A note on the break‐up of lakes and rivers as indicators of climate change

Abstract

A detailed examination has been made of the relationship between the space and time variations of the Indian summer monsoon rainfall and the equatorial eastern‐Pacific sea surface temperature (SST) anomaly in different seasons for the 108‐year period, 1871–1978. There is a strong inverse relationship between the two. The correlation coefficients between All‐India monsoon rainfall and the sea surface temperature anomaly for the concurrent season; June, July and August (JJA) and for the succeeding seasons; September, October and November (SON) and December, January and February (DJF) are consistently and highly significant. Even a random sample of 50 years gave values significant at the 0.1 percent level. The sliding window correlation analysis of 10‐, 20‐ and 30‐year widths indicates that the relationships between All‐India monsoon rainfall and the sea surface temperature anomaly for the concurrent JJA and the succeeding SON and DJF seasons exhibit stability and consistency in significance. For contiguous meteorological sub‐divisions west of longitude 80°E the relationship is highly significant for JJA and for succeeding SON and DJF seasons.     

Surface pressure and summer monsoon rainfall over India

The relationship between the all-India summer monsoon rainfall and surface pressure over the Indian region has been examined to obtain a useful predictor for the monsoon rainfall. The data series of all-India monsoon rainfall and the mean pressures of three seasons before and after the monsoon season as well as the winter-to-spring pressure tendency (MAM-DJF) at 100 stations for the period 1951-1980 have been used in the analysis. The all-India monsoon rainfall is negatively correlated with the pressure of the spring (MAM) season preceding the monsoon and winter-to-spring seasonal difference as pressure tendency (MAM-DJF), at almost all the stations in India, and significantly with the pressures over central and northwestern regions. The average mean sea level pressure of six stations (Jodhpur, Ahmedabed, Bombay, Indore, Sagar and Akola) in the Western Central Indian (WCI) region showed highly significant (at 1% level) and consistent CCs of -0.63 for MAM and -0.56 for MAM-DJF for the period 1951–1980. Thus, the pre-monsoon seasonal pressure anomalies over WCI could provide a useful parameter for the long-range forecasting scheme of the Indian monsoon rainfall.     

夏季中国华北降水、印度降水与太平洋海表面温度的耦合关系

本文基于1951~2014年的站点观测资料以及再分析资料,应用多变量经验正交分解法（MEOF）研究了年际尺度上华北夏季降水、印度夏季降水与海表面温度之间的耦合关系（主要模态）。结果表明:当印度夏季降水偏强时,若同期夏季赤道中东太平洋海温表现为La Ni?a位相,则西太平洋暖池对流加强,副热带高压偏西偏北,有利于华北夏季降水与印度夏季降水一致增强。反之,当印度大部降水偏弱时,若同期夏季赤道中东太平洋海温表现为El Ni?o位相,则华北夏季降水和印度夏季降水一致减弱。然而,两地夏季降水的协同变化关系并不总是成立。当赤道中东太平洋海温异常随时间演变表现为冬春El Ni?o衰减型时,伴随着印度洋偶极子（IOD）正位相的衰减过程,这会减弱东亚夏季风,使得华北夏季降水偏少。此时印度半岛夏季降水增强区集中在其西部,无法形成连接印度和华北夏季降水异常的环半球遥相关（CGT）波列,可能使得华北夏季降水异常与全印度夏季降水异常成相反形势。这些结论揭示了中国华北夏季降水、印度夏季降水和海表面温度之间的耦合关系,有助于进一步理解海温外强迫对两地夏季降水之间相关关系的作用,从而对华北夏季降水的预测具有参考意义。     

Coupling between the El-Niño and planetary-scale waves and their linkage with the Indian monsoon rainfall

