The severe winters of 1976–77 and 2013–14: Contrasting impacts on the Gulf Stream/North Atlantic Current

Changes in the path and strength of the Gulf Stream and the North Atlantic Current (GSNAC) are associated with strong wintertime air-sea interactions that can further influence the atmospheric storm track. The winter of 2013-14 broke records in both North America and the UK, through extremes of cold and storminess respectively. The last time the northeast of North America experienced such a winter was in 1976-77. In the following spring, Worthington (1977) observed near doubling of GSNAC transport at 72oW. He attributed this intensification to an “anti-cyclogenesis” mechanism, as follows: (1) an outbreak of cold polar continental air moved southeast over the warm Gulf Stream; (2) strong surface heat loss led to a deep mixed layer and thermocline across the Sargasso Sea; (3) strong horizontal temperature gradients on the offshore side of the Gulf Stream supported larger geostrophic transports. An optimally interpolated satellite SST dataset (AVHRR) at 0.25o resolution spanning 1982-2014 is used in conjunction with an objectively analysed temperature and salinity product (EN4) at 1o resolution spanning 1970-2014, to investigate the variability of the GSNAC. The maximum SST gradient is calculated using AVHRR and is used to define the GSNAC path. Temperature and salinity profiles, using EN4, are used to obtain the geostrophic transport along multiple transects across the GSNAC. Reanalysis products are used to investigate the air-sea heat flux over the GSNAC region alongside an analysis of the mixed layer depth, using EN4, to test Worthington's hypothesis. Case studies for springs of 1977 and 2014 are undertaken. Associated with spring 2014 is an anomalous SST field indicating that the GSNAC was shifted to the north. Geostrophic current and temperature anomalies in spring 2014 were similar to those of 1977 at 30oN and 70oW but dissimilar further downstream at 50oW and 30oW. In addition to observations, the use of model output from NEMO 1/4o and 1/12o will be used to investigate the role of air-sea fluxes in GSNAC transport. A modified version of Worthington's mechanism is proposed.