Local and remote SST variability contribute to the westward shift of the Pacific Walker circulation during 1979–2015

Abstract During the modern satellite era since 1979, the Pacific Walker circulation (PWC) experienced an intensification and a westward shift, which has broad impacts on the global climate variability. While the strengthening of the PWC has been shown to be driven by both the regional Pacific sea surface temperature (SST) and the remote forcing from other basins, its westward shift is primarily attributed to the phase change of the Atlantic Multidecadal variability. In this study, we investigate the potential effect of the remote SST forcing from the Atlantic and the Indian Oceans on the westward shift of the PWC, through statistical analysis and numerical experiments using atmospheric and coupled models. Results show that the tropical Atlantic warming plays a key (decisive) role in driving the PWC westward shift by triggering a Gill–Matsuno-type circulation anomaly in the tropics. This circulation response drives anomalous surface westerlies over the eastern Pacific and subsidence over the central Pacific that weakens the eastern part of the PWC, meanwhile generating easterly wind anomalies over the central-western Pacific and anomalous atmospheric convection over the western Pacific that intensifies the western part of the PWC. This direct forcing contributes ~ 32% of the observed PWC movement, while the Atlantic-induced inter-basin SST changes contribute another ~ 36% of its westward shift according to coupled model simulation results. Our results reinforce the importance of the inter-basin interactions in adjusting the tropical climate variabilities, and have broad implication for projecting the global climate.