A very likely weakening of Pacific Walker Circulation in constrained near-future projections

Mingna Wu,Lixia Zhang,Bo Wu,Chao Li,Xiaolong Chen,Tianjun Zhou,Wenxia Zhang,Hongmei Li

doi:10.1038/s41467-021-26693-y

Abstract

The observational records have shown a strengthening of the Pacific Walker circulation (PWC) since 1979. However, whether the observed change is forced by external forcing or internal variability remains inconclusive, a solid answer to more societal relevantly question of how the PWC will change in the near future is still a challenge. Here we perform a quantitative estimation on the contributions of external forcing and internal variability to the recent observed PWC strengthening using large ensemble simulations from six state-of-the-art Earth system models. We find the phase transition of the Interdecadal Pacific Oscillation (IPO), which is an internal variability mode related to the Pacific, accounts for approximately 63% (~51–72%) of the observed PWC strengthening. Models with sufficient ensemble members can reasonably capture the observed PWC and IPO changes. We further constrain the projection of PWC change by using climate models’ credit in reproducing the historical phase of IPO. The result shows a high probability of a weakened PWC in the near future.

Highlights

The observational records have shown a strengthening of the Pacific Walker circulation (PWC) since 1979
In this work, based on six sets of large ensemble simulations collected from the US CLIVAR Working Group on Large Ensembles[38], we investigate to what extent the recent strengthening of PWC is attributable to internal variability
Is the recent PWC strengthening a forced response or modulated by internal variability? To address this question, we examined the PWC change based on the sea-level pressure (SLP) anomaly over the equatorial Pacific during recent decades in Max Planck institute Grand Ensemble (MPI-GE) with 100 members

Summary

Introduction

The observational records have shown a strengthening of the Pacific Walker circulation (PWC) since 1979. The externally-forced PWC change, represented by the ensemble mean of all members, shows a slight increasing trend of 12.9 Pa (36 year)−1 during the period of 1980-2015 (Fig. 1a). The PWC in observation shows an averaged increasing trend of 99.9 Pa (36 year)−1 over 1980-2015, which is, within the range of member spread of the ensemble simulation of MPI-GE model (Fig. 1a).

Results

Conclusion