Tropical Atmospheric Circulation Research Articles

Drivers of the mean biases of the tropical atmospheric circulation in a moist static energy framework

AbstractIn this study, we apply the moist static energy for first baroclinic mode (MSEB) model to examine the drivers of the mean tropical atmospheric circulation biases over oceanic regions. The model diagnoses the vertical motion in an air column of the tropical regions based on net energy heat flux and advection of moisture or heat into the air column in relation to the stability of the air column due to the gradients in moist static energy. Analysis of Coupled Model Intercomparison Project (CMIP) and Atmospheric Model Intercomparison Project (AMIP) simulations helped to identified errors intrinsic to the atmospheric models or errors due to atmosphere–ocean coupling process. Despite some limitations of the MSEB model, our multi-model mean analysis over the entire tropical ocean reveals that the primary drivers of the tropical circulation biases mostly result from intrinsic atmospheric model errors in top of the atmosphere longwave radiation and surface latent heats fluxes, suggesting a link to biases in the hydrological cycle. Oceanic coupling significantly enhanced some of the biases. Biases in the advection of moist static energy also play an important role, while biases in the gross moist stability profiles play only a minor role. Further, we examine the inter-model variations in four main regional large-scale biases (double-ITCZ, Pacific cold tongue, southward shift of ITCZ over the Atlantic, and dipole bias over the Indian Ocean). The analysis suggests that regional bias patterns across general circulation models are primarily driven by coupling errors, except for the bias in the Indian Ocean, which is intrinsic to the atmospheric model but amplified by coupling. Notably, longwave radiation biases at the top of the atmosphere are prevalent among the four bias patterns, as well as biases in moisture advection over the Atlantic. Our results underscore the significant role of net longwave radiation at the top of the atmosphere, an aspect not sufficiently emphasized in previous studies.

Arctic summer precipitation variability and its association with Indian summer monsoon anomalies

In recent years, there has been an increased focus on changes in Arctic precipitation，however, dominant characteristics of Arctic summer precipitation variability and their relationship to atmospheric circulation anomalies in the mid-low latitudes remain debatable. Spatially, the leading mode of Arctic summer precipitation variability is positive in most regions of the Arctic, corresponding to a cold anomaly in the middle and lower troposphere of the Arctic. As the temperature gradient between the Arctic and high latitudes increases, the zonal westerlies over the Arctic enhanced and lead to increased local baroclinicity and moisture convergence over the Arctic. This creats favorable conditions for increased summer precipitation. Additionally, it is found that there is a robust correlation between the first mode of Arctic summer precipitation and the Indian summer monsoon. Specifically, the positive Arctic summer precipitation anomaly is accompanied by a systematic northward shift of the zonal westerlies in the mid-latitude. The northward shift of the Asian westerly jet stream is associated with the intensified tropical easterly jet in the upper-troposphere and low-stratosphere, resulting in the strengthening of the monsoon circulation over the Indian Peninsula and then facilitating the precipitation anomaly over India. Numerical experiments reproduce the major features of the observed Arctic-Indian summer monsoon association, implying that Arctic summer precipitation variability is dynamically linked to tropical atmospheric circulation anomalies.

Predicting the interannual variability of the subtropical high over the western Pacific Ocean based on the improved information diffusion model

Because of its special location and structure, the Western Pacific Subtropical High (WPSH) influences greatly the climate and weather in East Asia, especially the summer precipitation. To clarify how the interannual variability (IAV) of the WPSH is related to anomalies in the tropical sea surface temperature (SST) and atmospheric circulation, time series of the intensity index of the WPSH are subjected to wavelet analysis, showing IAV in the index. Characteristic indexes are defined for three key sea areas and the equatorial-latitude westerly region. After a continuous wavelet transform, the oscillation period of them is similar to that of the WPSH. The cross-wavelet transform of the four regional and two WPSH indexes is used to obtain the corresponding time-delay correlation. Regarding the potential correlation, WPSH weakening leads to strengthening of the westerly wind and then affects the rise of SST in the eastern equatorial Pacific Ocean. At the same time, warm water moves eastward. This gradually increases the SST in the equatorial central Pacific and warm pool area and then strengthens the WPSH under the action of the Hadley circulation. From the above analysis, a model for predicting the IAV of the WPSH intensity index is established based on the information diffusion model improved by a genetic algorithm. An experiment is conducted to predict the IAV of the WPSH intensity index, and the results show that the prediction model is accurate in predicting the IAV trend, with good prediction for 84 months. The mean absolute percentage error is 14.44% and the correlation coefficient is 0.8507. Also, the normal and abnormal years of the WPSH are used as different starting points for different prediction experiments. However, the different starting points have little influence on the predictions, showing the stability of the model. Studying the IAV of the WPSH provides a strong theoretical and scientific basis for predicting its abnormal interannual behavior and offers the prospect of socially important disaster prevention and mitigation.

