Monthly Characterization of the Tropospheric Circulation over the Euro-Atlantic Area in Relation with the Timing of Stratospheric Final Warmings

In recent decades, there has been a growing interest in the study of a possible active role of the stratosphere on the tropospheric climate. However, most studies have focused on this connection in wintertime. This paper deals with the possible relationship between variations in the timing of stratospheric final warmings (SFWs, observed in springtime) and monthly averaged changes in the Euro-Atlantic climate. On the basis of the date on which the SFW occurs, two sets of years have been selected for the period of study (1958-2002): "early years" and "late years," reflecting a very early or a very late breakup of the polar vortex. The statistical significance of the early-minus-late differences in the analyzed fields has been established by applying a nonparametric test based on a Monte Carlo-like technique. Using data from 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40), a dynamical study for March and April has shown important differences between both sets of years in stationary waves, especially ultralong ones (waves with k = 1 in March and k = 2 in April). Furthermore, the interannual variations in the stratospheric zonal wind seem to propagate downward as the spring progresses, in such a way that they reach tropospheric levels in April. Relevant differences between "early" and "late" years have been found in tropospheric monthly fields in the Euro-Atlantic area (geopotential, zonal wind, and storm-track activity), being at their most extensive in April.