Resilience of the North Atlantic Circulation on Decadal Timescales

The circulation of the North Atlantic Ocean plays a vital role in the Earth's climate system. Numerous studies, mainly through computer simulations, have examined the stability of the Atlantic Meridional Overturning Circulation (AMOC) in the context of a warming climate. Some of these studies predict a potential collapse of the AMOC in the foreseeable future, which would require a significant influx of freshwater into the subpolar North Atlantic (NA) and/or Nordic Seas. Paleoreconstructions of the NA circulation indicates a major shift in the position of the subpolar cold front either precedes or coincides with substantial changes in AMOC dynamics. These changes imply a significant alteration in circulation patterns, beginning with noticeable restructuring of the subtropical and subpolar gyres. This would lead to modifications in the Gulf Stream system and the North Atlantic Current (NAC), affecting the thermohaline fields as well as the position and strength of these two current systems. Although some models predict a significant slowdown or even collapse of the AMOC, recent observational studies offer a more cautious perspective. For instance, the Gulf Stream system exhibits high resilience to perturbations from ongoing sea-surface warming. In this study, we analyze the decadal variability of temperature and salinity from in situ observations, along with upper-ocean currents in the subpolar NA (SPNA). We find that the thermohaline pattern of the upper ocean layers in the SPNA and Nordic Seas has remained resilient for over 70 years. The deceleration of the AMOC is evident but relatively modest, with average velocities in the upper layers decreasing by less than 10-15% over 30 years. This deceleration is also not consistent throughout the NAC region. Furthermore, the subpolar front migration over 70 years is a maximum of 3° of latitude, with the spatial variability of the yearly 10°C isotherms substantially less than that. Overall, the conclusion about the resilience of the NAC aligns well with that of the Gulf Stream, with no substantial changes in the position or intensity of the subpolar gyre. We conclude that while the AMOC is susceptible to some deceleration due to ongoing surface warming and/or freshening at high latitudes, it may also be sufficiently resilient to withstand these changes. Although it cannot be entirely ruled out that the AMOC may reach its tipping point within this century, an analysis of data on decadal variability in the upper arm of the AMOC suggests that such a collapse is unlikely.