Recorded presentation11A.1
Ocean-atmosphere Coupling over Mid-latitude Ocean Fronts
W. Timothy Liu, JPL, Pasadena, CA; and X. Xie and P. P. Niiler
The Agulhas and Kuroshio Extensions have strong horizontal current shears and temperature gradients in their meanders. The spaceborne scatterometer, QuikSCAT, observes that, over these meanders, the variability of surface stress (momentum flux) is very different from those of surface wind. The geophysical product of QuikSCAT, the equivalent neutral wind, rotates in opposition to the surface current, which is a clear characteristic of turbulent stress generated by vertical shear; stress is the vector difference between wind and current. The ubiquitous spatial coherence between sea surface temperature and the equivalent neutral wind, found under a variety of atmospheric conditions, including the two ocean fronts, is a characteristics of turbulent stress generated by buoyancy. It is only at the small scales of turbulence that factors, such as pressure gradient force, Coriolis force, and large-scale advection, are not important, and the coherence is independent of large-scale atmospheric circulation. There was no continuous and large-scale stress measurement over ocean until the launch of the scatterometers and our knowledge of stress variability is largely based on wind. Scatterometer measurement has been used as the actual wind, particularly in operational weather applications. The difference between the variability of stress and wind is assumed to be negligible on the justification that the marine atmosphere has near-neural stratification, and the magnitude of ocean current is negligible compared with wind. Such assumption has to be vigorously examined over the ocean fronts, as they should be examined under the high wind conditions when flow separation occurs.
Most studies using atmospheric general circulation models fail to generate systematic response to prescribe midlatitude sea surface temperature anomalies. The lapse rate is believed to be too weak to support deep convection and the ocean effect is confined to the atmospheric boundary layer. We found strong spatial coherence among sea surface temperature and surface stress convergence at the surface, and cloud top temperature and cloud optical thickness provided by the International Satellite Cloud Climatology Project. Atmospheric Infrared Sounder temperature profiles show that the high and low temperatures penetrate from the ocean all the way up to 300 mb. The surface and cloud top temperatures are also spatially coherent with rainfall from the Tropical Rain Measuring Mission over the southern branch of the Kuroshio Extension, and TRMM radar confirms that rainfall anomalies up to 5 km, way above the top of the atmospheric boundary layer. The AIRS and TRMM data contradict the prevailing notion among atmospheric modeling community and present a challenge to model dynamic and the spatial versus temporal scale parameterization.
Session 11A, Air-Sea Interaction Over Eddies and Fronts (contributions from the Western Boundary Current Working Group)
Wednesday, 14 January 2009, 4:00 PM-5:30 PM, Room 128A
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