Local Hadley Cell Research Articles

What Controls the Interdecadal Enhancement in Interannual Variability of Summertime Intraseasonal Precipitation Over South China?

AbstractThis study focuses on the interannual variability of summertime intraseasonal precipitation intensity (SIPI) over South China (SC) in the 30–90‐day range. The results demonstrate a significant increase in the interannual variability of SIPI over SC around the mid‐1990s. By comparing atmospheric and oceanic anomalies between two periods, P1 (1984–1993) and P2 (1994–2003), we identify that both the Silk Road pattern (SRP) and the western Pacific subtropical high (WPSH) play a role in influencing this interdecadal increase in SIPI interannual variability. These system anomalies are modulated by sea surface temperature anomalies (SSTA) in the North Atlantic (NA) and equatorial East Pacific (EP). During years with positive precipitation anomalies in P2, warming EP SSTA induces anomalous Walker cell and local Hadley cell, weakening the WPSH and enhancing convective activity, and increasing moisture transport over SC. The warming NA SSTA near 45°N triggers a more southerly propagation of SRP, thereby providing additional disturbance energy that enhances summer precipitation over SC. Conversely, during years with negative precipitation anomalies in P2, these processes occur in reverse. The enhanced interannual variability of local convective activity from P1 to P2 leads to a corresponding increase in the interannual variability of the WPSH, which, coupled with an amplified interannual variability of SRP, results in heightened interannual variability of SIPI. Additionally, the composite analysis of 30–90‐day ISO events further substantiates the underlying mechanisms driving the interdecadal amplification of SIPI interannual variability. Diagnostic results reveal that the 30–90‐day horizontal wind induces heightened convergence of background moisture over SC.

Asian Anthropogenic Aerosol Forcing Played a Key Role in the Multidecadal Increase in Australian Summer Monsoon Rainfall

Abstract Observations show a significant increase in Australian summer monsoon (AUSM) rainfall since the mid-twentieth century. Yet the drivers of this trend, including the role of anthropogenic aerosols, remain uncertain. We addressed this knowledge gap using historical simulations from a suite of Coupled Model Intercomparison Project phase 6 (CMIP6) models, the CESM2 Large Ensemble, and idealized single-forcing simulations from the Precipitation Driver Response Model Intercomparison Project (PDRMIP). Our results suggest that Asian anthropogenic aerosol emissions played a key role in the observed increase in AUSM rainfall from 1930 to 2014, alongside the influence of internal variability. Sulfate aerosol emissions over Asia led to regional surface cooling and strengthening of the climatological Siberian high over eastern China, which altered the meridional temperature and sea level pressure gradients across the Indian Ocean. This caused an intensification and southward shift of the Australian monsoonal westerlies (and the local Hadley cell) and resulted in a precipitation increase over northern Australia. Conversely, the influence of increased greenhouse gas concentrations on AUSM rainfall was minimal due to the compensation between thermodynamically induced wettening and transient eddy-induced drying trends. At a larger scale, aerosol and greenhouse gas forcing played a key role in the climate response over the Indo-Pacific sector and eastern equatorial Pacific, respectively (coined the “tropical Pacific east–west divide”). These findings contribute to an improved understanding of the drivers of the multidecadal trend in AUSM rainfall and highlight the need to reduce uncertainties in future projections under different aerosol emission trajectories, which is particularly important for northern Australia’s agriculture. Significance Statement Australian summer monsoon (AUSM) rainfall plays a vital role in sustaining northern Australia’s unique biodiversity and extensive agricultural industry. While observations show a significant increase in AUSM rainfall since the mid-twentieth century, the causes remain uncertain. We find that anthropogenic aerosol emissions from Asia played a key role in driving this multidecadal AUSM rainfall trend by inducing dynamic adjustments over the Indo-Pacific sector. These findings highlight the need to consider different aerosol emission trajectories when assessing future projections of AUSM rainfall.

