Mid-depth ventilation in the western boundary current system of the sub-polar gyre

Abstract

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