Future change of Asian-Australian monsoon under RCP 4.5 anthropogenic warming scenario

We investigate the future changes of Asian-Australian monsoon (AAM) system projected by 20 climate models that participated in the phase five of the Coupled Model Intercomparison Project (CMIP5). A metrics for evaluation of the model’s performance on AAM precipitation climatology and variability is used to select a subset of seven best models. The CMIP5 models are more skillful than the CMIP3 models in terms of the AAM metrics. The future projections made by the selected multi-model mean suggest the following changes by the end of the 21st century. (1) The total AAM precipitation (as well as the land and oceanic components) will increase significantly (by 4.5 %/°C) mainly due to the increases in Indian summer monsoon (5.0 %/°C) and East Asian summer monsoon (6.4 %/°C) rainfall; the Australian summer monsoon rainfall will increase moderately by 2.6 %/°C. The “warm land-cool ocean” favors the entire AAM precipitation increase by generation of an east-west asymmetry in the sea level pressure field. On the other hand, the warm Northern Hemisphere-cool Southern Hemisphere induced hemispheric SLP difference favors the ASM but reduces the Australian summer monsoon rainfall. The combined effects explain the differences between the Asian and Australian monsoon changes. (2) The low-level tropical AAM circulation will weaken significantly (by 2.3 %/°C) due to atmospheric stabilization that overrides the effect of increasing moisture convergence. Different from the CMIP3 analysis, the EA subtropical summer monsoon circulation will increase by 4.4 %/°C. (3) The Asian monsoon domain over the land area will expand by about 10 %. (4) The spatial structures of the leading mode of interannual variation of AAM precipitation will not change appreciably but the ENSO-AAM relationship will be significantly enhanced.     

A climatological perspective of water vapor at the UTLS region over different global monsoon regions: observations inferred from the Aura-MLS and reanalysis data

The Aura-MLS observations of eight years from 2004 to 2011 have been utilized to understand the hydration and the dehydration mechanism over the northern and the southern hemispheric monsoon (NH and SH) regions. The monsoon regions considered are the Asian Summer Monsoon, East Asian Summer Monsoon, Arizona Monsoon (AM), North African Monsoon, South American Monsoon and the Australian Monsoon. The annual cycle of water vapor as expected shows maxima over the NH during June–August and during December–February over the SH. The time taken by the air parcels over the NH monsoon regions is found to be different compared to that over the SH monsoon regions. The analysis shows the concentration of water vapor in the upper troposphere and the lower stratosphere (UTLS) has not changed over these eight years in both the hemispheres during their respective monsoon seasons. The present analysis show different processes viz., direct overshooting convection, horizontal advection, temperature and cirrus clouds in influencing the distribution of water vapor to the UTLS over these different monsoon regions. Analysis of the UTLS water vapor with temperature and ice water content shows that the AM is hydrating the stratosphere compared to all the other monsoon regions where the water vapor is getting dehydrated. Thus it is envisaged that the present results will have important implications in understanding the exchange processes across the tropopause over the different monsoon regions and its role in stratosphere chemistry.     

RCP4.5情景下中国季风区及降水变化预估

本文使用国际耦合模式比较计划第五阶段(CMIP5)中共46个全球气候模式的数值试验结果,通过对中国区域的年、夏季和冬季降水气候态的模拟能力评估,择优选取了18个气候模式用来预估RCP4.5情景下21世纪中国季风区范围、季风降水及其强度变化。结果表明,相对于1986~2004年参考时段,RCP4.5情景下多数模式和所有模式集合平均在不同时段内均模拟出中国季风区面积、季风降水及其强度的增加趋势,最明显的时段出现在2081~2099年。其中,季风区面积扩张是导致季风降水增加的主要因素。在机制上,热力与动力条件变化均有利于季风降水强度的增加以及更多的水汽进入中国东部,从而引起季风区范围的扩大。     

Projected changes in South Asian summer monsoon by multi-model global warming experiments

