83rd Annual

Wednesday, 12 February 2003
Rich Structures of Cold Ocean-Atmosphere Interaction
Shang-Ping Xie, University of Hawaii, Honolulu, HI
Over most of the world ocean, sea surface temperature (SST) is below 26C and atmospheric deep convection rarely takes place. Our knowledge of ocean-atmosphere interaction over warm oceans (SST > 26C) has grown tremendously in the past two decades and ENSO prediction is now being carried out routinely. Air-sea interaction over cold oceans (SST < 26C), by contrast, is poorly understood. In particular, conclusive evidence for significant and robust atmospheric response to SST variations there is lacking, because of high levels of weather noise on one hand and insufficient observations on the other.

Using new satellite observations, the response of surface wind and clouds to SST changes is investigated over cold oceans, where the planetary boundary layer (PBL) is often capped by a temperature inversion. In contrast to the negative correlation often observed on the basin-scale that is indicative of one-way atmospheric forcing, clear wind response is detected with zero to positive correlation with the causal SST anomalies, depending on the dominant wind-adjustment mechanism. In particular, SST modulation of vertical momentum mixing is a ubiquitous mechanism for surface wind variability near oceanic fronts around the world. Several types of boundary-layer cloud response are found, whose correlation with SST varies from positive to negative, depending on the role of surface moisture convergence. Noting that the surface moisture convergence is strongly scale-dependent, we propose that the horizontal scale is important for setting the sign of this SST-cloud correlation.

Finally, the processes by which local shallow PBL response might lead to a deep, tropospheric-scale response and the implications for the study of extratropical air-sea interaction are discussed.

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