Wintertime Air-sea Interactions In The North Pacific Subtropical Oceanic Frontal Zone On Intraseasonal Time Scale

An oceanic frontal zone is a confluent region of warm and cool ocean currents,characterized by strong meridional gradient of sea surface temperature(SST),which is the key regions of extratropical air-sea interactions.In recent years,high-resolution SST observations show that the wintertime North Pacific exhibits unique double-oceanic-front structure,a subtropical oceanic frontal zone(STFZ)and a subarctic oceanic frontal zone(SAFZ).Investigating the air-sea interactions in the oceanic frontal zones not only helps understand extratropical air-sea interactions,but also has the important scientific meaning for climate prediction over the East Asia-North Pacific-North American region.Most of the previous studies focused on the air-sea interactions in SAFZ,however,this study mainly investigates the air-sea interactions in STFZ at the intraseasonal time scale with observational data and numerical experiments.The main conclusions of the study are listed below:(1)The characteristics of air-sea flux in the wintertime North Pacific subtropical region are revealed.Air-sea flux is the most direct variable in the air-sea interactions.The air-sea flux exhibits bimodal structure in the wintertime North Pacific.The strongest one is located in the Kuroshio-Oyashio extension(KOE)and the second one occurs in the subtropics,south of the STFZ.Compared to the air-sea flux in the KOE,which has been extensively studied,this study investigates the air-sea flux around the STFZ using observational data.Our results indicate that the air-sea flux in the wintertime North Pacific subtropical region is dominated by significiantly synoptic variabilitiy,which exhibits opposite changes with the air-sea flux around KOE,with enhancement(weakening)of air-sea flux in the subtropics and weakening(enhancement)of air-sea flux in the KOE.Our further results show that the Aleutian low and subtropical high display concurrent change in the winter,which induces the opposite changes of the air-sea flux in the subtropics and KOE through anomalous wind stress and specific humidity in the near-surface air.Moreover,the anomalous air-sea flux in the subtropics is always accompanied by the atmospheric convection anomalies in the subtropics.(2)The process of wintertime air-sea interactions in the North Pacific subtropical oceanic frontal zones on intraseasonal time scale is delineated.Based on the daily observational data,our study reveal that STFZ in the wintertime exhibits significiantly intraseasonal variability,which interacts with the atmosphere.The dipoar baroclinic structure of large-scale atmospheric circulations results in the air-sea flux anomalies in the both sides of STFZ through wind stress and difference of air-sea temperature,which further lead to the positive SST anomalies in the south flank of STFZ and negative SST anomalies in the north,indicating that the intensity of STFZ is gradually enhanced.As the intensity of STFZ is enhanced,the positive SST anomalies in the south flank of STFZ heat the atmosphere through upward air-sea flux,which maintains the positive low-level air temperature anomalies.The persistent anomalous meridional wind and downward aie-sea flux maintain the negative low-level air temperature in the north flank of STFZ.With the influence of STFZ and atmospheric circulation,the low-level atmospheric baroclinicity and storm track are maintained along STFZ.(3)The importance of subtropical oceanic frontal zone for the wintertime air-sea interactions in the North Pacific is identified.Through investigating the different impacts of STFZ and SAFZ on the wintertime atmospheric large-scale circulations by conducting groups of multi-year and ensemble simulations using high resolution WRF model,our results show that,though with weaker intensity,the impacts of STFZ on the atmosphere are at least as strong as the SAFZ.The STFZ shows an evident and consistent influence on the upper and lower tropospheric circulations in each year,acting to intensify and extend the jet stream and Aleutian low more eastward.The impacts of SAFZ on the atmospheric circulations,though with consistent impacts on the lower-level baroclinicity and storm track,exhibit strong year-by-year differences.Our study further shows the important role of diabatic heating for the different impacts of STFZ and SAFZ on the wintertime atmospheric large-scale circulations.The STFZ can induce deep diabatic heating,reaching up to the upper-level troposphere.The diabatic heating induced by STFZ changes the baroclinicity in the whole troposphere,which results in anomalous transient eddies in the whole troposphere.However,the diabatic heating and the baroclinicity induced by SAFZ is relatively shallow,which is confined in the lowerlevel.We argue that the atmospheric responses to this shallow diabatic heating depend on the background mean states,especially the background westerly jet.