Mean transport of mass, heat, salt and nutrients in southern California coastal waters: implications for primary production and nutrient cycling

Abstract

A close balance is demonstrated between geostrophic convergence, relative to 500 m, and Ekman divergence, in a control volume defined by the modern (1984–1987) CalCOFI surveys of southern California coastal waters. Both the geostrophic and Ekman calculations yield an area mean vertical velocity of about 3.5 × 10 −4 cm s −1 upward at the base of the Ekman layer. Cruise-to-cruise and year-to-year variations in geostrophic transport are large and a long time series is required for a good estimate of the mean field. With the mass field balanced by geostrophic and Ekman transport, the resulting oceanic heat flux is equivalent to 68 W m −2 of heat gain by the ocean and is in good agreement with calculations of air-sea heat transfer. Agreement in the heat budget sets a limit on possible errors due to ambiguity in the reference level of the geostrophic velocity field. Mean annual and seasonal fluxes of nutrients, oxygen, chlorophyll a, and salinity are calculated, together with error bars, and are used to estimate primary production as well as rates of air-sea oxygen exchange and net water vapor loss to the atmosphere. The nitrate import, converted to equivalent carbon by means of a Redfield ratio, and including local estimates of nitrogen recycling, yields an estimate of total primary production of roughly 600 mg C m −2 d −1, consistent with direct measurements of carbon uptake. The geostrophic convergence and subsequent upwelling of subsurface nitrate-rich waters is the dominant physical mechanism for nutrient importation, with a much smaller contribution due to alongshore advection of nutrients in the surface layer. It is suggested that some additional measurements would permit the flux calculations to be made with a substantially greater degree of accuracy and that the seasonal and interannual variations of the mass transport field could be determined together with rates of biologically and physically important processes.