An Atmospheric Bridge Between the Subpolar and Tropical Atlantic Regions: A Perplexing Asymmetric Teleconnection

The largest sea surface temperature (SST) anomalies associated with Atlantic Multidecadal Variability (AMV) occur over the Atlantic subpolar gyre, yet it is the tropical Atlantic from where the global impacts of AMV originate. Processes that communicate SST change from the subpolar Atlantic gyre to the tropical North Atlantic thus comprise a crucial mechanism of AMV. Here we use idealized model experiments to show that such communication is accomplished by an "atmospheric bridge." Our results demonstrate an unexpected asymmetry: the atmosphere is effective in communicating cold subpolar SSTs to the north tropical Atlantic, via an immediate extratropical atmospheric circulation change that invokes slower wind-driven evaporative cooling along the Eastern Atlantic Basin and into the tropics. Warm subpolar SST anomalies do not elicit a robust tropical Atlantic response. Our results highlight a key dynamical feature of AMV for which warm and cold phases are not opposites. Plain Language Summary To investigators of Atlantic Multidecadal Variability (AMV), one outstanding conundrum is the discrepancy between where the largest AMV changes occur (subpolar North Atlantic) and where the remote impacts of AMV originate (tropical North Atlantic). Processes that communicate sea surface temperature (SST) changes from the subpolar gyre region to the tropical North Atlantic are a potentially crucial but poorly understood aspect of AMV. Here, using idealized model experiments, we investigate the idea that such communication is accomplished by an "atmospheric bridge"-a fast, basin-wide atmospheric response to subpolar gyre SST anomalies that produces a tropical SST change. Our results demonstrate an unexpected, but robust asymmetry: The atmosphere is effective in communicating, via a fast circulation change that invokes wind-driven evaporative cooling along the Eastern Basin, only cold SST anomalies from the Atlantic subpolar gyre to the tropical North Atlantic. In contrast, imposed warm subpolar SST anomalies do not elicit a tropical Atlantic response. Our results highlight a key dynamical feature of AMV for which warm and cold phases are not opposites.