Characteristics And Influences Of The Northwest Pacific Subtropical Sea Surface Temperature Front

Based on the high-resolution and long time-scale reanalysis data,the characteristics of the intensity and position of the Northwest Pacific Subtropical Sea Surface Temperature(SST)front(NPSTF)are investigated.Moreover,the importance of the net heat flux term and the meridional oceanic temperature advection term(including the Ekman convergence)in the NPSTF frontogenesis is examined quantitatively by the mixed-layer energy budget equation,and the roles of the atmosphere in both terms are further identified.Additionally,the interannual responses of the East Asian Jet Stream(EAJS)and El Nino-Southern Oscillation(ENSO)to the spring NPSTF and the underlying mechanism are revealed by the reanalysis data and model simulations.The primary conclusions are as follows:(1)The NPSTF appears in December and enhances with southward shift in January and February to reach the strongest in March and April(the magnitude is about 0.8-1.0 ?/100 km).Then,it weakens and shifts northward in May and June,finally disappearing in July.In winter,the climatological magnitude of the NSPTF is 0.71 ?/100 km,the meridional(zonal)position is 28.1°N(168.9°E),and the slope is 0.065,while in spring is 0.79?/100 km,29.0°N(166.0°E)and 0.065,respectively.The intensity variation characteristics of NPSTF in both winter and spring are relatively consistent in the three reanalysis data with different resolutions.However,those of the position manifest much differences,especially the ERSST data with low resolution.In addition,the intensity and position indexes all have an interannual period of 2-4 or 4-8 years and an interdecadal period of 8-16 years.(2)The frontogenesis of the NPSTF occurs from the previous October to February.The net heat flux term dominates the frontogenesis from October to December,while the meridional temperature advection term in the ocean plays a comparable role as the net heat flux term in January and February.The atmosphere can work directly in the frontogenesis through the net heat flux term and indirectly through the Aleutian low.The components of the net heat flux term combine for the frontogenesis,in which the latent heat flux(shortwave radiation)term determines in October(from November to February).The meridional temperature advection term in the ocean is related to the Aleutian low.Climatologically,the strengthening and southward migration of the Aleutian low from October to February are characterized by the acceleration and southward shift of the westerly wind to the south,respectively,which can drive anomalous southward ocean currents.Correspondingly,the southward ocean currents give the cold meridional advection over the north of the NPSTF in January and February,favoring the frontogenesis.Moreover,the decreasing net heat flux term from Octomber to February is also related to the reducing meridional gradient of 10 m wind speed associated with the southward shifted Aleutian low.(3)A strong spring NPSTF can induce an anomalous cyclone(anticyclone)to its north(south)through the transient eddy activity and atmospheric heat source,leading to an enhanced EAJS in spring and a La Nina event in the following winter.The enhanced air temperature gradient due to a strong NPSTF increases the atmospheric baroclinicity,favoring an enhanced transient eddy activity over the NPSTF and its north.The enhanced transient eddy activity triggers a significant negative height anomaly to its north via the high-frequency transient eddy feedback forcing,which is also conductive to an anticyclonic circulation over the subtropical North Pacific through the downstream effect of Rossby waves.On the other hand,the convergence and increased atmospheric baroclinicity related to the strong NPSTF act to enhance precipitation over the NPSTF and its north,corresponding to the increased atmospheric heat source,which can excite an anomalous cyclonic(anticyclonic)circulation to the northwest(southeast)of the NPSTF.The associated anomalous westerly wind south of the cyclone anomaly at 200 hPa benefits the enhancement of the EAJS.The northeasterly wind anomaly associated with the anticyclone anomaly at 1000 hPa increases the climatological northeasterly wind over the subtropical Northeast Pacific,resulting in S ST decrease via the wind-evaporation-SST feedback.The SST and wind perturbations propagate southward from the subtropical to the eastern-central equatorial Pacific in summer and grow via the Bjerknes feedback from summer to winter,leading to a La Nina event eventually.