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Abstract:
Sea surface temperature increase resulting from greenhouse gas emissions is not fully understood. In high latitudes, extreme warming has been responsible for melting glaciers and sea ice, augmenting freshwater inputs in the sub-polar North Atlantic. In particular, this region has shown persistent cooling, contrasting sharply with overall global warming trends. In many studies, this sea surface cooling has been associated with a weakening of the Atlantic Meridional Overturning Circulation (AMOC) partially induced by the increasing freshwater input from Greenland resulting from global warming. To investigate this relationship, we employed non-linear techniques which reveal a long-term inverse relationship between increasing freshwater input and cooling, whereby freshwater transport reduces sea surface salinity, increases surface water stability, and impedes vertical mixing. Therefore, surface waters require further cooling to reach the density where vertical mixing can occur, producing, in the long-term, negative anomalies of both surface temperature and salinity, which is reflected in changes in the timing of the seasonal cycles of sea surface temperature in some regions of the sub-polar North Atlantic. This much more straightforward and novel explanation is entirely independent of AMOC ́s long-term variability and is consistent with historical and current observations. Therefore, on the basis of our analysis, we can explain the emergence and maintenance of the observed long-term cooling and the freshening in the sub-polar North Atlantic, regardless of the AMOC's state.
