Moored wind, temperature, and current observations made during Coastal Ocean Dynamics Experiments 1 and 2 over the Northern California Continental Shelf and upper slope

Abstract

Moored observations of winds, currents, and temperature made over the northern California shelf and upper slope during the 1981 and 1982 upwelling seasons as part of the Coastal Ocean Dynamics Experiment (CODE) are described. In both field experiments (CODE 1 and CODE 2), the onset of the upwelling season is triggered by the strengthening of the North Pacific high off northern California and signalled by the presence of water colder than 9°C near the bottom over the shelf. Vertical temperature stratification over the shelf is reduced during active upwelling. Local winds and subtidal currents over the shelf are strongly polarized in the alongshelf direction, and the mean wind stress is directed equatorward. Response of alongshelf currents to wind forcing is similar at all depths over the shelf. The response of the cross‐shelf current is more complex. In the surface layer, offshelf flow is significantly and positively correlated with the upwelling‐favorable alongshelf wind stress. Beneath this layer a weak return flow exists. The thickness of the upper mixed layer varies greatly, reaching 60 m during upwelling events. Thus direct wind forcing affects a substantial fraction of the water over the shelf. Low‐frequency wind stress, current, and temperature fields are adequately described by a few energetic empirical orthogonal functions with simple spatial structure. The most energetic mode typically accounts for 75% or more of the variance of wind stress and temperature. Over the middle and inner shelf, the most energetic current mode accounts for 90% or more of the variance, is dominated by he alongshelf component, and veers to the left with depth. The low‐frequency alongshelf flow fluctuations are quasi‐barotropic, with some vertical shear but no reversal with depth; vertical shear in alongshelf current and the cross‐shelf density gradient obey the thermal wind relation. The simple structure of the subdiurnal currents allow estimation of the alongshelf volume transport between the 60 and 130 m isobaths through three cross‐shelf transects in CODE 2. Volume transport over the shelf is often nearly two‐dimensional, but occasional large differences indicate significant mass exchange between the shelf and the adjacent upper slope. Correlation of low‐frequency currents and alongshelf wind stress shows the response of alongshelf current to be maximum near the surface and near midshelf, where a 15 cm−1 alongshelf flow lags a 1 dyn cm−2 alongshelf wind stress by the order of 10 hours. Currents over the upper slope are not significantly correlated with local wind. Alongshelf transport over the shelf is highly correlated with local wind stress, explaining 80% of the variance in the low‐frequency transport during CODE 2. Volume transport is best correlated with wind stress approximately 30 km to the south. Winds in the CODE area during spring and summer are characterized by strong and persistent upwelling‐favorable winds interrupted by shorter periods of weak winds. Regression analysis indicates that in the absence of wind, a poleward barotropic current jet exists over the shelf with a maximum amplitude over the inner shelf in the CODE area. An upwelling‐favorable wind counters this jet over the shelf, so that the alongshore flow is everywhere equator‐ward during strong wind events. Since the maximum response to wind forcing is found at midshelf and near surface, the mean current over the upwelling season is strongly sheared in both the vertical and cross‐shelf directions. Mean flow over the outer shelf is surface intensified and is directed offshelf and equatorward, while the deeper flow at midshelf and through most of the water column over the inner shelf is directed poleward.