Wind Stress Curl and Ocean Conditions in the Northeast Pacific: A Mechanism for Ocean Climate Change

Abstract

Wind stress and wind stress curl strongly shape upper ocean conditions in the North Pacific on a full range of time scales. The NCEP reanalysis surface daily winds have been processed to define climatologies and monthly values and anomalies of wind stress curl over the Pacific Basin. Prior work by Bakun and Nelson have shown that cyclonic wind stress in coastal regions coupled with alongshore equatorward flow can affect upwelling and therefore coastal temperatures, stratification, and productivity. By comparing the evolution of wind stress and curl to SST and subsurface temperature we are able to investigate the impact of ekman processes on the seasonal progression of ocean conditions. Climatologies show zonal bands of positive curl from the equator to 20N in winter and from 20N to 35N in summer. From climatologies, the time of maximum annual wind curl occurs later in the season with distance from the equator along the West Coast of North America spring and fall. Isotherms shoal seasonally in response to the annual wind stress curl cycles. Because the ocean response to atmospheric forcing is "dynamically similar" on annual and longer periods, the seasonal evolution of the wind field and the ocean's response will provide insight on the development of ocean anomalies during extreme events such as El Niño and decadal oscillations.