Surface drifter exchange between the North Atlantic subtropical and subpolar gyres

Abstract

Surface drifters deployed in the subtropical and subpolar North Atlantic from 1990 to 2002 show almost no connection between the subtropical and subpolar gyres; only one drifter crosses the intergyre boundary even though other data types (e.g., dynamic topography and tracers) suggest a major connection. Two of several possible causes for the lack of intergyre connectivity in this two‐dimensional data set are examined: (1) undersampling and short drifter lifetime leading to underestimation of the northward flow, and (2) the southward mean Ekman velocity. Advection of a large number of long‐lived synthetic drifters through the observed mean velocity results in a 5% increase in cross‐gyre flux compared with that for synthetic drifters with realistic lifetimes. By further advecting synthetic drifters through the observed mean velocity field with and without the Ekman component, estimated from the wind field associated with the actual drifters, it is shown that removal of the Ekman component further increases the intergyre flux by up to 6%. With a turbulent component added to the mean velocity field to simulate the eddy field, there is a further increase in connection by 5%. Thus the Ekman and eddy contributions to the drifter trajectories nearly cancel each other. Consideration of three‐dimensional processes (subduction and obduction) is reserved for complete modeling studies.