Observations of Surface Forcing from the Subduction Experiment: A Comparison with Global Model Products and Climatological Datasets

Abstract

Reliable estimates of the exchange of heat, moisture, and momentum across the air?sea interface are essential in assessing the local ?representativeness? of the surface forcing fields depicted by global model and climatological datasets. The reliability and extended length of the in situ data collected by a large-scale array of buoys deployed during the Subduction Experiment make this dataset particularly well suited to providing such an assessment. The Subduction Experiment was designed to explore the process by which the mixed layer waters of the eastern subtropical North Atlantic are incorporated into the top of the main thermocline. To this end, an array of five buoys was maintained between 18°?33°N and 22°?34°W from June 1991 to June 1993. In situ dynamic, thermodynamic, and radiometric measurements are utilized along with a state-of-the-art bulk flux algorithm to estimate the time-dependent surface forcing at each of the buoys. The resulting air?sea fluxes are compared to similar quantities offered by the Isemer and Hasse (Bunker) and the Wright?Oberhuber Comprehensive Ocean?Atmosphere Data Set climatological datasets, and global model forecasts from the European Centre for Medium-Range Weather Forecasts and the National Centers for Environmental Prediction. Some substantial differences are exhibited between the surface forcing components garnered from the Subduction Experiment buoys and those of the climatological and model products. The mean net heat flux from the Subduction Experiment buoys exhibits a qualitatively similar spatial gradient to that of the climatological and model products across the array, but generally reflects a greater oceanic heat gain in summer and a smaller oceanic heat loss in winter. On shorter timescales, the models? inability to replicate the heat and radiometric fluxes of the buoys is reflected in large mean standard deviations of the differences between the buoy and model fluxes at 6-h intervals. Some of the observed differences are attributed to differences in bulk formulas and/or differences in the mean variables from which the bulk air?sea fluxes are derived, while others are simply an artifact of the spatial and temporal filtering inherent within the climatological and model products.