Simulation by CMIP5 Models of the Atlantic Multidecadal Oscillation and Its Climate Impacts

This study focuses on the climatic impacts of the Atlantic Multidecadal Oscillation （AMO） as a mode of internal vari- ability. Given the difficulties involved in excluding the effects of external forcing from internal variation, i.e., owing to the short record length of instrumental observations and historical simulations, we assess and compare the AMO and its related climatic impacts both in observations and in the ＂Pre-industrial＂ experiments of models participating in CMIP5. First, we evaluate the skill of the 25 CMIP5 models＇ ＂Historical＂ simulations in simulating the observational AMO, and find there is generally a considerable range of skill among them in this regard. Six of the models with higher skill relative to the other models are selected to investigate the AMO-related climate impacts, and it is found that their ＂Pre-industrial＂ simulations capture the essential features of the AMO. A positive AMO favors warmer surface temperature around the North Atlantic, and the Atlantic ITCZ shifts northward leading to more rainfall in the Sahel and less rainfall in Brazil. Furthermore, the results confirm the existence of a teleconnection between the AMO and East Asian surface temperature, as well as the late withdrawal of the Indian summer monsoon, during positive AMO phases. These connections could be mainly caused by internal climate variability. Opposite patterns are true for the negative phase of the AMO.

The Influence of the Indian Ocean Dipole on Atmospheric Circulation and Climate

The SST variation in the equatorial Indian Ocean is studied with special interest in analyzing its dipole oscillation feature. The dipole oscillation appears to be stronger in September-November and weaker in January-April with higher SST in the west region and lower SST in the east region as the positive phase and higher SST in the east region and lower SST in the west region as the negative phase. Generally, the amplitude of the positive phase is larger than the negative phase. The interannual variation (4-5 year period) and the interdecadal variation (25-30 year period) also exist in the dipole. The analyses also showed the significant impact of the Indian Ocean dipole on the Asian monsoon activity, because the lower tropospheric wind fields over the Southern Asia, the Tibetan high in the upper troposphere and the subtropical high over the northwestern Pacific are all related to the Indian Ocean dipole. On the other, the Indian Ocean dipole still has significant impact on atmospheric circulation and climate in North America and the southern Indian Ocean region (including Australia and South Africa).

Subseasonal mode of cold and wet climate in South China during the cold season: a climatological view

The authors investigate the dominant mode of climatological intraseasonal oscillation(CISO) of surface air temperature(SAT) and rainfall in China, and discuss the linkage of cold and wet climate in South China(SC) with the Arctic circulation regime during the cold season(from November to March). Results show that a positive CISO displays a cold-dry climate in North China,whereas a cold-wet pattern prevails in SC with a quasi-30-day oscillation during the peak winter season. In SC, the intraseasonal variability of SAT plays a leading role, altering the cold-wet climate by the southward shift of a cold front. Evidence shows that the circulation regime related to the cold and wet climate in SC is mainly regulated by a pair of propagating ISO modes at the500-hPa geopotential height in the negative phase of Arctic Oscillation. It is demonstrated that the local cyclonic wave activity enhances the southward movement of the Siberian high, favoring an unstable atmosphere and resulting in the cold-wet climate over SC. Therefore, the cold-air activity acts as a precursor for subseasonal rainfall forecasting in SC.

Effect of El Nino Southern Oscillations on the incidence of enteric fever in Ahmedabad,India from 1985 to 2017

Objective:To explore the relationship between climate variables and enteric fever in the city of Ahmedabad and report preliminary findings regarding the influence of El Nino Southern Oscillations and Indian Ocean Dipole over enteric fever incidence.Method:A total of 29808 Widal positive enteric fever cases reported by the Ahmedabad Municipal Corporation and local climate data in 1985-2017 from Ahmedabad Meteorology Department were analysed.El Nino,La Nina,neutral and Indian Ocean Dipole years as reported by the National Oceanic and Atmospheric Administration for the same period were compared for the incidence of enteric fever.Results:Population-normalized average monthly enteric fever case rates were the highest for El Nino years(25.5),lower for La Nina years(20.5)and lowest for neutral years(17.6).A repeated measures ANOVA analysis showed no significant difference in case rates during the three yearly El Nino Southern Oscillations categories.However,visual profile plot of estimated marginal monthly means showed two distinct characteristics:an early rise and peaking of cases in the El Nino and La Nina years,and a much more restrained rise without conspicuous peaks in neutral years.Further analysis based on monthly El Nino Southern Oscillations categories was conducted to detect differences in median monthly case rates.Median case rates in strong and moderate El Nino months and strong La Nina months were significantly dissimilar from that during neutral months(P<0.001).Conclusions:El Nino Southern Oscillations events influence the incidence of enteric fever cases in Ahmedabad,and further investigation from more cities and towns is required.

