Air-ocean Characteristics Associated With Large-scale Sea-warming Anomalies In The North Pacific In Autumn And Winter On An Intramonthly Scale

Previous studies on large-scale air-sea interaction are mainly based on monthly average data,but whether there is ocean forcing on the atmosphere in a submonthly or shorter timescale remains to be studied.To provide directly observational evidence of large-scale oceanic forcing on the atmosphere on a submonthly timescale in midlatitude,using the National Centers for Environmental Prediction/Department of Energy reanalysis 2 and the National Oceanic and Atmospheric Administration sea surface temperatures(SSTs)during the1985–2015 period,eight warm events in winter and ten warm events in autumn over the North Pacific are selected based on the definition of large-scale SST anomaly events.The dynamic composite method,following the SST anomaly center,is employed to investigate the large-scale SST warm anomalies with a lifespan of 50 days over the wintertime and autumntime North Pacific and associated characteristics of the air–sea interaction on submonthly timescales before and after their peak stages.The results show the following:(1)There are different air-sea forcing relationships in the evolution of large-scale SST anomalies over the wintertime and autumntime North Pacific in mid-latitudes.In particular,the early stage of the large-scale SST warm anomalies is mainly characterized by the forcing of the atmosphere on the ocean,whereas the late stage is dominated by the forcing of the ocean on the atmosphere.(2)The atmospheric structure associated with the SST warm anomalies changes significantly from the early to late stages.In winter,the early stage shows an equivalent barotropic dipole pattern of pressure anomalies above the warmer SSTs,with an anomalous high in the northeast and an anomalous low in the southwest,corresponding to the anomalous easterly wind over SST anomalies.At the late stage,an equivalent barotropic anomalous cyclone is located to the north of warmer SSTs,with a weak anomalous anticyclone to the south,corresponding to the anomalous westerly wind over SST anomalies.In autumn,the early stage shows an equivalent barotropic anomalous anticyclone above the warmer SSTs,corresponding to the anomalous easterly wind over SST anomalies.At the late stage,an equivalent barotropic anomalous cyclone occurs over the warmer SSTs,corresponding to the anomalous westerly wind over SST anomalies.(3)The cyclonic circulation anomaly occurs at the late stage mainly due to the high-frequency transient eddy vorticity feedback forcing in wintertime,which acts as the major contributing factor.However,in autumn,the effects of high-frequency transient eddy vorticity feedback forcing and atmospheric heat source force the atmospheric circulation to change from an anomalous anticyclone to an anomalous cyclone,and the condensation latent heat released by convective precipitation plays a major role in the atmospheric heating field.Therefore,their joint efforts make the response of cyclonic circulation anomaly over the SST warm anomalies region at the late stage in autumn stronger than that in winter.(4)The oceanic structure associated with the SST warm anomalies changes significantly from the early to late stages.A mixed layer heat budget analysis shows that the ocean dynamic process is not conducive to maintaining the SST warm anomalies at the early stage.At the late stage,three-dimensional temperature advection positive anomalies act to maintain the SST warming and thus its influence on the atmosphere.