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 degrees m
ode water is formed in a region of high subduction rates. In this region th
ere is high mesoscale eddy activity. In the present study the role of eddie
s in modifying the large-scale subduction into mode water is investigated.
An eddy-resolving isopycnic ocean model with mixed layer physics included a
nd coupled to an atmospheric anomaly model is used for this purpose. The ge
ometry and forcing of the model are idealized.
Annual mean subduction rates into mode water up to 200 m yr(-1) are found s
outh of the Gulf Stream extension. The eddy contribution to the annual subd
uction 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 detr
ainment rates over the period when fluid is irreversibly detrained. Eddy su
bduction 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, Th
e results show an eddy-induced enhancement of the Ekman overturning. Conclu
sions are drawn with regard to parameterization of eddy subduction and eddy
-induced changes of the Ekman overturning.