Irminger Current Anticyclones in the Labrador Sea observed in the hydrographic record, 1990-2004

Abstract

A significant fraction of the lateral heat transport into the Labrador Sea's interior, needed to balance the net heat loss to the atmosphere, is attributed to the Irminger Current Anticyclones. These mesoscale eddies advect warm, salty boundary current water, of subtropical origin, from the boundary current to the interior— but when or how they release their anomalous heat content has not been previously investigated. In this study, we discuss the seasonal and interannual evolution of these anticyclones as inferred from the analysis of hydrographic data from the Labrador Sea from 1990 to 2004. The 29 identified anticyclones fall into two categories, which we refer to as unconvected and convected. Unconvected anticyclones have properties that are close to those of the boundary current, including a fresh surface layer, and they are found near the boundaries and never observed in winter. Convected anticyclones, on the other hand, contain a mixed layer, lack a freshwater cap and are observed throughout the year. Using a one-dimensional mixing model, it is shown that the convected eddies are those Irminger Current Anticyclones that have been modified by the large winter buoyancy loss of the region. This provides evidence that such eddies can survive the strong winter buoyancy loss in the Labrador Sea and that their anomalous heat and salt content is not trivially mixed into the Sea's interior. Finally, we observe a clear trend in the eddies' properties toward warmer and saltier conditions after 1997 reflecting changes in the source waters and the reduced atmospheric forcing over the Labrador Sea.