Climate variability and predictability associated with the Indo-Pacific Oceanic Channel Dynamics in the CCSM4 Coupled System Model

An experiment using the Community Climate System Model(CCSM4), a participant of the Coupled Model Intercomparison Project phase-5(CMIP5), is analyzed to assess the skills of this model in simulating and predicting the climate variabilities associated with the oceanic channel dynamics across the Indo-Pacific Oceans. The results of these analyses suggest that the model is able to reproduce the observed lag correlation between the oceanic anomalies in the southeastern tropical Indian Ocean and those in the cold tongue in the eastern equatorial Pacific Ocean at a time lag of 1 year. This success may be largely attributed to the successful simulation of the interannual variations of the Indonesian Throughflow, which carries the anomalies of the Indian Ocean Dipole(IOD) into the western equatorial Pacific Ocean to produce subsurface temperature anomalies, which in turn propagate to the eastern equatorial Pacific to generate ENSO. This connection is termed the "oceanic channel dynamics" and is shown to be consistent with the observational analyses. However, the model simulates a weaker connection between the IOD and the interannual variability of the Indonesian Throughflow transport than found in the observations. In addition, the model overestimates the westerly wind anomalies in the western-central equatorial Pacific in the year following the IOD, which forces unrealistic upwelling Rossby waves in the western equatorial Pacific and downwelling Kelvin waves in the east. This assessment suggests that the CCSM4 coupled climate system has underestimated the oceanic channel dynamics and overestimated the atmospheric bridge processes.

赤道印度洋海温偶极子型振荡及其气候影响

对近百年观测资料的分析表明赤道印度洋海温（ＳＳＴ）确实存在着偶极子型振荡的变化特征，它在９～１１月最强，而在１～４月最弱；年际变化（４～５年周期）和年代际变化（主要为２０～２５年周期）也十分清楚。这个偶极子主要有正位相型（海温西高东低）和负位相型（海温东高西低）；一般正位相型的振幅强于负位相型。尽管在极个别年赤道印度洋海温偶极子似乎与太平洋ＥＮＳＯ无关，但总体而论，赤道印度洋海温偶极子与赤道太平洋海温偶极子（类似ＥＮＳＯ）有很好负相关。它们的联系主要是赤道大气纬向（Ｗａｌｋｅｒ）环流。资料分析表明，赤道印度洋海温偶极子与亚洲南部流场、青藏高压和西太平洋副高都有明显关系，表明它对亚洲季风活动有重要影响。

赤道印度洋海温偶极子的气候影响及数值模拟研究

在分析研究印度洋海温变化的基本特征,尤其是在分析赤道印度洋海温偶极子及其影响的基础上,利用IAP9L大气环流模式模拟研究了赤道印度洋海温偶极子异常对亚洲季风区气候变化的影响.其结果表明,印度洋、亚洲南部和东部地区的流场和降水都对印度洋海温异常的强迫作用比较敏感.正位相印度洋偶极子的作用使得赤道东印度洋-印度次大陆南部-阿拉伯海一带出现距平东风,孟加拉湾-中南半岛出现异常反气旋性环流,从而对减少印度南部和中南半岛南部、印度尼西亚地区的夏季降水,以及增加中国南部和东非的夏季降水有十分重要的作用.与此相反,负位相印度洋偶极子的作用将使赤道东印度洋附近出现西风异常,孟加拉湾-中南半岛存在异常气旋性环流,从而使印度次大陆和中南半岛南部、印度尼西亚地区的降水增加,使中国西部和孟加拉湾的降水减少.数值模拟结果与资料分析相互映证,切实地揭示了印度洋海温偶极子对亚洲季风区的气候变化有重要影响.

