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Upper branch of the North Atlantic meridional overturning circulation, using observed and synthetic drifters
Elena Brambilla, Lynne D. Talley
Scripps Institution of Oceanography
ebrambilla@ucsd.edu(Abstract received 04/18/2005 for session A)
ABSTRACT
A transport of 15-20 Sv from the Gulf Stream should flow to the subpolar gyre as the warm water source for the dense water formation. The surface flow at 15 m, based on Lagrangian drifters, identifies the Gulf Stream system and the North Western Corner of the North Atlantic Current, but it does not provide an accurate description of the water mass exchange between the two gyres. Most of the drifters are trapped in the subtropical gyre and only one reaches the subpolar gyre. Considering the goal of analyzing the apparent lack of connection between subtropical and subpolar gyre showed by drifter observations, we initially verify the adequacy of the dataset used. Secondly, we used synthetic drifters advected through the observed mean velocity field to investigate the influence of the Ekman velocity on the drifter trajectories. The results from this simple numerical experiment show that removing the Ekman component increases the number of the drifter trajectories that flow into the subpolar gyre, but not by enough to represent the 15-20 Sv that should flow from low to high latitudes in the North Atlantic. Since neither Ekman or sampling issues can explain the low number of Lagrangian drifters entering the subpolar gyre, other processes must be involved. They could include: depth of the core of the northward flow, shear between surface and subsurface flow, obduction or subduction events.
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2005 LAPCOD Meeting, Lerici, Italy, June 13-17, 2005