The Jetstream Orientation and Weather over the North Atlantic

Abstract:

We study the effect of interannual variability of the jetstream orientation on weather systems over the North Atlantic basin (NAB) using reanalysis data. The daily transient kinetic energy of the geostrophic wind (GTKE) is taken as a measure of the collective strength of the weather systems, i.e. of storm activity over the North Atlantic. We partition the NAB (85 W–0 W, 15 N–65 N) into four rectangular regions, divided by the 45 W meridian and the 40 N parallel, and calculate the winter (DJFM) average of GTKE for each quadrant at 400hPa. The spatial average of GTKE over each region shows prominent year-to-year variability that is strongly correlated with the NAO; the highest correlations occur in the eastern half of the NAB. The number of winter days when the average wind is faster than 30 m/s is 13–103 days in the NE quadrant and 0–2 days in the SE quadrant over the 1948–2012 period. The GTKE strength in the NE quadrant is a result of the orientation angle of the jetstream, with larger values when the jet is more zonal. To gain insight into the relation between the orientation angle and its downstream impact, we used a quasi-geostrophic, baroclinic model in a β-channel. The results show that an initially zonal jet persists at its initial latitude until the end of the integration, while a tilted jet propagates meridionally according to the Rossby wave group velocity, unless there is external forcing. We found an analytical steady-state solution to this problem with a forcing by a Gulf Stream–like narrow SST front. This front influences the atmospheric jet hundreds of kilometers further north, in the NW quadrant: it both strengthens the jet and tilts it northward at higher levels, as confirmed by thereanalysis data, while it weakens and turns the flow southward at lower levels. These results suggest that the interannual variability found in the angle of the jet stream and the GTKE are due to the interannual variability of the Gulf Stream’s SST front.