Causes of the Unprecedented Month‐To‐Month Persistent Extreme Heat Event Over South China in Early Summer 2020: Role of Sea Surface Temperature Anomalies in the Tropical Indo‐Pacific Region

AbstractAn unprecedented month‐to‐month persistent extreme heat event attacked South China in early summer (May–July) 2020, with an extremely and persistently stronger‐than‐normal intensity and higher‐than‐normal number of extreme heat days. The underlying mechanisms were investigated from the month‐to‐month perspective using observations, reanalysis data, and numerical experiments. The results showed that a local anomalous anticyclone in the mid‐lower troposphere over South China and the South China Sea directly contributed to this persistent extreme heat event via anomalous warm advection heating and diabatic warming at near‐surface level. Further investigation revealed that the rapid transition from El Niño to La Niña with subsequent development in the central and eastern tropical Pacific, along with persistent warming in the tropical Indian Ocean in the early summer of 2020, were crucial to the maintenance and reinforcement of the local anomalous anticyclone, with greater importance of the latter. On the one hand, rapid cooling in the central and eastern tropical Pacific enhanced the Walker circulation and further triggered an abnormal local Hadley cell, leading to anomalous subsidence over South China and the South China Sea. On the other hand, the persistent warming in the tropical Indian Ocean resulted in an enhanced and westward‐extended western North Pacific subtropical high via eastward‐propagating Kelvin waves and an intensified and eastward‐extended South Asian high by heating the troposphere. These two aspects of atmospheric circulation configuration favored the maintenance and reinforcement of the local anomalous anticyclone in the mid‐lower troposphere over South China and the South China Sea.

Impact of solar variability on Indian summer monsoon through large scale circulations

Via large-scale circulations, the role of solar variability on the Indian summer monsoon is investigated. Standardized anomaly is used to identify years of solar maxima and minima. Statistical analysis such as moving mean, empirical mode decomposition, and wavelet analysis are used to determine the plausible relationship between solar variability, large scale circulations such as El- Niño 3.4 SST anomaly, Dipole Mode Index (DMI), and Atlantic Multidecadal Oscillation (AMO), and Indian summer monsoon rainfall (ISMR). In addition, the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP-NCAR) reanalysis data collection is used to assess time-averaged composite anomalies of wind, latent heat, geopotential height during maximum/minimum solar activity years.The 31-years moving mean of DMI shows significant negative correlation (−0.35) with sunspot number and with 31-years moving mean ISMR (−0.28). Similarly, 31-years moving mean of AMO exhibits a substantial positive connection with ISMR (0.68) and Niño 3.4 SST shows significant negative correlation (−0.62) with ISMR. Using intrusive mode function analyses it has noted that India receives rainfall for both phase of solar forcing but from wind studies at 850 hPa and 200 hPa, it is found that the phenomena for ISMR is different during solar maximum and minimum conditions. During solar maximum, a strong Low Level Jet is observed, and during solar minimum, a strong Tropical Easterly Jet is observed. Also, the area of influences of ISMR is different for solar maximum and solar minimum. The mean difference plot of seasonal rainfall shows that during solar maximum, north India receives reasonable amount of rainfall however during solar minimum, south India receives comparatively more rainfall than north India. A weakening of the local Hadley cell during solar minima is also observed.

Recent Early-Spring Drying Trend over Southern China Associated with Changes in the Zonal Thermal Contrast over the Pacific

Abstract Southern China has experienced a discernible early-spring drying trend in recent decades. This drying trend is a direct result of a severe deficiency of water vapor, which is induced by local descent anomalies and strong anomalous northerlies from the northwestern Pacific. The predominant anomalous northerlies are directly associated with two anomalous warm highs over the North Pacific (NPH) and East Asia (EAH), respectively. Anomalous easterlies along the southern edge of the NPH divert southwestward over southern Japan and form a branch of the northerlies, which merges into strong northerlies east of the EAH to construct the prevalent and deep northerly anomalies along the coast of East Asia. The NPH is related to changes in the zonal contrast of the Pacific sea surface temperature (SST-Tri), whose linear trend is dominated by warming in the western Pacific. The change in SST-Tri causes more precipitation in the tropical North Pacific, where strong ascent occurs and further facilitates robust descent over the extratropical North Pacific, facilitating the formation of the NPH. The EAH results from the interaction of tropical and higher latitudinal forcing in the context of global warming. On the one hand, enhanced precipitation over the Maritime Continent associated with the change in SST-Tri induces descent anomalies of the EAH over southern China via a local Hadley cell circulation. On the other hand, a “positive–negative–positive” tripolar tendency over the North Atlantic forces a wave train, which propagates eastward, becomes intensified over the Ural Mountains, and then moves farther eastward to affect the EAH. In addition, changes in SST-Tri can also modulate the tripole pattern over the North Pacific to further contribute to the EAH.

