89th American Meteorological Society Annual Meeting

Wednesday, 14 January 2009: 4:45 PM
Air-sea heat exchanges characteristic to a prominent midlatitude oceanic front in the South Indian Ocean as simulated in a high-resolution coupled GCM
Room 128A (Phoenix Convention Center)
Masami Nonaka, JAMSTEC, Yokohama, Kanagawa, Japan; and H. Nakamura, B. Taguchi, N. Komori, A. Kuwano-Yoshida, and K. Takaya
Through analysis of an integration of a high-resolution coupled general circulation model whose ocean component is eddy-permitting and thus able to reproduce sharp sea surface temperature (SST) fronts, air-sea heat exchanges characteristic to midlatitude oceanic frontal zone are investigated. Our focus is placed on a prominent SST front in the South Indian Ocean, which is collocated with the core of the Southern Hemisphere storm track. Time-mean distribution of sensible heat flux is characterized by a distinct cross-frontal contrast. It is upward and downward on the warmer and cooler flanks, respectively, of the SST front, acting to maintain the sharp gradient of surface air temperature (SAT) that is important for anchoring the storm-track. Induced by cross-frontal advection of cold (warm) air associated with migratory atmospheric disturbances, the surface flux is highly variable with intermittent enhancement of the upward (downward) flux only on the warmer (cooler) flank of the front. This anti-symmetric behavior yields the sharp cross-frontal gradient in the time-mean flux. Since the flux intensity is strongly influenced by local magnitude of the SST-SAT difference that tends to increase with the SST gradient, the concentration of the flux variance to the frontal zone and cross-frontal contrasts in the mean and skewness of the flux all become stronger during the spin-up of the SST front. Synoptically, the enhanced sensible heat flux near the SST front can restore SAT to the underlying SST effectively with a time scale of a day, to maintain a frontal SAT gradient against the relaxing effect of atmospheric disturbances. In addition to the concentration of moisture supply from the ocean, the restoring can be a key process through which oceanic frontal zones influence atmospheric eddy activity and mean circulation.

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