89th American Meteorological Society Annual Meeting

Tuesday, 13 January 2009: 9:15 AM
Impact of midlatitude oceanic frontal zones on the atmospheric general circulation and its annular variability: Importance of "oceanic baroclinic adjustment"
Room 128B (Phoenix Convention Center)
Hisashi Nakamura, Univ. of Tokyo, Tokyo, Japan; and T. Sampe, A. Goto, B. Taguchi, M. Nonaka, N. Komori, A. Kuwano-Yoshida, W. Ohfuchi, and S. P. Xie
A close association among a midlatitude storm-track, a westerly polar-front jet (PFJ) and an underlying oceanic frontal zone is observed most typically in the South Indian Ocean or in the North Atlantic. Along a surface baroclinic zone anchored around a frontal zone, enhanced storm-track activity maintains a well-defined PFJ with strong surface westerlies. It is this eddy-driven jet whose axial migrations are manifested as the annular mode. These important aspects of the atmospheric general circulation are well reproduced in a perpetual "aqua-planet experiment" conducted with a relatively high-resolution AGCM (AFES), where zonally-uniform frontal SST gradient is prescribed at 45 deg. latitude as observed. The main storm-track is anchored firmly around the frontal zone and a well-defined PFJ forms slightly poleward, and the most dominant mode of variability in the extra-tropical zonal-mean zonal wind resembles the observed Southern Hemisphere annular mode. In another experiment where the frontal SST gradient is eliminated, the storm-track activity and PFJ both weaken substantially, leading to unrealistic equatorward displacement of a subtropical high-pressure belt and weakening of the Trades. The amplitude of the model annular mode is also reduced substantially, and its meridional structure is apparently distorted especially in the presence of the intensified wintertime STJ. Though idealized, our AGCM experiments suggest a particular importance of midlatitude oceanic frontal zones in the formation and maintenance of the extratropical atmospheric circulation and its dominant eddy-driven annular variability.

In the particular aqua-planet experiment, the surface baroclinicity is restored effectively via differential heat supply from the ocean across the oceanic frontal zone, despite the relaxing effect by atmospheric disturbances migrating along the storm-track. More realistic model simulations also reproduce the differential heat supply across such an oceanic frontal zone as in the South Indian Ocean or in the Kuroshio-Oyashio Extension, as long as persistent monsoonal winds are absent. Induced by cross-frontal thermal advection associated with atmospheric disturbances, surface sensible heat flux in the frontal zone is highly variable and skewed in the opposing manner between the warmer and cooler flanks of the frontal zone, leaving sharp cross-frontal contrasts in its time mean field. With this "oceanic baroclinic adjustment", the surface air temperature gradient can be effectively restored, which is necessary for the recurrent development of atmospheric disturbances and thereby for anchoring a storm-track firmly along the frontal zone.

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