THE EASTERN BOUNDARY OF THE GULF-STREAM RECIRCULATION

A meridionally aligned thermocline front near 60W in the subtropical N orth Atlantic is revealed by the 1992 Trident data set. The front sepa rates saltier thermocline water to the east from less salty water to t he west. The eastern water is subjected to excess evaporation of the s ubtropics, while the western water is fed by lower salinity Gulf Strea m water, which derives water from the wet tropical Atlantic. It is sug gested that the front marks the eastern edge of the Gulf Stream recirc ulation cell, hence refer to it as the recirculation front. ?The surfa ce layer displays a fan-like TIS scatter above the 18 degrees C Subtro pical Mode Water, with the fresher surface water located west of the r ecirculation front, and a subsurface salinity maximum to the east. In the lower thermocline (8 to 12 degrees C) there is a step-like salinit y increase of about 0.04 toward the east as measured along isotherms, producing two modes in the TIS scatter. At the intermediate water leve l (approximately in the 4 to 8 degrees C range) the extent of the low salinity Antarctic Intermediate Water and salty Mediterranean outflow water are also reflected in the position of the recirculation front. T hat the front marks the easternmost extent of the Gulf Stream recircul ation is supported by the potential vorticity, which reveals a region of high homogeneous values within the recirculation cell. East of the front, the potential vorticity field is sloped along isopycnals indica ting the meridional flow of the Sverdrup interior. Mapping of the reci rculation front using archived data reveals that it extends deep into the subtropical convergence zone (STCZ), a region whose fronts have al l been previously attributed to Ekman convergence.