South Asian summer monsoon (June through September) rainfall simulation and its potential future changes are evaluated in a multi-model ensemble of global coupled climate models outputs under World Climate Research Program Coupled Model Intercomparison Project (WCRP CMIP3) dataset. The response of South Asian summer monsoon to a transient increase in future anthropogenic radiative forcing is investigated for two time slices, middle (2031–2050) and end of the twenty-first century (2081–2100), in the non-mitigated Special Report on Emission Scenarios B1, A1B and A2 .There is large inter-model variability in the simulation of spatial characteristics of seasonal monsoon precipitation. Ten out of the 25 models are able to simulate space–time characteristics of the South Asian monsoon precipitation reasonably well. The response of these selected ten models has been examined for projected changes in seasonal monsoon rainfall. The multi-model ensemble of these ten models projects a significant increase in monsoon precipitation with global warming. The substantial increase in precipitation is observed over western equatorial Indian Ocean and southern parts of India. However, the monsoon circulation weakens significantly under all the three climate change experiments. Possible mechanisms for the projected increase in precipitation and for precipitation–wind paradox have been discussed. The surface temperature over Asian landmass increases in pre-monsoon months due to global warming and heat low over northwest India intensifies. The dipole snow configuration over Eurasian continent strengthens in warmer atmosphere, which is conducive for the enhancement in precipitation over Indian landmass. No notable changes have been projected in the El Niño–Monsoon relationship, which is useful for predicting interannual variations of the monsoon.     

Temperature and precipitation in the context of the annual cycle over Asia: Model evaluation and future change

Since the early or late arrival of monsoon rainfall can be devastating to agriculture and economy, the prediction of the onset of monsoon is a very important issue. The Asian monsoon is characterized by a strong annual cycle with rainy summer and dry winter. Nevertheless, most of monsoon studies have focused on the seasonal-mean of temperature and precipitation. The present study aims to evaluate a total of 27 coupled models that participated in phase 5 of the Coupled Model Intercomparison Project (CMIP5) for projection of the time evolution and the intensity of Asian monsoon on the basis of the annual cycle of temperature and precipitation. And future changes of onset, retreat, and intensity of monsoon are analyzed. Four models for good seasonal-mean (GSM) and good harmonic (GH) groups, respectively, are selected. GSM is based on the seasonal-mean of temperature and precipitation in summer and winter, and GH is based on the annual cycle of temperature and precipitation which represents a characteristic of the monsoon. To compare how well the time evolution of the monsoon is simulated in each group, the onset, retreat, and duration of Asian monsoon are examined. The highest pattern correlation coefficient (PCC) of onset, retreat, and duration between the reanalysis data and model outputs demonstrates that GH models’ MME predicts time evolution of monsoon most precisely, with PCC values of 0.80, 0.52, and 0.63, respectively. To predict future changes of the monsoon, the representative concentration pathway 4.5 (RCP 4.5) experiments for the period of 2073-2099 are compared with historical simulations for the period of 1979-2005 from CMIP5 using GH models’ MME. The Asian monsoon domain is expanded by 22.6% in the future projection. The onset date in the future is advanced over most parts of Asian monsoon region. The duration of summer Asian monsoon in the future projection will be lengthened by up to 2 pentads over the Asian monsoon region, as a result of advanced onset. The Asian monsoon intensity becomes stronger with the passage of time. This study has important implication for assessment of CMIP5 models in terms of the prediction of time evolution and intensity of Asian monsoon based on the annual cycle of temperature and precipitation.     

CMIP5 simulated change in the intensity of the Hadley and Walker circulations from the perspective of velocity potential

Based on the simulations of 31 global models in CMIP5, the performance of the models in simulating the Hadley and Walker circulations is evaluated. In addition, their change in intensity by the end of the 21 st century(2080–2099) under the RCP4.5 and RCP8.5 scenarios, relative to 1986–2005, is analyzed from the perspective of 200 h Pa velocity potential.Validation shows good performance of the individual CMIP5 models and the multi-model ensemble mean(MME) in reproducing the meridional(zonal) structure and magnitude of Hadley(Walker) circulation. The MME can also capture the observed strengthening tendency of the winter Hadley circulation and weakening tendency of the Walker circulation. Such secular trends can be simulated by 39% and 74% of the models, respectively. The MME projection indicates that the winter Hadley circulation and the Walker circulation will weaken under both scenarios by the end of the 21 st century. The weakening amplitude is larger under RCP8.5 than RCP4.5, due to stronger external forcing. The majority of the CMIP5 models show the same projection as the MME. However, for the summer Hadley circulation, the MME shows little change under RCP4.5 and large intermodel spread is apparent. Around half of the models project an increase, and the other half project a decrease. Under the RCP8.5 scenario, the MME and 65% of the models project a weakening of the summer southern Hadley circulation.     

