北半球夏季海洋性大陆区域气候与EP型ENSO:直接与间接联系

利用1979—2009年的NECP资料、Hadley海温月平均资料和CMAP降水资料,采用Kao and Yu(2009)的方法定义了夏季EP型ENSO指数EPI,用合成分析的方法分析了东部型ENSO与海洋性大陆降水的关系。结果表明:EPI与MC(Maritime Continent,海洋性大陆)区域降水变化间存在非常弱的负相关。造成这一弱相关的原因是EPI与MC区域降水在某些年份存在同号变化。在剔除Nio4信号后,海洋性大陆区域降水序列与EPI与存在着同号和反号两种关系。反号关系是通常所认为的,当经典的El Nio(La Nia)发生时MC区域降水出现显著地减少(增多)。此时,沿赤道的异常Walker环流建立了EP型ENSO与MC区域气候间的直接联系。而在同号关系时,菲律宾以东异常加热和SPCZ区域异常冷却引起的西北—东南走向的垂直环流圈削弱了MC区域与赤道东太平洋之间的异常Walker环流所建立的直接联系,或者说,赤道东太平洋区域SSTA与MC区域降水异常的形成是通过SPCZ区域SST的反号异常而产生间接联系的。这种机制的揭示为深刻认识ENSO影响海洋性大陆区域甚至东亚地区气候变动的联系提供了新的线索。

Influences of Eastern Pacific-type ENSO on climate variations over the Maritime Continent region:Direct and indirect connections

ENSO is the strongest interannual variability in our climate system.It has profound impacts on global climate.Since El Niño-Modoki was revealed in 2007,more and more efforts have been made in the climate community to investigate the different types of ENSO events and their influences.Those different ENSO events are known as the EP-type(Eastern Pacific-type) and CP-type(central Pacific-type) ENSO.Following established methodology,we adopt the EP-type ENSO index(EPI) for boreal summer in the present paper.By employing methods including composite analysis,linear regression,and EOF decomposition,we investigate the simultaneous relationships between EP-type ENSO and precipitation anomalies in the Maritime Continent(MC) by using the monthly mean data from the NECP/NCAR reanalysis,global SST data from the Hadley Center,and CAMP monthly mean precipitation data for the period 1979-2013.The results demonstrate that EPI and CPI in boreal summer are able to describe the main features of EP-type and CP-type ENSO,respectively,although the EPI and CPI are not perfectly independent of each other in a statistical sense because the correlation coefficient of EPI with CPI was found to be 0.30.During some years,EPI is relatively smaller.But,the spatial pattern of anomalous SST still looks like EP-type ENSO.In these years,the anomalous precipitation averaged over the MC region is negative.Usually,we expect a strong negative correlation between canonical ENSO and rainfall anomalies in Indonesia.However,we find that the time series of the regional mean precipitation anomaly over the MC is weakly correlated with the EPI.These results are inconsistent with one another.By examining the anomalous circulations case by case,we find that this weak correlation is induced by some co-occurrences of the EP-type El Niño(La Niña) with more(less) than normal rainfall in the Indonesian region in some years.After removing the Niño4 signal from SST anomalies,we find that both the SST anomaly(SSTA) and rainfall anomalies are independent of the Niño4 signal.The EPI independent of Niño4 is calculated.Then,the co-occurrences of positive EPI with positive anomalous rainfall in the MC can still be observed in some years.By performing composite analyses,we found that there are two mechanisms that link the SSTAs in the eastern equatorial Pacific to precipitation anomalies in the MC region.One is canonical,which is characterized by the anomalous Walker circulation with its ascending(descending) branch over the east equatorial Pacific and the descending(ascending) branch over the MC region in years when EP-type El Niño(La Niña) events occur.We refer to this as the direct connection mechanism.The other is quite different from the direct connection mechanism.When an EP-type El Niño event occurs in certain years,significant anomalous heating is observed in both the MC and the region east of the Philippines,and significant anomalous cooling in the South Pacific Convergence Zone(SPCZ),respectively.These two anomalous diabatic forcing centers force the atmosphere to respond,inducing a northwest-southeast vertical circulation bridging these two centers,which anomalously weakens the descent of the atmosphere in the MC and intensifies the descending motion in the SPCZ,resulting in the anomalous Walker circulation along the equator being stopped in the SPCZ east of 160°E.In this way,positive anomalous rainfall events occur in the MC when the EPI is positive.This anomalous northwest-southeast vertical circulation,with its upward motion branches over the MC and the region east of the Philippines,and a downward motion branch over the SPCZ,connects the EP-type ENSO-induced Walker circulation,with its downward motion over the SPCZ and upward motion over the east equatorial Pacific.The anomalous adiabatic forcing in the SPCZ is crucial in interfering with the influences of EP-type ENSO on rainfall variations in the MC region.This linkage of EP-type ENSO with rainfall anomalies in the MC via anomalous forcing in the SPCZ is referred to as the indirect connection mechanism,in relation to EP-type ENSO with rainfall variations in the MC.The results in the present paper are highly meaningful in terms of improving our understanding of the mechanisms behind the relationships of ENSO with climate variations in the MC region,and even in East Asia.