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
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摘要
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Nio4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 Nio(La Nia)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。
ENSO is the strongest interannual variability in our climate system.It has profound impacts on global climate.Since El Nino-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 Nino(La Nina) with more(less) than normal rainfall in the Indonesian region in some years.After removing the Nino4 signal from SST anomalies,we find that both the SST anomaly(SSTA)and rainfall anomalies are independent of the Nino4 signal.The EPI independent of Nino4 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 Nino(La Nina) 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 Nino 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.
ENSO is the strongest interannual variability in our climate system.It has profound impacts on global climate.Since El Nino-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 Nino(La Nina) with more(less) than normal rainfall in the Indonesian region in some years.After removing the Nino4 signal from SST anomalies,we find that both the SST anomaly(SSTA)and rainfall anomalies are independent of the Nino4 signal.The EPI independent of Nino4 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 Nino(La Nina) 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 Nino 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.
引文
Alexander M A,BladéI,Newman M,et al.2002.The Atmospheric Bridge:The influence of ENSO teleconnections on air-sea interaction over the global oceans[J].J Climate,15(16):2205-2231.
Ashok K,Behera S K,Rao S A,et al.2007.El Nio Modoki and its possible teleconnection[J].J Geophys Res,112(C11):C11007.doi:10.1029/2006JC003798.
Chang C P,Wang Z,Mc Bride J,et al.2005.Annual cycle of Southeast Asia-Maritime continent rainfall and the asymmetric monsoon transition[J].J Climate,18(2):287-301.
Feng J L,Li J.2011.Influnce of Elnino Modoki on spring rainfall over south China[J].J Geophys Res,116,D13102.doi:10.1029/2010JD015160.
Feng J L,Wang W,Chen C S,et al.2010.Different impacts of two types of pacific ocean warming on the Southeast Asian rainfall during Boreal Winter[J].J Geophys Res,115,D24122.doi:10.1029/2010JD014761.
冯娟,管兆勇,王黎娟,等.2010.夏季热带中太平洋SST异常型与中国东部夏季气候异常的关系[J].大气科学学报,33(5):547-554.Feng J,Guan Z Y,Wang L J,et al.2010.The summer in the tropical Pacific SST anomaly type relationship with summer climate anomalies in the east of China[J].Trans Atmos Sci,33(5):547-554.(in Chinese).
Gill A.1980.Some simple solutions for heat-induced tropical circulation[J].Quart J Roy Meteor Soc,106(449):447-462.
Haylock M,Mc Bride J.2001.Spatial coherence and predictability of Indonesian wet season rainfall[J].J Climate,14(18):3882-3887.
Hendon H H.2003.Indonesian rainfall variability:Impacts of ENSO and local air-sea interaction[J].J Climate,16(11):1775-1790.
Kalnay E,Kanamitsu M,Kistler R,et al.1996.The NCEP/NCAR 40-year reanalysis project[J].Bull Amer Meteor Soc,77(3):437-471.
Kao H Y,Yu J Y.2009.Contrasting eastern-Pacific and central-Pacific types of ENSO[J].J Climate,22(3):615-632.
Lau K M,Chan P H.1983.Short-term climate variability and atmospheric teleconnections from satellite-observed outgoing longwave radiation.Part II:Lagged correlations[J].J Atmos Sci,40(12):2751-2767.
李晓燕,翟盘茂.2000.ENSO事件指数与指标研究[J].气象学报,58(1):102-109.Li X Y,Zhai P M.2000.On indices and indicators of ENSO episodes[J].Acta Meteorological Sinica,58(1):102-109.(in Chinese).
Luo H,Yanai M.1984.The large-scale circulation and heat sources over the Tibetan Plateau and surrounding areas during the early summer of 1979.Part II:Heat and moisture budgets[J].Mon Wea Rev,112(5):966-989.
Ramage C S.1968.Role of a tropical“maritime continent”in the atmospheric circulation[J].Mon Wea Rev,96(6):365-370.
Ren L,Jin F F.2011.Nio indices for two types of ENSO[J].Geophys Res Lett,38,L04704.doi:10.1029/2010GL046031.
唐卫亚,孙照渤.2005.印度洋海温偶极振荡对东亚环流及降水影响[J].大气科学学报,28(3):316-322.Tang W Y,Sun Z B.2005.SST in the Indian Ocean dipole oscillation influence on east Asian circulation and precipitation[J].Trans Atmos Sci,28(3):316-322.(in Chinese).
Trenberth K E,Stepaniak D P.2001.Indices of El Nio evolution[J].J Climate,14(8):1697-1701.
吴国雄,孟文.1998.赤道印度洋-太平洋地区海气系统的齿轮式耦合和ENSO事件Ⅰ.资料分析[J].大气科学,22(4):470-480.Wu G X,Meng W.1998.Gearing between the indo-monsoon circulation and the pacific-walker circulation and the ENSO.PartⅠ:Data analyses[J].Chin J Atmos Sci,22(4):470-480.(in Chinese).
