Accueil > Actualités > Séminaires > Séminaire de Shoshiro Minobe au LMD-ENS


Titre : Atmospheric response to the Gulf Stream: Numerical diagnostics on the mechanism of surface wind convergence and observational analysis on seasonal variations.
Nom du conférencier : Shoshiro MINOBE
Son affiliation : Graduate School of Science, Hokkaido University, Sapporo, Japan
Laboratoire organisateur : LMD
Date et heure : 13-07-2010 11h00
Lieu : Salle E314 (ex-316) du Dept des Géosciences, LMD-ENS, 24 rue Lhomond, 75005 Paris
Résumé :

In this seminar, following a brief review of recent studies for ocean-to-atmosphere influences along SST fronts, I would like tointroduce two recent studies of our group about the atmospheric response to the Gulf Stream.

In the first part of this talk, which is studied by my PhD student Kohei Takatama, I would like to explain the mechanism about the surface wind convergence, which is collocated with the Gulf Stream rain band and the middle-tropospheric ascent. In the previous studies, mainly two mechanisms are investigated; one is the downward momentum mixing (Wallace et al. 1989) and the other is the pressure adjustment mechanism (Lindzen and Nigam 1987). In order to identify the mechanism, several studies conducted momentum budget analyses. Because these analyses were not conclusive, because they did not separate two roles of vertical stresses in the atmosphere. Wallace et al. (1989) actually discussed the two roles; one is the downward momentum mixing and the other is drag due to the surface stress. In our new diagnostics, we successfully separate these roles. By applying this diagnostics to our five year integration of a regional atmospheric model, we have found that the pressure adjustment mechanism primarily explains the surface wind convergence (70%) and the downward momentum mixing mechanism is secondary (30%). The numerical model underestimates the wind convergence, but this model deficiency is not likely to alter our major conclusion.

In the second part of this talk, the atmospheric response to the Gulf Stream front in sea surface temperature is investigated using high-resolution data from satellite observations and operational analysis and forecast. Two types of atmospheric response are observed with different seasonality and spatial distribution. In winter, atmospheric responses are prominent from the surface to the lower-middle troposphere after the separation of the Gulf Stream at Cape Hatteras, including strong surface wind convergence, frequent occurrence of middle-level cloud. In summer, atmospheric response are more prominent from the middle troposphere to the upper troposphere before and just after the separation, including enhanced high clouds and increased lightning. Consistently, an analysis of atmospheric heating of a Japanese reanalysis reveals that in winter, a"shallow-heating mode" dominates with strong sensible heating in the marine atmospheric boundary layer and latent heating in the lower troposphere. In summer, a "deep-heating mode" is pronounced, characterized by latent heating in the middle and upper troposphere due to deep convection. Possible occurrences of these heating modes in other regions are discussed.

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