Comparison of Intensity Changes in Pacific and Atlantic Meridional Modes Under Global Warming

Abstract The Pacific Meridional Mode (PMM) and the Atlantic Meridional Mode (AMM) are key modes of interannual sea surface temperature (SST) variability in the Pacific and Atlantic Oceans, respectively. Analysis of CMIP6 model outputs reveals a robust intensification of the PMM under global warming, whereas the AMM exhibits no consensus among models. These different responses are attributed to mid-to-high latitude atmospheric forcing and subtropical feedback mechanisms. Changes in the upper-level westerly jet drive distinct atmospheric variability over the North Pacific and Atlantic, amplifying sea-level pressure variations associated with the PMM but weakening those linked to the AMM. The SST response to atmospheric forcing shows an increase in the Pacific and a decrease in the Atlantic, both of which are significantly positively correlated with the respective changes in each mode. The enhanced wind-evaporation-SST (WES) feedback, primarily driven by rising background SSTs, positively impacts the intensification of both modes. In the subtropical Pacific, the PMM is further strengthened by an increasing latent heat flux response. The enhancement of the PMM is principally connected to intensified atmospheric forcing and strengthened subtropical feedback. Although the WES feedback is enhanced to some extent, wind anomalies that oppose the climatological state reduce latent heat flux. Combined with the weakening of atmospheric forcing over the Atlantic, this phenomenon contributes to the uncertainty in the AMM’s response to global warming.