Synergistic Effects Of Atmospheric Upstream Disturbances And Oceanic Subtropical Front Intensity Variability On North Pacific Jet Stream In Winter

Both the atmospheric upstream disturbances and oceanic subtropical front intensity variability can influence the North Pacific jet stream(NPJS),and have great impact on climate change.NCEP/CFSR dataset and WRF 3.5 model were used to investigate the synergistic effects of atmospheric upstream disturbances and oceanic subtropical front intensity variability on NPJS.Effects of different configurations of atmospheric upstream disturbances and subtropical frontal zone(STFZ)on the strength and location of NPJS were discussed,and the mechanism was shown.Besides,reasons for the intensity variability of STFZ with the transport of subsurface currents were studied.The conclusions are as follows:When analyzing the atmospheric anomalies associated with composed strong and weak years,it is found that the differences of NPJS are more southerly of the large value region of climatological mean NPJS and northerly of the STFZ,which reflects that the atmospheric upstream disturbances can influence the midlatitude atmosphere,in comparison with the effect of STFZ intensity variability.The averaged eddy kinetic energy(EKE)within 20°N-55°N,127°E-129°E and 100 h Pa-700 h Pa is defined as the intensity index of atmospheric upstream disturbances.Composite analysis is used to find out the synergistic effects of atmospheric upstream disturbances and STFZ intensity variability on NPJS in observation.Strong atmospheric upstream disturbances can anchor the NPJS at about 25°N-45°N,corresponding to the large value region of climatological mean NPJS.The strength of NPJS is affected by STFZ intensity,while intensified(weakened)STFZ corresponds to stronger(weaker)NPJS.However,when the atmospheric upstream disturbances are weakened,the latitudinal position of NPJS is changed by STFZ.The enhancement of STFZ makes NPJS move southward to about 20°N-35°N,corresponding to the location of STFZ,more southerly of the large value region of climatological mean NPJS.The phenomenon is verified in model experiments.Further study shows that strong atmospheric disturbances make the lower atmosphere obtaining baroclinic energy over both SAFZ and STFZ,thereby anchor the NPJS between the two oceanic fronts.At this time,the intensified(weakened)STFZ influence the NPJS strength by increasing(decreasing)the upward baroclinic energy,but the latitudinal position of the upward baroclinic energy is anchored.When the atmospheric upstream disturbances are weakened,the enhancement of STFZ corresponds to the southward movements of upward baroclinic energy and the NPJS.In addition,changes in the strength and location of NPJS can influence the precipitation along the North American coast,which may be beneficial for weather forecast and disaster prevention in North America.Vertical distribution of meridional temperature gradient of the STFZ is analyzed to find out the reason for the intensity variability of STFZ.The sea subsurface(80-160m)temperature gradient is found to be significantly larger than the surface(0-80m)temperature gradient.Empirical orthogonal function(EOF)analysis of oceanic temperature gradient in the STFZ is performed and the first two modes may correspond to different air-sea coupled progresses.Lead-lag regression is used to analyze the airsea coupled processes the two modes correspond to.The results suggest that the first EOF mode shows a process in which the atmosphere affects the ocean.The increase of the westerly wind in day-8 causes a cold sea surface temperature anomaly(SSTA)at day 0 by affecting the air-sea heat flux,and it passes down after day 0,affecting the temperature of the subsurface ocean.In this process,the enhanced westerly jet strengthens the surface subtropical front,while the downward cold oceanic temperature anomaly weakens the surface subtropical front.The second EOF mode shows a process in which the subsurface ocean affects the sea surface and atmosphere.The cold oceanic temperature anomaly generates in the subsurface layer at day-8 because of the increasing eastward flow and southward flow.Then it passes up,making the sea surface temperature(SST)decreasing and SST gradient increasing.At day 0,the baroclinic eddy growth rate shows a positive anomaly near the surface.Then the anomaly extends from lower troposphere to upper troposphere gradually,leading to the acceleration of the zonal wind.After that,the enhanced westerly wind leads to more heat fluxes from ocean to the atmosphere,which results in a colder SSTA and a larger SST gradient.In this process,the upward cold oceanic temperature anomaly and the enhanced westerly jet after day 0 makes the surface subtropical front intensifying,a positive oceanatmosphere feedback then persists.