Causes of Interannual Variability of Summer Precipitation Intraseasonal Oscillation Intensity over Southwest China

AbstractThe interannual variability in summer precipitation intraseasonal oscillation intensity over southwest China (SWC) is investigated in this study. The results indicate that the 7–20-day period dominates the intraseasonal variability in summer SWC precipitation. The leading mode of summer SWC precipitation 7–20-day oscillation intensity (SPOI) is a north–south dipole pattern with prominent interannual variability. The atmospheric circulation anomalies from both tropics and mid- to high latitudes are responsible for the interannual variability in the dipole pattern. In the tropics, an enhanced local Hadley cell and an anomalous anticyclone over southern China and the northwest Pacific contribute to the north-positive–south-negative pattern of SPOI by inducing moisture convergence (divergence) over northern (southern) SWC in the background state. In the mid- to high latitudes, the 7–20-day Rossby wave trains along the subtropical jet are crucial for the 7–20-day precipitation over northern SWC. Further analyses suggest that the sea surface temperature (SST) anomalies over the Maritime Continent (MC) and the North Atlantic (NA) are associated with the SPOI dipole pattern. The MC SST warming causes convection anomalies over the tropical Indo-Pacific, consequently triggering a Matsuno–Gill-type atmospheric response conducive to the north-positive–south-negative pattern of SPOI. The NA SST tripole triggers a Rossby wave train across Eurasia that strengthens the East Asian westerly jet and enhances 7–20-day atmospheric variability, consequently favoring the variability of 7–20-day precipitation over northern SWC. Diagnoses of moisture and vorticity budgets further indicate the importance of the interaction between intraseasonal fluctuations and atmospheric background in the formation of the north–south difference in 7–20-day precipitation variability over SWC.

Zonal wave 3 pattern in the Southern Hemisphere generated by tropical convection

A distinctive feature of the Southern Hemisphere extratropical atmospheric circulation is the quasi-stationary zonal wave 3 pattern. This pattern is present in both the mean atmospheric circulation and its variability on daily, seasonal and interannual timescales. While the zonal wave 3 pattern has substantial impacts on meridional heat transport and Antarctic sea ice extent, the reason for its existence remains uncertain, although it has long been assumed to be linked to the presence of three major landmasses in the Southern Hemisphere extratropics. Here we use an atmospheric general circulation model to show that the stationary zonal wave 3 pattern is instead driven by zonally asymmetric deep convection in the tropics, with little influence from extratropical orography or landmasses. Localized regions of deep convection in the tropics form a local Hadley cell, which in turn creates a wave source in the subtropics that excites a poleward- and eastward-propagating wave train, forming quasi-stationary waves in the Southern Hemisphere high latitudes. Our findings suggest that changes in tropical deep convection, either due to natural variability or climate change, fundamentally control the zonal wave 3 pattern, with implications for southern high-latitude climate, ocean circulation and sea ice. The zonal wave 3 circulation pattern in the Southern Hemisphere is driven by tropical convection, according to results from an atmospheric general circulation model.

Reexamining the Mechanisms of East Asian Summer Monsoon Changes in Response to Non–East Asian Anthropogenic Aerosol Forcing

AbstractThis study examines the mechanisms by which the East Asian summer monsoon (EASM) changes in response to non–East Asian (NEA) anthropogenic aerosol forcing by distinguishing the fast direct atmospheric response and slow ocean-mediated response to forcing using a global aerosol–climate coupled model. The results show that NEA aerosol forcing significantly exacerbates the weakening of the EASM due to local aerosol forcing. The fast response is dominant in the weakening of the EASM and an anomalous precipitation pattern over eastern China resembling the “southern flood and northern drought” pattern in the total response to NEA aerosol forcing. Changes in upper-tropospheric temperature caused by the fast response play a major role in the impact of NEA aerosol forcing on the EASM. Anomalous cooling occurs during summer in the upper troposphere (at ~40°N) over East Asia caused by the fast response. This is due to the combined effects of strong eastward cold advection in the Northern Hemisphere midlatitudes caused by increased aerosol loading in Europe and the resulting change in local meridional heat transport in East Asia. Subsequently, the zonal wind speed changes on either side of the anomalous cooling, and the East Asian subtropical jet shifts equatorward, thereby weakening the EASM. The changes in atmospheric temperature and the local Hadley cell caused by the slow response to NEA aerosol forcing are conducive to strengthening the southwesterly winds over eastern China. Our study suggests the importance of NEA aerosol forcing in driving changes in the EASM on a fast time scale.

