两类中太平洋El Nio特征的对比分析

Comparative analysis of characteristics of two types of central Pacific El Nio

利用1979—2012年逐月Hadley中心海表温度、欧洲中期天气预报中心次表层海温、NCEP/NCAR风场再分析资料,对两类中太平洋（CP）El Nino及耦合的大气环流特征进行分析。结果表明,第一类CP El Nino（CP-ⅠEl Nino）增暖中心位于Nino4区且关于赤道对称;第二类CP El Nino（CP-ⅡEl Nino）的Nino4区与热带东北太平洋区域（NEP,130-110°W、15-25°N）同位相变化,冬季成熟后形成关于赤道非对称的带状增暖结构。进一步的研究表明,两类CP El Nino次表层结构存在差异：CP-ⅠEl Nino冬季次表层海温异常（SOTA）在中东太平洋与西太平洋呈显著偶极分布;CP-ⅡEl Nino在中、西太平洋位相相反但东太平洋异常较弱,且经向异常主要在赤道及其以北。两类CP El Nino耦合的大气环流特征不同：CP-ⅠEl Nino冬季异常Walker环流上升中心位于赤道上空,经向风向赤道辐合,低纬地区Hadley环流加强;CP-ⅡEl Nino冬季低层向北越赤道气流加强,Walker环流上升中心移到赤道以北,低纬地区Hadley环流减弱。

The newEl Nino phenomenons named central Pacific（ CP） El Nino are discussed all over the world in the most nearly three decades. Specially,two types of CP El Nino which have different sea surface temperature anomalies（ SSTA） structures are investigated in recent years. In order to recognize the different sea-air interaction processes of two types of CP El Nino comprehensively,in this research,we base on monthly Hadley Center sea surface temperature（ Had ISST）,European Centre for Medium-Range Weather Forecasts（ ECM WF） ocean subsurface temperature and NCEP/ NCAR reanalysis datasets for 1979—2012,the characteristics of two types of central Pacific（ CP） El Nino and coupled atmospheric processes are studied. The results showthat there are two types of CP El Nino,the difference lies in sea surface temperature anomalies distribution in the tropical pacific. A type of CP El Nino（ CP-Ⅰ El Nino） events have symmetrical structure about the equator in the winter,the warming center locate at Nino4 regions and stretch eastward into eastern pacific. Another type of CP El Nino（ CP-Ⅱ El Nino）,the sea surface temperature anomalies change in phase between Nino4 regions and tropical northeast pacifc areas（ NEP,15- 25° N,130- 110° W）,a banded and asymmetric structure of SSTA forms in the winter and shows horseshoe-shaped cold SSTA distribution in the western pacific. Further research indicates that the subsurface temperature anomalies（ SOTA） of two types of El Nino are different. In the CP-Ⅰ El Nino winter,a zonal dipole mode forms between western and central-eastern pacific,the subsurface temperature of central-eastern pacific raise significantly,the meridional structure are also symmetric,a major positive SOTA center locates at the equator. However,in the CP-Ⅱ El Nino winter,a zonal dipole mode confines in the western and central pacific,the warming develop at the upper subsurface in the central pacific,the SOTA in the eastern pacific are tiny. M eridional SOTA also confine in the north hemisphere,two major positive anomalies center locate at the the equator and near 10°N. Characteristics of atmospheric circulation have distinct difference among CP-Ⅰ El Nino and CP-Ⅱ El Nino. In the CP-Ⅰ El Nino winter,the abnormal upwelling branch of walker circulation are over equator near the longitude of 180°,the abnormal downwelling branch locate at equatorial western and eastern pacific which responded to symmetric SSTA. In the lowlevel atmosphere,there are significant westerly wind anomalies and convergent meridional wind anomalies in the equatorial area. The Hadley circulation intensify in the lowlatitude regions. However the mass transport between 20°N and 30°N are reduced which lead to southward move of downwelling branch of the Hadley circulation. In the CP-Ⅱ El Nino winter,the cross-equatorial southwest flowstrengthen in the lowlevel atmosphere,the upwelling center of Walker circulation move to north hemisphere which responded to asymmetric SSTA,the Hadley circulation weak in the lowlatitude regions to the contrary. In addition,the sea surface heat flux are investigated preliminary. In the August to December,the positive sea surface heat flux anomalies are observed in the CP-Ⅱ El Nino,which contribute to the warming in the central pacific and tropical northeast pacific. It shows that the wind-evaporation-SST processes exist in the tropical ocean. contrarily,in the CP-Ⅱ El Nino,negative sea surface heat flux anomalies damp the developments of SSTA.