Differences between CMIP6 and CMIP5 Models in Simulating Climate over China and the East Asian Monsoon

We compare the ability of coupled global climate models from the phases 5 and 6 of the Coupled Model Intercomparison Project(CMIP5 and CMIP6, respectively) in simulating the temperature and precipitation climatology and interannual variability over China for the period 1961–2005 and the climatological East Asian monsoon for the period1979–2005. All 92 models are able to simulate the geographical distribution of the above variables reasonably well.Compared with earlier CMIP5 models, current CMIP6 models have nationally weaker cold biases, a similar nationwide overestimation of precipitation and a weaker underestimation of the southeast–northwest precipitation gradient, a comparable overestimation of the spatial variability of the interannual variability, and a similar underestimation of the strength of winter monsoon over northern Asia. Pairwise comparison indicates that models have improved from CMIP5 to CMIP6 for climatological temperature and precipitation and winter monsoon but display little improvement for the interannual temperature and precipitation variability and summer monsoon. The ability of models relates to their horizontal resolutions in certain aspects. Both the multi-model arithmetic mean and median display similar skills and outperform most of the individual models in all considered aspects.     

Variability and teleconnections of South and East Asian summer monsoons in present and future projections of CMIP5 climate models

Coupled Model Inter-comparison Project Phase 5 (CMIP5) model outputs of the South and East Asian summer monsoon variability and their tele-connections are investigated using historical simulations (1861-2005) and future projections under the RCP4.5 scenario (2006-2100). Detailed analyses are performed using nine models having better representation of the recent monsoon teleconnections for the interactive Asian monsoon sub-systems. However, these models underestimate rainfall mainly over South Asia and Korea-Japan sector, the regions of heavy rainfall, along with a bias in location of rainfall maxima. Indeed, the simulation biases, underestimations of monsoon variability and teleconnections suggest further improvements for better representation of Asian monsoon in the climate models. Interestingly, the performance of Australian Community Climate and Earth System Simulator version 1.0 (ACCESS1.0) in simulating the annual cycle, spatial pattern of rainfall and multi-decadal variations of summer monsoon rainfall over South and East Asia appears to more realistic. In spite of large spread among the CMIP5 models, historical simulations as well as future projections of summer monsoon rainfall indicate multi-decadal variability. These rainfall variations, displaying certain epochs of more rainfall over South Asia than over East Asia and vice versa, suggest an oscillatory behaviour. Teleconnections between South and East Asian monsoon rainfall also exhibit a multi-decadal variation with alternate epochs of strengthening and weakening relationship. Furthermore, large-scale circulation features such as South Asian monsoon trough and north Pacific subtropical high depict zonal oscillatory behaviour with east-west-east shifts. Periods with eastward or westward extension of the Mascarene High, intensification and expansion of the upper tropospheric South Asian High are also projected by the CMIP5 models.     

基于转移累计概率分布统计降尺度方法的未来降水预估研究:以湖南省为例

基于最新一代CMIP5（Coupled Model Intercomparison Project Phase 5）模式历史情景和未来RCP4.5情景下的模式逐日降水数据,使用转移累计概率分布（CDF-t）统计降尺度方法,从空间变化和时间变率两个方面评估该降尺度方法对湖南日降水量模拟能力的改善效果,并在此基础上对未来降水量变化进行预估。结果表明, CMIP5气候模式由于分辨率较低,无法细致反映湖南地形变化和大气环流影响导致的区域降水变化特征。经过CDF-t统计降尺度处理之后,模式对湖南降水的时、空分布模拟与实况更为接近,绝大部分模式对降水空间结构的模拟能力都有显著提高。基于CDF-t统计降尺度的多模式集合预估结果表明,21世纪湖南省日降水量呈弱的增多趋势（0.95%/（10 a））。21世纪初、中和末期相对于1986—2005年的气候平均态,湖南省日降水量分别增加了4.6%、5%和5.2%。3个时期湖南省日平均降水变化的空间分布存在较强的一致性,皆表现为湖南西北、东北和东南3个地区降水增幅最为显著,且随着辐射强迫的增大,3个地区降水增幅也呈递增趋势。需要指出的是,预估结果在模式之间存在一定差异,并且这种差异随着辐射强迫的增大而增大。      

Water vapor transport and cross-equatorial flow over the Asian-Australia monsoon region simulated by CMIP5 climate models

