1.College of Atmospheric Science, Nanjing University of Information Science and Technology;2.Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters (CICFEMD) / Key Laboratory of Meteorological Disaster, Ministry of Education (KLME) / Joint International Research Laboratory of Climate and Environment Change (ILCEC), Nanjing University of Information Science and Technology
Using the National Centers for Environmental Prediction/Department of Energy (NCEP/DOE) reanalysis 2 and the National Oceanic and Atmospheric Administration (NOAA) Sea Surface Temperatures (SSTs) during the period of 1985－2015, eight warm events in the North Pacific are selected based on the definition of large-scale SST anomalies. The dynamic composite method following the SST anomaly center is used to study the large-scale SST warm anomalies with a lifespan of 50 days over the wintertime North Pacific and associated characteristics of the air-sea interaction on submonthly timescales before and after their peak stages. The results show that: (1) the early stage of the large-scale SST warm anomalies is mainly characterized by the forcing of the atmosphere on the ocean, while the forcing of the ocean on the atmosphere dominates the late stage. (2) The atmospheric structure associated with the SST warm anomalies changes significantly from the early to late stages. 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, which corresponds to the anomalous easterly wind over SST anomalies. At the late stage, the equivalent barotropic anomalous cyclone is located to the north of warmer SSTs, with a weak anomalous anticyclone to the south, which corresponds to the anomalous westerly wind over SST anomalies. (3) The cyclonic circulation anomaly occurs at the late stage due mainly to the high frequency transient eddy feedback forcing, in which the forcing of transient eddy vorticity feedback acts as the major contributing factor. (4) The structure of ocean current is also different between the early and late stages. At the early stage, the ocean dynamic process is not conducive to maintaining the SST warm anomalies. At the late stage, both anomalous warm advection and anomalous downwelling act to maintain the SST warming and thus its influence on the atmosphere.