Two Competing Drivers of the Recent Walker Circulation Trend

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. Plain Language Summary The large‐scale overturning circulation in the tropical Pacific known as the Pacific Walker circulation is the heart of the general circulation of the atmosphere. Most of the future climate projections by climate models suggest that the Walker circulation weakens with surface warming. However, observations show that the circulation has strengthened over the past decades despite ongoing global warming. We show, using atmosphere model simulations for 1979–2013 that the Walker circulation strengthening is well reproduced and it weakens when a hypothetical uniform surface warming trend was imposed on the sea surface temperature (SST) data that force the model. The weakening of the Walker circulation occurs at a rate of about 8% per degree warming, but this effect cannot overcome the SST pattern effect that intensifies the circulation. Further attribution simulations show that the past strengthening is explained directly by the SST warming pattern in the narrow equatorial band, about one‐third of which is induced by the warming of the Indian Ocean. Key Points The past strengthening trend of the Pacific Walker circulation (PWC) is well reproduced in the AMIP simulations PWC weakens with surface warming consistent with global energy constraints, but the effect cannot overcome the pattern effect on circulation The pattern effect originates directly from the SST trend in the equatorial band, to which Indian Ocean warming contributes by about 30%