The role of AMOC weakening in future Euro-Atlantic atmospheric circulation
Andrea Vito
Vacca
Politecnico di Torino
Talk
Understanding how a warmer climate will affect large-scale atmospheric circulation in the Northern mid-latitudes is a major challenge due to the lack of robustness in climate model projections, with implications for regional impacts and extremes. While previous research has shown the importance of ocean forcing on extratropical atmospheric dynamics at multiple timescales, the impact of changes in ocean circulation on the inter-model spread in future projections of large-scale atmospheric patterns is unknown.
In this study, we investigate the atmospheric response to the future AMOC slowdown in the Euro-Atlantic region (EAT) employing the Weather Regimes framework. Weather Regimes are recurrent and quasi-stationary patterns of daily geopotential height associated with the eddy-driven jet configuration. Out of the main four wintertime Euro-Atlantic regimes, three are considered “blocked” regimes (Scandinavian Blocking, Atlantic Ridge, NAO-), while one is “unblocked” (NAO+) (Madonna et al., 2017; Fabiano et al., 2020).
Analysing two coordinated experiments from the CMIP6 archive, we find that models that simulate a larger AMOC decline feature a net increase in the NAO+ regime frequency and persistence compared to models that simulate a smaller AMOC decline, while Scandinavian Blocking more strongly decreases. Mechanistically, this is due to the influence of a reduced warming of the subpolar North Atlantic (SPNA) on mean geopotential height. A relative lowering of the geopotential height leads to the strengthening of the mid-latitude jet stream, which appears in its climatology to be more zonal and elongated eastward in models predicting a larger AMOC decline. We show that, consistently, the storm track strengthens due to an increased baroclinicity of the atmosphere in the region, with repercussions on future extreme events in Europe.
We conclude that the weakening of the AMOC is a crucial ingredient in the evolution of the large-scale Euro-Atlantic circulation and a key source of uncertainty in future projections of climate change impacts in Europe.
In this study, we investigate the atmospheric response to the future AMOC slowdown in the Euro-Atlantic region (EAT) employing the Weather Regimes framework. Weather Regimes are recurrent and quasi-stationary patterns of daily geopotential height associated with the eddy-driven jet configuration. Out of the main four wintertime Euro-Atlantic regimes, three are considered “blocked” regimes (Scandinavian Blocking, Atlantic Ridge, NAO-), while one is “unblocked” (NAO+) (Madonna et al., 2017; Fabiano et al., 2020).
Analysing two coordinated experiments from the CMIP6 archive, we find that models that simulate a larger AMOC decline feature a net increase in the NAO+ regime frequency and persistence compared to models that simulate a smaller AMOC decline, while Scandinavian Blocking more strongly decreases. Mechanistically, this is due to the influence of a reduced warming of the subpolar North Atlantic (SPNA) on mean geopotential height. A relative lowering of the geopotential height leads to the strengthening of the mid-latitude jet stream, which appears in its climatology to be more zonal and elongated eastward in models predicting a larger AMOC decline. We show that, consistently, the storm track strengthens due to an increased baroclinicity of the atmosphere in the region, with repercussions on future extreme events in Europe.
We conclude that the weakening of the AMOC is a crucial ingredient in the evolution of the large-scale Euro-Atlantic circulation and a key source of uncertainty in future projections of climate change impacts in Europe.
Presentation file
vacca-andrea-vito-blocking.pdf
(2.32 MB)
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