Intensification of the Gulf Stream after the winter of 1976-77

Abstract

THE eastward volume transport of the Gulf Stream south of New England varies within a wide range. Within a triangle whose apex is at Bermuda and whose base extends from Cape Hatteras to Cape Sable, Nova Scotia, there have been 32 reliable oceanographic sections made across the Stream between 1932 and 1968. Geostrophic volume transports for these sections, computed relative to 2,000 m, are shown in Fig. 1. This 2,000 m surface is necessary if any seasonal change in transport is to be detected; only 10 of these sections reach to the ocean bottom (∼4,500 m), and these were all made between April and August. These results tend to confirm an earlier suggestion¹ that the Gulf Stream is strongest in late winter and weakest in late autumn; this was attributed to the strengthening of the zonal winds in the winter. More recently, I have postulated a different mechanism for this annual variation in transport. The two most intense ocean currents, the Gulf Stream and the Kuroshio, are found on the western side of northern hemisphere oceans. These oceanic regions are visited throughout the winter by frequent outbreaks of frigid polar continental air. As a result of these outbreaks, very deep mixed layers are formed immediately south of these currents in late winter. The greatest thermocline depths are always found directly beneath the most deeply mixed layers, and it is primarily the variations in thermocline depth which account for the variations in computed transport shown in Fig. 1; the deeper the thermocline south of the Stream, the larger the transport. I suggest that the Gulf Stream receives a new charge of energy at the end of each winter due to this deepening of the thermocline. Here I discuss how the cold winter of 1976–77 influenced the Gulf Stream transport.