Tropical Surface Temperature and Atmospheric Latent Heating: A Whole-Tropics Perspective Based on TRMM and ERA5 Datasets

Tropical surface temperature (TST) and its connection with atmospheric heating, including tropical latent heating (TLH), is essential to the interannual variability of tropical atmospheric circulation and global teleconnection. Utilizing seasonally averaged satellite-based TRMM precipitation data as a proxy of TLH and ERA5-based TST data from 1998 to 2018, we reveal some new features in terms of cross-hemispheric connection in the TLH and TST variability by decomposing them into equatorially symmetric and antisymmetric components. We find surprisingly that the spatial patterns of TLH projected upon the first principal components (PC1) of symmetric and antisymmetric TSTs over the whole-tropics, are very similar to each other, seemingly at odds with the classic Mastuno–Gill theory. The similarity in the projected TLH patterns is mainly because the PC1s of symmetric and antisymmetric TSTs co-vary temporally with a very high correlation. We use the spatial pattern of local correlation between symmetric and antisymmetric components, for both TST and TLH to depict geographic dependence of the symmetric–antisymmetric connection. We suggest that a whole-tropics perspective, which takes the different but connected nature of equatorially symmetric and antisymmetric modes across the whole-tropics into consideration, may well be useful in understanding and predicting tropical climate variability because clarifying the puzzle raised in this research from such a perspective about the consistency between the observation and the classic Mastuno–Gill theory is directly related to the fundamental dynamics of tropical systems, such as Walker circulation, monsoons, and their relationship with underlying land and sea conditions.

Weak Equatorial Superrotation during the Past 250 Million Years

Abstract For modern Earth, the annual-mean equatorial winds in the upper troposphere are flowing from east to west (i.e., easterly winds). This is mainly due to the deceleration effect of the seasonal cross-equatorial Hadley cells, against the relatively weaker acceleration effect of coupled Rossby and Kelvin waves excited from tropical convection and latent heat release. In this work, we examine the evolution of equatorial winds during the past 250 million years using one global Earth system model, the Community Earth System Model version 1.2.2 (CESM1.2.2). Three climatic factors different from the modern Earth—solar constant, atmospheric CO2 concentration, and land–sea configuration—are considered in the simulations. We find that the upper-tropospheric equatorial winds change sign to westerly flows (called equatorial superrotation) in certain eras, such as 250–230 and 150–50 Ma. The strength of the superrotation is below 4 m s−1, comparable to the magnitude of the present-day easterly winds. In general, this phenomenon occurs in a warmer climate within which the tropical atmospheric circulation shifts upward in altitude, stationary and/or transient eddies are relatively stronger, and/or the Hadley cells are relatively weaker, which in turn are due to the changes of the three factors, especially CO2 concentration and land–sea configuration.

The Zonal Patterns in Late Quaternary Tropical South American Precipitation

AbstractSpeleothem oxygen isotope records (δ18O) of tropical South American rainfall in the late Quaternary show a zonal “South American Precipitation Dipole” (SAPD). The dipole is characterized by opposing east‐west precipitation anomalies compared to the present—wetter in the east and drier in the west at the mid‐Holocene (∼7 ka), and drier in the east and wetter in the west at the Last Glacial Maximum (∼21 ka). However, the SAPD remains enigmatic because it is expressed differently in western versus eastern δ18O records and isotope‐enabled climate model simulations usually misrepresent the magnitude and/or spatial pattern of δ18O change. Here, we address the SAPD enigma in two parts. First, we re‐interpret the δ18O data to account for upwind rainout effects that are known to be pervasive in tropical South America, but are not always considered in Quaternary paleoclimate studies. Our revised interpretation reconciles the δ18O data with cave infiltration and other proxy records, and indicates that the centroid of tropical South American rainfall has migrated zonally over time. Second, using an energy balance model of tropical atmospheric circulation, we hypothesize that zonal migration of the precipitation centroid can be explained by regional energy budget shifts, such as changing Saharan albedo associated with the African Humid Period, that have not been modeled in previous SAPD studies. This hypothesis of a migrating precipitation centroid presents a new framework for interpreting δ18O records from tropical South America and may help explain the zonal rainfall anomalies that predate the late Quaternary.