Quantifying the dynamics of the interannual variabilities of the wintertime East Asian Jet Core

The East Asian jet has evident interannual variability in strength and position and has broad impacts on weather and climate in the Asian-Pacific-American region. In this study, we quantify the relative contributions of distinct dynamical processes responsible for the wintertime interannual variability of the East Asian jet core. The quantification is based on analyzing the zonal momentum budgets. Our budget analyses show distinct up- and downstream controls for the interannual intensification of the jet. Over the upstream, the jet intensification is predominantly driven by enhanced upper-level angular momentum transport associated with local convective-driven Hadley-cells. In addition to convections over the Tropical Pacific, those over the Bay of Bengal–South China Sea are found responsible for a distinct local Hadley cell over 80^circ–100^circ E, which accelerates the jet from its very upstream. Over the downstream, angular momentum advected by the mean flow from the upstream is the first-order term responsible for the jet intensification, whereas synoptic eddies play a secondary role. For the interannual meridional displacement of the jet, synoptic eddy activities over the North Pacific are found to contribute predominantly. For both the intensification and the meridional displacement, the upper-tropospheric jet covaries closely with changes in the Subtropical Oceanic Frontal Zone in the North Pacific. Such a strong covariation implies the contributions of the air–sea interactions to the downstream jet variability.

Granger causality of the local Hadley cell and large-scale cloud cover over South Africa

This study demonstrates that Hadley cell dynamics could be used as a proxy to determine cloud cover and thus solar energy potential over South Africa. Granger causality was used to investigate causal interactions between the Hadley cell and cloud cover for the period 1980–2015, and such links were established. Areas of strong causality are found over the northwestern parts of South Africa. Moreover, weak causality from cloud cover to the Hadley cell does exist, with vertical velocity being the main variable responsible for this causality, which hence indirectly links cloud cover to Hadley cell causality.
 Significance:
 
 Hadley cell dynamics may be used to identify regions of cloudlessness over South Africa.
 Hadley cell dynamics may further be used as a proxy for cloud cover towards understanding the solar energy potential in South Africa within the context of climate variability and change.

Wintertime Modulation of the Local Cloud and Diagnostic Fields by the Hadley Cell Subsiding Boundary Over the Western North Pacific

AbstractThe responses of cloud and diagnostic fields to the poleward shift of the local Hadley cell subsiding edge are examined using observational and reanalysis data from December 1982 to February 2016. Over the western North Pacific, the interannual variability of the local Hadley cell subsiding edge is marked by an anomalous rising motion, decreased large‐scale static stability, and corresponding increases of the midlevel cloud fraction over the climatological sinking zone around 35°N. The most sensitive cloud type is identified to be cumulus congestus, with cloud top pressures in the range of 440–680 hPa and optical thicknesses in the range of 23–60. This kind of cloud is estimated to be the main contributor to the net negative top‐of‐atmosphere radiation anomaly and to constrain the distribution of the anomalous precipitation associated with the local HC expansion.

Extremely Late Onset of the 2018 South China Sea Summer Monsoon Following a La Niña Event: Effects of Triple SST Anomaly Mode in the North Atlantic and a Weaker Mongolian Cyclone

AbstractThe onset of South China Sea (SCS) summer monsoon (SCSSM) in June 2018 was extremely late following a La Niña event, indicating a breakdown in the traditional positive relationship between ENSO and SCSSM onset. Although the weaker western Pacific subtropical high in early May facilitated the SCSSM establishment, the persistent weaker Mongolian cyclone featured by an anomalous barotropic anticyclone over Northeast Asia during mid–late May dramatically postponed the SCSSM onset in 2018. This not only reduced upper‐tropospheric divergence over the SCS, which suppressed monsoon convection by attenuating the South Asian High, but cooled the mid–upper troposphere to decrease the in‐situ vertical easterly shear. The convection over the Maritime Continent was therefore trapped in the tropics, which further delayed the SCSSM onset via an enhanced local Hadley cell. It suggests a potential effect of triple SSTA mode in North Atlantic on the SCSSM onset by changing the mid‐latitudinal circulation.

