Modelling changes in North Atlantic circulation under the NAO‐minimum wind forcing of 1877–81

Abstract

Proxy evidence suggests a historical episode of extreme cold in the shelf waters off southeast Canada and the northeastern United States between April and August of 1882. This event is hypothesized to be the consequence of enhanced equatorward transport of cold water in the Labrador Current (LC), coincident with strongly negative North Atlantic Oscillation (NAO) indices in preceding winters (notably 1878/79 and 1880/81), and driven by associated anomalies in large‐scale wind forcing. In a model sensitivity experiment, the reconstructed wind forcing of 1877–82 is applied to a General Circulation Model (GCM) of the North Atlantic. Under this wind forcing, the model subpolar gyre strengthens and expands to the south. The offshore, wind‐driven component of the LC accordingly strengthens and extends around the Tail of the Grand Banks. During periods of relatively strong transport around the Tail, sub‐surface temperatures fall by up to 6°C in a region inshore of the Gulf Stream and to the south and west of the Grand Banks. At a depth of 200 m, in waters off the Scotian Shelf, temperature anomalies of –2°C extend westward in “spring 1882” of the sensitivity experiment. Through the enhanced equatorward export of Labrador Sea Water (LSW) in the offshore LC, near‐surface densities are correspondingly higher, increasing the cross‐shelf density gradient which is thought to drive baroclinic transport of the shelf‐break LC (unresolved in the GCM). By transporting more cold LSW into the Grand Banks region, and possibly intensifying the shelf‐break LC, the large‐scale, wind‐driven ocean circulation may play a crucial role in regional cold episodes such as that of 1882.