Future Indian Ocean warming patterns

Most future projections conducted with coupled general circulation models simulate a non-uniform Indian Ocean warming, with warming hotspots occurring in the Arabian Sea (AS) and the southeastern Indian Ocean (SEIO). But little is known about the underlying physical drivers. Here, we are using a suite of large ensemble simulations of the Community Earth System Model 2 to elucidate the causes of non-uniform Indian Ocean warming. Strong negative air-sea interactions in the Eastern Indian Ocean are responsible for a future weakening of the zonal sea surface temperature gradient, resulting in a slowdown of the Indian Ocean Walker circulation and the generation of southeasterly wind anomalies over the AS. These contribute to anomalous northward ocean heat transport, reduced evaporative cooling, a weakening in upper ocean vertical mixing and an enhanced AS future warming. In contrast, the projected warming in the SEIO is related to a reduction of low-cloud cover and an associated increase in shortwave radiation. Therefore, the regional character of air-sea interactions plays a key role in promoting future large-scale tropical atmospheric circulation anomalies with implications for society and ecosystems far outside the Indian Ocean realm.

Relationship between South China Sea Summer Monsoon and Western North Pacific Tropical Cyclones Linkages with the Interaction of Indo-Pacific Pattern

The South China Sea (SCS) summer monsoon (SCSSM) and Western North Pacific tropical cyclones (TCs) are both tropical systems that interact with each other on multiple scales. This study examines the differences in TCs activity characteristics between anomalous strong and weak SCSSM years, and explores the possible mechanisms behind these differences through the coupling relationship between tropical atmospheric circulation and oceanic surface conditions. Results show that the destructiveness of TCs over the Western North Pacific is stronger during weak SCSSM years than in strong years, whereas the opposite occurs for TCs over the SCS. The interaction between the tropical Indo-Pacific ocean and atmosphere plays a key role in the relationship between SCSSM intensity and TCs activity. In strong (weak) SCSSM years, the sea surface temperature anomaly (SSTA) in the tropical Pacific Ocean tends to correspond to a La Niña-like (El Niño-like) distribution, whereas the tropical Indian Ocean shows an Indian Ocean dipole-negative (positive) phase distribution. Moreover, Walker circulations in both the Indian and Pacific Oceans are coupled during these years, which creates a seesaw-like relationship in the conditions for TCs formation between the SCS and the Western Pacific Ocean. During weak SCSSM years, the formation and activity of TCs over the SCS are suppressed due to the weakened water vapor transport caused by abnormal easterly winds from the eastern Indian Ocean to the southern SCS. Meanwhile, the higher SSTA in the Western Pacific Ocean enhances the TCs activity. In strong SCSSM years, the enhanced monsoon drives a stronger monsoon trough, improving the convective environment over the SCS, whereas in contrast, the Western Pacific Ocean is covered by colder water, resulting in poorer conditions for TCs genesis.

Meridional response of Western North Pacific paleocyclone activity to tropical atmospheric circulation variability over the past millennium

Sedimentary records of past tropical cyclone (TC) activity indicate large variability and spatial heterogeneity on the interannual-century scales. However, the meridional variation patterns and climatic forcing mechanisms for regional TC activity are not well understood because of the short and incomplete observational record. Here, we present a near-annually resolved record of intense TC activity derived from a back-reef lagoon at Anle Atoll in the southern South China Sea since the Little Ice Age (LIA). Our reconstruction indicates that TC activity was higher during the early LIA than what has been observed throughout the instrumental record. Basin-wide compiled records of the western North Pacific (WNP) cyclogenesis revealed strong evidence of a meridional seesaw pattern for storm activity between relatively low and high latitudes over the past millennium. This pattern suggests a centennial-scale link with Pacific hydroclimate atmospheric circulation shifts. A combination of southward displacement of the Intertropical Convergence Zone and westward migration of the Pacific Walker Circulation, as well as damped El Niño Southern Oscillation variability, likely contributed to enhanced TC activity during the LIA in the low latitudes of the WNP. External forcing of solar irradiance budgets profoundly affects the hydroclimate and TC climatology in the WNP.