Preferred solar signal and its transfer in the Asian–Pacific subtropical jet region

Solar impact on the tropospheric subtropical jet (SJ) has been identified previously from a zonally averaged perspective. The SJ was observed to be weaker in the high solar activity winters. However, some regional features of solar-induced SJ variations might remain unrecognized. Here it is found that the regional solar signal in wintertime Asian–Pacific zonal wind at 200 hPa, which exhibits a tripolar banded structure, greatly resembles the second internal mode of zonal wind within the same sector. Significant response of the Asian–Pacific SJ (APSJ) to increased solar forcing in boreal winter exclusively marks its center region, showing a deceleration in westerlies. Further exploration suggests two possible top–down routes to interpret this particular manifestation of solar signal in APSJ center, a tropical route and a middle–high latitude route. Regarding the tropical route, during the cold season, driven by the solar-associated reduction in Brewer–Dobson circulation, ozone concentration in tropical lower stratosphere increases notably and merely within the zonal range of APSJ center. This heats the air here and the tropical tropospheric regional upwelling is thereby suppressed. Consequently, a significant weakened APSJ center is produced via local Hadley cell. Regarding the middle–high latitude route, in early winter, solar-related pronounced westerly anomalies in the mid-latitude stratosphere only appear in the longitudinal range of APSJ center. Meanwhile, the upward propagating planetary waves from the troposphere could be reflected back downward by this intensified stratospheric westerlies. As winter progresses, through wave mean flow interactions, a resultant weakened APSJ center markedly presents in the middle of winter.

Diurnal Cycle of the ITCZ in DYNAMO

Abstract During the 2011 special observing period of the Dynamics of the Madden–Julian Oscillation (DYNAMO) field experiment, two sounding arrays were established over the central Indian Ocean, one north and one south of the equator, referred to here as the NSA and SSA, respectively. Three-hourly soundings from these arrays augmented by observations of radiation and rainfall are used to investigate the diurnal cycle of ITCZ convection during the MJO suppressed phase. During the first half of October, when convection was suppressed over the NSA but prominent over the SSA, the circulation over the sounding arrays could be characterized as a local Hadley cell. Strong rising motion was present within the ITCZ extending across the SSA with compensating subsidence over the NSA. A prominent diurnal pulsing of this cell was observed, impacting conditions on both sides of the equator, with the cell running strongest in the early morning hours (0500–0800 LT) and notably weakening later in the day (1700–2000 LT). The declining daytime subsidence over the NSA may have assisted the moistening of the low to midtroposphere there during the pre-onset stage of the MJO. Apparent heating Q1 within the ITCZ exhibited a diurnal evolution from early morning bottom-heavy profiles to weaker daytime top-heavy profiles, indicating a progression from convective to stratiform precipitation. Making use of the weak temperature gradient approximation, results suggest that both horizontal radiative heating gradients and direct cloud radiative forcing have an important influence on diurnal variations of vertical motion and convection within the ITCZ.

Month-to-month variability of Indian summer monsoon rainfall in 2016: role of the Indo-Pacific climatic conditions