Summary The relationships between the El-Niño phenomenon and the planetary-scale waves, and the interannual variations in the Indian monsoon (June–September) rainfall have been analysed in order to investigate how the sea surface temperature (SST) in the equatorial eastern Pacific associated with the El-Niño can produce reduced monsoon rainfall over India by teleconnections.The longitude of ridge location over the Indian region of the integrated planetary waves (numbers 1–3) along 15° N latitude circle in the height field of 200 mb pressure level in May is significantly (r=0.93, significant at 98% CL) related to the May SST anomaly at Puerto Chicama. This implies that warmer (colder) SST anomalies are associated with eastward (westward) longitude of the ridge location. The variations of the ridge location in May appear to be significantly inversely (r=–0.95, significant at 99% CL) related to the Indian monsoon rainfall, with rainfall tending to be less (more) than normal during eastward (westward) longitude of the ridge location suggesting some predictive value for the Indian monsoon rainfall. The Indian monsoon rainfall and May SST anomaly at Puerto Chicama are inversely related (r=–0.90, significant at 96% CL).In terms of the observed relationships, a plausible mechanism for linking El-Niño with the reduced Indian monsoon rainfall is discussed. The relationships noted suggest that excessive warm SST anomalies associated with El-Niño induce an eastward shift in the planetary waves which in turn reduce the Indian monsoon rainfall.With 8 Figures     

热带海面温度对亚马逊旱季降水年际变率的影响及机制

用偏最小二乘(Partial Least Square,PLS)回归方法分析了 1979～2018年影响亚马逊旱季(6～8月)降水年际变率的热带海面温度模态.第一海面温度模态解释了总方差的64％,主要表现为前期亚马逊雨季(12月至次年2月)至旱季(6～8月)热带东太平洋La Ni?a型海面温度异常演变.12月至次年2月...     

Impacts of Indian Ocean SST biases on the Indian Monsoon: as simulated in a global coupled model

In this study, the impact of the ocean–atmosphere coupling on the atmospheric mean state over the Indian Ocean and the Indian Summer Monsoon (ISM) is examined in the framework of the SINTEX-F2 coupled model through forced and coupled control simulations and several sensitivity coupled experiments. During boreal winter and spring, most of the Indian Ocean biases are common in forced and coupled simulations, suggesting that the errors originate from the atmospheric model, especially a dry islands bias in the Maritime Continent. During boreal summer, the air-sea coupling decreases the ISM rainfall over South India and the monsoon strength to realistic amplitude, but at the expense of important degradations of the rainfall and Sea Surface Temperature (SST) mean states in the Indian Ocean. Strong SST biases of opposite sign are observed over the western (WIO) and eastern (EIO) tropical Indian Ocean. Rainfall amounts over the ocean (land) are systematically higher (lower) in the northern hemisphere and the south equatorial Indian Ocean rainfall band is missing in the control coupled simulation. During boreal fall, positive dipole-like errors emerge in the mean state of the coupled model, with warm and wet (cold and dry) biases in the WIO (EIO), suggesting again a significant impact of the SST errors. The exact contributions and the distinct roles of these SST errors in the seasonal mean atmospheric state of the coupled model have been further assessed with two sensitivity coupled experiments, in which the SST biases are replaced by observed climatology either in the WIO (warm bias) or EIO (cold bias). The correction of the WIO warm bias leads to a global decrease of rainfall in the monsoon region, which confirms that the WIO is an important source of moisture for the ISM. On the other hand, the correction of the EIO cold bias leads to a global improvement of precipitation and circulation mean state during summer and fall. Nevertheless, all these improvements due to SST corrections seem drastically limited by the atmosphere intrinsic biases, including prominently the unimodal oceanic position of the ITCZ (Inter Tropical Convergence Zone) during summer and the enhanced westward wind stress along the equator during fall.     

Phase transition of the Pacific decadal oscillation and decadal variation of the East Asian summer monsoon in the 20th century

