The Impacts of the Atlantic Multidecadal Variability on Tropical Pacific as Assessed from CMIP6/DCPP-C Idealized Simulations

Over the 1980-2011 period, the eastern tropical Pacific SST shows a cooling trend that appears to be one of the causes of the global surface warming slowdown, the so-called hiatus period. This cooling contrasts with the climate warming response expected from the increase in anthropogenic greenhouse gases. Intensification of the Walker Circulation and of the eastern tropical Pacific easterlies appears to be the causes of this eastern tropical Pacific cooling. Such changes are partly attributable to the accelerated SST warming trend of the tropical Atlantic during the considered period. The mechanism proposed is that the warmer tropical Atlantic drives changes of the Walker Circulation in boreal summer, which modifies the Pacific surface wind and eventually leads to La Niña-like conditions in boreal winter. Yet, it still remains to evaluate the robustness and strength of this tropical Atlantic - tropical Pacific connection.

Here we investigate this teleconnection in 20 idealized GCM simulations in which the coupled model North Atlantic SST is restored through the observed Atlantic Multidecadal Variability anomalies (CMIP6/DCPP-C-type protocol). In response to a North Atlantic warming, all models simulate a tropical Pacific cooling. However, the amplitude of this response varies by a factor of 10 across the models. Using energy constraint analyses, we track back the origin of the inter-model spread. The more a model simulates an Atlantic ITCZ northward shift (i.e. southward anomalous atmospheric heat transport) in response to the SST warming, the more the tropical Pacific cools down. We find that this inter-model spread is ultimately controlled by the way ocean model components are releasing top layer heat anomalies to the atmosphere.