Mechanism Of Influences Of Anomalous Thermal Forcings In Tropical Indo-pacific Oceans On Variations Of Western Pacific Subtropical High During Boreal Summer

Using NCEP/NCAR monthly reanalysis,NOAA sea surface temperature(SST),Hadley SST,Climate Prediction Center Nino 3.4 index,station observation data and the characteristic index of the Western Pacific Subtropical High(WPSH for short)provided by the National Climate Center,the mechanism of influences of anomalous thermal forcings in tropical IndoPacific Oceans on variations of WPSH during boreal Summer was studied.Empirical Orthogonal Function(EOF),wavelet analysis,power spectrum analysis and regression analysis are used in the reaserch.The main results are as follows:Indian Ocean(IO)and the Pacific Ocean(PO)have significant combined thermal forcings on the area variation of the WPSH in summer.The central tropical Indian Ocean(CTI)and the central equatorial Pacific(CEP)are two key oceanic regions that affect the summertime WPSH.During autumn and winter,warm SST anomalies(SSTA)in CEP force the Walker circulation to change anomalously,resulting in divergence anomalies over the western Pacific and the Maritime Continent(MC).Due to the Gill-type response,the abnormal anticyclonic circulation is generated over the western Pacific and South China Sea(SCS).In the subsequent spring,the warm SSTA in CEP weaken,while the SST over CTI demonstrates a lagged response to Pacific SSTA.The warm CTI-SSTA and CEP-SSTA cooperate with the eastward propagation of cold Kelvin waves in the western Pacific,leading to the eastward shift of the abnormal divergence center that originally locates at the western Pacific and the Maritime Continent.The anticyclone forced by this divergence subsequently moves eastward,leading to the intensification of the negative vorticity there.Meanwhile,warm SSTA in CTI triggers eastward propagating Kelvin waves,which leads to easterly anomalies over the equatorial Indian Ocean and Indonesia,being favorable for maintenance and intensification of the anticyclone over the South China Sea and western Pacific.The monsoonal meridional-vertical circulation strengthens,which is favorable for the intensification of the WPSH and the increase in the area of the WPSH.The Indian Ocean Basin Mode(IOBM)under ENSO background in summer is different from that of the IOBM independent of ENSO(IOBM?P)in the mechanism of the influence on WPSH.When the preceding El Ni(?)o signal is removed from SSTA,it is warmer than normal in Inidan Ocean while cooler in the MC in summer.The anomalous anticyclonic circulation appears in the northwest side of the cold SSTA center.At the same time,the warm IO SSTA triggers the Kelvin wave,mainly affecting the region west of MC.The abnormal atmospheric circulations look roughly symmetrical about the equator in the western Pacific.Meanwhile,the abnormal cooling over the South China Sea and the northwest Pacific caused by less laten heat release leads the WPSH to intensify and expand southwestward.However,in the background of early El Ni(?)o,the SSTA is warm in both IO and MC,resulting in more eastward propagation of Kelvin waves,leading to stronger circulation anomalies above north of the equator.The diabatic cooling and the Ekman pumping generate abnormal negative vorticity in the lower troposphere,being favorable for intensification and the southwest expansion of the WPSH.Moreover,in the background of ENSO,the WPSH is forced to vary by the remote forcing of IO SSTA,which actually includes both the lagging effects of the signals with a short period of 3-7 years from the central-east Pacific and the simultaneous impacts of local SSTA in 2yr and 8yr oscillation from the tropical India Ocean.Super El Ni(?)o events are quit different from regular ones in surface and subsurface characteristics.For super El Ni(?)o events,positive sea surface temperature anomalies appear relatively earlier and develop faster with a stronger zonal gradient of SSTA.The maximum of positive SSTA is located in NINO3 region.However,for regular El Ni(?)o events,positive SSTA are located more westward and the zonal gradient of SSTA is weaker.During the El Ni(?)o episodes,the subsurface ocean temperature anomalies(SOTA)propagate eastward with Kelvin waves and reach up to the sea surface along the thermocline.The vertical currents in front of Kelvin waves play an important role in the change of SOTA.Stronger oceanic waves can be excited in the thermocline when air-sea couples more intensively,which are favorable for higher SOTA and thus possibly induce a super El Ni(?)o event.In both types of El Ni(?)o events,abnormal anticyclonic circulations are generated over the western Pacific and South China Sea due to the Gill-type responses.As the intensity of the anomalous atmospheric circulation forced by SSTA is closely related to the intensity of the forcing sources,the WPSH is stronger in super El Ni(?)o cases than in regular ones.Besides,the lagged warming of SST in the Indian Ocean in response to El Ni(?)o is favorable for the persistence of atmospheric circulation anomalies in the AsianPacific region.The anomalous gradient of SSTA in the tropical southeastern Indian Ocean and the western Pacific Ocean contributes to the anomalous variation of WPSH ridge position.WPSH activity is extremely northward in this extreme heat wave event in summer of 2018,which makes the inter-annual positive temperature anomalies most obvious in North China,Northeast China,North Korea and South Japan.The abnormal convergence over the western part of the northwest Pacific Ocean(NWPO?W)caused by remote forcing of the Southeast Indian Ocean(SEIO)and the ascending motion anomalies caused by warm SSTA in the eastern part of the northwest Pacific Ocean(NWPO?E)can enhance the latent heat release over the northwest Pacific Ocean(NWPO),thus forcing an cyclonic circulation anomaly in the west side of the anomalous heat source.The anomalous cyclone can transfer Rossby wave perturbation energy to the north,and then motivates an anomalous anticyclone around North China and Northeast China,resulting in the extremely northward WPSH activity and anomalous subsidence and warming in the north.Besides,the SSTA gradient between NWPO?E and SEIO in summer is highly correlated with the position of the WPSH ridge line.Using the SSTA index of key sea areas and the typical SSTA models of tropical IndoPacific Oceans,which are closely related to WPSH anomalies in summer,the prediction models of the area,intensity and Western ridge point index of WPSH are constructed respectively.According to the SSTA index of the key sea areas which have important forcing effects on WPSH variation in the tropical Indo-Pacific Oceans,and the typical SSTA modes of the tropical Indo-Pacific Oceans,which are closely related to the changes of WPSH intensity and location in summer,the regression models of the characteristic index of WPSH are constructed.It can fit the characteristics of WPSH area and intensity,as well as west ridge point well in summer 1981-2010,by using these models.However,compared with the prediction based on the SSTA index,the prediction based on the typical modes of the tropical Indo-Pacific Oceans is more stable.The simulation results for 2011-2018 are closer to the actual situation.Among them,the west ridge points are best predicted.It shows that these models have important practical values in forecasting the WPSH in summer.