Strengthening of the Walker Circulation in recent decades and the role of natural sea surface temperature variability

In response to global warming, climate model simulations suggest a weakening of the Walker Circulation (WC), which is supported by long-term sea level pressure observations over the 20th century. Here, we show the observations and multiple reanalyses yield the opposite trend from 1979 to present-a WC intensification. Atmosphere-only simulations driven by the real-world evolution of sea surface temperatures (SSTs) simulate this observed intensification, whereas coupled ocean atmosphere simulations do not. Thus, the recent WC intensification is related to real-world SST evolution. Assuming the multi-model mean SSTs from 20th century coupled climate model simulations accurately represent the externally forced response, the observed SSTs can be decomposed into a forced and an unforced component. Idealized Community Atmosphere Model version 5 (CAM5) simulations driven by the unforced component of SSTs yield significant WC strengthening, whereas negligible WC changes occur when driven by the forced component of SSTs. Although coupled climate models may be deficient in their tropical response to anthropogenic warming, our results suggest natural SST variability, and in particular a La Nina-like SST pattern, is primarily responsible for the strengthening of the WC since 1979.