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 20^th century. Here, we show that observations and multiple reanalyses yield the opposite trend from 1979 to present a WC intensification.Atmosphere-only (AMIP) and coupled ocean atmosphere (CMIP) simulations from CMIP5 are used. Analogous sets of AMIP simulations with NCAR'S CAM5 and CMIP simulations with CESM LENS are also analyzed. AMIP simulations driven by the real-world evolution of sea surface temperatures (SSTs) simulate this observed intensification, whereas CMIP simulations do not. Thus, the recent WC intensification is related to real-world SST evolution. Idealized simulation conducted with NCAR's CAM5 driven by the unforced component of SSTs yield significant WC strengthening, whereas negligible WC changes occur when driven by the forced component of SSTs. Our results suggest natural SST variability, and in particular a La Niña-like SST pattern, is primarily responsible for the strengthening of the WC since 1979.