The summer mean water vapor transport (WVT) and cross-equatorial flow (CEF) over the Asian-Australian monsoon region simulated by 22 coupled atmospheric-oceanic general circulation models (AOGCMs) from the World Climate Research Programme’s Coupled Model Intercomparison Project Phase 5 (CMIP5) were evaluated. Based on climatology of the twentieth-century simulations, most of models have a reasonably realistic representation of summer monsoon WVT characterized by southeast water vapor conveyor belt over the South Indian Ocean and southwest belt from the Arabian Sea to the East Asian. The correlation coefficients between NCEP reanalysis and simulations of BCC-CSM1-1, BNU-ESM, CanESM2, FGOALS-s2, MIROC4h and MPI-ESM-LR are up to 0.8. The simulated CEF depicted by the meridional wind along the equator includes the Somali jet and eastern CEF in low atmosphere and the reverse circulation in upper atmosphere, which were generally consistent with NCEP reanalysis. Multi-model ensemble means (MME) can reproduce more reasonable climatological features in spatial distribution both of WVT and CEF. Ten models with more reasonable WVT simulations were selected for future projection studies, including BCC-CSM1-1, BNU-ESM, CanESM2, CCSM4, FGOALS-s2, FIO-ESM, GFDL-ESM2G, MRIOC5, MPI-ESM-LR and NorESM-1M. Analysis based on the future projection experiments in RCP (Representative Concentration Pathway) 2.6, RCP4.5, RCP6 and RCP8.5 show that the global warming forced by different RCP scenarios will results in enhanced WVT over the Indian area and the west Pacific and weaken WVT in the low latitudes of tropical Indian Ocean.     

Connections Between the South Asian Summer Monsoon and the Tropical Sea Surface Temperature in CMIP5

The South Asian summer monsoon (SASM) precipitation is analyzed based on reanalysis datasets and historical simulation results from 23 climate models of the Coupled Model Intercomparison Project phase ...     

How Frequently Will the Persistent Heavy Rainfall over the Middle and Lower Yangtze River Basin in Summer 2020 Happen under Global Warming?

The middle and lower Yangtze River basin (MLYRB) suffered persistent heavy rainfall in summer 2020, with nearly continuous rainfall for about six consecutive weeks. How the likelihood of persistent heavy rainfall resembling that which occurred over the MLYRB in summer 2020 (hereafter 2020PHR-like event) would change under global warming is investigated. An index that reflects maximum accumulated precipitation during a consecutive five-week period in summer (Rx35day) is introduced. This accumulated precipitation index in summer 2020 is 60% stronger than the climatology, and a statistical analysis further shows that the 2020 event is a 1-in-70-year event. The model projection results derived from the 50-member ensemble of CanESM2 and the multimodel ensemble (MME) of the CMIP5 and CMIP6 models show that the occurrence probability of the 2020PHR-like event will dramatically increase under global warming. Based on the Kolmogorov–Smirnoff test, one-third of the CMIP5 and CMIP6 models that have reasonable performance in reproducing the 2020PHR-like event in their historical simulations are selected for the future projection study. The CMIP5 and CMIP6 MME results show that the occurrence probability of the 2020PHR-like event under the present-day climate will be double under lower-emission scenarios (CMIP5 RCP4.5, CMIP6 SSP1-2.6, and SSP2-4.5) and 3–5 times greater under higher-emission scenarios (3.0 times for CMIP5 RCP8.5, 2.9 times for CMIP6 SSP3-7.0, and 4.8 times for CMIP6 SSP5-8.5). The inter-model spread of the probability change is small, lending confidence to the projection results. The results provide a scientific reference for mitigation of and adaptation to future climate change.     

Projected Changes in Asian Summer Monsoon in RCP Scenarios of CMIP5

Responses of the Asian Summer Monsoon(ASM) in future projections have been studied based on two core future projections of phase five of the Coupled Model Intercomparison Project(CMIP5) coordinated experiments with the IAP-coupled model FGOALS_s2(the Flexible Global Ocean-Atmosphere-Land System Model).The projected changes of the ASM in climatological mean and interannual variability were respectively reported.Both the South Asian Summer Monsoon(SASM) and the East Asian Summer Monsoon(EASM) were intensified in their climatology,featuring increased monsoon precipitation and an enhanced monsoon lower-level westerly jet flow.Accordingly,the amplitude of the annual cycle of rainfall over East Asia(EA) is enhanced,thereby indicating a more abrupt monsoon onset.After the EA monsoon onset,the EASM marched farther northward in the future scenarios than in the historical runs.In the interannual variability,the leading pattern of the EASM,defined by the first multi-variable EOF analysis over EA,explains more of the total variances in the warmest future scenario,specifically,Representative Concentration Pathway(RCP8.5).Also,the correlation coefficients analysis suggests that the relationship between the EASM interannual variations and ENSO was significantly strengthened in the future projections,which may indicate improved predictability of the EASM interannual variations.     