This paper focuses on the relationship between the phase transition of the Pacific decadal oscillation (PDO) and decadal variation of the East Asian summer monsoon (EASM) in the twentieth century. The first transition occurred in the 1940s, with an enhanced SST in the North Pacific and reduced SST in the tropical eastern Pacific and South Indian Ocean. In agreement with these SST changes, a higher SLP was found in most parts of the Pacific, while a lower SLP was found in the North Pacific and most parts of the Indian Ocean. In this case, the EASM was largely enhanced with a southerly anomaly in the lower troposphere along the east coast of China. Correspondingly, there was less rainfall in the Yangtze River valley and more rainfall in northern and southern China. An opposite change was found when the PDO reversed its phase in the late 1970s. In the tropical Indian Ocean and western Pacific, however, the SST was enhanced in both the 1940s and 1970s. As a result, the western Pacific subtropical high (WPSH) tended to extend westward with a larger magnitude in the 1970s. The major features were reasonably reproduced by an atmospheric general circulation model (IAP AGCM4.0) prescribed with observed SST and sea ice. On the other hand, the westward extension of the WPSH was exaggerated in the 1970s, while it was underestimated in the 1940s. Besides, the spatial pattern of the simulated summer rainfall in eastern China tended to shift southward compared with the observation.     

Tanzanian rainfall responses to El Niño and positive Indian Ocean Dipole events during 1951–2015

The relative impacts of Indian and Pacific Ocean processes on Tanzanian rainfall was evaluated using composite and correlation analyses. It was found that the seasonal responses of rainfall to positive Indian Ocean Dipole (pIOD) and El Niño events are substantial from September–October–November (SON) to December–January–February (DJF), whereas the Indian Ocean Dipole (IOD) exerts more control than El Niño–Southern Oscillation (ENSO) in both seasons. The associated relationship with the sea surface temperature (SST) and large-scale atmospheric circulations revealed distinct features. For the pure pIOD years, there is above-normal rainfall over the entire country. A strong rainfall condition is evident over the Lake Victoria basin and coastal and northeastern highland parts of the country during SON, while areas of the central and southern highlands exhibit substantial rains during DJF. For the pure El-Niño events, Tanzania has suffered from insignificant, weak, and non-coherent rainfall conditions during SON. However, a contrasting insignificant rainfall signature is found between the northern and southern parts of the country during the subsequent DJF season. For the co-occurrence of pIOD and El Niño, significant, excessive rainfall conditions are restricted to over the northern coast and northeastern areas of the country during SON, consistent with the rainfall pattern for pIOD. A weak, positive rainfall condition is observed over the entire country in the following season of DJF. Generally, in terms of Tanzanian rainfall, the IOD/ENSO variability and the associated impacts can be explained by the anomalous SST and circulation anomalies.     

The role of northern Arabian Sea surface temperature biases in CMIP5 model simulations and future projections of Indian summer monsoon rainfall

Many climate models have problems simulating Indian summer monsoon rainfall and its variability, resulting in considerable uncertainty in future projections. Problems may relate to many factors, such as local effects of the formulation of physical parametrisation schemes, while common model biases that develop elsewhere within the climate system may also be important. Here we examine the extent and impact of cold sea surface temperature (SST) biases developing in the northern Arabian Sea in the CMIP5 multi-model ensemble, where such SST biases are shown to be common. Such biases have previously been shown to reduce monsoon rainfall in the Met Office Unified Model (MetUM) by weakening moisture fluxes incident upon India. The Arabian Sea SST biases in CMIP5 models consistently develop in winter, via strengthening of the winter monsoon circulation, and persist into spring and summer. A clear relationship exists between Arabian Sea cold SST bias and weak monsoon rainfall in CMIP5 models, similar to effects in the MetUM. Part of this effect may also relate to other factors, such as forcing of the early monsoon by spring-time excessive equatorial precipitation. Atmosphere-only future time-slice experiments show that Arabian Sea cold SST biases have potential to weaken future monsoon rainfall increases by limiting moisture flux acceleration through non-linearity of the Clausius–Clapeyron relationship. Analysis of CMIP5 model future scenario simulations suggests that such effects are small compared to other sources of uncertainty, although models with large Arabian Sea cold SST biases may suppress the range of potential outcomes for changes to future early monsoon rainfall.     

