Study On The Impact Of Polar Vortex On North Pacific Storm Track And Its Possible Mechanism During Winter

In the context of US NCEP/NCAR reanalysis data, and NOAA Extended Reconstructed global monthly sea surface temperature data, and northern polar vortex index data of China National Climate Center from1951to2011, by using several statistical methods, the spatio-temporal evolutions of the north Pacific storm track and the northern polar vortex in recent60winters were studied, and compared with the previous results. On this basis, also analyzed were the impact of polar vortex on storm track and its possible physical mechanism, as well as the possible effects of sea surface temperature on this relationship, in which the coupled characteristics among the polar vortex, teleconnection patterns, storm track and SST were revealed. The major conclusions are drawn as follows:1. The north Pacific storm track experiences significant seasonal, interannual and interdecadal variability. As viewed from month to month, storm track exhibits midwinter suppression. The minimum of storm track intensity appears in the July, and the maximum of intensity appears in the November. As viewed from season to season, the maximum of storm track intensity appears in the winter. During the60years, the storm track shows a weak increasing trend, and extending westward and southward. The annual variability of the storm track exhibits two major patterns, i.e., a monopole pattern with the track showing their change in strength and position in the vicinity of its long-term mean location, and a dipole pattern depicting the northward or southward migration of the track.2. The enhanced period of polar vortex is from previous August to this January, and the weakened period is from February to July. Polar vortex area weakens from early1950s to early1960s, and it shows a gradually increasing trend after1960s. During the60winters, the polar vortex area and intensity all have a decreasing trend. The Empirical Orthogonal Function (EOF) of the polar vortex shows:the first mode depicts a meridional seesaw variation of geopotential height between the polar region and to the south, and the second mode depicts the difference between the ocean and continent.3. By calculating the correlation coefficient between the index of polar vortex and those of the storm track, it shows that in the year while the northern polar vortex is stronger (weaker), the north Pacific storm track is also stronger (weaker). Singular Value Decomposition (SVD) shows that there are two major coupled modes between the polar vortex and storm track. The first coupled mode depicts that while the geopotential height to the north of50°N is weaker (stronger), the polar vortex is stronger (weaker) in the polar region, and the storm track is stronger (weaker) in the vicinity of its long-term mean locations. The second mode depicts that while the geopotential height to the north of the Eurasian continent and north Pacific is weaker, the polar vortex increases and shifts to the north Pacific, then, the storm track is stronger and shifts to the south of its long-term mean locations. On the contrary, while the geopotential height in the north America continent is weaker, the polar vortex increases and moves to the north America, then the storm track increases and shifts to the north of its long-term mean locations.4. Composite analysis and regression analysis show that in the years when the polar vortex is stronger (weaker), the intensity of the storm track, as well as the synoptic-scale kinetic energy, meridional and vertical heat transport, and transport of westerly momentum, is also stronger (weaker). And the anomalous change of polar vortex may give rise to change in the upper level geopotential height over the Eurasian continent and its downstream Pacific ocean, then change the East Asia westerly jet and the baroclinicity in the upstream of the storm track, and finally the storm track itself.5. In addition, the impact of the polar vortex on the storm track can also be modulated by the external thermal forcing from the SSTA in the Kuroshio and equatorial central and eastern Pacific ocean. There is a coupled interaction among the polar vortex, teleconnection, storm track and SST. When the northern polar vortex strengthens (weakens) in the polar region, in the same time, the SST of Kuroshio is warmer (colder) than ordinary, a positive (negative) WP teleconnection pattern appears in the middle troposphere, the pressure gradient is increased (decreased) in the vicinity of50°N leading to a remarkable zonal (meridional) anomaly, and the wind velocity and baroclinicity all strengthen (weaken) between40°N and60°N, thus all of which, causing the north Pacific storm track strengthened (weakened) in the vicinity of its long-term mean location. However, when the SST of equatorial central and eastern Pacific ocean is warmer (colder) than ordinary, a positive (negative) PNA teleconnection pattern appears in the middle troposphere, the polar vortex strengthens and moves to the north Pacific (north America), the pressure gradient is increased (decreased) in the vicinity of40°N in north Pacific, and the wind velocity and baroclinicity all strengthen to the south (north) of45°N, thus, the north Pacific storm track strengthens and trends to move to the south (north) of45°N.