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

Abstract

Abstract. There is ample evidence that the state of the northern polar stratospheric vortex in boreal winter influences tropospheric variability. Therefore, the main teleconnection patterns over the North Atlantic are defined separately for winter episodes in which the zonal mean wind at 50 hPa and 65° N is above or below the critical Rossby velocity for zonal planetary wave one. It turns out that the teleconnection structure in the middle and upper troposphere differs considerably between the two regimes of the polar vortex, while this is not the case at sea level. If the "polar vortex is strong", there exists "one" meridional dipole structure of geopotential height in the upper and middle troposphere, which is situated in the central North Atlantic. If the "polar vortex is weak", there exist "two" such dipoles, one over the western and one over the eastern North Atlantic. Storm tracks (and precipitation related with these) are determined by mid and upper tropospheric conditions and we find significant differences of these parameters between the stratospheric regimes. For the strong polar vortex regime, in case of a negative upper tropospheric "NAO" index we find a blocking height situation over the Northeast Atlantic and the strongest storm track of all. It is reaching far north into the Arctic Ocean and has a secondary maximum over the Denmark Strait. Such storm track is not found in composites based on a classic NAO defined by surface pressure differences between the Icelandic Low and the Azores High. Our results show that it is essential to include the state of the upper dynamic boundary conditions (the polar vortex strength) in any study of the variability over the North Atlantic. Climate forecast based solely on the forecast of a "classic NAO" and further statistical downscaling may lead to the wrong conclusions if the state of the polar vortex is not considered as well.

Stratospheric polar vortex as a possible reason for temporal variations of solar activity and galactic cosmic ray effects on the lower atmosphere circulation

“…Many recent studies showed that the stratospheric polar vortex is implicated in a variety of atmospheric processes including the formation of ozone holes in the Antarctic, the North-Atlantic Oscillation (NAO) and the Arctic Oscillation (AO), interannual and secular climate variability (e.g., Solomon, 1990;Tompson and Wallace, 1998;Baldwin and Dunkerton, 2001;Walter and Graf, 2005;Frolov et al, 2009;Gudkovich et al, 2009, etc.). In particular, the temperature drop below À80°C inside the vortex produces favorable conditions for the formation of polar stratospheric clouds (PSC).…”

“…According to Baldwin and Dunkerton (2001), more positive values of the NAO and AO indices are associated with strong vortex conditions. In turn, the changes of the NAO under strong and weak vortex regimes influence the average latitude of cyclone tracks, which are shifted to the north when the NAO is positive, and, respectively, temperature and precipitation over Europe (Baldwin and Dunkerton, 2001;Walter and Graf, 2005). The data presented by Frolov et al (2009) andGudkovich et al (2009) show that the rotation of cold and warm epochs in the Arctic region is related to the changes of the polar vortex state, warm and cold periods being associated with a strong and weak vortex, respectively.…”

Strength and spatial patterns of the Holocene wintertime westerlies in the NE Atlantic region

“…The strength of the polar vortex has been discussed as a forcing mechanism on the climate variability over the North Atlantic region and also as a mechanism to explain large-scale teleconnection patterns Walter and Graf, 2005). The polar vortex is a persistent large-scale cyclonic circulation pattern in the middle and upper troposphere and the stratosphere, centred generally in the polar regions of each hemisphere.…”

“…During the latest years, however, it is shown that one of the two polarities of the leading mode of the North Atlantic storm-track variability corresponds to a blocking situation of the NE Atlantic with a strong north-eastward tilt of the storm-track axis and reduced precipitation in western Europe. This situation is, however not captured by standard NAO indices (Rogers, 1997;Shabbar et al, 2001), and reanalyses using data of geopotential height, air temperature, wind and precipitation for the high-latitude Northern Hemisphere suggests that it is necessary to include the state of the polar vortex in any study of North Atlantic climate variability Walter and Graf, 2005). Walter and Graf (2005) suggest that precipitation anomalies at the west coast of Greenland and the anomalies at the Norwegian coast could serve as a proxy for the strength of the polar vortex.…”

“…This situation is, however not captured by standard NAO indices (Rogers, 1997;Shabbar et al, 2001), and reanalyses using data of geopotential height, air temperature, wind and precipitation for the high-latitude Northern Hemisphere suggests that it is necessary to include the state of the polar vortex in any study of North Atlantic climate variability Walter and Graf, 2005). Walter and Graf (2005) suggest that precipitation anomalies at the west coast of Greenland and the anomalies at the Norwegian coast could serve as a proxy for the strength of the polar vortex. Their reanalysis data shows enhanced precipitation in these regions during periods with a strong state of the polar vortex, and a significant reduced precipitation in south-western Norway during periods with a weaker state.…”