeq and, conversely, substantial variations in the magnitude of the AHTeq while the ITCZ is stationary at its northernmost position. Variations both in the net vertical energy input at the ITCZ location and in the vertical profile of ascent play a role in setting the model behavior apart from the conceptual framework. Nevertheless, a linear fit to the ITCZ–AHTeq relationship captures a substantial fraction of the seasonal variations in these quantities as well as the intermodel or across-climate variations in their annual mean values. The slope of the ITCZ–AHTeq linear fit for annual mean changes across simulations with different forcings and configurations varies in magnitude and even sign from model to model and we identify variations in the vertical profile of ascent as a key factor. A simple sea surface temperature–based index avoids the complication of changes in the vertical structure of the atmospheric circulation and provides a more reliable diagnostic for the ITCZ position.

Large domains in regional climate models are known to provide a better representation of the mesoscale circulation and hence the precipitation field. To evaluate the model performance in reproducing the tropical circulation as well as the severe drought event (2012–2016) in the northeastern Brazil, simulations for the 2012–2016 period were performed using the tropical band version of the Regional Climate Model (RegT-Band). The RegT-Band simulation were driven by the ERA-Interim data for the initial and lateral boundary conditions and sea surface temperature was obtained from NOAA_OISST_V2 reanalysis. The domain covers the entire tropical belt between 30°S and 30°N with grid spacing of about 30 km. Results showed that the general features of the tropical circulation and associated precipitation pattern over the entire domain are well captured by the model compared to the observations. However, systematic biases, mainly related to the double ITCZ bias are observed, which provide an unrealistic peak of double precipitation outside the equator and insufficient precipitation at the equator. In the NEB subdomain, RegT-Band shows weakness and significant model error to represent the subregion where the drought event was more severe. However, the model satisfactorily represents the precipitation anomalies in the driest years. Therefore, this study shows results encouraging the use of RegT-Band to study extreme events associated with droughts conditions in the Northeastern Brazil.

Aerosol radiative forcing can influence climate both locally and far outside the emission region. Here we investigate black carbon (BC) aerosols emitted in four major emission areas and evaluate the importance of emission location and magnitude as well as the concept of the absolute regional temperature-change potentials (ARTP). We perform simulations with a climate model (NorESM) with a fully coupled ocean and with fixed sea surface temperatures. BC emissions for year 2000 are increased by factors of 10 and 20 in South Asia, North America, and Europe, respectively, and by 5 and 10 in East Asia (due to higher emissions there). The perturbed simulations and a reference simulation are run for 100 years with three ensemble members each. We find strikingly similar regional surface temperature responses and geographical patterns per unit BC emission in Europe and North America but somewhat lower temperature sensitivities for East Asian emissions. BC emitted in South Asia shows a different geographical pattern in surface temperatures, by changing the Indian monsoon and cooling the surface. We find that the ARTP approach rather accurately reproduces the fully coupled temperature response of NorESM. Choosing the highest emission rate results in lower surface temperature change per emission unit compared to the lowest rate, but the difference is generally not statistically significant except for the Arctic. An advantage of high-perturbation simulations is the clearer emergence of regional signals. Our results show that the linearity of normalized temperature effects of BC is fairly well preserved despite the relatively large perturbations but that regional temperature coefficients calculated from high perturbations may be a conservative estimate. Regardless of emission region, BC causes a northward shift of the ITCZ, and this shift is apparent both with a fully coupled ocean and with fixed sea surface temperatures. For these regional BC emission perturbations, we find that the effective radiative forcing is not a good measure of the climate response. A limitation of this study is the uncertainties in BC–cloud interactions and the amount of BC absorption, both of which are model dependent.

Arctic and Antarctic sea-ice extent are both projected to dramatically decline over the coming century. The effects of Arctic sea-ice loss are not limited to the northern high latitudes, and reach deep into the tropics. Yet little is known about the effects of future Antarctic sea-ice loss outside of the southern high latitudes. Here, using a fully coupled climate model, we investigate the tropical response to Antarctic sea-ice loss and compare it with the response to Arctic sea-ice loss. We show that Antarctic sea-ice loss, similar to Arctic sea-ice loss, causes enhanced warming in the eastern equatorial Pacific and an equatorward intensification of the Intertropical Convergence Zone. We demonstrate that Antarctic sea-ice loss causes a mini global warming signal comparable to the one caused by Arctic sea-ice loss, and reminiscent of the response to greenhouse gases. We also show that ocean dynamics are key to capturing the tropical response to sea-ice loss. In short, we find that future Antarctic sea-ice loss will exert a profound influence on the tropics. Combined Arctic and Antarctic sea-ice losses will account for 20–30% of the projected tropical warming and precipitation changes under the high-emissions scenario Representative Concentration Pathway 8.5. Antarctic sea-ice loss causes enhanced warming in the eastern equatorial Pacific, and together with Arctic sea-ice loss accounts for 20–30% of projected warming and rainfall changes in the tropics, suggest climate model simulations.