Hydroclimatic record from an Altiplano cushion peatland (24°S) indicates large-scale reorganisation of atmospheric circulation for the late Holocene.

The hydroclimate of South America is characterized by the South American summer monsoon (SASM), a tropical atmospheric circulation that induces a summer precipitation regime, and the Southern Hemisphere Westerlies (SHW), an extratropical atmospheric circulation that induces a winter precipitation regime. Stretched between these two systems is a NW-SE-oriented region dominated by descending air masses, resulting in the South American subtropical dry zone (SASDZ), also known as the arid diagonal. We investigated the Cerro Tuzgle cushion peatland (CTP) located on the Argentine Altiplano, north of the present-day SASDZ. Previous work revealed that the CTP was consistently in the SASM regime during the last 2900 cal yr BP. Here, we extend the CTP record to the middle Holocene covering the last 7200 cal yr BP to gain further knowledge of the Holocene development of the SASM and potential modulations of the SASDZ. The prominent feature of the entire record is a distinct and lasting transition centred around 3100 cal yr BP characterized by declining minerogenic content, increasing organic carbon content, rising stable carbon isotope values of organic matter and cellulose, and increasing stable oxygen isotope values of cellulose. We interpret this specific proxy pattern as a hydroclimatic transition towards less arid conditions at the CTP after 3100 cal yr BP. The transition corresponds with the end of the continuous Holocene strengthening of the SASM between 3500 cal yr BP and 3000 cal yr BP indicated by proxy records from north and east of the CTP. The CTP does not reflect this strengthening of the SASM and rather exhibits a threshold response indicating the effective establishment of the SASM summer precipitation regime at 24°S. This suggests that moisture supply during a more arid middle Holocene was provided by isotopically depleted precipitation, while moisture supply after the transition originated from isotopically enriched SASM summer precipitation. Concurrent hydroclimatic changes in the SHW winter precipitation regime south of the SASDZ are documented in a distinct lake level rise of Laguna Aculeo (33°50´S) around 3200 cal yr BP. These coinciding hydrological changes of the SASM and the SHW precipitation regimes indicate larger scale reorganisations of atmospheric circulation components, potentially connected to major modulations of the SASDZ. Thus, our CTP record sheds light on the middle to late Holocene development of the SASM at its southern limit and corroborates connections between the tropical and extratropical hydroclimate of South America.

Influence of the Atlantic Multidecadal Oscillation on South American Atmosphere Dynamics and Precipitation

The Atlantic Multidecadal Oscillation (AMO) is coherently linked to climate variations over many parts of the globe. Despite recent achievements, the mechanism by which the AMO influences regional precipitation over South America is not well understood. In this study, we isolate the atmospheric response to the AMO using a water isotope-enabled version of the Community Earth System Model version 1.2 (iCESM1.2) and determine its influence on (sub)tropical South American regional precipitation and atmospheric circulation. The results suggest an interhemispheric seesaw in Hadley circulation strength and that the section of the Atlantic Hadley cell is marked by a stronger upward air component south of the equator during the cold AMO phase. We also find that the precipitation anomalies over (sub)tropical South America during AMO phases are mainly related to changes in the Atlantic Intertropical Convergence Zone (ITCZ) core strength, where in the cold (warm) AMO phase the core region strengthens (weakens) from February to July, while from July to November the core region weakens (strengthens). Our results stress the importance of acknowledging the dynamics of season- and regional-dependent ITCZ responses, as they are sufficient to produce observed AMO-related signals even in the absence of marked changes in the ITCZ position.

How Does Sea Surface Temperature Drive the Intertropical Convergence Zone in the Southern Indian Ocean?

