秋季北极海冰对EP型ENSO的非线性响应

Nonlinear response of Arctic sea ice to EP ENSO in autumn

利用1961—2015年Hadley中心逐月海表温度资料、海冰密集度资料以及NCEP/NCAR再分析资料,探讨了秋季北极海冰对于EP型ENSO事件的异常响应,并进一步研究了这种异常响应的可能原因。结果表明,秋季北极海冰对EP型ENSO的响应具有非线性,特别是喀拉海海域(60°～90°E,70°～80°N)海冰无论在EP型El Ni?o或是La Ni?a位相,均表现为显著的负异常。进一步研究发现,不同ENSO位相造成该区域海冰异常偏少的机制有明显不同。EP型El Ni?o年秋季菲律宾附近海域对流活动被抑制,所激发的经向波列在高纬地区形成异常反气旋环流,其南风分量向喀拉海输送暖平流,造成海冰异常偏少。而EP型La Ni?a年喀拉海海域则主要受到来自大西洋开放性海域西风异常的影响,合成结果和个例年均显示EP型La Ni?a年秋季北大西洋上空存在一个显著的西风急流中心,有利于北大西洋开放性海域较暖海水向下游输送,进而影响喀拉海海冰。这些结果表明,热带外地区大气环流场对EP型ENSO的非线性响应导致了喀拉海海冰对EP型ENSO事件的响应也表现出明显的非线性。

Based on the monthly Hadley Center sea surface temperature data,Hadley Center Arctic sea ice concentration data,and NCEP/NCAR reanalysis data during 1961-2015,this paper analyzed the abnormal response of Arctic sea ice to EP (eastern Pacific) ENSO events in autumn,and further discussed the possible causes of such an abnormal response.Results show that the response of Arctic sea ice to EP ENSO in autumn is nonlinear.Particularly,the sea ice concentration anomalies in the Kara Sea area (70°-80°N,60°-90°E) are significantly negative both in the EP El Nino and EP La Nina phases.Further analyses reveal that the mechanisms of sea ice anomalies in different EP ENSO phases are significantly different.For EP El Nino years,the suppression of convective activity in the waters near the Philippines stimulates the meridional wave train in the extratropical area.Meanwhile,the southward wind component of the anomalous anticyclonic circulation in response to the wave train in the high latitudes forms warm convection transport to the Kara Sea area and causes sea ice retreat as a result.However,the Kara Sea area is mainly influenced by the westerly anomalies from the open Atlantic Ocean during EP La Nina years.Moreover,both the composite results and the analysis results of individual EP La Nina event show that there is a remarkable westerly jet center over the North Atlantic in autumn,which is conducive to downstream transport of warmer seawater from the open North Atlantic,affecting sea ice in the Kara Sea as a result.The above results show that the nonlinear response of circulation field in the extratropical areas to EP ENSO leads to a significant nonlinear response of Arctic sea ice to EP ENSO in turn.