Atmospheric circulation characteristics during some El-Nino years in relation to Indian summer monsoon rainfall

Summary Based on observed rainfall data of India Meteorological Department (IMD), correlation coefficients (CCs) have been computed between Indian summer monsoon rainfall (ISMR) and sea surface temperature (SST) anomalies over different Nino regions and standardised pressure difference between Tahiti and Darwin. Significant positive CCs are found between the Southern Oscillation Index (SOI) in winter and subsequent June rainfall over India. Concurrent with and subsequent to Indian summer monsoon, SOI shows significant positive CC with the mean rainfall of July to September (JAS). Significant negative CCs are found between JAS mean rain and the concurrent and following SST anomalies over Nino-3.4 region. On the basis of these correlations, it is proposed that the entire period of summer monsoon from June to September could be divided into two sub-periods such as: early summer (June) and mid-late summer (July to September) monsoon for prediction of ISMR in the extended range.In order to examine the characteristics of atmospheric circulation during some El-Nino years, divergent flow at 200hPa and omega field at 500hPa based on NCEP/NCAR reanalysis have been studied in detail. Major significant southward shift of upper level divergent field from India are related to El-Nino and this shift may be responsible for causing droughts during several El-Nino years over India. Also vertical wind fields at 500hPa show sinking motion over large parts of India and west Pacific and ascending motion over southern Indian Ocean, central and eastern Pacific during major drought years.     

Influence of the Preceding Austral Summer Southern Hemisphere Annular Mode on the Amplitude of ENSO Decay

There is increasing evidence of the possible role of extratropical forcing in the evolution of ENSO.The Southern Hemisphere Annular Mode(SAM) is the dominant mode of atmospheric circulation in the Southern Hemisphere extratropics.This study shows that the austral summer(December–January–February; DJF) SAM may also influence the amplitude of ENSO decay during austral autumn(March–April–May;MAM).The mechanisms associated with this SAM–ENSO relationship can be briefly summarized as follows:The SAM is positively(negatively) correlated with SST in the Southern Hemisphere middle(high) latitudes.This dipole-like SST anomaly pattern is referred to as the Southern Ocean Dipole(SOD).The DJF SOD,caused by the DJF SAM,could persist until MAM and then influence atmospheric circulation,including trade winds,over the Nio3.4 area.Anomalous trade winds and SST anomalies over the Nio3.4 area related to the DJF SAM are further developed through the Bjerkness feedback,which eventually results in a cooling(warming) over the Nio3.4 area followed by the positive(negative) DJF SAM.     

Dependence of Indian monsoon rainfall on moisture fluxes across the Arabian Sea and the impact of coupled model sea surface temperature biases

The Arabian Sea is an important moisture source for Indian monsoon rainfall. The skill of climate models in simulating the monsoon and its variability varies widely, while Arabian Sea cold sea surface temperature (SST) biases are common in coupled models and may therefore influence the monsoon and its sensitivity to climate change. We examine the relationship between monsoon rainfall, moisture fluxes and Arabian Sea SST in observations and climate model simulations. Observational analysis shows strong monsoons depend on moisture fluxes across the Arabian Sea, however detecting consistent signals with contemporaneous summer SST anomalies is complicated in the observed system by air/sea coupling and large-scale induced variability such as the El Ni?o-Southern Oscillation feeding back onto the monsoon through development of the Somali Jet. Comparison of HadGEM3 coupled and atmosphere-only configurations suggests coupled model cold SST biases significantly reduce monsoon rainfall. Idealised atmosphere-only experiments show that the weakened monsoon can be mainly attributed to systematic Arabian Sea cold SST biases during summer and their impact on the monsoon-moisture relationship. The impact of large cold SST biases on atmospheric moisture content over the Arabian Sea, and also the subsequent reduced latent heat release over India, dominates over any enhancement in the land-sea temperature gradient and results in changes to the mean state. We hypothesize that a cold base state will result in underestimation of the impact of larger projected Arabian Sea SST changes in future climate, suggesting that Arabian Sea biases should be a clear target for model development.     

