Ocean general circulation model sensitivity to forcing from scatterometer winds

Authors

  • Ralph F. Milliff,

  • William G. Large,

  • Jan Morzel,

  • Gokhan Danabasoglu,

  • Toshio M. Chin


Abstract

Enhanced global surface wind fields are constructed from a blend of NASA scatterometer (NSCAT) and ERS 2 scatterometer data and National Centers for Environmental Prediction (NCEP) analyses, at 6-hour intervals, for a repeatable annual cycle from August 1996 through July 1997. Wind field properties (wind speed, zonal and meridional wind stresses, wind stress curl, and kinetic energy input) for the enhanced winds are compared with the NCEP analyses for the same time period. Large-scale, zonal patterns dominate annual average difference maps for wind speed and both components of wind stress. Wind stress curl differences are largest in the subpolar North Atlantic and in the midlatitudes of the southern hemisphere. The importance of wind field differences are measured by their impacts on the response of an ocean general circulation model (OGCM). Twin experiments are performed using the National Center for Atmospheric Research Climate System Model ocean component in stand-alone mode. The annual mean OGCM responses to the enhanced winds and to surface winds from the NCEP analyses are compared for barotropic stream function, surface velocities, upper ocean upwelling, sea surface temperature (SST), surface heat flux, meridional overturning stream function, and total northward heat transport. Differences in the annual mean responses are attributable to differences in the mean forcing and not to the mesoscale signal that is present in the enhanced winds but not resolved by the OGCM. Differences in SST and surface heat flux are partitioned between local thermodynamic balances and balances involving ocean dynamics as well. Northward heat transport differences of order −0.2 PW in the southern hemisphere midlatitudes are consistent with weaker eastward and southward stresses in the westerlies and a 27% reduction in kinetic energy input from enhanced winds. This difference in northward heat transport is compensated in the southern hemisphere tropical Pacific where surface stresses and the OGCM surface response to enhanced winds are more westward and more divergent, resulting in 27% greater kinetic energy input. An extensive appendix details the realistic high-wavenumber character of the enhanced winds.

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