Atkinson, Christopher Paul
Variability of the Atlantic Meridional Overturning Circulation at 26N
University of Southampton, School of Ocean and Earth Science,
The Atlantic Meridional Overturning Circulation (AMOC) and its variability has received considerable attention, motivated by its major role in the global climate system. Observations of AMOC strength at 26?N made by Rapid-WATCH (Rapid climate change - Will the Atlantic Thermohaline Circulation Halt) provide our best current estimate of the state of the AMOC. This study aims to improve understanding of ongoing measurements of AMOC variability made by Rapid-WATCH, and provide context for the 5-year timeseries presently available. The Rapid-WATCH system combines Gulf Stream transport, zonally integrated Ekman transport and mid-ocean transport observations at 26?N to assess AMOC strength. The Gulf Stream is found to possess a smooth annual cycle set by regional meridional wind stress, which is obscured on a year to year basis by internal ocean variability. Ekman transport possesses no dominant annual or semi-annual periodicity, and the seasonal cycle is obscured on a year to year basis by internal atmospheric variability largely related to fluctuations of the Azores high and the North Atlantic Oscillation. Sverdrup transport shows a dominant semi-annual periodicity which is strongly persistent from year to year but not seen in Rapid-WATCH measurements due to barotropic adjustment at these timescales. At interannual timescales, Sverdrup transport variability estimated from wind stress climatologies is found to exceed that of Gulf Stream variability, suggesting AMOC fluctuations of several Sverdrups have occurred over the past 30 years, partly related to the North Atlantic Oscillation. At decadal timescales, six hydrographic sections spread over half a century show that high latitude deep water mass changes have consistently spread to 24?N, with the DWBC their principal conduit into the 24?N section. Decadal changes in deep water transport of several Sverdrups are balanced by dynamic height changes close to the western boundary whilst water mass changes are largely density compensated.
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