MID-DEPTH VENTILATION IN THE WESTERN BOUNDARY CURRENT SYSTEM OF THE SUBPOLAR GYRE

Two processes are investigated that result in the rapid (order of mont hs) export of newly-ventilated water from the sub-polar north Atlantic . Both mechanisms involve mid-depth water mass formation within the we stern boundary current system, which leads to such rapid spreading. Th e first mechanism, which apparently occurs every winter, forms upper L abrador Sea water (LSW), which is a source of the high CFC layer of th e upper deep western boundary current (DWBC). A mixed-layer model show s that this water mass can be formed by convection in the main branch of the Labrador Current. Strong heat loss near the boundary together w ith the existing potential vorticity structure of the current enables overturning to 1000 m. A regional numerical model of the circulation n ear Flemish Cap reveals how eddies of upper LSW are then shed by the b aroclinically unstable Labrador Current. The eddies become detached fr om the boundary at the entrance to Flemish Cap and are entrained in to the offshore (barotropic) branch of the Labrador Current, which bring s them seaward of Flemish Cap (where they have been previously observe d). The second mechanism presented occurs only under extreme winter fo rcing, such as that experienced in the Labrador Sea in recent years. T he enhanced heat loss forms classical LSW south of the cyclonic gyre, where the DWBC and North Atlantic Current can then quickly transport t he water away from the Labrador Sea. It is shown that newly-ventilated lenses of classical LSW observed in the DWBC likely originate from th is southern region, consistent with their sudden appearance downstream in the early 1990s. (C) 1997 Elsevier Science Ltd.