Intensification of Interannual Cross-Basin SST Interaction between the North Atlantic Tripole and Pacific Meridional Mode since the 1990s

Abstract

The cross-basin interaction of the second EOFs of the interannual SST in the North Atlantic and North Pacific—the North Atlantic tripole (NAT) SST and Pacific meridional mode (PMM)—is discussed. Observations revealed that the total variances of the NAT and PMM have simultaneously experienced interdecadal enhancement since the 1990s. Wavelet analysis indicated that this enhancement was associated with the interdecadal variations (8–16 years) of the NAT and PMM, which have become significantly and positively coherent since the 1990s. This interdecadal variation also changed the interannual NAT–PMM relationship from negative to positive. The regression analysis indicated that the NAT forced a Matsuno–Gill circulation anomaly, which had a substantial lag impact on the PMM SST through wind–evaporation–SST feedback. Additionally, the NAT induced oceanic temperature advection, which also partially contributed to the PMM SST. On the other hand, the PMM-associated middle–upper atmospheric teleconnection, a North Atlantic Oscillation (NAO)-like circulation anomaly in the North Atlantic, gave positive feedback to the NAT. The numerical experiments suggest that the enhancement of the NAT–PMM interaction since the 1990s was associated with the eastward shift of PMM-associated convection, which was further enhanced by eastward extension of the upper-level extratropical jet in the North Pacific.