Indian summer monsoon (ISM) rainfall during 2016 exhibited a prominent month-to-month fluctuations over India, with below normal rainfall in June and August and above normal rainfall in July. The factors determining the month-to-month fluctuations in ISM rainfall during 2016 are investigated with main focus on the Indo-Pacific climatic anomalies. Warm sea surface temperature (SST) anomalies associated with super El Nino 2015 disappeared by early summer 2016 over the central and eastern Pacific. On the other hand, negative Indian Ocean dipole (IOD) like SST anomaly pattern over the equatorial Indian Ocean and anomalous anticyclonic circulation over the western North Pacific (WNP) are reported in summer 2016 concurrently with decaying El Nino/developing La Nina phase. Observations revealed that the low rainfall over central north India in June is due to moisture divergence caused by the westward extension of ridge corresponding to WNP anticyclone and subsidence induced by local Hadley cell partly related to negative IOD. Low level convergence of southeasterly wind from Bay of Bengal associated with weak WNP anticyclone and northwesterly wind corresponding to anticyclonic circulation over the northwest India remarkably contributed to positive rainfall in July over most of the Indian subcontinent. While reduced rainfall over the Indian subcontinent in August 2016 is associated with the anomalous moisture transport from ISM region to WNP region, in contrast to July, due to local cyclogenesis corroborated by number of tropical cyclones in the WNP. In addition to this, subsidence related to strong convection supported by cyclonic circulation over the WNP also resulted in low rainfall over the ISM region. Coupled General Circulation model sensitivity experiments confirmed that strong convective activities associated with cyclonic circulation over the WNP is primarily responsible for the observed negative ISM rainfall anomalies in August 2016. It is noted that the Indo-Western Pacific circulation anomalies in August 2016 are well predicted when the coupled model is initiated with initial conditions from end of July and beginning of August compared to May. This analysis suggests the importance of the WNP circulation in forcing strong sub-seasonal/month to month rainfall variations over India.

Reduced connection between the East Asian Summer Monsoon and Southern Hemisphere Circulation on interannual timescales under intense global warming

Previous studies show a close relationship between the East Asian Summer Monsoon (EASM) and Southern Hemisphere (SH) circulation on interannual timescales. In this study, we investigate whether this close relationship will change under intensive greenhouse-gas effect by analyzing simulations under two different climate background states: preindustrial era and Representative Concentration Pathway (RCP) 8.5 stabilization from the Community Climate System Model Version 4 (CCSM4). Results show a significantly reduced relationship under stabilized RCP8.5 climate state, such a less correlated EASM with the sea level pressure in the southern Indian Ocean and the SH branch of local Hadley Cell. Further analysis suggests that the collapse of the Atlantic Meridional Overturning Circulation (AMOC) due to this warming leads to a less vigorous northward meridional heat transport, a decreased intertropical temperature contrast in boreal summer, which produces a weaker cross-equatorial Hadley Cell in the monsoonal region and a reduced Interhemispheric Mass Exchange (IME). Since the monsoonal IME acts as a bridge connecting EASM and SH circulation, the reduced IME weakens this connection. By performing freshwater hosing experiment using the Flexible Global Ocean—Atmosphere—Land System model, Grid-point Version 2 (FGOALS-g2), we show a weakened relationship between the EASM and SH circulation as in CCSM4 when AMOC collapses. Our results suggest that a substantially weakened AMOC is the main driver leading to the EASM, which is less affected by SH circulation in the future warmer climate.

Interdecadal change on the relationship between the mid-summer temperature in South China and atmospheric circulation and sea surface temperature

South China suffers from high temperature frequently in mid-summer and this study aims to explore the interdecadal change of interannual variation of the mid-summer temperature in South China. It is revealed that the relationship between South China temperature and atmospheric circulation and sea surface temperature anomaly (SSTA) experiences an interdecadal change around the early 1990s. Before the early 1990s, warmer summer in South China is associated with the mid-latitude teleconnection featured by higher pressure over the Ural Mountains and the Korean Peninsula and lower pressure around the Lake Baikal. South China is located at the southern flank of an anomalous high pressure. After the early 1990s, South China temperature is prominently influenced by the tropical SSTA, and meanwhile the mid-latitude teleconnection becomes much weaker. Warmer summer is associated with higher pressure centered over South China and the El Nino to La Nina transition phase. The higher pressure influencing South China is located more southwards after the early 1990s, and it is favored by the tropical SSTA. The warmer SST in summer over the western tropical Pacific enhances the local convection and triggers an anomalous local Hadley cell with stronger subsidence over South China, directly leading to higher pressure over South China. Moreover, the colder SST over the central–eastern Pacific induces an anomalous Walker circulation and further strengthens the convection over the western tropical Pacific, exerting an indirect impact on the higher pressure over South China. The relative role of the western Pacific warming and central–eastern Pacific cooling is verified by CAM4 simulations. The intimate relationship between the tropical SSTA and South China temperature occurs during the El Nino to La Nina transition phase, which is the case after the early 1990s and suggests higher predictability for South China temperature in the recent decades.