Yeh S W,Kug J S,Dewitte B,et al.2009:El Nio in a changing climate[J].Nature,461:511-514.doi:10.1038/nature 08316.
祝从文,何敏.1998.热带环流指数与夏季长江中下游旱涝的年限变化[J].大气科学学报,21(1):15-22.Zhu C W He M.1998.Tropical circulation index and the middle and lower reaches of the Yangtze river in summer drought and flood of change[J].Trans Atmos Sci,21(1):15-22.(in Chinese).
Ashok K,Behera S K,Rao S A,et al.2007.El Nio Modoki and its possible teleconnection[J].J Geophys Res,112(C11):C11007.doi:10.1029/2006JC003798.
Chang C P,Wang Z,Mc Bride J,et al.2005.Annual cycle of Southeast Asia-Maritime continent rainfall and the asymmetric monsoon transition[J].J Climate,18(2):287-301.
Feng J L,Li J.2011.Influnce of Elnino Modoki on spring rainfall over south China[J].J Geophys Res,116,D13102.doi:10.1029/2010JD015160.
Feng J L,Wang W,Chen C S,et al.2010.Different impacts of two types of pacific ocean warming on the Southeast Asian rainfall during Boreal Winter[J].J Geophys Res,115,D24122.doi:10.1029/2010JD014761.
冯娟,管兆勇,王黎娟,等.2010.夏季热带中太平洋SST异常型与中国东部夏季气候异常的关系[J].大气科学学报,33(5):547-554.Feng J,Guan Z Y,Wang L J,et al.2010.The summer in the tropical Pacific SST anomaly type relationship with summer climate anomalies in the east of China[J].Trans Atmos Sci,33(5):547-554.(in Chinese).
Gill A.1980.Some simple solutions for heat-induced tropical circulation[J].Quart J Roy Meteor Soc,106(449):447-462.
Haylock M,Mc Bride J.2001.Spatial coherence and predictability of Indonesian wet season rainfall[J].J Climate,14(18):3882-3887.
Hendon H H.2003.Indonesian rainfall variability:Impacts of ENSO and local air-sea interaction[J].J Climate,16(11):1775-1790.
Kalnay E,Kanamitsu M,Kistler R,et al.1996.The NCEP/NCAR 40-year reanalysis project[J].Bull Amer Meteor Soc,77(3):437-471.
Kao H Y,Yu J Y.2009.Contrasting eastern-Pacific and central-Pacific types of ENSO[J].J Climate,22(3):615-632.
Lau K M,Chan P H.1983.Short-term climate variability and atmospheric teleconnections from satellite-observed outgoing longwave radiation.Part II:Lagged correlations[J].J Atmos Sci,40(12):2751-2767.
李晓燕,翟盘茂.2000.ENSO事件指数与指标研究[J].气象学报,58(1):102-109.Li X Y,Zhai P M.2000.On indices and indicators of ENSO episodes[J].Acta Meteorological Sinica,58(1):102-109.(in Chinese).
Luo H,Yanai M.1984.The large-scale circulation and heat sources over the Tibetan Plateau and surrounding areas during the early summer of 1979.Part II:Heat and moisture budgets[J].Mon Wea Rev,112(5):966-989.
Ramage C S.1968.Role of a tropical“maritime continent”in the atmospheric circulation[J].Mon Wea Rev,96(6):365-370.
Ren L,Jin F F.2011.Nio indices for two types of ENSO[J].Geophys Res Lett,38,L04704.doi:10.1029/2010GL046031.
唐卫亚,孙照渤.2005.印度洋海温偶极振荡对东亚环流及降水影响[J].大气科学学报,28(3):316-322.Tang W Y,Sun Z B.2005.SST in the Indian Ocean dipole oscillation influence on east Asian circulation and precipitation[J].Trans Atmos Sci,28(3):316-322.(in Chinese).
Trenberth K E,Stepaniak D P.2001.Indices of El Nio evolution[J].J Climate,14(8):1697-1701.
吴国雄,孟文.1998.赤道印度洋-太平洋地区海气系统的齿轮式耦合和ENSO事件Ⅰ.资料分析[J].大气科学,22(4):470-480.Wu G X,Meng W.1998.Gearing between the indo-monsoon circulation and the pacific-walker circulation and the ENSO.PartⅠ:Data analyses[J].Chin J Atmos Sci,22(4):470-480.(in Chinese).
Yeh S W,Kug J S,Dewitte B,et al.2009:El Nio in a changing climate[J].Nature,461:511-514.doi:10.1038/nature 08316.
祝从文,何敏.1998.热带环流指数与夏季长江中下游旱涝的年限变化[J].大气科学学报,21(1):15-22.Zhu C W He M.1998.Tropical circulation index and the middle and lower reaches of the Yangtze river in summer drought and flood of change[J].Trans Atmos Sci,21(1):15-22.(in Chinese).