Variations Of Summer Marine Heat Waves In Northwest Pacific And Associated Air-sea Interaction

Spatiotemporal distribution of summer marine heat waves(MHWs)in Northwest Pacific and associated air-sea interaction were systematically studied based on daily Optimum Interpolation Sea Surface Temperature(OI SST)data set from National Oceanic and Atmospheric Administration(NOAA)and reanalysis data set from National Centers for Environmental Prediction(NCEP)during the period of 1982-2017.(1)Empirical orthogonal function(EOF)analysis of MHWs’ frequency shows a linear increasing trend except Kuroshio extension region in the first mode,while a north-south ’dipole’ pattern appears in the second mode,with characteristic of inter-decadal oscillation.Maximum intensity,duration,and cumulative intensity of MHWs also shows a linear increasing trend.Maximum intensity and cumulative intensity rise most in high latitudes,and duration varies significantly in Kuroshio extension region.Analysis of air-sea interaction illustrates that,when MHWs occur in middle or low latitudes,ocean forcing to the overlying atmosphere plays a dominant role while the atmospheric forcing is more significant as MHWs occur in high latitudes.(2)The trend analysis and composite analysis of marine heat wave events at different latitudes show that summer marine heat wave events at different latitudes in the Northwest Pacific show a certain linear trend,with the lowest linear trend coefficient in the low latitude region,the second highest in the high latitude region and the lowest in the mid-latitude region.This upward trend is closely related to the increase of the regional average SST.From the perspective of the atmospheric circulation,when the marine heat waves in different latitudes are formed,there are positive potential height anomalies and anti-cyclonic anomalies over the corresponding areas,while negative potential height anomalies and cyclonic anomalies or no obvious anomalies over the corresponding areas during the recession phase.From the perspective of the air-sea flux,when the marine heat waves are formed,the corresponding area tends to receive more abnormal solar radiation,release less latent heat flux,and the upward long-wave radiation flux and sensible heat flux tend to decline.In addition,the formation process of marine heat waves in low latitudes and high latitudes is related to the P-J teleconnection pattern to a certain extent.(3)A continuous marine heat wave event occurred in the Kuroshio region in the summer of 2004,during which the air-sea interaction was significant.From the perspective of the ocean,the weak El Ni?o in the early winter and the large-scale easterly anomaly led to an abnormal SST increase in the equatorial region.Subsequently,the SST anomaly moved with the northern equatorial current and reached the Kuroshio region in June.The anticyclone circulation over the area converges the local warm seawater caused the SST in the Kuroshio region to abnormally increase.From the perspective of the atmospheric circulation,the P-J teleconnection pattern was established in East Asia in June.The Kuroshio region is characterized by positive potential height anomalies and anti-cyclonic circulation anomalies,with subsidence airflow anomalies,lower surface wind speeds,and more downward short-wave radiation,which exacerbates local SST anomalies.