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Advection from the North Atlantic as the forcing of winter greenhouse effect over Europe
Author(s) -
Otterman J.,
Angell J.,
Atlas R.,
Bungato D.,
Schubert S.,
Starr D.,
Susskind J.,
Wu M.L. C.
Publication year - 2002
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2001gl014187
Subject(s) - advection , climatology , environmental science , precipitable water , forcing (mathematics) , cloud cover , troposphere , atmospheric sciences , atmosphere (unit) , precipitation , sea surface temperature , meteorology , geology , geography , physics , cloud computing , computer science , thermodynamics , operating system
In winter, large interannual fluctuations in the surface temperature are observed over central Europe. Comparing warm February 1990 with cold February 1996, a satellite‐retrieved surface (skin) temperature difference of 9.8 K is observed for the region 50–60°N; 5–35°E. Previous studies show that advection from the North Atlantic constitutes the forcing to such fluctuations. The advection is quantified by Index I na , the average of the ocean‐surface wind speed over the eastern North Atlantic when the direction is from the southwest (when the wind is from another direction, it counts as a zero speed to the average). Average I na for February 1990 was 10.6 m s −1 , but for February 1996 I na was only 2.4 m s −1 . A large value of I na means a strong southwesterly flow which brings warm and moist air into central Europe at low level, producing a steeper tropospheric lapse rate. Strong ascending motions at 700 mb were observed in association with warm and moist advection from the ocean in February 1990, producing clouds and precipitation. Total precipitable water and cloud‐cover fraction have larger values in February 1990 than in 1996. The difference in the greenhouse effect between these two scenarios, this reduction in the heat loss to space, can be translated into a virtual radiative heating of 2.6 W m −2 above the February 1990 surface/atmosphere system, which contributes to a warming of the surface on the order of 2.6 K. Accepting this estimate as quantitatively meaningful, we evaluate the direct effect, the rise in the surface temperature in Europe as a result of the maritime‐air inflow, as 7.2 K (9.8 K–2.6 K). Thus, fractional reinforcement by the greenhouse effect is 2.6/7.2, or 36%, a substantial positive feedback.