Sea surface temperature gradients, baroclinicity, and vegetation gradients in the sea

Abstract

We describe the results of an analysis of the 1984 CalCOFI data set, in which sea surface temperature (SST) gradients were found to be well correlated to isopycnal slope (baroclinicity) at the mesoscale. A link was established between the SST gradients and gradients of integral biomass [mg chlorophyll (Chl) a m-2] and integral primary production (mg C m-2 day-1). The water column photosynthetic efficiency, Ψ, was calculated by combining the CalCOFI data base with satellite-based estimates of the surface irradiance. It was found that Ψ was rather stable, but not constant, for a wide range of isopycnal slopes, in which integrated nitrate concentrations varied by up to 500x. Variability in Ψ was most strongly dependent on calendar date, with the highest values in winter months and the lowest values in the summer, which was probably due to an effect of day length or irradiance. The relationship between baroclinicity and SST gradients was highly scale dependent; from 0 to 200 km, the data were randomly distributed, then as the length scales exceeded 400 km, the relationship between baroclinicity and the SST gradient showed a significantly negative slope, with steadily improving correlation coefficient. Such relationships for individual cruises generally showed improved correlation over the generic annual relationship, which highlights the dynamic hydrography of the region. Baroclinicity/SST gradient relationships also improved in waters of increasing depth. A technique is described for calculating net integral biomass or production gradients from SST imagery. The gradients in biomass and productivity showed increasingly strong ties to isopycnal slope at larger size scales. Although this topic has long been described by oceanographers, this paper is the first to demonstrate a quantitative coupling between baroclinicity and primary production gradients as a function of length scale. Also significant is the observation that the relationships between baroclinicity versus gradients of integral chlorophyll or integral productivity are consistent over seasons and over an area exceeding 1 million km2. We conclude that for satellite algorithms to account for >50% of the variance in primary production, the nutrient term must be considered. Information on baroclinicity is relevant to estimates of the along-isopycnal nutrient flux to the euphotic zone. Merging this with the more traditional light-based estimates of photosynthesis will improve algorithm performance since the methods utilize entirely independent sources of information.