A heat budget for the Northern California Shelf during CODE 2

Abstract

Moored current, water temperature, and meteorological observations made during the summer of 1982 as part of the Coastal Ocean Dynamics Experiment (CODE 2) are used to study both the mean and fluctuating heat budget for the northern California shelf. The volume considered extends from the sea surface to the bottom, from the coast to near the shelf break (13 to 23 km offshore), and 56 km alongshore. The largest terms in the mean heat budget over a 105‐day period spanning the summer season are a surface heat flux of 1.6 × 1011 W into the ocean and an offshore heat flux of 2.4 × 1011 W due primarily to an offshore flow of relatively warm water in the upper 30 m. Cooling due to an alongshore heat flux divergence is 0.6 × 1011 W. Fluctuations with time scales from days to weeks are large relative to the means. The correlation between observed changes in the heat content of the volume and the changes predicted by estimating the heat fluxes into and out of the volume is 0.82. The primary balance in the fluctuating heat budget is heating and cooling of the volume in response to on/offshore fluxes of heat in the upper 30 m. A simple two‐dimensional Ekman model reproduces the major features of the observed offshore heat flux fluctuations, correlation 0.82, indicating that this offshore heat flux is consistent with wind‐driven upwelling. However, at alongshore scales less than 56 km there is considerable alongshore variability in the cross‐shelf heat flux, particularly over the outer shelf. This spatial variability is presumably due to eddy motions. Standard deviations of the alongshore heat flux are about 50% of the cross‐shelf heat flux. Two modes of alongshore heat flux variability are observed. The dominant mode is characterized by large equatorward heat fluxes over the outer shelf during the early part of the upwelling season. This mode is associated with strong vertical shears in alongshore velocity in the vicinity of the upwelling front which results in equatorward flow of the warm surface water relative to the colder deep water. The second mode of alongshore heat flux variability is concentrated over the inner half of the shelf and is related to the wind such that poleward heat fluxes occur in the northern portion of the volume during wind relaxations (periods of weak winds). The presence of an anomalous lens of warm water over the shelf during CODE 2 is also examined.