Two Competing Drivers of the Recent Walker Circulation Trend

Abstract

The Pacific Walker circulation (PWC) weakens under global warming in climate change projections, supported by a global hydrological constraint. However, the PWC has strengthened over the past decades despite ongoing global warming, and the cause has been a puzzle. Because PWC is coupled with the pattern of sea surface temperature (SST) in the tropical Pacific, quantifying the relative impact of SST pattern change and global warming on the past PWC trend is important. We show, using an atmosphere model driven by observed boundary conditions for 1979–2013 and a hypothetical uniform surface warming trend with varying magnitude, that the PWC scales with warming and weakens by 8% per °C, but this effect cannot overcome the SST pattern effect that intensifies the circulation. Further attribution experiments show that the past strengthening of PWC is explained directly by the SST warming pattern in the narrow equatorial band, about 30% of which is induced by the Indian Ocean.