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Zusammenfassung:
Over the past century, a large region of the subpolar North Atlantic experienced a slight cooling, dubbed the North Atlantic Warming Hole (NAWH). A similar warming hole also emerges in the global warming projections for this region. The causes of the NAWH remain under debate as both oceanic (the Atlantic meridional overturning circulation) and atmospheric processes can potentially explain this phenomenon. Here, we compare the results of several recent studies investigating the mechanisms of the NAWH. In the future climate the warming hole appears to be strongly linked to the AMOC weakening as seen for example in large-ensemble simulations using CESM1 (Liu et al. 2020). However, sensitivity experiments with coupled GCMs also show that a warming hole can be induced by remotely-induced wind changes in the North Atlantic. Specifically, Arctic sea ice decline is shown to cause a strong warming hole within the southern part of the subpolar gyre without any significant change in the AMOC (Ferster et al. 2022). Likewise, the enhanced warming of the tropical Indian Ocean of the past 50 years is also shown to affect the North Atlantic through atmospheric teleconnections driven by atmospheric planetary waves (Hu and Fedorov 2020; Ferster et al. 2021). While these two mechanisms have very different origins, their effect is similar – changes in westerly winds south of Greenland cause a pronounced warming hole. These results generally agree with slab-ocean model experiments of He et al. (2022). Thus, both oceanic and atmospheric processes can indeed contribute to the NAHW.