EFFECT OF WINTER MONSOON ON SUMMER MONSOON THROUGH AIR-SEA INTERACTION*

In this paper the relationships between the sea surface temperature (SST) of Xisha and that in the northern Indian and northern Pacific Oceans,the geopotential height at 500 hPa level of the Northern Hemisphere,and rainfall in China are studied statistically using data in the period of 1961-1992.Results show that in winter,the interannual variation in SST of Xisha describes that for a large oceanic region off the East Asia coast,and is closely related to the activity of East Asia winter monsoon.On the other hand,there exist very high values of auto-correlation of Xisha SST anomaly from December through the following July,but the anomalous condition is hardly correlated to that in the preceding autumn.The winter monsoon related anomalous SST condition in Xisha has a strong tendency to persist through the succeeding summer monsoon season with the same sign.In addition,correlation maps of monthly mean rainfall in China with respect to Xisha SST of the same month show positive correlations with confidence level above 95% to the east of 110°E and to the south of Changjiang (Yangtze) River during the months of October through April;the region becomes smaller in May and changes correlation sign in June;the positive correlation region is located in the middle and lower reaches of Changjiang River from July to September.The air-sea interaction plays an important role in these processes.     

Roles of Indian and Pacific Ocean air–sea coupling in tropical atmospheric variability

Sea surface temperature (SST) variations include negative feedbacks from the atmosphere, whereas SST anomalies are specified in stand-alone atmospheric general circulation simulations. Is the SST forced response the same as the coupled response? In this study, the importance of air–sea coupling in the Indian and Pacific Oceans for tropical atmospheric variability is investigated through numerical experiments with a coupled atmosphere-ocean general circulation model. The local and remote impacts of the Indian and Pacific Ocean coupling are obtained by comparing a coupled simulation with an experiment in which the SST forcing from the coupled simulation is specified in either the Indian or the Pacific Ocean. It is found that the Indian Ocean coupling is critical for atmospheric variability over the Pacific Ocean. Without the Indian Ocean coupling, the rainfall and SST variations are completely different throughout most of the Pacific Ocean basin. Without the Pacific Ocean coupling, part of the rainfall and SST variations in the Indian Ocean are reproduced in the forced run. In regions of large mean rainfall where the atmospheric negative feedback is strong, such as the North Indian Ocean and the western North Pacific in boreal summer, the atmospheric variability is significantly enhanced when air–sea coupling is replaced by specified SST forcing. This enhancement is due to the lack of the negative feedback in the forced SST simulation. In these regions, erroneous atmospheric anomalies could be induced by specified SST anomalies derived from the coupled model. The ENSO variability is reduced by about 20% when the Indian Ocean air–sea coupling is replaced by specified SST forcing. This change is attributed to the interfering roles of the Indian Ocean SST and Indian monsoon in western and central equatorial Pacific surface wind variations.     

Surface and upper air temperatures over India in relation to monsoon rainfall

Summary The relationship between the all-India summer monsoon rainfall and surface/upper air (850, 700, 500 and 200 mb levels) temperatures over the Indian region and its spatial and temporal characteristics have been examined to obtain a useful predictor for the monsoon rainfall. The data series of all-India and subdivisional summer monsoon rainfall and various seasonal air temperatures at 73 surface observatories and 9 radiosonde stations (1951–1980) have been used in the analysis. The Correlation Coefficients (CCs) between all-India monsoon rainfall and seasonal surface air temperatures with different lags relative to the monsoon season indicate a systematic relationship.The CCs between the monsoon rainfall and surface-air temperature of the preceding MAM (pre-monsoon spring) season are positive over many parts of India and highly significant over central and northwestern regions. The average surface air temperature of six stations i.e., Jodhpur, Ahmedabad, Bombay, Indore, Sagar and Akola in this region (Western Central India, WCI) showed a highly significant CC of 0.60 during the period 1951–1980. This relationship is also found to be consistently significant for the period from 1950 to present, though decreasing in magnitude after 1975. WCI MAM surface air temperature has shown significant CCs with the monsoon rainfall over eleven sub-divisions mainly in northwestern India, i.e., north of 15 °N and west of 80 °E.Upper air temperatures of the MAM season at almost all the stations and all levels considered show positive CCs with the subsequent monsoon rainfall. These correlations are significant at some central and north Indian stations for the lower and middle tropospheric temperatures.The simple regression equation developed for the period 1951–1980 isy = – 183.20 + 8.83x, wherey is the all-India monsoon rainfall in cm andx is the WCI average surface air temperature of MAM season in °C. This equation is significant at 0.1% level. The suitability of this parameter for inclusion in a predictive regression model along with five other global and regional parameters has been discussed. Multiple regression analysis for the long-range prediction of monsoon rainfall, using several combinations of these parameters indicates that the improvement of predictive skill considerably depends upon the selection of the predictors.With 9 Figures     

