MESOSCALE VARIABILITIES AND GULF-STREAM BIFURCATION IN THE NEWFOUNDLAND BASIN OBSERVED BY THE GEOSAT ALTIMETER DATA

Three years of analysed Geosat altimeter data exhibit mesoscale meande rs and eddies in the bifurcation region of the Gulf Stream (GS) into t he North Atlantic (NAC) and Azores (AZC) currents over the Newfoundlan d ridge and Basin. The decorrelation scales of the sea-surface height variabilities are approximately 50 d in time, approximately 120 km in the along-track direction and approximately 150 km in the cross-track direction. Although significant westward propagation is observed in Fo urier analysis at speeds of 3 to approximately 15 cm s-1 near the curr ent axes, some individual mesoscale features are chaotic: they are som etimes persistent for several repeat cycles (approximately 100 d) and then suddenly propagate westward or disappear within one cycle. The GS bifurcation is enhanced by cross-track smoothing, which minimizes mes oscale features resolved by only one track but retains the larger scal e features. The surface geostrophic velocity field with the cross-trac k smoothing provides the following description of the bifurcation: the GS starts to split near the ridge, including dipole-eddy structures, and turns into two branches over the Basin while it flows eastward. Th e northern branch (the NAC) is persistent with a well-defined axis, wh ose position oscillates north and south by 300-400 km twice in the 3-y ear time series. The southern branch or AZC with a broader core appear s intermittently and has a smaller surface transport than the NAC Eddy -eddy interactions may play a role in accelerating/decelerating the NA C when it varies slowly.