Local atmospheric forcing during the Coastal Ocean Dynamics Experiment: 1. A description of the marine boundary layer and atmospheric conditions over a northern California upwelling region

Abstract

As part of the Coastal Ocean Dynamics Experiment (CODE), meteorological instruments were deployed on buoys and at coastal stations and instrumented aircraft flights and coastal soundings were made to study the three‐dimensional structure of the marine boundary layer over the continental shelf off northern California during the 1981 and 1982 upwelling seasons. These measurements show that after the atmospheric spring transition the airflow in the marine layer is dominated by the North Pacific high, and the surface wind field over the shelf is characterized by periods of strong (7–15 m/s), upwelling‐favorable alongshelf winds lasting for up to 30 days, interrupted by shorter periods of much weaker winds directed either equatorward or poleward. These periods of weak or reversed winds typically last several days and are called wind relaxations, even though they are primarily associated with coastally trapped perturbations of the marine layer along the central and northern California coast and not with a large‐scale weakening of the North Pacific high. The atmospheric boundary layer measurements made in CODE suggest a simple conceptual model which can explain much of the physiology or structure of the marine layer and associated surface wind field during periods of persistent upwelling‐favorable winds. During these periods, which represent the quasi steady state regime during the upwelling season, the inversion base of the marine layer drops eastward toward the coast until it intersects the coastal mountain range at a height of several hundred meters, and the associated thermal wind produces an alongshelf wind jet which has a maximum speed just below the inversion base. Turbulent mixing tends to homogenize any stratification in the marine layer beneath the jet and couple the jet to the ocean surface, producing strong upwelling‐favorable winds over the shelf. Day/night heating/cooling over the narrow coastal strip beneath the marine layer generates a weak cross‐coast secondary circulation which causes the core of the alongshelf jet to drop in elevation and shift onshore. This diurnal change in the marine layer structure explains both the daytime acceleration of the surface winds observed over and near the coast and its offshore decay and the associated offshore increase in the subdiurnal alongshelf wind. Thus the quasi‐steady component of the wind stress has a significant curl over the inner shelf during periods of active upwelling. This mean summer atmospheric boundary layer regime is occasionally interrupted by synoptic and/or mesoscale events or anomalous conditions. Analysis of the CODE observations suggests five types of events, two primarily synoptic‐scale conditions which lead to stronger‐than‐normal upwelling‐favorable winds over the shelf and three primarily mesoscale events which lead to wind relaxation. About half of the wind relaxation events observed in 1981 and 1982 are believed to be associated with either coastal‐trapped gravity currents or internal Kelvin waves which propagate north‐ward in the marine layer along the central and northern California coastal mountain range.