The Indo-Australian monsoon and its relationship to ENSO and IOD in reanalysis data and the CMIP3/CMIP5 simulations

A large spread exists in both Indian and Australian average monsoon rainfall and in their interannual variations diagnosed from various observational and reanalysis products. While the multi model mean monsoon rainfall from 59 models taking part in the Coupled Model Intercomparison Project (CMIP3 and CMIP5) fall within the observational uncertainty, considerable model spread exists. Rainfall seasonality is consistent across observations and reanalyses, but most CMIP models produce either a too peaked or a too flat seasonal cycle, with CMIP5 models generally performing better than CMIP3. Considering all North-Australia rainfall, most models reproduce the observed Australian monsoon-El Niño Southern Oscillation (ENSO) teleconnection, with the strength of the relationship dependent on the strength of the simulated ENSO. However, over the Maritime Continent, the simulated monsoon-ENSO connection is generally weaker than observed, depending on the ability of each model to realistically reproduce the ENSO signature in the Warm Pool region. A large part of this bias comes from the contribution of Papua, where moisture convergence seems to be particularly affected by this SST bias. The Indian summer monsoon-ENSO relationship is affected by overly persistent ENSO events in many CMIP models. Despite significant wind anomalies in the Indian Ocean related to Indian Ocean Dipole (IOD) events, the monsoon-IOD relationship remains relatively weak both in the observations and in the CMIP models. Based on model fidelity in reproducing realistic monsoon characteristics and ENSO teleconnections, we objectively select 12 “best” models to analyze projections in the rcp8.5 scenario. Eleven of these models are from the CMIP5 ensemble. In India and Australia, most of these models produce 5–20 % more monsoon rainfall over the second half of the twentieth century than during the late nineteenth century. By contrast, there is no clear model consensus over the Maritime Continent.     

Role of thermocline–SST coupling in the evolution of IOD events and their regional impacts

The evolution of sea surface temperature (SST) and thermocline (represented by 20 °C isotherm depth, D20) in the east equatorial Indian Ocean (EIO) associated with the Indian Ocean Dipole (IOD) years is studied for the period of 50 years from 1958 to 2007. A new IOD index based on combined anomalies of surface winds, D20 and SST over the equatorial Indian Ocean is defined to identify strong and weak IOD events. It is found that the evolution of strong IOD events is driven by ocean dynamics in the form of thermocline–SST coupling and is strongly interactive with the atmosphere, whereas the weak IOD events are mere response to surface winds without such dynamical coupling. The easterly wind anomalies extend up to the western equatorial Indian Ocean (WIO) during strong IOD years and support enhanced EIO air–sea interactions. On the other hand, the evolution of zonal wind anomalies is weak during the weak IOD years. Thermocline–SST coupling is robust in both EIO and WIO during strong IOD years, which is primarily responsible for the enhanced SST gradient, strong enough to establish anomalous Walker circulation within the Indian Ocean. The strong convection over the WIO associated with the Indian Ocean Walker cell triggers a secondary cell with subsidence over the African landmass. This double cell structure over the equatorial Indian Ocean is not reported before. Such double cell structure is not evident in weak IOD years and instead the convection over WIO extends up to African landmass. These are well supported by the spatial pattern of anomalous precipitable water during strong and weak IOD years. Strengthening of monsoon flow and local Hadley cell associated with strong IOD events enhances precipitation over the Indian subcontinent, whereas weak IOD years have less impact on the Indian summer monsoon circulation and rainfall. Analysis reveals that the EIO thermocline index and combined index could be potential predictors for the central Indian rainfall during summer.     

