Short communication are stronger north-atlantic southwesterlies the forcing to the late-winter warming in Europe?

摘要

We examine a possible mechanism leading to lae-winter araing, and thus to an early spring in Europe. From the National Centers for Environmental prediction reanalysis, we extract for the years 1948-99 ocean-surface winds over the eastern North Atlantic, and air temperatures at the surface T_s, and at the 500 hPa level T_(500) in late-winter and spring. T_s is extracted at six European locations, all at 50.5°N, ranging in longitude from 1.9°E (northeastern France) to 26.2°E (Ukraine). To quantify the advection of maritime air into Europe, we evaluate for three-pentad groups the index I_(na) of the southwesterlies at 45°N, 20°W; I_(na) is the average wind sped at this point if the direction is from the quadrant 180-270°(when the direction is different, the contribution counts as zero). In late winter, correlations C_(it) between I_(na) and T_s are substantial, up to the 0.6 level in western Europe (but weaker correlations for Poland and Ukraine). C_(it) drops sharply by mid-March, occasionally taking negative values subsequently. This drop in C_(it) indicates that maritime air advection is no longer associated closely with the surface-air warming; the role of insolation becomes important, and thus the drop in C_(it) marks the arrival of spring. Correlations C_(iΔ) between I_(na) and our lapse-rate parameterΔ, the difference between T_s and T_(500), indicate that the flow of warm maritime-air from the North Atlantic into this 'corridor' at 50.5°N is predominantly at lower tropospheric level. By computing the best linear fit to I_(na) and T_s, the trends for the period 1948-99 are evaluated. The trends are appreciable in the second half of February and the first half of March: for I_(na), the trends are 0.41 m s~(-1) and 0.15 m s~(-1) per decade in pentad groups 10-12 and 13-15 respectively (I_(na) increased from 1948 to 1999 by 2.10 m s~(-1) and 0.77 m s~(-1)); for T_s, the trends for western Germany are 0.36 ℃ and 0.43 ℃ per decade in these two respective pentad groups (T_s in this location increased from 1948 to 1999 by 1.86 ℃ and 2.19 ℃). Such higher near-surface temperatures would markedly influence snow-melt, and thus absorption of insolation by the surface. Our three-pentad analysis points to the interval from mid-February to mid-March as the end of-winter period in which the southwesterlies over the eastern North Atlantic become stronger and the surface-air temperatures in Europe rise markedly, the lapse rate becomes steeper, and concurrently the longitudinal temperature gradient between the Somme (France) and the Oder (Germany-Poland border) (about -4 ℃ in 1948 for the 10°longitude distance) is reduced by 0.8 ℃, i.e. by 20% of its 1948 value. Our thesis, that the observed late-winter warming and the concomitant advancement of spring in Europe results, at least in part, from stronger southwesterlies over the North Atlantic, merits further investigations.