CMIP5 simulated climate conditions of the Greater Horn of Africa (GHA). Part II: projected climate

This is the second of the two-part paper series on the analysis and evaluation of the Fifth phase of Coupled Model Intercomparison Project (CMIP5) simulation of contemporary climate as well as IPCC, AR5 Representative Concentrations Pathways (RCP), 4.5 and 8.5 scenarios projections of the Greater Horn of Africa (GHA) Climate. In the first part (Otieno and Anyah in Clim Dyn, 2012) we focused on the historical simulations, whereas this second part primarily focuses on future projections based on the two scenarios. Six Earth System Models (ESMs) from CMIP5 archive have been used to characterize projected changes in seasonal and annual mean precipitation, temperature and the hydrological cycle by the middle of twenty-first century over the GHA region, based on IPCC, 5th Assessment Report (AR5) RCP4.5 and RCP8.5 scenarios. Nearly all the models outputs analyzed reproduce the correct mean annual cycle of precipitation, with some biases among the models in capturing the correct peak of precipitation cycle, more so, March–April–May (MAM) seasonal rainfall over the equatorial GHA region. However, there is significant variation among models in projected precipitation anomalies, with some models projecting an average increase as others project a decrease in precipitation during different seasons. The ensemble mean of the ESMs indicates that the GHA region has been experiencing a steady increase in both precipitation and temperature beginning in the early 1980s and 1970s respectively in both RCP4.5 and RCP8.5 scenarios. Going by the ensemble means, temperatures are projected to steadily increase uniformly in all the seasons at a rate of 0.3/0.5 °C/decade under RCP4.5/8.5 scenarios over northern GHA region leading to an approximate temperature increase of 2/3 °C by the middle of the century. On the other hand, temperatures will likely increase at a rate of 0.3/0.4 °C/decade under RCP4.5/8.5 scenarios in both equatorial and southern GHA region leading to an approximate temperature increase of 2/2.5 °C by the middle of twenty-first century. Nonetheless, projected precipitation increase varied across seasons and sub-regions. With the exception of the equatorial region, that is projected to experience precipitation increase during DJF season, most sub-regions are projected to experience precipitation increase within their peak seasons, with the highest rate of increase experienced during DJF and OND seasons over southern and equatorial GHA regions respectively. Notably, as precipitation increases, the deficit (E < P) between evaporation (E) and precipitation (P) increased over the years, with a negatively skewed distribution. This generally implies that there is a high likelihood of an increased deficit in local moisture supply. This remarkable change in the general hydrological cycle (i.e. deficit in local moisture) is projected to be also coincident with intensified westerly anomaly influx from the Congo basin into the region. However, better understanding of the detailed changes in hydrological cycle will require comprehensive water budget analyses that require daily or sub-daily variables, and was not a specific focus of the present study.     

Recent change of the global monsoon precipitation (1979–2008)

The global monsoon (GM) is a defining feature of the annual variation of Earth’s climate system. Quantifying and understanding the present-day monsoon precipitation change are crucial for prediction of its future and reflection of its past. Here we show that regional monsoons are coordinated not only by external solar forcing but also by internal feedback processes such as El Ni?o-Southern Oscillation (ENSO). From one monsoon year (May to the next April) to the next, most continental monsoon regions, separated by vast areas of arid trade winds and deserts, vary in a cohesive manner driven by ENSO. The ENSO has tighter regulation on the northern hemisphere summer monsoon (NHSM) than on the southern hemisphere summer monsoon (SHSM). More notably, the GM precipitation (GMP) has intensified over the past three decades mainly due to the significant upward trend in NHSM. The intensification of the GMP originates primarily from an enhanced east–west thermal contrast in the Pacific Ocean, which is coupled with a rising pressure in the subtropical eastern Pacific and decreasing pressure over the Indo-Pacific warm pool. While this mechanism tends to amplify both the NHSM and SHSM, the stronger (weaker) warming trend in the NH (SH) creates a hemispheric thermal contrast, which favors intensification of the NHSM but weakens the SHSM. The enhanced Pacific zonal thermal contrast is largely a result of natural variability, whilst the enhanced hemispherical thermal contrast is likely due to anthropogenic forcing. We found that the enhanced global summer monsoon not only amplifies the annual cycle of tropical climate but also promotes directly a “wet-gets-wetter” trend pattern and indirectly a “dry-gets-drier” trend pattern through coupling with deserts and trade winds. The mechanisms recognized in this study suggest a way forward for understanding past and future changes of the GM in terms of its driven mechanisms.     

