Hindcasting Wind-Driven Anomalies Using Reduced-Gravity Global Ocean Models with 1/2 deg and 1/4 deg Resolution.

Global versions of the Navy Layered Ocean Model are used to hindcast wind-driven oceanic anomalies. These versions are reduced-gravity with the lowest layer infinitely deep and at rest Grid resolutions of 0.5 12 deg and 0.25 14 deg are used. Winds at the 1000-mbar level from the European Centre for Medium-Range Weather Forecasts are used as forcing functions for the models over the 1981-1990 time frame. The models ability to reproduce wind-forced anomalies on intraseasonal to interannual time scales is studied by comparing the model solutions with various obsersational data sets. These include satellite altimetry, drifting buoys, and islandcoastal sea-level stations. The effects of varying horizontal and veflical resolution are also investigated. The models are able to hindcast many of the wind-driven anomalies. the best correlation is found in the tropical regions where the oceanic response to atmospheric forcing is the most rapid. At midlatitudes, mesoscale flow instabilities are a major source of oceanic anomalies. However, the resolution of the global models used here is not adequate for these instabilities. In addition, the reduced.gravity models lack the barotropic mode and the realistic bottom topography, both of which can play an important role in the flow instabilities. A technique that gives maps showing the fraction of variability in the hindcast that is deterministic in response to atmospheric forcing and the fraction that is not deterministic due to flow instabilities is also demonstrated.