厄尔尼诺/拉尼娜事件对山西省北部地区气候的影响

通过对山西省北部50a来气象资料的搜集和整理,研究了厄尔尼诺/拉尼娜(El Nino—SouthernOscillation,ENSO)事件对山西省北部地区气候的影响。结果表明,大同、朔州、忻州地区厄尔尼诺事件发生年降水减少,平均气温降低,与正常年份相比年降水量分别减少了46.86,46.44和80.96mm,年平均气温分别降低了0.03,0.17和0.22℃;拉尼娜年降水减少,平均气温降低,与正常年份相比年降水量分别减少了21.27,26.32和25.30mm,平均气温分别降低了0.03,0.18和0.04℃,厄尔尼诺事件年平均气温变化与黄土高原相反。ENSO暖事件对山西省北部地区气温的影响具有从大同向朔州和忻州依次逐渐增强的特点,表明随着纬度的降低厄尔尼诺事件对气温的影响增强,对降水也有类似的影响。ENSO暖事件年发生旱灾的可能性较大,通常会给山西省北部地区的工农业生产带来不利影响。

亚洲—太平洋地区冬季海平面气压异常偶极模与我国冬季气候异常的关系

利用全球月平均海平面气压资料和各标准等压面的位势场、风场,以及我国160站的降水和温度资料,研究了亚洲—太平洋(20°N～70°N,40°E～120°W)地区(简称亚太地区)冬季(12～2月)海平面气压异常偶极型振荡(偶极模)的时间变化特征,以及该异常偶极模与我国冬季气候异常的关系。结果表明:近半个世纪以来,亚太地区冬季海平面气压场异常以经向型(ME)偶极模负异常和纬向型(ZO)偶极模正异常为主,其中又以ZO正异常发生的频数较高。对应ME模,其各空间层的高度场异常形势和高度场与MEI(经向型指数)的相关系数分布形势十分类似;对应ZO模则主要表现为与阿留申地区高相关中心的一致性特征,说明阿留申低压的变化对其起着重要作用。ME模与我国冬季东部的降水异常和气温异常关系密切,ZO模仅对东北地区的降水异常有影响,因为ZO模的异常对我国上空大气环流异常的影响相对较小;这一结果同我们传统上在冬季比较关注纬向气压差的变化(即ZO模)有些相左,提示我们在冬季更要关注ME模的情况。大气环流形势的分析表明,MEI正异常时,即西伯利亚—蒙古冷高压的北半部气压异常偏高,副热带西北太平洋地区气压偏低时,利于东亚大槽的加深加强,东亚冬季风因而也会偏强;但是由于此时我国东部主要为下沉气流控制,暖湿气流难于源源不断地输送到我国境内,从而造成此时冬季我国东部尤其是长江中下游地区降水偏少。反之,当MEI负位相时,大气环流异常利于造成我国东部大范围的冬季降水。

春季北极涛动对南海气候的影响

利用1979～2013年月降水量和NCEP再分析大气环流资料,分析了春季（3～5月）北极涛动对南海气候变率的可能影响.结果表明,春季南海多气候要素主导模态的时间系数与同期北极涛动指数相关系数达？0.4,超过95%的信度水平,二者反位相的特征在年际尺度上更加明显.当北极涛动指数为正异常时,西太平洋副热带高压减弱,热带西太平洋有异常气旋式环流,南海地区偏南风减弱,南海大部分地区降水增多,北部到华南地区降水减少,北极涛动指数为负异常时情况则相反.北极涛动可由两条途径影响南海地区气候,一是通过青藏高原南侧西风急流的波列,二是通过北太平洋西风急流区南北侧的偶极型环流异常.南海地区环流和气候的异常与这两条途径的相对强弱和配置有关,当高原南侧波列异常偏强同时北太平洋偶极型异常偏弱时,二者共同作用可增强西太平洋副热带高压和南海地区异常反气旋环流,导致降水的显著减少.当北极涛动指数为正异常时,北太平洋偶极型南支的气旋还可触发热带地区海气相互作用间接影响南海气候.