Strength of the North African monsoon in the Last Interglacial and under future warming

The authors explore the response of the Northern African (NAF) monsoon to orbital forcing in the Last Interglacial (LIG) compared with its response to greenhouses gas (GHG) forcing under the SSP5-8.5 scenario simulated in CMIP6. When the summer surface air temperature increases by 1 °C over the Northern Hemisphere, the NAF monsoon precipitation and its variability during the LIG increase by approximately 51% and 22%, respectively, which is much greater than under SSP5-8.5 (2.8% and 4.3%, respectively). GHG forcing enhances the NAF monsoon mainly by increasing the atmospheric moisture, while the LIG's orbital forcing intensifies the NAF monsoon by changing the monsoon circulation. During the LIG, models and data reconstructions indicate a salient hemispheric thermal contrast between the North and South Atlantic, strengthening the mean-state NAF monsoon precipitation. The interhemispheric temperature contrast enhances atmosphere–ocean interaction and the covariability of the northward sea surface temperature gradient and Saharan low, strengthening the NAF monsoon variability.摘要与人为强迫引起的全球变暖相比, 末次间冰期是轨道强迫引起的过去80万年来最暖的一个间冰期, 但鲜有人研究末次间冰期中北非季风的响应. 因此, 本文基于CMIP6多模式模拟结果对比研究了末次间冰期和SSP5–8.5情景下北非季风的响应, 发现末次间冰期下北非季风平均降水及其降水变率均远大于SSP5–8.5情景下的结果. 轨道强迫导致的北大西洋暖于南大西洋增加了北非季风环流和平均降水, 同时, 南北大西洋海温梯度变化通过增强热带北大西洋的海气相互作用增大了海温梯度和撒哈拉低压的变率, 从而增强了北非季风降水变率.     

CMIP5模式对我国西南地区干湿季降水的模拟和预估

利用降水观测资料, 评估了参加国际耦合模式比较计划第五阶段(CMIP5)的34个全球模式对1986～2005年我国西南地区干湿季降水的模拟能力。结果表明, 34个CMIP5模式中分别有30和25个模式模拟的干季和湿季降水偏多。34个模式对我国西南地区干湿季降水的模拟能力差异较大, 大约半数模式的模拟值与观测值的空间相关系数通过了99%的信度检验, 且标准差之比小于2。利用两个技巧评分标准, 分别挑选出了对干湿季降水模拟最优的9个模式。最优模式集合平均结果要优于34个模式的集合平均, 更要优于大多数单个模式。进一步利用最优的9个模式的集合平均对RCP4.5和RCP8.5两种典型浓度路径下我国西南地区干湿季降水的变化进行了预估。相对于1986～2005年气候平均态, 在21世纪初期(2016～2035年), 我国西南地区干季降水变化表现为川西高原降水增多, 而四川盆地及攀西地区、重庆、贵州和云南的大部分地区降水减少;湿季降水变化表现为川西高原、贵州和广西大部分地区降水增多, 而四川盆地及攀西地区和云南降水减少。在21世纪中期(2046～2065年)和末期(2080～2099年), 西南地区干湿季降水普遍增多。在RCP8.5情景下, 降水的变化幅度要强于RCP4.5情景。     

Responses of global monsoon and seasonal cycle of precipitation to precession and obliquity forcing

