The relationship between synoptic weather systems and meteorological forcing on the North Carolina inner shelf

Abstract

A strong relationship is observed between synoptic weather systems and atmospheric forcing of the ocean as estimated from buoy measurements made on the North Carolina inner shelf during August and October‐November 1994 as part of the Coastal Ocean Processes (CoOP) Inner Shelf Study. Synoptic variation (timescales of days to weeks) in the meteorological time series was primarily associated with the passage of atmospheric frontal systems. The most common synoptic weather pattern observed was the passage of a low‐pressure center to the north of the study site, which caused the associated cold front to pass over the study region. Before passage of the cold front, warm, moist northeastward winds increased the heat flux into the ocean, whereas after the cold front passed, cold, dry southwestward winds decreased the heat flux into the ocean. In addition, in the presence of oceanic stratification, northeastward winds drove coastal upwelling, bringing colder water to the surface, further increasing the air‐sea temperature contrast and hence the heat flux into the ocean inshore of the surface front between cool upwelled water and warmer water offshore. The decrease in surface heat flux during the passage of a cold front was of order 400 W m−2, due primarily to a decrease in latent heat flux. Although other synoptic patterns were observed, including one warm front passage and two tropical storm systems, the dominance of cold fronts as a source of variability resulted in a strong positive correlation between the along‐shelf component of wind stress and the surface heat flux. To address the issue of spatial variation in the surface heat fluxes, data from several different sources located along a cross‐shelf transect were analyzed. This analysis suggests that the temperature of the atmospheric boundary layer undergoes adjustment when warm air blows over cold water but not when cold air blows over warm water. This produces cross‐shelf gradients in the bulk estimates of turbulent heat fluxes during offshore winds but not during onshore winds.