气候系统模式FGOALS-s1.1对热带降水年循环模态的模拟

文中评估了中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室(LASG/IAP)新一代耦合气候模式Fgoals_s1.1对热带降水年循环模态的模拟能力。通过与观测表层海温(SST)强迫的大气模式SAMIL试验结果比较,分析了海气耦合过程对年循环模态模拟效果的影响。结果表明Fgoals_s1.1能合理再现热带地区降水年循环模态的基本特征。Fgoals_s1.1模拟出了年平均降水场中的主要降水中心,但模拟的赤道和南太平洋降水偏多,而北太平洋降水则偏少。Fgoals_s1.1的季风模态降水呈现与观测一致的关于赤道反对称的特征,其模拟偏差大部分来自大气分量,尤其是在赤道外。Fgoals_s1.1的主要缺陷在于它对春秋非对称模态模拟能力低于单独大气模式,这主要是由于耦合模式模拟的SST距平的年循环位相与观测相反。SST纬向梯度的位相偏差使得太平洋沃克环流和印度洋的反沃克环流在春季强于秋季,最终导致模拟的春秋非对称模态的偏差。Fgoals_s1.1模拟的季风区范围接近观测,存在的问题在于模拟的西北太平洋季风区、东亚季风区都偏小。本文结果表明,大气模式偏差仅是Fgoals_s1.1在降水年循环模态模拟上的偏差的部分来源,改进模式模拟的SST,特别是赤道地区SST季节循环,是今后Fgoals_s1.1发展过程中急需解决的问题。     

我国夏季降水与全球海温的耦合关系分析

利用我国160个台站从1951~2000年的月降水观测资料和NCEP/NCAR的全球海表温度(SST)资料,分析了我国夏季(6、7、8月)降水的时空变化特征及其与海温的相关,并应用奇异值分解(SVD)方法研究了我国夏季降水分布异常与海温变化的耦合关系。结果表明,我国夏季降水异常的雨型分布主要有3种,这些雨型的时间变化除了有明显的年际变化外,还存在显著的年代际变化。尤其是华北地区的降水从1965年左右开始减少,特别是大约1976年后有显著的减少。SVD分析揭示的我国夏季降水和全球海温异常的耦合关系表明,这种耦合关系最主要的时空变化特征表现在年代际变化的时间尺度上。我国华北和东北南部的夏季降水从1976年前后明显减少,与之显著关联的海温异常的关键区包括太平洋、印度洋以及热带和南大西洋。特别是热带中、东太平洋,印度洋,以及热带和南大西洋海水,从1976年前后也明显增暖。本研究揭示的华北持续干旱与印度洋和大西洋海温的年代际变化的耦合关系,在以往的研究中还未见到,因而有必要在今后的研究中加以重视。我国夏季降水和海温的耦合关系,还表现在长江中下游地区的降水异常与太平洋和大西洋海温异常的显著相关上。当南海和黑潮区域以及相邻的热带西太平洋海区海温为正异常时,热带和北大西洋海温也为正异常;而热带中、东太平洋海温为负异常时,长江中下游地区往往偏涝;反之,该地区则偏旱。     

A reconstructed dynamic Indian monsoon index extended back to 1880

The authors present a reconstruction of summer (June–July–August) mean dynamic Indian monsoon index (DIMI) back to 1880 based on a large number of historical surface observation data as well as information from the upper air data. The reconstruction shows a satisfying skill in terms of both the value of reduction of error and an evaluation against other independent monsoon indices. The skill of reconstruction increases over time with more predictor data (in particular upper-level data) becoming available. A comparison with the observed all Indian summer monsoon rainfall index (AIRI) shows a high consistence in both inter-decadal and inter-annual variability. The reconstruction shows stronger than normal monsoon during the 1880s, 1915–1925 (around 1920) and 1930–1945 (around 1940) as the AIRI. The El Nino/Southern Oscillation (ENSO)—monsoon relationship is reasonably captured in the reconstruction. Powers concentrating within quasi-biennial band stand out in the reconstruction as well as in the AIRI. A comparison of the reconstruction against an atmospheric general circulation model simulation with specified SST and external forcing agents spanning 1901–1999 indicates a slightly higher reproducibility of monsoon circulation than monsoon rainfall in terms of interannual variability. The relationship between the Asian continent warming and the ENSO–monsoon connection is also discussed by using the new dynamic index.