Eddy Subduction in a Model of the Subtropical Gyre

Abstract

Subduction is the process by which fluid transfers from the mixed layer to the interior of the ocean. South of the Gulf Stream extension, 18° mode water is formed in a region of high subduction rates. In this region there is high mesoscale eddy activity. In the present study the role of eddies in modifying the large-scale subduction into mode water is investigated. An eddy-resolving isopycnic ocean model with mixed layer physics included and coupled to an atmospheric anomaly model is used for this purpose. The geometry and forcing of the model are idealized. Annual mean subduction rates into mode water up to 200 m yr?1 are found south of the Gulf Stream extension. The eddy contribution to the annual subduction is estimated by comparing the annual mean subduction rates, obtained from monthly mean quantities, with the total subduction rates (i.e., eddy plus mean contributions). The latter are determined by integrating the detrainment rates over the period when fluid is irreversibly detrained. Eddy subduction rates up to 100 m yr?1 are found. The high-frequency variability enhances the annual mean subduction by almost a factor 2. The subduction is compared to annual net detrainment. The latter is related to the so-called shallow Ekman overturning. The eddy contribution to this overturning is determined by calculating divergence of the eddy transports in mode water as well as comparing results from the eddy-resolving version of the model with results from a coarse resolution version of the model. The results show an eddy-induced enhancement of the Ekman overturning. Conclusions are drawn with regard to parameterization of eddy subduction and eddy-induced changes of the Ekman overturning.