Sea surface temperatures in the subpolar North Atlantic over the last 230 years and their relation to the North Atlantic Oscillation and great salinity anomalies

August Sea Surface Temperatures (aSSTs) based on fossil diatom assemblages are generated from a 230-year-long sediment core (Rapid 21-12B), from the southern limb of the Gardar Drift on the eastern flank of the Reykjanes Ridge in the subpolar North Atlantic. The reconstructed record has two years average resolution and represents the highest resolution diatom SST reconstruction from the subpolar North Atlantic. The results indicate a warming trend of ~0.5 °C of the surface waters in the Reykjanes Ridge area for the last 230 years. Superimposed on this warming trend there is a multidecadal to decadal aSST variability of up to 1 °C. The interval from the 1770s to the 1830s represents the coldest period, whereas ~1860-1880 represents the warmest period during the last 230 years. The last 25 years is characterized by a warming trend showing strong decadal aSST variability with several warm years, but also the coldest years since the 1820s. The time of these cold years in the mid-1970s, -1980s and -1990s correspond with the documented great salinity anomalies (GSA) in the North Atlantic suggesting increased fluxes of cold, low-salinity waters from the Arctic during the last decades. Results suggest that the wind driven variations in volume fluxes of the North Atlantic surface waters could be the major mechanism behind the observed aSST trends, because the aSST record and the August North Atlantic Oscillation (aNAO) index show similar multidecadal-scale variability indicating a close coupling between the oceanic and atmospheric patterns. Their relationship however is negative. On the long time scale, the aSST record shows an increasing (warming) trend, whereas the aNAO index shows a decreasing trend. On the multidecadal scale, smoothed records show warm aSST periods during the negative aNAO trends and cold aSSTs during the positive aNAO trends. This suggests that the strong westerlies during the positive aNAO phase drive the main part of the Atlantic surface water towards north. During the negative aNAO phase, the westerly winds over the Atlantic are weakened and could cause a remarkable increase in the westwards flowing warm Atlantic water across the Reykjanes Ridge area. The observed relationship between aSST and the aNAO, however, does not hold for the most recent period from the 1960s onwards, where aSSTs and the aNAO show co-phased variations at a decadal scale. This change in the observed SST/NAO system may be caused by the anthropogenic forcing and/or global warming.