NSFGEO-NERC:Assessing the influence of sub-annual variability in the Atlantic Meridional Overturning Circulation on the Gulf Stream and the atmosphere

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We hypothesize that the Atlantic Meridional Overturning Circulation (AMOC) is a control on weather and climate in the Northern Hemisphere. The reasoning leading from the cause (AMOC) to effect (weather/climate control) consists of three components which, in turn, are the foci of this proposal. (1) Extreme atmospheric events control mean atmospheric fields through anomalously large, but rare, contributions to the storm track. (2) These extreme atmospheric events are governed by heat exchange with the Gulf Stream (GS), and are sensitive to whether the GS is in a meandering or non-meandering state. (3) The (non)-meandering state of the GS responds to AMOC variability. We propose here to support or refute this hypothesis by means of a combined numerical-observational study of the ocean-atmosphere system over the North Atlantic Gulf Stream.The role of the Gulf Stream (GS) in modulating the weather and climate of the Northern Hemisphere has received considerable recent support. Interest in part lies in the relative persistence of GS variability compared to the atmosphere, which offers the potential for increased predictability of catastrophic weather events. Observations show significant variability in GS meandering on monthly timescales. Recent observational experiments have also confirmed a high degree of variability in the AMOC on these monthly timescales. This provides a potential connection between the AMOC and mid-latitude weather via the GS. Modelling studies, however, have yet to agree on the relationship between the AMOC and the GS. There are many causes for these discrepancies, but an overarching explanation appears to involve (the lack of) eddy-scale resolution (< 1/10o). We here at Florida State University, under prior NSF funding, have generated an ensemble of eddy resolving North Atlantic simulations that, due to their construction, appear to be ideally designed to address the above issues. In fact, currently, this is the only eddy-resolving ensemble suite of North Atlantic model simulations in the United States. We propose to leverage these runs to systematically evaluate the relationship between the AMOC and GS variability with the influence of ocean eddies explicitly resolved. Our focus will primarily be on those Gulf Stream attributes that have been shown to significantly influence the atmosphere locally and globally. Furthermore, additional modelling work will assess how changes in the AMOC, through its influence on GS variability, imprint on the atmosphere. This will be achieved by prescribing GS conditions in high-resolution atmosphere-only model simulations to systematically assess the dependence of atmospheric variability and surface heat fluxes on the underlying ocean. Lastly, coupled high-resolution runs will be conducted to elucidate the contribution of atmospheric feedbacks on the ocean.