Abstract The Indian Ocean has an intriguing intertropical convergence zone (ITCZ) south of the equator year-round, which remains largely unexplored. Here we investigate this Indian Ocean ITCZ and the mechanisms for its origin. With a weak semiannual cycle, this ITCZ peaks in January–February with the strongest rainfall and southernmost location and a northeast–southwest orientation from the Maritime Continent to Madagascar, reaches a minimum around May with a zonal orientation, grows until its secondary maximum around September with a northwest–southeast orientation, weakens slightly until December, and then regains its mature phase in January. During austral summer, the Indian Ocean ITCZ exists over maximum surface moist static energy (MSE), consistent with convective quasi-equilibrium theory. This relationship breaks up during boreal summer when the surface MSE maximizes in the northern monsoon region. The position and orientation of the Indian Ocean ITCZ can be simulated well in both a linear dynamical model and the state-of-the-art Community Atmosphere Model version 6 (CAM6) when driven by observed sea surface temperature (SST). To quantify the contributions of the planetary boundary layer (PBL) and free-atmosphere processes to this ITCZ, we homogenize the free-atmosphere diabatic heating over the Indian Ocean in CAM6. In response, the ITCZ weakens significantly, owing to a weakened circulation and deep convection. Therefore, in CAM6, the SST drives the Indian Ocean ITCZ directly through PBL processes and indirectly via free-atmosphere diabatic heating. Their contributions are comparable during most seasons, except during the austral summer when the free-atmosphere diabatic heating dominates the mature-phase ITCZ. Significance Statement The intertropical convergence zone (ITCZ) is the globe-encircling band where trade winds converge and strong rainfall occurs in the tropics. Its rains provide life-supporting water to billions of people. Its associated latent heating invigorates the tropical atmospheric circulation and influences climate and weather across the planet. The ITCZ is located north of the equator in most tropical oceans, except in the Indian Ocean where it sits south of the equator year-around. In contrast to the well-known northern ITCZs, the origin of the southern ITCZ in the Indian Ocean remains unknown. This work provides the first explanation for how ocean surface temperature works together with processes in the lower and upper atmosphere to shape the unique ITCZ in the Indian Ocean.

Coupled dynamics of the North Equatorial Countercurrent and Intertropical Convergence Zone with relevance to the double-ITCZ problem

The Intertropical Convergence Zone (ITCZ) is a salient feature of tropical atmospheric circulation characterized by intense convective clouds and rainfall north of the equator. Climate models, however, commonly experience the double-ITCZ problem-the tendency to produce another strong precipitation band but south of the equator. Here, we describe coupled interaction between the ITCZ and the North Equatorial Countercurrent (NECC) that is relevant to this problem. This current is a major component of the tropical Pacific upper-ocean circulation, which flows against easterly trade winds and transports warm water from the western Pacific eastward. Its core follows the latitudinal position of the ITCZ. Trade winds converge toward the ITCZ, creating a local minimum in zonal winds and generating positive wind stress curl that maintains an eastward current despite westward winds. We show that interaction between the ITCZ and the NECC involves positive feedback: a stronger NECC advects warm water from the western Pacific to the colder east thus increasing sea surface temperature (SST) along its path, intensifying convection within the ITCZ and hence strengthening wind stress curl, further strengthening the NECC. To demonstrate this wind stress curl-advection-SST-precipitation (WASP) feedback, we conduct climate model experiments in which we progressively strengthen the surface Ekman component of the NECC and observe the intensification of the ITCZ and the entire NECC. Consequently, a weak NECC leads to a weak ITCZ, which can contribute to the double-ITCZ problem in climate models, since weak wind convergence north of the equator enables stronger convergence in the south.

