Disagreement on the North Atlantic Cold Blob Formation Mechanisms among Climate Models

Abstract Despite global warming, sea surface temperature (SST) in the subpolar North Atlantic has decreased since the 1900s. This local cooling, known as the North Atlantic cold blob (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 its constraints on climate models leave the North Atlantic cold blob formation mechanism inconclusive. Using simulations from the Coupled Model Intercomparison Project Phase 6, we assess the primary processes driving the North Atlantic cold blob within individual models and the consistency of mechanisms across models. We show that 11 out of 32 models, which we call “Cold Bold” models, simulate subpolar North Atlantic cooling over 1900–2014. Further analyzing the heat budget of subpolar North Atlantic SST shows that models have distinct mechanisms of cold blob formation. Whereas four out 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 are comparable in the remaining three models. Such a model spread in the mechanism of cold blob formation may be associated with distinct base-state Atlantic Meridional Overturning Circulation (AMOC) strength, which explains about 39% of the inter-model 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 radiative processes are more responsible. This model discrepancy suggests that the cold blob formation mechanism diagnosed from single models should be interpreted with caution.