Mechanisms for vertical nutrient transport within a North Atlantic mesoscale eddy

Prompted by observational evidence for an enhanced source of surface nutrients within an anticyclonic eddy in the NE Atlantic, we investigate vertical transport processes that may produce such a phenomenon. For the eddy investigated, the dominant mechanism is found to be ageostrophic circulation resulting from a perturbation of the circular flow of the eddy. This can produce upwelling velocities of order 10 m d(-1) Ekman pumping due to wind stress on the eddy also produces upwelling within, but on a smaller scale of similar to 0.5 m d(-1). There is no evidence that self-propagation of the eddy leads to an enhanced nutrient flux by displacement upwards of nutrient-laden deeper isopycnals over the core of the eddy. Deeper winter mixing within the eddy relative to the surroundings, however, may contribute to the elevated nutrient levels. (C) 2001 Elsevier Science Ltd. All rights reserved.