Simulation of the long-term variability of the Hadley circulation in CMIP6 models

• Published in: Atmospheric research 2023-05, Vol.287, p.106716, Article 106716
• Main Authors: Wang, Shuang, Feng, Juan, Liu, Xiaohan, Ji, Xuanliang
• Format: Article
• Language: English
• Subjects: CMIP6 models; ENSO; Equatorial asymmetric mode; Hadley circulation; Meridional gradient
• ISSN: 0169-8095; 1873-2895
• DOI: 10.1016/j.atmosres.2023.106716

Based on observations and reanalyses, the ability of 24 models from the Coupled Model Inter-comparison Project Phase 6 (CMIP6) in simulating the Hadley circulation's (HC) long-term variability was evaluated, and possible reasons for the simulation uncertainties were analyzed. The results showed that each model utilized in this study can reproduce the HC climatological pattern well, but the simulated HC variability shows large inter-model uncertainties. The first principle mode of the HC's long-term variability (EOF1) exhibited an equatorial asymmetric structure (i.e., asymmetric mode, AM) in observations. The CMIP6 models were divided into two groups according to their abilities to reproduce the AM: models that could simulate the AM well were classified as Type I, and models that could not simulate the AM and exhibited an approximately equatorial symmetric structure of the EOF1 were classified as Type II. The results showed that the EOF1 spatial structure is determined by variations in the meridional gradient of the tropical ocean sea surface temperature (SST). Type I models can significantly reproduce the relationship between the EOF1's variation and the meridional gradient of tropical SST, so as to reproduce the HC's long-term variability characteristics. In Type II models, the EOF1's variation is tightly related to SST distribution, similar to the El Niño-Southern Oscillation (ENSO). Type II models exaggerate the variation of ENSO, which is associated with equatorial symmetric structure of SST anomalies, resulting in enhanced variability in the HC of the equatorial symmetric part. This is the reason why EOF1 of the HC in Type II models exhibits an equatorial symmetric structure. The results point out the possible reasons for the different simulations of CMIP6 models in the HC variability and provide a reference for future climate models to be developed in this area. •CMIP6 models reproduce the HC climatological pattern well, but the HC variability shows large inter-model uncertainties.•A model reproduction of the tropical SST meridional gradient is a key factor in determining the HC variability simulation.•The unsatisfactory performance of the variability of the HC is due to the exaggeration of the ENSO.