Wind stress curl and coastal upwelling in the area of Monterey Bay

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. 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. Wind stress and speed maxima frequently were found to be non-collocated as bulk parameterizations imply probably due to significant stability and non-homogeneity effects at cold SST pools. The analyses revealed that complicated processes with different time scales (wind stress field variation, ocean response and upwelling, sea surface currents, heating by solar radiation) affect the coastal sea surface temperature. It was found that the stress curl induced coastal upwelling only dominates in events with wind speed peaks greater than about 8-10 ms-1 and the background upwelling due to Ekman transport is weak, during which positive curl extended systematically over a significant area (scales larger than 20 km). These events include cases with northerly wind which resulted in an expansion fan downstream from Point Año Nuevo and cases with offshore/onshore flow. However, in general cold pools of sea surface temperature in the central area of Monterey bay seem to have been advected by ocean surface currents from persistent upwelling regions. Inside the bay aircraft vertical soundings showed the dominant effects of the lee wave sheltering of coastal 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. The sea surface temperature is generally warm at this part of the Bay because of the shallow ocean mixed layer and the solar heating of the upper ocean.