A COMPARISON OF OCEANIC CIRCULATION IN THE NORTH PACIFIC FORCED BY SCATEROMETER AND REANALYSIS WINDS

Xiaojun Yuan & Enrique Curchitser

Lamont-Doherty Earth Observatory of Columbia University 61 Rt. 9W, Palisades, NY 10964

Space-based scatterometer instruments provide crucial surface wind measurements with high spatial and temporal resolutions over global oceans, which open many new opportunities in climate and oceanography studies. The objective of this study is to evaluate the impact of QuikSCAT winds on the North Pacific ocean circulation as simulated by the Regional Ocean Modeling System (ROMS). Here we contrast two simulations of the forced by QuikSCAT and NCEP/NCAR reanalysis winds, respectively, for the period from 2000 through 2001. ROMS is a free-surface hydrostatic, primitive equation ocean model that uses stretched, terrain-following coordinates in the vertical and orthogonal curvilinear coordinates (c-grid) in the horizontal. ROMS uses state-of-the-art boundary layer schemes for air/sea/benthic boundary layers based upon Fairall et al. (1996), Large et al. (1994) and Styles and Glenn (2000). ROMS employs a high-order, quasi-monotone advection scheme and accurate pressure gradient algorithms implying low implicit dissipation in the model.

For this comparison the ROMS is forced with NCEP daily fluxes and either NCEP daily, or QuikSCAT twice a day, winds. Wind stresses are calculated through a bulk flux formulation (Fairall et al.). The model domain extends from 30S to 65N, and from 100E to 70W at a resolution of 0.4 degrees. To evaluate the effects of the different wind forcing we examine the following features in the Pacific Ocean: 1. Overall basin energetics, 2. Equatorial upwelling, 3. Strength of the Alaska Gyre. A recent study showed that clear differences exist between QuikSCAT and NCEP/NCAR reanalysis wind speeds (Yuan, 2004). The differences in the strength of wind forcing produce clear discrepancies in these two model simulations.

The preliminary results show that the QuikSCAT forced simulation is overall more energetic and produces significantly different results in regions where oceanic upwelling exists. Although the model is not eddy-resolving at the current resolution, it does permit some variability to develop which is sensitive to the strength of wind forcing. In particular, the QuikSCAT run produces temperature anomalies of nearly 4 degrees Celsius in the equatorial region relative to the NCEP run (figure 1). The difference is also significant along the west coast of North America, where the QuikSCAT forced simulation better represents the basic current and density structure of the coastal areas. Comparisons with available in-situ data are used to show the relative skill of both simulations.

Figure 1 Sea surface temperature (°C) differences between ROMS simulations in September 2, 2000 forced by QuikSCAT and NCEP/NCAR reanalysis winds, respectively.

REFERENCES

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