Are stronger North-Atlantic southwesterlies the forcing to the late-winter warming in Europe?

✍ Scribed by J. Otterman; R. Atlas; S.-H. Chou; J. C. Jusem; R. A. Pielke Sr; T. N. Chase; J. Rogers; G. L. Russell; S. D. Schubert; Y. C. Sud; J. Terry

• Publisher: John Wiley and Sons
• Year: 2002
• Tongue: English
• Weight: 177 KB
• Volume: 22
• Category: Article
• ISSN: 0899-8418
• DOI: 10.1002/joc.681

✦ Synopsis

We examine a possible mechanism leading to late-winter warming, 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 speed 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 i between I na and our lapse-rate parameter , 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 °C and 0.43 °C per decade in these two respective pentad groups (T s in this location increased from 1948 to 1999 by 1.86 °C and 2.19 °C). 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 °C in 1948 for the 10 °longitude distance) is reduced by 0.8 °C, 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.