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Abstract Despite global warming, the sea surface temperature (SST) in the subpolar North Atlantic has decreased since the 1900s. This local cooling, known as the North Atlantic cold blob, signifies a unique role of the subpolar North Atlantic in uptaking heat and hence impacts downstream weather and climate. However, a lack of observational records and their constraints on climate models leave the North Atlantic cold blob formation mechanism inconclusive. Using simulations from phase 6 of Coupled Model Intercomparison Project, we assess the primary processes driving the North Atlantic cold blob within individual models and whether the mechanisms are consistent across models. We show that 11 out of 32 models, which we call “Cold Blob” models, simulate the subpolar North Atlantic cooling over 1900–2014. Further analyzing the heat budget of the subpolar North Atlantic SST shows that models have distinct mechanisms of cold blob formation. While 4 of the 11 Cold Blob models indicate decreased oceanic heat transport convergence (OHTC) as the key mechanism, another four models suggest changes in radiative processes making predominant contributions. The contribution of OHTC and radiative processes is comparable in the remaining three models. Such a model disagreement on the mechanism of cold blob formation may be associated with simulated base-state Atlantic meridional overturning circulation (AMOC) strength, which explains 39% of the intermodel spread in the contribution of OHTC to the simulated cold blob. Models with a stronger base-state AMOC suggest a greater role of OHTC, whereas those with a weaker base-state AMOC indicate that radiative processes are more responsible. This model discrepancy suggests that the cold blob formation mechanism diagnosed from single model should be interpreted with caution. Significance StatementThe mechanisms driving sea surface temperatures over the subpolar North Atlantic to cool since the 1900s remain uncertain due to the lack of direct observations. Here, we use a temperature change decomposition framework to dissect the historical trend of surface temperature simulated in multiple global climate models. The models diverge on whether the subpolar North Atlantic cooling is induced by reduced ocean heat transport convergence or altered radiative processes. Notably, the importance of ocean heat transport convergence is influenced by the simulated base-state strength of Atlantic meridional overturning circulation and the Irminger Sea’s mixed layer depth. This finding cautions against concluding the cooling mechanism from a single model and highlights a need for ongoing observations to constrain AMOC-related climate projection in the subpolar North Atlantic.
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Indian Ocean warming as a driver of the North Atlantic warming hole
Abstract Over the past century, the subpolar North Atlantic experienced slight cooling or suppressed warming, relative to the background positive temperature trends, often dubbed the North Atlantic warming hole (NAWH). The causes of the NAWH remain under debate. Here we conduct coupled ocean-atmosphere simulations to demonstrate that enhanced Indian Ocean warming, another salient feature of global warming, could increase local rainfall and through teleconnections strengthen surface westerly winds south of Greenland, cooling the subpolar North Atlantic. In decades to follow however, this cooling effect would gradually vanish as the Indian Ocean warming acts to strengthen the Atlantic meridional overturning circulation (AMOC). We argue that the historical NAWH can potentially be explained by such atmospheric mechanisms reliant on surface wind changes, while oceanic mechanisms related to AMOC changes become more important on longer timescales. Thus, explaining the North Atlantic temperature trends and particularly the NAWH requires considering both atmospheric and oceanic mechanisms.
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- PAR ID:
- 10193518
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Nature Communications
- Volume:
- 11
- Issue:
- 1
- ISSN:
- 2041-1723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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