Long-Term Coordinated Changes in the Convective Activity of the North Atlantic

Abstract

The North Atlantic is a peculiarly convective ocean. The convective renewal of intermediate and deep waters in the Labrador Sea and Greenland/Iceland Sea both contribute significantly to the production and export of North Atlantic Deep Water, thus helping to drive the global thermohaline circulation, while the formation and spreading of 18-degree water at shallow-to-intermediate depths off the US eastern seaboard is a major element in the circulation and hydrographic character of the west Atlantic. For as long as time-series of adequate precision have been available to us, it has been apparent that the intensity of convection at each of these sites, and the hydrographic character of their products have been subject to major interannual change, as shown by Aagaard (1968), Clarke et al (1990), and Meincke et al (1992) for the Greenland Sea, in the OWS BRAVO record from the Labrador Sea, (eg Lazier,1980 et seq.), and at the PANULIRUS / Hydrostation “S” site in the Northern Sargasso off Bermuda (eg Jenkins, 1982, Talley and Raymer, 1982). This paper reviews the recent history of these changes showing that the major convective centres of the Greenland- and Labrador Seas are currently at opposite convective extrema in our postwar record, with vertical exchange at the former site limited to 1000 m or so, but with Labrador Sea convection reaching deeper than previously observed, to over 2300 m. As a result, Greenland Sea Deep Water has become progressively warmer and more saline since the early ‘70’s due to increased horizontal exchange with the Arctic Ocean through Fram Strait, while the Labrador Sea Water has become progressively colder and fresher over the same period through increased vertical exchange; most recently, convection has become deep enough there to reach into the more saline NADW which underlies it, so that cooler, but now saltier and denser LSW has resulted.