Impacts of the extratropical North Pacific on boreal summer Arctic circulation

北极夏季大气环流的一个显着特征是以北冰洋为中心的异常反气旋环流,与北极增暖存在紧密的联系.先前的研究已将其形成的潜在机制与早春欧亚融雪,热带太平洋海表温度异常联系起来.本研究发现北极夏季反气旋环流异常与春季副热带北太平洋海表温度异常(SSTA)存在显着联系,表明副热带北太平洋与北极之间存在遥相关.该SSTA的特点是北太平洋中纬度地区有暖的SSTA,,周围有明显的冷SSTA,类似于太平洋年代际振荡(PDO)的负位相,但热带地区没有明显的信号(本研究称之为类负位相PDO),5月份,这种类负位相PDO可以激发从白令海传播到北极的罗斯贝波,导致北极出现反气旋环流异常.这种反气旋环流异常可以持续到夏季.同时,这种类负位相PDO的SSTA可以持续到夏季,并在夏季引起白令海上空的低压异常.这种低层的低压异常会导致异常的上升运动,并引起高层的辐散异常,从而进一步加剧夏季北极对流层上层的反气旋环流异常。这种反气旋环流异常可迫使北极上空出现异常绝热下沉运动,导致北极出现显著的绝热加热.于是,北极上空出现显著的暖异常,其暖中心位于北极对流层中部。数值模拟试验证实了5月份类负位相PDO的SSTA与夏季北极反气旋环流异常之间的联系.

A distinct characteristic of the Arctic summer atmospheric circulation is the anomalous anticyclonic circulation centered over the Arctic Ocean associated with significant Arctic warming.Previous studies have related the underlying mechanisms to the earlier spring Eurasian snowmelt and the tropical Pacific forcing.Here,the authors show that the Arctic summer anomalous anticyclonic circulation is significantly related to the extratropical North Pacific sea surface temperature anomalies(SSTAs)in spring,indicating a teleconnection from the extratropical North Pacific to the Arctic.The SSTA pattern is characterized by warm anomalies in the midlatitudes of the extratropical North Pacific surrounded by significant cold anomalies,resembling the negative phase of the Pacific Decadal Oscillation(PDO)but without significant signals in the tropics(referred to as the negative PDOlike pattern).This negative PDO-like pattern in May can stimulate a Rossby wave propagating from the Bering Sea to the Arctic and lead to an anomalous Arctic anticyclone which can be sustained during summer.Meanwhile,the negative PDO-like pattern can persist to summer and induce an anomalous surface low pressure over the Bering Sea in summer.This anomalous surface low causes anomalous rising motions and induces upper-level divergence anomalies which further intensify the summer Arctic anticyclone.The upper-level tropospheric Arctic anticyclone can force anomalous adiabatic descent over the Arctic and sub-Arctic,leading to significant adiabatic heating in the Arctic.As a result,a significant warming emerges over the Arctic with the center located in the middle troposphere.The connection between the negative PDO-like SSTAs and the summer Arctic anticyclone has been confirmed by numerical experiments.