What controls the mean state, variability and change of the Atlantic meridional overturning circulation (AMOC) is a major question of climate science, and yet, while continuous progress has been made over the past decades in addressing this question, we still cannot predict the strength of AMOC and its other characteristics in newly designed or even slightly modified models. This is true for the Energy Exascale Earth System Model (E3SM) v1 and v2, which has too weak AMOC in its lower-resolution configuration. It is true for other Earth system models across CMIP5 and CMIP6, which exhibit a broad range of AMOC strengths, spatial characteristics, and variability. The rates of simulated AMOC slowdown with anthropogenetic warming vary broadly as well. Constraining those rates would be of significant practical value because the AMOC affects the climate of different regions of the globe, including North America, Europe, and Africa. In this project, we propose to investigate the impacts of ocean basin inter-connections, specifically the impacts of tropical Indian Ocean (TIO) temperature and change, on the mean state, variability, and past and future changes of the AMOC and Atlantic climate in E3SM and other Earth system models.

It is well established that the TIO strongly affects regional climate variability. However, the role it plays in the global climate system has received much less attention than that of the Pacific or Atlantic Ocean. The TIO sea surface temperature (SST) has risen steadily since the 1950s, more than in the tropical Pacific and Atlantic, amplifying inter-basin zonal asymmetries in the tropical climate system. In fact, the enhanced TIO warming has been a salient feature of anthropogenic global warming. However, Earth system models are unable to capture this trend. Hence, there is an urgent need to investigate how the TIO influences global climate variability and change. Emergent evidence suggests that the TIO could actively interact with other ocean basins through atmospheric and oceanic teleconnections, affecting a broad range of climate phenomena in other basins. In particular, recent studies have identified previously overlooked climate links from the TIO to the Atlantic Ocean (Hu and Fedorov 2019, 2020): First, enhanced TIO warming can induce a positive North Atlantic Oscillation (NAO)-like response accompanied by enhanced surface westerly winds over the subpolar North Atlantic, cooling the underlying ocean and potentially explaining the observed development of the North Atlantic “warming hole” (NAWH). Second, latent heat release caused by enhanced TIO warming can reduce tropical Atlantic rainfall via the reorganization of the atmospheric Walker Circulation, and the resultant positive sea surface salinity anomalies can act to accelerate the AMOC after being transported to the subpolar North Atlantic over several decades. Those two mechanisms were initially identified within the Community Earth System Model (CESM1), and later confirmed by experiments using another, very different coupled model, IPSL-CM6. 

The proposed project will bridge a broad range of climate phenomena across various timescales and investigate the impacts of TIO warming on the Atlantic climate state, including the AMOC, Atlantic salinity, the jet stream, and NAWH. Those TIO-Atlantic links will be thoroughly explored in terms of climate mean state, variability, and historical and future changes, using sensitivity experiments with E3SM v2. The proposed E3SM simulations will include (i) A hierarchy of idealized TIO-warming experiments, (ii) TIO mean state bias correction experiments, (iii) TIO bias corrected historical experiments, (iv) TIO pacemaker historical experiments, and (v) TIO adjusted future warming experiments, amongst others. Observational and CMIP6 model analyses will also be conducted as additional tests of the proposed mechanisms. In summary, the proposed work will contribute to the emerging, potentially transformative ideas that ocean inter-basin connections play a key role in shaping and driving climate variability across the globe and influence how the global climate responds to external radiative forcing. Our experiments will help understand some of the key factors that control the AMOC, and its variability and change, in E3SM. The proposed project will involve cross-institutional collaboration and support one postdoctoral fellow.