On the Atlantic extratropical-tropical teleconnection in response to external freshwater forcing

The weakening of the Atlantic Meridional Overturning Circulation (AMOC) leads to a distinct horseshoe pattern of colder sea surface temperatures (SST) anomalies in the North Atlantic as found in many modeling studies. This SST horseshoe pattern is a characteristic feature of the Atlantic extratropical-tropical teleconnection leading to the tropical atmospheric response associated with the AMOC weakening, such as the southward shift of the Atlantic Intertropical Convergence Zone (ITCZ). A similar SST horseshoe pattern associated with the Atlantic Multidecadal variability (AMV) has also been observed in modern climate, with the SST anomalies propagating from the extratropical North Atlantic into the tropical North Atlantic along the horseshoe pathway. Despite its importance, the mechanisms of the Atlantic extratropical-tropical teleconnection associated with the AMOC weakening remain poorly understood. Previous studies suggest the Wind-Evaporation-SST (WES) feedback as a plausible mechanism. Here, we conduct water hosing experiments using a high-resolution fully coupled climate model to elucidate the mechanisms responsible for the Atlantic extratropical-tropical teleconnection associated with the AMOC weakening. Our analysis, focusing on boreal summer, suggests that the WES feedback is not the primary mechanism for the Atlantic extratropical-tropical teleconnection. By examining the transient response as the AMOC weakens, we identify the key mechanisms responsible and reveal the important role of the oceanic and atmospheric circulations involved in the SST horseshoe pattern formation. We also illustrate how the relative importance of the oceanic and atmospheric processes in the Atlantic extratropical-tropical teleconnection changes under different amplitudes of the freshwater forcing applied in the water hosing experiments. The mechanisms of the Atlantic extratropical-tropical teleconnection are crucial for the development of the tropical atmospheric response associated with the AMOC weakening (e.g. the southward shift of the Atlantic ITCZ).