Wind stress and stress curl from aircraft measurements and their connection to local coastal upwelling

Aircraft measurements obtained during the Autonomous Ocean Sampling Network 2003-2004 (AOSN-II) project were used to study the effect of small scale variation of near surface wind stress on coastal upwelling in the area of Monterey Bay. Using 5 km long segments of measurement at 35 m above the sea surface, wind stress and its curl were calculated with an estimated accuracy of 0.02-0.03 Nm^-2 and 0.1-0.2 Nm^-2 per 100 km, respectively, which is sufficient for the purpose of this study. The spatial distribution of wind speed, wind stress, stress curl, and sea surface temperature were analyzed for four general wind conditions: northerly or southerly wind along the coastline, onshore flow, and offshore flow. In general, positive wind stress curl does not seem to be connected with cold pools of sea surface temperature which seem to have been advected by ocean surface currents from persistent upwelling regions. However, there are events with significant localized acceleration of wind speed during which wind stress curl pattern is highly coherent and enhances local upwelling. Inside the bay aircraft vertical soundings showed the dominant effects of the lee wave sheltering of costal mountains that resulted in weak atmospheric turbulence and affected the development of the atmospheric boundary layer. This effect causes low wind stress that limits upwelling especially at the north part of the Monterey Bay.