The North Atlantic variability structure, storm tracks, and precipitation depending on the polar vortex strength

[摘要] Motivated by the strong evidence that the state of the northern hemispherevortex in boreal winter influences tropospheric variability, teleconnectionpatterns over the North Atlantic are defined separately for winter episodeswhere the zonal wind at 50hPa and 65° N is above or below the criticalvelocity for vertical propagation of zonal planetary wave 1. We argue thatthe teleconnection structure in the middle and upper troposphere differsconsiderably between the two regimes of the polar vortex, while this is notthe case at sea level. If the polar vortex is strong, there exists one meridional dipole structure ofgeopotential height in the upper and middle troposphere, which is situatedin the central North Atlantic. If thepolar vortex is weak, there existtwo such dipoles, one overthe western and one over the eastern North Atlantic. Storm tracks (andprecipitation related with these) are determined by mid and uppertropospheric conditions and we find significant differences of theseparameters between the stratospheric regimes. For the strong polar vortexregime, in case of a negative upper tropospheric "NAO" index we find ablocking height situation over the Northeast Atlantic and the strongeststorm track of all. It is reaching far north into the Arctic Ocean and has asecondary maximum over the Denmark Strait. Such storm track is not found incomposites based on a classic NAO defined by surface pressure differencesbetween the Icelandic Low and the Azores High. Our results suggest that itis important to include the state of the polar vortex strength in any studyof the variability over the North Atlantic.

[学科分类] 大气科学