Examining the Regional Co‐Variability of the Atmospheric Water and Energy Imbalances in Different Model Configurations—Linking Clouds and Circulation

Clouds are a key player in the global climate system, affecting the atmospheric water and energy budgets, and they are strongly coupled to the large‐scale atmospheric circulation. Here, we examine the co‐variability of the atmospheric energy and water budget imbalances in three different global model configurations–radiative‐convective equilibrium, aqua‐planet, and global simulations with land. The gradual increase in the level of complexity of the model configuration enables an investigation of the effects of rotation, meridional temperature gradient, land‐sea contrast, and seasonal cycle on the co‐variability of the water and energy imbalances. We demonstrate how this co‐variability is linked to both the large‐scale tropical atmospheric circulation and to cloud properties. Hence, we propose a co‐variability‐based framework that connects cloud properties to the large‐scale tropical circulation and climate system and is directly linked to the top‐down constrains on the system—the water and energy budgets. In addition, we examine how the water and energy budget imbalances co‐variability depends on the temporal averaging scale, and explain its dependency on how stationary the circulation is in the different model configurations. Finally, we demonstrate the effect of an idealized global warming and convective aggregation on this co‐variability.

Dynamical‐statistical long‐term prediction for tropical cyclone landfalls in East Asia

AbstractThis study develops a statistical‐dynamical seasonal typhoon forecast model (SDTFM) that utilizes the statistical correlation between East Asia (EA) tropical cyclone (TC) landfall and atmospheric circulation predicted by a coupled general circulation model for seasonal prediction and its predictability is verified. A total of 40 ensemble members produced through different data assimilation and time‐lag methods introduced as a way to reduce the initial condition error and model uncertainty enabled the development of the new SDTFM. According to the results, the SDTFM developed in this study showed significant predictability in TC landfall prediction when using the month of May for the initial conditions for the entire East Asia (EEA) and its three sub‐domains: Northern East Asia (NEA), Middle East Asia (MEA), and Southern East Asia (SEA). The predicted TC season is July–September (JAS), and only for SEA, including South China, the Philippines, and Vietnam, it is July–November (JASON) considering the relatively long landfall period. The models developed for each domain significantly predict the interannual variability of TC landfall at the 99% confidence level. The cross‐validated results are still significant at the 99% confidence level in NEA and SEA and the 95% confidence level in MEA and EEA.

Structure of the Pacific Walker Circulation Depicted by the Reanalysis and CMIP6

The Pacific Walker circulation (PWC) is one of the most important components of large-scale tropical atmospheric circulations. The PWC and its influences have been studied extensively by numerical models and reanalysis. The newly released ERA5 and NCEP2 are the most widely used reanalysis datasets and serve as benchmarks for evaluation of model simulations. If the results of these datasets differ significantly, this could lead to a bias in projected long-term climate knowledge. For better understanding of future climate change, it is necessary to evaluate PWC reanalysis productions. As a result, we compared the PWC structures between the ERA5 and NCEP2 datasets from month to seasonal time scales. We used the zonal mass streamfunction (ZMS) over the equatorial Pacific to indicate the strength of the PWC. The PWC’s average monthly or seasonal cycle peaks around July. From February to June, the NCEP2 shows a higher PWC intensity, whereas the ERA5 shows greater intensity from July to December. The circulation center in the NCEP2 is generally stronger and wider than in the ERA5. The ERA5, however, revealed that the PWC’s west edge (zero line of ZMS over the western Pacific) had moved 10 degrees westward in comparison to the NCEP2. In addition, we compared the PWC mean state in the reanalysis and CMIP6 models; the mean state vertical structures of the tropical PWC in the CMIP6 multi-model ensemble (MME) are similar to those of the reanalyses in structure but weaker and wider than in the two reanalysis datasets. The PWC is broader in CMIP6, and the western boundary is 7 and 17 degrees farther west than in the ERA5 and NCEP2, respectively. This study suggests that, when using reanalysis datasets to evaluate PWC structural changes in intensity and western edge, extreme caution should be exercised.

Glacial‐Interglacial Shifts Dominate Tropical Indo‐Pacific Hydroclimate During the Late Pleistocene

AbstractThe climatic drivers of tropical rainfall and atmospheric circulation in the late Pleistocene are still debated. Some studies suggest that tropical precipitation primarily responded to precession (23–19 ky cycle), whereas others propose that glacial‐interglacial (100 ky) changes in ice sheets and sea level dominate. Here, we reexamine orbital influences on tropical‐to‐subtropical precipitation isotopes using singular spectrum analysis to isolate leading oscillatory modes from proxy records across the Indo‐Pacific Warm Pool (IPWP) and Asian monsoon domain. We find that the IPWP, Bay of Bengal, and South China Sea are dominated by the 100 ky glacial‐interglacial mode of variability, whereas eastern China clearly follows precession, suggesting that precipitation isotopes over the mid‐latitude Asian continent respond to different mechanisms than those in the IPWP or Indian and East Asian monsoon regions. This study demonstrates that glacial cycles, rather than changes in local insolation, are the dominant drivers of Pleistocene IPWP hydroclimate.

