Abstract

An analysis is presented of the mean coastal upwelling circulation over the wide shelf off Peru based on the current and temperature profiling measurements described in Shaffer (1982). The natural coordinate conservation method (NCCM) in temperature (T) space is applied to boxes formed from stations along two lines with 60 km alongshore separation. Mean distributions in x (cross-shelf coordinate) and T are obtained for diapycnal advection and mixing. The vertical component of isopycnal flow is also calculated and found to be considerably less than vertical, diapycnal flow in this region of strong upwelling and internal mixing.Richardson numbers (Ri) are also studied as an independent way of looking at the internal mixing. Both the x-T distributions of Ri statistics and of "mean" Ri calculated as the ratio of the mean square bouyancy frequency to the mean square current shear show similar structure. Both the results for Ri and K (the coefficient of diapycnal turbulent diffusion of heat) from the NCCM calculation indicate strongest internal mixing and upwelling at the base of an inclined frontal zone.General K(Ri) relationships are discussed and a particular form is chosen and "calibrated" with the mean Ri and mean K results from a number of common bins in x-T space. The "best choice" K(Ri) is then used to calculate new, "improved" distributions of the diapycnal exchanges. Maximum values for K and upwelling at the base of the frontal layer are ∼5 cm2 s−1 and ∼7 × 10−3 cm−1. A procedure is described for calculating mean upwelling circulation from known distributions of diapycnal exchanges. As expected, the circulation based on the diapycnal exchanges from the K(Ri) model agrees best with observed current structure, calculated Ekman transport in the surface layer and heat balance requirements. The heat balance calculation indicated that shoreward eddy heat flux supplies more heat to the nearshore surface layers of northern Peru than direct surface heating. It is concluded that the K(Ri) results obtained here may have a certain universal character and one of the best ways to accurately estimate coastal upwelling circulation may be to observe the Richardson number.

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