Evaluation of intra‐seasonal oscillation simulations in IPCC AR5 coupled GCMs associated with the Asian summer monsoon

ABSTRACTIn this study, we systematically evaluate the intra‐seasonal oscillation (ISO) associated with the Asian summer monsoon region in coupled general circulation models (GCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5). Results show that most of the models simulate reasonable climatological circulation patterns in boreal summer, but insufficient precipitation over the Bay of Bengal (BOB) and western North Pacific summer monsoon region. Most models underestimate the variance of 12–90‐day ISO mode but overestimate the percent variance of 12–90‐day ISO accounting for raw precipitation anomalies. There remains a wide gap between model simulations and observations in the simulations of eastward‐propagating ISO (30–80‐day), while the models perform better in the northward propagation of the 30–80‐day mode and the westward propagation of the 12–24‐day mode.In addition, mean state circulation, atmospheric internal dynamic process, air–sea interactions and model resolution have varying degrees of impacts on the ISO simulation. The local Hadley and Walker cell biases are investigated in this study, which might relate well with the precipitation bias in models. Within the similar background of vertical easterly shear, few models have the capacity to simulate the equivalent barotropic vorticity to the north of the convection centre. Furthermore, many models have difficulty in reproducing the strong air–sea interactions over the Indian Ocean and the BOB. Our results indicate that simply improving model resolution is not an effective method to obtain more reasonable propagation characteristics of the ISO, especially for eastward propagation. Therefore, further improving the capacity of the ISO simulation remains a great challenge for future development of climate system models.

Attribution of the local Hadley cell widening in the Southern Hemisphere

AbstractThis study conducts an attribution analysis of long‐term changes in the southern edge of the local Hadley cell (HC) during austral summer for the past three decades (1979–2009). The southern edges of the local overturning circulations (local HC) are defined as the latitudes of maximum sea level pressure in the Southern Hemisphere subtropics, and the long‐term variations of local HC edges from multireanalyses are compared with those from Coupled Model Intercomparison Project Phase 5 (CMIP5) multimodel simulations by using the optimal fingerprinting technique. The observed local HC exhibits a poleward expansion in the Atlantic and Indian Ocean regions, which is successfully reproduced by the CMIP5 models including anthropogenic forcing (ANT) but with a weaker amplitude. The detection analyses further show that ANT signals are detected robustly in both Atlantic and Indian HC trends. More importantly, anthropogenic forcings other than greenhouse gas forcing are found to be clearly detected in isolation, indicating a possible attribution of the observed local HC widening over these regions to stratospheric ozone depletion.

CMIP5 Projections of Two Types of El Niño and Their Related Tropical Precipitation in the Twenty-First Century

Abstract Future projections of the eastern-Pacific (EP) and central-Pacific (CP) types of El Niño in the twenty-first century, as well as their associated tropical circulation and precipitation variability, are investigated using historical runs and representative concentration pathway 8.5 (RCP8.5) simulations from 31 coupled models in phase 5 of the Coupled Model Intercomparison Project (CMIP5). As inferred from CMIP5 models that best capture both El Niño flavors, EP El Niño sea surface temperature (SST) variability will become weaker in the future climate, while no robust change of CP El Niño SST is found. Models also reach no consensus on the future change of relative frequency from CP to EP El Niño. However, there are robust changes in the tropical overturning circulation and precipitation associated with both types of El Niño. Under a warmer climate, magnitudes of precipitation anomalies during EP El Niño are projected to increase, presenting significant enhancement of the dry (wet) signal over the western (central–eastern) Pacific. This is consistent with an accelerated hydrological cycle in the deep tropics; hence, a “wet get wetter” picture appears under global warming, accompanied by a weakened anomalous Walker circulation. For CP El Niño, drier-than-normal conditions will be intensified over the tropical central–eastern Pacific in the future climate, with stronger anomalous sinking related to the strengthened North Pacific local Hadley cell. These results suggest that, besides the enhanced basic-state hydrological cycle over the tropics, other elements, such as the anomalous overturning circulation, might also play a role in determining the ENSO precipitation response to a warmer background climate.