A hybrid statistical–dynamical model is developed to predict multiyear variability of Atlantic tropical cyclone (TC) activity. A Poisson model takes sea surface temperature (SST) averaged over the Atlantic main development region (MDR) and the Atlantic subpolar gyre region (SPG) from the initialized CESM prediction as predictors, and skillfully predicts the basinwide TC frequency, accumulated cyclone energy (ACE), landfalling TC frequency, and hurricane and major hurricane days. Further analysis shows that the SPG SST is a more important source of predictability than the MDR SST for multiyear Atlantic TC activity. The comparison between the uninitialized and initialized CESM predictions suggests that the SPG SST is better predicted by the initialized CESM owing to the better prediction of Atlantic meridional overturning circulation, which contributes to the overall more skillful TC predictions. On the other hand, the skillful prediction of the basinwide TC frequency by the uninitialized CESM suggests the role of external forcing in the variability of Atlantic TC activity. The dependence of the hybrid prediction skills on the dynamic model ensemble size is also explored, and an ensemble size of ~20 is suggested as optimal. Further analysis shows that the SPG SST is associated with the variability of vertical wind shear and precipitable water over the tropical Atlantic even when the influence of the MDR SST is controlled. The spatial patterns of vertical wind shear and precipitable water suggest a strong modulation of ACE and hurricane frequency but a relatively weak influence on the basinwide TC frequency. The physical mechanisms between the SPG SST and Atlantic TC activity are discussed.

The climate of tropical regions is significantly affected by migration of the intertropical convergence zone (ITCZ) whose location can be understood based on an energetic perspective. The ITCZ location depends on the cross-equatorial atmospheric energy transport (AET) and the equatorial net energy input ($$\hbox {NEI}_0$$). Using ERA-Interim dataset for the period 1980–2018, the ITCZ location over the Indian Ocean in different seasons and its migration are investigated. The cross-equatorial AET and the $$\hbox {NEI}_0$$ over the Indian Ocean are analyzed to determine the location of the ITCZ based on the atmospheric energy balance. Based on zonal mean maximum precipitation, the annual mean location of the ITCZ over the Indian Ocean is found in the Southern Hemisphere (approximately at 5.5$${}^{\circ }$$ S), while in some seasons, a double ITCZ forms. During the period 1980–2018, the annual mean location of the ITCZ over the Indian Ocean has migrated toward the equator by approximately 1$${}^{\circ }$$, although it is not statistically significant. The largest meridional migration of the ITCZ over the Indian Ocean occurs with the period of 12 months, showing seasonal migration of the ITCZ associated with the monsoon circulation. The location of maximum precipitation over the Indian Ocean is different from that of the energy flux equator (EFE), such that the EFE is always located north of the location of maximum precipitation. This is partly due to the fact that only the mean meridional circulation (MMC) component of the energy flux is considered to compute the EFE. Variations of the cross-equatorial AET and the Oceanic Nino Index (ONI) are not consistently correlated, while variations of the $$\hbox {NEI}_0$$ over the Indian Ocean and the ONI are quite consistent. Over the Indian Ocean, variation of the cross-equatorial AET dominates interannual variation of the ITCZ location, while variation of the $$\hbox {NEI}_0$$ dominates seasonal variation of the ITCZ location.

Observational records and climate model projections reveal a considerable decline in the Atlantic Meridional Overturning Circulation (AMOC). Changes in the AMOC can have a significant impact on the global climate. Sustained warming due to increased greenhouse gas emissions is projected to weaken the AMOC, which in turn can lead to changes in the location of Inter-tropical convergence zone (ITCZ), oceanic and atmospheric large-scale circulation, tropical precipitation and regional monsoons. Using proxy records, observations and CMIP6 simulations of IITM Earth System Model (IITM-ESM), we investigate the changes in the AMOC and associated changes in the large-scale circulation and precipitation patterns over the South Asian monsoon region. Transient CO2 simulation and additional model sensitivity experiments with realistic surface heat and freshwater perturbation anomalies under the experimental protocol of Flux Anomaly Forcing Model Intercomparison Project (FAFMIP) performed with IITM-ESM reveal a decline in the strength of AMOC. The weakening of AMOC is associated with enhanced heat and freshwater forcing in the North Atlantic resulting in the reduction of northward oceanic heat transport and an enhanced northward atmospheric heat transport. Changes in AMOC lead to weakening of large-scale north–south temperature gradient and regional land-sea thermal gradient, which in turn weaken the regional Hadley circulation and, monsoon circulation over the South Asian region. Both the FAFMIP and transient CO2 experiments reveal consistent results of weakening South Asian Monsoon circulation with a decline of AMOC, while precipitation exhibits contrasting responses as precipitation changes are dominated by the thermodynamic response. The suite of observational and numerical analysis provides a mechanistic hypothesis for the weakening of South Asian monsoon circulation concomitant with a weakening of AMOC in a warming climate.

Rapid monsoon changes since the last deglaciation remain poorly constrained due to the scarcity of geological archives. Here we present a high-resolution scanning X-ray fluorescence (XRF) analysis of a 13.5-m terrace succession on the western Chinese Loess Plateau (CLP) to infer rapid monsoon changes since the last deglaciation. Our results indicate that Rb/Sr and Zr/Rb are sensitive indicators of chemical weathering and wind sorting, respectively, which are further linked to the strength of the East Asia summer monsoon (EASM) and the East Asia winter monsoon (EAWM). During the last deglaciation, two cold intervals of the Heinrich event 1 and Younger Dryas were characterized by intensified winter monsoon and weakened summer monsoon. The EAWM gradually weakened since the beginning of the Holocene, while the EASM remained steady till 9.9 ka and then grew stronger. Both the EASM and EAWM intensity were relatively weak during the middle Holocene, indicate a mid-Holocene climatic optimum. Rb/Sr and Zr/Rb exhibit an anti-phase relationship between the summer and winter monsoon changes on centennial timescale during 16~1 ka BP. Comparison of these monsoon changes with solar activity and North Atlantic cooling events reveals that both factors can lead to abrupt changes on the centennial timescale in the early Holocene. During the late Holocene, North Atlantic cooling became the major forcing of centennial monsoon events.