Interannual variability of the Atlantic water layer in the West Spitsbergen Current at 76.5∘N76.5∘N in summer 1991–2003

The Atlantic water (AW) layer in the West Spitsbergen Current (WSC) is an important source of heat and salt for the Arctic Ocean and the areas of deep convection in the Greenland Sea. In the southern Fram Strait, the WSC has two main branches, an eastern core on the continental slope and a western one in the Knipovich Ridge area. Here we analyze interannual variability of the heat content and geostrophic flow (level of no motion at 1000 m) in the AW layer (temperature above 2∘C and salinity above 34.9) in both branches of the WSC using summer hydrographic data along a zonal section from 6∘6∘ to 15∘E15∘E at 76.5∘N76.5∘N occupied by R/V Oceania   from 1996 to 2003. In the eastern branch, our time series go back to 1991. In particular, we make inferences of correlation with the winter North Atlantic Oscillation (NAO) index. The AW temperature series in the eastern core show two warm periods, one at the beginning of the 1990s and one at the beginning of the 2000s, separated by a cold event in 1997–1998. There is a competition between a lagged and non-lagged response to the NAO. While the cold event lags the spectacular drop of the NAO index in 1996, the temperature extremes in the warm periods are concomitant with the extremes of the NAO index in these periods. However, if we consider the average thickness of the AW layer on the whole section in the 1996–2003 period, we obtain a high correlation (r=0.94r=0.94) in which the NAO index leads by 1 yr. In contrast, the geostrophic flow shows a close relation to the NAO only in limited areas. The most striking result is a nearly perfect correlation for the strength of the western branch of the WSC, which evolves without a lag and in opposite phase to the NAO index (r=-0.96r=-0.96). Possible causes of links between the evolution of the AW layer and the NAO are discussed based on information available from the literature. It is suggested that both the eastern and western branches undergo interannual variations which are coherent from the northern North Atlantic to Fram Strait. A conclusion is drawn that the summer AW thickness in the eastern branch may be a proxy for the annual mean volume transport in the WSC.