The role of the alongshore wind stress in the heat budget of the North Carolina inner shelf

The heat budget of a cross-shelf section extending 16 km offshore of the ou ter banks of North Carolina is studied during two time periods: August 1994 and October 1994, using data collected as part of the Coastal Ocean Proces ses (CoOP) Inner Shelf Study. Heat budgets are computed on tyro different t imescales: monthly averages over August and October, which reflect seasonal variations, and a fluctuation budget, which reflects variation on daily to weekly timescales. During August, a period of strong stratification, the i ncrease in the area-averaged water temperature (approximately 3.2 degrees C ) was due primarily to the surface heat flux. Fluctuations in temperature d uring August were caused primarily by the cross-shelf heat flux, due to win d-driven upwelling and downwelling circulation. In October, the area-averag ed shelf temperature dropped by approximately 3.5 degrees C due to both sur face heat loss and the alongshore transport of heat. Weak vertical stratifi cation in October led to small cross-shelf heat fluxes, and temperature flu ctuations in October were due primarily to fluctuations in the surface and alongshore heat fluxes. In both August and October, variation on daily to w eekly timescales of the area-averaged temperature of the shelf tvas strongl y correlated with the alongshore component of the wind stress. In August, a longshore poleward winds caused upwelling and the area-averaged temperature decreased; conversely, equatorward winds caused downwelling and warming. I n October, although the variations in temperature were smaller, alongshore winds were positively correlated with alongshore currents and the surface h eat flux (for reasons discussed by Austin and Lentz [this issue]), so that poleward winds resulted in warming; conversely, equatorward winds resulted in cooling. Therefore the dependence of the change in heat content on the a longshore wind stress changed sign between August and October. A simple dyn amical model was constructed to relate changes in heat content to the along shore wind stress. The model results were compared to 12 years of meteorolo gical records from the Coastal Engineering Research Center's Field Research Facility, directly onshore of the experimental site. The results suggest a seasonal cycle in the dominant fluctuating heat balance, consistent with t he field results found for August and October 1994. In May through August, cross-shelf flux dominates variation in the heat content. In October throug h March, the surface heat flux and alongshore heat flux dominate the variat ion.