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1. Indian Ocean warming as a driver of the North Atlantic warming hole

   https://doi.org/10.1038/s41467-020-18522-5  

   Hu, Shineng; Fedorov, Alexey V. (December 2020, Nature Communications)

   null (Ed.)

   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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2. The Time-Dependent Response of a Two-Basin Ocean to a Sudden Surface Temperature Change

   https://doi.org/10.1175/JCLI-D-21-0821.1  

   Chang, Chiung-Yin; Jansen, Malte F. (July 2022, Journal of Climate)

   Abstract Building on previous work using single-basin models, we here explore the time-dependent response of the Atlantic meridional overturning circulation (AMOC) to a sudden global temperature change in a two-basin ocean–ice model. We find that the previously identified mechanisms remain qualitatively useful to explain the transient and the long-term time-mean responses of the AMOC in our simulations. Specifically, we find an initial weakening of the AMOC in response to warming (and vice versa for cooling), controlled by the mid-depth meridional temperature contrast across the Atlantic basin. The long-term mean response instead is controlled primarily by changes in the abyssal stratification within the basin. In contrast to previous studies we find that for small-amplitude surface temperature changes, the equilibrium AMOC is almost unchanged, as the abyssal stratification remains similar due to a substantial compensation between the effects of salinity and temperature changes. The temperature-driven stratification change results from the differential warming/cooling between North Atlantic Deep Water and Antarctic Bottom Water, while the salinity change is driven by changes in Antarctic sea ice formation. Another distinct feature of our simulations is the emergence of AMOC variability in the much colder and much warmer climates. We discuss how this variability is related to variations in deep-ocean heat content, surface salinity, and sea ice in the deep convective regions, both in the North Atlantic and in the Southern Ocean, and its potential relevance to past and future climates. 

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3. Pantropical Indo-Atlantic temperature gradient modulates multi-decadal AMOC variability in models and observations

   https://doi.org/10.1038/s41612-023-00489-x  

   Ferster, Brady S; Borchert, Leonard F; Mignot, Juliette; Menary, Matthew B; Cassou, Christophe; Fedorov, Alexey V (December 2023, npj Climate and Atmospheric Science)

   Abstract Interconnections between ocean basins are recognized as an important driver of climate variability. Recent modeling evidence suggests that the North Atlantic climate can respond to persistent warming of the tropical Indian Ocean sea surface temperature (SST) relative to the rest of the tropics (rTIO). Here, we use observational data to demonstrate that multi-decadal changes in pantropical ocean temperature gradients lead to variations of an SST-based proxy of the Atlantic Meridional Overturning Circulation (AMOC). The largest contribution to this temperature gradient-AMOCconnection comes from gradients between the Indian and Atlantic Oceans. TherTIOindex yields the strongest connection of this tropical temperature gradient to theAMOC. Focusing on the internally generated signal in three observational products reveals that an SST-basedAMOCproxy index has closely followed low-frequency changes ofrTIOtemperature with about 26-year lag since 1870. Analyzing the pre-industrial control simulations of 44 CMIP6 climate models shows that theAMOCproxy index lags simulated mid-latitudeAMOCvariations by 4 ± 4 years. These model simulations reveal the mechanism connectingAMOCvariations to pantropical ocean temperature gradients at a 27 ± 2 years lag, matching the observed time lag in 28 out of the 44 analyzed models. rTIO temperature changes affect the North Atlantic climate through atmospheric planetary waves, impacting temperature and salinity in the subpolar North Atlantic, which modifies deep convection and ultimately the AMOC. Through this mechanism, observed internalrTIOvariations can serve as a multi-decadal precursor ofAMOCchanges with important implications forAMOCdynamics and predictability. 

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4. Stratospheric control of the linkage between the AMOC and Atlantic multidecadal variability

   https://doi.org/10.1175/JCLI-D-24-0154.1  

   Studholme, Joshua; Fedorov, Alexey; Ferster, Brady S (May 2025, Journal of Climate)

   Abstract The ocean’s role in Atlantic Multidecadal Variability (AMV) remains intensely debated. The core issue is whether AMV, as an internal climate mode, is driven by variations in Atlantic Meridional Overturning Circulation (AMOC) or by atmospheric processes. Climate models exhibit wide diversity in AMOC-AMV linkages, producing temporal correlations between 0.3-0.8, but no robust explanation for these differences exists. Here, using multi-model intercomparison and perturbation experiments, we propose a dynamical mechanism relating the strength of AMOC-AMV linkage in climate models to stratospheric temperature. This mechanism includes (1) tropospheric midlatitude jet response to stratospheric mean-state temperature anomalies in mid-latitudes and (2) resulting ocean surface density changes that alter the spatial structure of deep-water formation in the subpolar North Atlantic and hence AMOC-AMV connection. Specifically, colder stratospheric temperatures produce tighter linkage through the northward jet shifts and a stronger AMOC, with enhanced deep-water formation in the Labrador and Irminger Seas relative to the Nordic Seas. Models with a warm stratospheric bias tend to produce weaker linkage. Perturbation experiments imposing stratospheric cooling at mid to high latitudes within two independent climate models support these conclusions. Furthermore, we find that models with stronger AMOC-AMV linkage predict a stronger North Atlantic “warming hole” and weaker 21st-century Arctic amplification. We conclude that these results have significant implications for climate prediction and projections. 

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5. Impacts of Atlantic meridional overturning circulation weakening on Arctic amplification

   https://doi.org/10.1073/pnas.2402322121  

   Lee, Yu-Chi; Liu, Wei; Fedorov, Alexey V; Feldl, Nicole; Taylor, Patrick C (September 2024, Proceedings of the National Academy of Sciences)

   Enhanced warming of the Arctic region relative to the rest of the globe, known as Arctic amplification, is caused by a variety of diverse factors, many of which are influenced by the Atlantic meridional overturning circulation (AMOC). Here, we quantify the role of AMOC changes in Arctic amplification throughout the twenty-first century by comparing two suites of climate model simulations under the same climate change scenario but with two different AMOC states: one with a weakened AMOC and another with a steady AMOC. We find that a weakened AMOC can reduce annual mean Arctic warming by 2 °C by the end of the century. A primary contributor to this reduction in warming is surface albedo feedback, related to a smaller sea ice loss due to AMOC slowdown. Another major contributor is the changes in ocean heat uptake. The weakened AMOC and its associated anomalous ocean heat transport divergence lead to increased ocean heat uptake and surface cooling. These two factors are inextricably linked on seasonal timescales, and their relative importance for Arctic amplification can vary by season. The weakened AMOC can also abate Arctic warming via lapse rate feedback, creating marked cooling from the surface to lower-to-mid troposphere while resulting in relatively weaker cooling in the upper troposphere. Additionally, the weakened AMOC increases the low-level cloud fraction over the North Atlantic warming hole, causing significant cooling there via shortwave (sw) cloud feedback despite the overall effect of sw cloud feedback being a slight warming of the average temperature over the Arctic. 

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