In this paper, the response of global monsoon to changes in orbital forcing is investigated using a coupled atmosphere–ocean general circulation model with an emphasis on relative roles of precession and obliquity changes. When precession decreases, there are inter-hemispheric asymmetric responses in monsoonal precipitation, featuring a significant increase over most parts of the Northern Hemisphere (NH) monsoon regions and a decrease over the Southern Hemisphere (SH) monsoon regions. In contrast, when obliquity increases, global monsoon is enhanced except for the American monsoon. Dynamic effects (caused by changes in winds with humidity unchanged) dominate the monsoonal precipitation response to both precession and obliquity forcing, while thermodynamic effects (caused by changes in humidity with winds unchanged) is related to the northward extension of the North African summer monsoon. During minimum precession, the seasonal cycle of tropical precipitation is advanced with respect to the maximum precession. The rainfall increase in the transitional season (April-June in the NH and October-December in the SH) is dominated by the dynamic component. From an energetics perspective, the southward (northward) cross-equatorial energy transport during April-June (October-December) corresponds to a northward (southward) shift of tropical precipitation, which results in a seasonal advance in the migration of tropical precipitation. Nonetheless, there is no significant change in the seasonal cycle in response to obliquity forcing.

     

What drives the global summer monsoon over the past millennium?

The global summer monsoon precipitation (GSMP) provides a fundamental measure for changes in the annual cycle of the climate system and hydroclimate. We investigate mechanisms governing decadal-centennial variations of the GSMP over the past millennium with a coupled climate model’s (ECHO-G) simulation forced by solar-volcanic (SV) radiative forcing and greenhouse gases (GHG) forcing. We show that the leading mode of GSMP is a forced response to external forcing on centennial time scale with a globally uniform change of precipitation across all monsoon regions, whereas the second mode represents internal variability on multi-decadal time scale with regional characteristics. The total amount of GSMP varies in phase with the global mean temperature, indicating that global warming is accompanied by amplification of the annual cycle of the climate system. The northern hemisphere summer monsoon precipitation (NHSMP) responds to GHG forcing more sensitively, while the southern hemisphere summer monsoon precipitation (SHSMP) responds to the SV radiative forcing more sensitively. The NHSMP is enhanced by increased NH land–ocean thermal contrast and NH-minus-SH thermal contrast. On the other hand, the SHSMP is strengthened by enhanced SH subtropical highs and the east–west mass contrast between Southeast Pacific and tropical Indian Ocean. The strength of the GSMP is determined by the factors controlling both the NHSMP and SHSMP. Intensification of GSMP is associated with (a) increased global land–ocean thermal contrast, (b) reinforced east–west mass contrast between Southeast Pacific and tropical Indian Ocean, and (c) enhanced circumglobal SH subtropical highs. The physical mechanisms revealed here will add understanding of future change of the global monsoon.     

Boreal summer continental monsoon rainfall and hydroclimate anomalies associated with the Asian-Pacific Oscillation

With the twentieth century analysis data (1901–2002) for atmospheric circulation, precipitation, Palmer drought severity index, and sea surface temperature (SST), we show that the Asian-Pacific Oscillation (APO) during boreal summer is a major mode of the earth climate variation linking to global atmospheric circulation and hydroclimate anomalies, especially the Northern Hemisphere (NH) summer land monsoon. Associated with a positive APO phase are the warm troposphere over the Eurasian land and the relatively cool troposphere over the North Pacific, the North Atlantic, and the Indian Ocean. Such an amplified land–ocean thermal contrast between the Eurasian land and its adjacent oceans signifies a stronger than normal NH summer monsoon, with the strengthened southerly or southwesterly monsoon prevailing over tropical Africa, South Asia, and East Asia. A positive APO implies an enhanced summer monsoon rainfall over all major NH land monsoon regions: West Africa, South Asia, East Asia, and Mexico. Thus, APO is a sensible measure of the NH land monsoon rainfall intensity. Meanwhile, reduced precipitation appears over the arid and semiarid regions of northern Africa, the Middle East, and West Asia, manifesting the monsoon-desert coupling. On the other hand, surrounded by the cool troposphere over the North Pacific and North Atlantic, the extratropical North America has weakened low-level continental low and upper-level ridge, hence a deficient summer rainfall. Corresponding to a high APO index, the African and South Asian monsoon regions are wet and cool, the East Asian monsoon region is wet and hot, and the extratropical North America is dry and hot. Wet and dry climates correspond to wet and dry soil conditions, respectively. The APO is also associated with significant variations of SST in the entire Pacific and the extratropical North Atlantic during boreal summer, which resembles the Interdecadal Pacific Oscillation in SST. Of note is that the Pacific SST anomalies are not present throughout the year, rather, mainly occur in late spring, peak at late summer, and are nearly absent during boreal winter. The season-dependent APO–SST relationship and the origin of the APO remain elusive.