Atlantic Induced Climate Changes in the Tropical Ocean and Atmosphere

Previous studies revealed many atmospheric bridges between the Pacific, the Atlantic, and the Indian Ocean. In particular, several recent works indicate that the Atlantic warming can contribute to the cooling trend over the Eastern Pacific. Our study is inspired by these previous studies, and aims at investigating the impact of the tropical Atlantic on the entire tropical climate systems on multi-decadal time scales. We first performed a ‘pacemaker' simulation using a fully coupled model – the Community Earth System Model (CESM1), by restoring the tropical Atlantic sea surface temperature (SST) trend during the satellite era. Results reveal that the Atlantic warming heats the Indo-Western Pacific and cools the Eastern Pacific, enhances the equatorial easterly wind and the Walker circulation, and drives the subsurface Pacific to a La Niña mode. Ensemble simulations show that the Atlantic warming can contribute to 60-70% of the above tropical changes during the last three decades. The same pan-tropical teleconnections have been validated by the statistics of observations and 106 CMIP5 control simulations. We then performed a series of simulations using a hierarchy of atmospheric and oceanic models with different complexities, to single out the roles of atmospheric dynamics, atmosphere-ocean energy fluxes, and oceanic dynamics in these teleconnections. Through these simulations, we established a two-step mechanism for the Atlantic to impact on the entire tropics, as summarized in the schematic figure: 1) Atlantic warming first generates a regional deep convection and induces a Gill-type convective circulation anomaly over the entire tropical atmosphere. This circulation changes force the Indian Ocean and the Pacific with wind-evaporation-SST (WES) effect, and forms a temperature gradient over the Indo-Pacific basins. 2) The Atlantic-induced Indo-Pacific temperature gradient further generates a secondary atmospheric deep convection over the Indo-Western Pacific warm pool region, which reinforces the easterly wind anomalies over the equatorial Pacific and enhances the Walker circulation, triggering the Pacific to a La Niña mode with Bjerknes ocean dynamical feedback. This mechanism indicates that the three tropical ocean basins are linked more closely than previously thought, and on decadal time scales the tropical oceans should be considered as a single entirety. In addition to the well-known ENSO-induced tropical-wide response that is dominant on inter-annual time scales, this study highlights the role of the tropical Atlantic in initiating a different pan-tropical dipole pattern that is important on decadal timescales. This Atlantic-induced pan-tropical variability has broad impacts on the global climate variability. In particular, the Atlantic contributes to the Eastern Pacific cooling, which is considered as an important source of the recent global warming hiatus. The tropical Atlantic warming is likely due to radiative forcing and Atlantic Multi-decadal Oscillation, the latter tied to the Atlantic meridional overturning circulation (AMOC). Our study suggests that the AMOC may force the pan-tropical decadal variability, and the slow time scales of the AMOC may contribute to the decadal predictability of the tropical-wide SST pattern.