Interannual Variability of the Altimetry-Derived Eddy Field and Surface Circulation in the Extratropical North Atlantic Ocean in 1993-2001

摘要

The distribution of surface eddy kinetic energy (EKE) depicts main oceanic surface circulation features. The interannual variability of EKE and associated geostrophic velocity anomalies in the North Atlantic Ocean were analyzed to describe the variations in oceanic currents between 1993 and 2002. The sea level anomaly maps of the combined TOPEX/Poseidon + ERS-1/2 and TOPEX/Poseidon-alone satellite altim-etry data were used to derive EKE. The study focused on the areas of the Gulf Stream extension (GS), North Atlantic Current (NAC), Azores Current (AC), and the northeastern (Rockall Channel and Iceland Basin) and northwestern (Irminger Basin and Labrador Sea) North Atlantic. The interannual variability of the altimetry-derived EKE field in the GS extension area reflected the meridional displacements of the GS core described in earlier studies. The interannual change of EKE in the AC was characterized by high values in 1993-95 followed by lower EKE in subsequent years. The interannual variability of EKE in the NAC area west of the Mid-Atlantic Ridge exhibited an out-of-phase change between the band centered at ～47°N and two bands on either side centered at ～43° and ～50°N. In the Rockall Channel the geostrophic velocity anomalies indicated an intensified northeastward flow in 1993-95 followed by a relaxation in 1996-2000. The EKE band associated with the NAC branch in the Iceland Basin was found to be extended farther west after 1996, possibly following the North Atlantic Oscillation (NAO)-induced shift of the subpolar front. A rise of EKE was observed in the Irminger Basin from 1995 to 1999. This rise may have been associated with large anticyclonic geostrophic velocity anomalies, which indicated significant weakening of the cyclonic circulation in the Irminger Basin after 1996, and/or with possibly intensified eddy generation mechanisms due to the NAO-induced approach of the subpolar front. The interannual change of EKE in the Labrador Sea did not appear to always follow the atmospheric forcing expressed by NAO. Therefore other eddy generation mechanisms in the Labrador Sea can be important.