Climate Coffee with Valentin Portmann

Please join us for this Climate Coffee with Valentin Portmann on

Comparison of different observational constraint methods applied to the AMOC projections

For a given greenhouse gas emission scenario, climate models are showing very significant differences for the future climate, due to processes complex to simulate. Among them, the Atlantic Meridional Overturning Circulation (AMOC) fate has been shown to be very uncertain, explaining a large amount of the differences in climate projections in the North Atlantic region. Indeed, under the ssp2-4.5 scenario, CMIP6 models show an AMOC maximum at 26°N that goes from 17.8 +- 4.0 Sv (1 Sv=106 m3/s, ensemble mean of 32 models +- one standard deviation) during the period 1850-1900, to 11.9 +- 3,9 Sv in the last decade of 2100, with AMOC ranging from 4.3 to 21.3 Sv depending on the model. There is thus a clear need to improve estimates of the AMOC in the future. In this respect, methods called emergent or observational constraint (OC) have been recently developed. They use observed predictors to constrain the distribution of an ensemble of model projections. Both the predictors and OC method choices can be key. What are the best choices to reduce most of the distribution spread of the AMOC at the end of the 21st century? To answer such a question, this study compares two possible cases: it uses either only one predictor, the past AMOC, or the following set of predictors: the past AMOC and the sea surface temperature and salinity from various regions in the world, which are known to impact on-going and future fate of the AMOC. Moreover, this study compares five different state-of-the-art OC methods. The performances are evaluated given the spread reduction of the future projection’s distribution, and using cross-validation. The linear regression OC method shows the lowest cross-validation error, using a Ridge regularization that limits overlearning. This method estimates future AMOC under ssp2-4.5 scenario, constrained by the observed AMOC over the period 2005-2021, at 11,5 +- 2,3 Sv. When constrained by the larger set of predictors, it is 8.9 +- 1.0 Sv. This strong AMOC reduction might have therefore considerable impacts on future adaptation plans within the North Atlantic regions.

About Valentin

Valentin studied signal processing in a French engineering school. Valentin uses these statistical skills to climate science in his thesis, about understanding and reducing the uncertainty in CMIP6 on the Atlantic Meridional Overturning Circulation. He is currently in his second year of his PhD in Bordeaux, France.

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