On the Multidecadal SST Variability in the Tropical North Atlantic

Observational sea surface temperature (SST) data exhibit pan-North Atlantic multidecadal variability, often called Atlantic multidecadal variability (AMV). It is generally considered, largely based on coupled model simulations, that the AMV is driven by changes in northward heat transport associated with fluctuations in the Atlantic meridional overturning circulation (AMOC). However, while the AMV signals in mid to high latitudes are robustly tied to the AMOC fluctuations in coupled model simulations, their link in the tropical North Atlantic (TPNA) appears to be weak. In addition, some recent studies argue that the multidecadal SST variability in the TPNA during the instrumental period is a result of combined influences of anthropogenic greenhouse gases and aerosols.

In this study, we investigate the cause of the multidecadal SST variability in the TPNA, focusing on a recent warming period (1984-2009) where satellite-derived surface radiation data are available. Multiple surface heat flux data sets have been analyzed, along with CESM large ensemble simulations to infer influences of external forcing. We found no evidence that either anthropogenic greenhouse gasses or aerosols have dominantly driven the warming in the TPNA during this period. Also, all the surface heat flux data sets agree that the net surface heat flux has acted to cool the TPNA SST, indicating that oceanic processes should have driven the warming during this period in the heat budget point of view. We further attempt to identify which oceanic process(es) is responsible for this warming using decadal prediction runs showing North Atlantic SST changes, which both temporally and spatially agree reasonably well with those found in observational SST data during 1984-2009.