Atmospheric Responses To Oceanic Front And Eddies In The Kuroshio Extension Region

Based on high-resolution satellite datasets,NCEP Climate Forecast System Reanalysis(CFSR)data,eddy detection dataset,and WRF(Weather Research and Forecasting)model,the atmospheric responses to the oceanic eddies in the Kuroshio Extension(KE)region and the underlying mechanism are investigated.Additionally,the seasonal difference in the atmospheric responses to oceanic eddies has been revealed.The meridional displacement and intensity variation characteristics of springtime oceanic front in the KE region and their impact on the East Asian subtropical jet stream(EASJ),storm track,and precipitation have also been examined.Furthermore,the oceanic front in 20 models with different ocean model resolutions from the Coupled Model Intercomparison Project Phase 5(CMIP5)is evaluated.The association between the SST bias and EASJ bias has been identified,based on which the importance of better representations of oceanic front in models has been emphasized.The primary conclusions are as follows:(1)Cold(warm)eddies in the KE region cause surface winds to decelerate(accelerate),and reduce(increase)latent and sensible heat fluxes,cloud liquid water,water vapor content,and rain rate.The linear correlation between wind divergence and downwind sea surface temperature(SST)gradient supports the vertical momentum mixing mechanism,which indicates that SST perturbations modify surface winds by changing the vertical turbulent mixing in the marine atmospheric boundary layer(MABL).Vertical velocity anomalies over oceanic eddies penetrate beyond the MABL into free atmosphere;there exists a positive correlated relationship between SST and convective rain rate anomalies,indicative of ocean eddies' impact on the free troposphere.A case study about the atmospheric responses to mesoscale oceanic eddies over the KE region in winter shows that mesoscale oceanic eddies can exert influence on atmospheric transient disturbance intensity in both the boundary layer and the mid-lower troposphere.Baroclinic energy conversion and advection play critical roles in the oceanic forcing of atmospheric transient disturbance.(2)High-resolution satellite observations and reanalysis data reveal distinct seasonal variations in atmospheric responses to oceanic eddies in the KE region,characterized by much stronger surface wind speed and heat flux responses in the cold seasons(winter and spring)than in the warm seasons(summer and autumn).Cloud liquid water and rain rate also display seasonally dependent characteristics,with more deficit(surplus)in winter than in summer over the cold(warm)oceanic eddies.Reanalysis data reveal remarkable seasonal variations in tropospheric responses,with eddy-induced wind speed(vertical velocity)anomalies reaching as high as 900-hPa(800-hPa)in winter,but near the sea surface in summer.Both the CFSR data and WRF model simulations indicate that the seasonal variations can be attributed to the variations in background atmospheric stability during different seasons.(3)There exist significant interannual and interdecadal changes in meridional position of the oceanic front in KE region during spring.A close relationship is found between meridional position variation of the oceanic front and the storm track in Pacific region,jet stream in East Asia.As the oceanic front is shifted northward in spring,storm track,jet stream,and rain band are shifted northward in June accordingly,and vice versa.The interannual variation in the intensity of the oceanic front in the KE region is evident.Since the late 1990s,the amplitude of the intensity variation increases significantly.Corresponding to a stronger oceanic front,SST to the north(south)of the KE decreases(increases);an anomalous anticyclone occurs over the East Asia-Northwestern Pacific region;precipitation enhances considerably over the middle and eastern China,Korean Peninsula,and middle Japan,and vice versa.The baroclinicity changes associated with temperature variations induced by the intensity changes in the oceanic front exert impact on the atmospheric circulation.(4)In the coupled general circulation models(CGCMs)with relatively coarse(fine)ocean model resolutions,the simulated oceanic fronts in the Kuroshio-Oyashio Extension(KOE)region are shifted southward(northward)remarkably in all four seasons,indicating that the simulated meridional location of the oceanic fronts is closely related to the ocean model horizontal resolution.The CGCMs with relatively low(high)ocean model resolutions generally simulate negative(positive)SST biases over the KOE region with an SST decrease(increase)of about-3K(+2K)in comparison with the MME.The SST differences between the models with coarse and fine ocean components are mainly attributed to the ocean heat transport effect,which may be related to the oceanic front simulation bias.The EASJs are also shifted southward(northward)in the CGCMs with relatively coarse(fine)ocean model resolutions,closely related to the cooler(warmer)SST in the KOE region,indicating that the better representation of the KOE mean state may markedly improve the simulation of the EASJ.