Contrasting responses of the extended Gulf Stream to intense air-sea exchange

Changes in the path and strength of the extended Gulf Stream, downstream of Cape Hatteras, and the North Atlantic Current (GSNAC), are associated with strong wintertime air-sea interactions. The GSNAC response to anomalous air-sea heat fluxes in particular is dependent on the location of excess heat loss, in turn related to meteorological circumstances. Outbreaks of cold continental air may lead to excess cooling over the Sargasso Sea, as in 1976-77. Under these circumstances, the Gulf Stream may intensify through a steepening of cross-stream density gradients. An alternative scenario prevailed during the cold outbreak of 2013-14 where excess cooling occurred over the central subpolar gyre. An eddy-resolving model hindcast spanning 1970-2013, is used to characterise GSNAC transport variability, allowing an extensive assessment of the relationship between the winter surface heat flux (SHF), the vertical temperature structure, horizontal temperature gradients and the GSNAC geostrophic transport. Significant relationships have been revealed between these variables with winters of anomalously high heat loss tending to lead to deeper end-of-winter mixed layers, stronger horizontal temperature gradients and, consequently, greater GSNAC transports. A Lagrangian analysis has been conducted to assess the effect of winter SHF on the downstream destination of GS water. Preliminary results suggest that in years where an anomalously high heat loss acts to intensify the transport of the GS, more GS water travels northward in the NAC. In contrast to this, in years where an anomalously low heat loss acts to reduce the GS transport, more water travels southward in the southern recirculation gyre.