The impact of tropical Atlantic SST variability on the tropical atmosphere during boreal summer

AbstractThe interannual variability of boreal summer sea surface temperature (SST) in the tropical Atlantic displays two dominant modes, the Atlantic zonal mode highlighting SST variations in the equatorial–southern tropical Atlantic (ESTA) region and the northern tropical Atlantic (NTA) mode focusing on SST fluctuations in the NTA region except in the Gulf of Guinea. Observational evidence indicates that both the boreal summer ESTA and NTA warming are accompanied by a pair of anomalous low-level anti-cyclones over the western tropical Pacific, and the NTA-related anti-cyclone is more obvious than the ESTA-related one. Both atmosphere-only and partially coupled experiments conducted with the Community Earth System Model Version 1.2 support the observed NTA–Pacific teleconnection. In contrast, the ESTA-induced atmospheric circulation response is negligible over the tropical Pacific in the atmosphere-only experiments, and though the response becomes stronger in the partially coupled experiments, obvious difference still exists between the simulations and observation. The ESTA-induced atmospheric circulation response is featured by an anomalous low-level cyclone over the western tropical Pacific in the partially coupled experiments, opposite to its observed counterpart. It is found that the ESTA warming coincides with significantly La Niña-like SST anomalies in the central–eastern equatorial Pacific, the influence of which on the tropical atmospheric circulation is opposite to that of the ESTA warming, and therefore contributes to difference between the ESTA-related simulations and observation. Moreover, the cold climatological mean SST in the ESTA region is unfavourable to enhancing the ESTA–Pacific teleconnection during boreal summer.

Characterizing Drying in the South American Monsoon Onset Season with the Moist Static Energy Budget

AbstractThe tropical atmospheric circulation and attendant rainfall exhibit seasonally dependent responses to increasing temperatures. Understanding changes in the South American monsoon system is of particular interest given the sensitivity of the southern Amazon rainforest to changes in dry season length. We utilize the latest Geophysical Fluid Dynamics Laboratory Atmospheric Model (GFDL AM4) to analyze the response of the South American monsoon to uniform sea surface temperature (SST) warming. SST warming is a poorly understood yet impactful component of greenhouse gas–induced climate change. Region-mean rainfall declines by 11%, and net precipitation (precipitation minus evaporation) declines by 40%, during the monsoon onset season (September–November), producing a more severe dry season. The column-integrated moist static energy (MSE) budget helps elucidate the physical mechanisms of the simulated drying. Based on the seasonal analysis, precipitation reductions tend to occur when 1) a convecting region’s climatological MSE export is dominated by horizontal rather than vertical advection, and 2) the horizontal MSE advection increases in the perturbed climate, impeding ascent. On a synoptic scale, the South American low-level jet strengthens and exports more moisture from the monsoon sector, exacerbating spring drying.

Predicting Daily Mean Wind Speed in Europe Weeks ahead from MJO Status

ABSTRACTThe Madden–Julian oscillation (MJO), a prominent feature of the tropical atmospheric circulation at subseasonal time scales, is known to modulate atmospheric variability in the Euro-Atlantic region. However, current subseasonal prediction systems fail to accurately reproduce the physical processes involved in these teleconnection mechanisms. This paper explores the observed impact of strong MJO events on surface wind speed over Europe. It is found that some MJO phases are accompanied by strong wind anomalies in Europe. After showing that this teleconnective mechanism is not present in the predictions of the ECMWF monthly forecasting system, a methodology to reconstruct forecasts of daily mean wind speed in the continent weeks ahead is proposed. This method combines MJO forecasts from the S2S project database and the observed teleconnection impacts in the historical records. Although it is found that strong MJO events cannot be skillfully predicted more than 10 days ahead with current prediction systems, a theoretical experiment shows that this method can effectively transform a dynamical MJO forecast into a probabilistic wind speed prediction in Europe.