Variability Of Winter North Pacific Storm Track And Its Associations With The Mid-high Latitude Atmospheric Circulations

Over the North Pacific Ocean,active transient waves conduct important roles for the formation and regulation of the atmosphere circulation in the mid-high latitude.It can transport heat,momentum and vorticity meridionally and vertically,and is a main source of the latent heat in the mid-latitude.On the other hand,the storm track could conduct influence towards the time-mean atmospheric circulation through eddy energy transportation,and bring about forcing effort,which could alter the steady wave as well as the basic flow.Year-to-year variations of the storm track exhibit two principal modes,i.e.the monopole intensity change and the meridional shift of the storm track,respectively.Based on the daily NCEP/NCAR reanalysis data and monthly Sea surface temperature data from Hadley Center during 1950-2015,this study investigated the inter-atmospheric mechanisms of the variability of the North Pacific storm track during the winter season.The external forcing effect of sea surface temperature anomalies towards storm track are then investigated applying the NCAR CAM 3.0 atmosphere model.Finally,daily teleconnection indexes are used to composite the development of the circulation anomalies associated with the Western Pacific teleconnection and the Pacific-North American teleconnection patterns.The possible dynamic mechanisms are investigated through the point of heat and vorticity forcing effect of transient eddies towards mean flow.The major conclusions are as follows:Over the entrance region of the storm track,accompanied by the enhanced baroclinity,more energy is converted from mean available potential energy to eddy available potential energy and then transferred to eddy kinetic energy,which is favorable for the overall enhancement of the storm track intensity.For the meridional shift mode,more energy is transformed from mean available potential energy to eddy available potential energy and further transferred to eddy kinetic energy over the southern(northern)areas of the storm track,contributing to the southward(northward)shift of the storm track.Additionally,the increased conversion from mean-flow kinetic energy to eddy kinetic energy over the north-eastern Pacific region is in favor of the northward shift of the storm track.Changes of the storm track are tightly correlated with the sea surface temperature.A series of numerical simulations results indicate that winter EP-P(N)SST anomaly could influence the zonal distribution of horizontal wind,and cause the off-phase anomalies on the two sides of the jet over its exit region,which is also crucial for the meridional shifting of the NPST over the Northeastern Pacific ocean.Moreover,the KC-P(N)SST anomaly plays an important role in enhancing(weakening)the intensity of storm track in the climate average position.Numerical simulations results based on the concurrent variabilities of the SST over the two key regions show that,the positive SSTA over the EP area conducts a broad influence on the north hemisphere atmosphere.Compared with EP,atmospheric response caused by KC's SSTA is more local.However,the negative SSTA over the EP area is less significant,and mainly influence the middle-west region of storm track,so that the contribution of the KC region is more important.The positive feedback between time-mean flow and the transient eddies travelling along the storm track is helpful for sustain and enhance of the low-frequency teleconnection signals.Composited results of the development of the circulation anomalies associated with the WP and the PNA teleconnection patterns show that,teleconnection patterns undergo about 6-day fast growth period before attaining their maximum amplitudes,during which the synoptic scale wave excite a rapid growth,which is also the crucial period for the feedback between transient eddies and teleconnections.During the development stage of the Western Pacific teleconnection pattern,the eddy kinetic energy over western region of the Pacific decline,which is favorable for the sustainment of the mean flow energy,in the meanwhile,the geopotential tendency induced by vorti city fluxes of the transient eddies is in accordance with the teleconnection.When the Pacific-North American accidents occur,associated with variations in intensity and position of the transient eddies,its forcing effort towards the mean flow is also favorable for the development of the Pacific-North American pattern.