vanWestern and two co-authors reported in early 2024 that one of the most prominent climate tipping elements is the Atlantic meridional overturning circulation (AMOC) (for the numerous positions on the status of the AMOC), which nonetheless might just collapse because of the input of fresh water in the North Atlantic. Although AMOC collapses have been induced in complex global climate models by strong freshwater forcing, the processes of an AMOC tip-ping event have so far not been investigated.
As a brief summary discussed in this article, here are the explanations for the above figure:
(A) Spatially averaged SST over the subpolar region (blue outlined region in inset) and AMOC strength (similar to A) before the AMOC collapse. The SST time series is averaged over the months November to May, where first the global mean SST [November to May] and then the time mean over the first 50 years are subtracted.
(B) Variance and (lag-1) autocorrelation of the SST subpolar region for three 300-year periods. For each 300-year period, the linear trend is removed [red lines in (A)] before determining the variance and autocorrelation. The variance and autocorrelation are determined over a 70-year sliding window, and the significance of their trends (dashed lines) is indicated for each period [see references for more details]. (C and D) Similar to (B) but now for the (C) AMOC strength (A) and (D).
The above illustrates the results for the first tipping event in the Community Earth System Model, (for the numerous positions on the status of CESM), including the articles on the possible large climate impacts of the collapse. Using these results, the authors developed a physics-based and observable early warning signal of AMOC tipping: the minimum of the AMOC-induced freshwater transport at the southern boundary of the Atlantic.
Re-analysis products indicate that the present-day AMOC is on route to tipping. The early warning signal is a useful alternative to classical statistical ones, which, when applied to our simulated tipping event, turn out to be sensitive to the analyzed time interval before tipping
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