Mechanisms of tropical Indian Ocean circulation biases in CESM historical experiments
Abstract
The study explores the mechanisms responsible for circulation biases over the tropical Indian Ocean (TIO) in Phase 6 of the Coupled Model Intercomparison Project (CMIP6) historical simulations, using four configurations of the Community Earth System Model (CESM2): CESM2, CESM2-FV2, CESM2-WACCM, and CESM2-WACCM-FV2. The dominant equatorial circulation feature, the Wyrtki Jet (WJ), is significantly underestimated across all models, primarily due to prevailing easterly wind bias. Notably, the fall WJs exhibit the lowest simulation skill, especially in CESM2-WACCM-FV2, owing to a strong underestimation of equatorial westerlies. All CESM2 configurations simulate a persistent eastward Equatorial Undercurrent (EUC) along the equatorial Indian Ocean, with the summer–fall EUC being strongly overestimated and most prominently in CESM2-WACCM-FV2, followed by CESM2-FV2. This overestimation is attributed to a westward tilt of the thermocline induced by southeasterly winds, operating through a weakened Bjerknes feedback mechanism. Additionally, strong easterly wind stress over the southern TIO contributes to the northward intensification and deepening of the South Equatorial Current. Among the four configurations, CESM2-WACCM, which features a high-top atmospheric component and enhanced horizontal resolution, shows the best performance in simulating surface circulation in the TIO, likely due to improved air–sea coupling. The equatorial easterly wind bias in the CESM2 models originates from southeasterly wind errors in their atmospheric component during June to August, with coupled feedbacks further amplifying the bias along the equator. Moreover, the overly strong southeasterlies and intensified midlatitude westerlies in them contribute to the broadening and poleward shift of the subtropical gyre in CESM2 models. These findings provide key insights into model deficiencies across CESM2 variants and can guide future improvements in simulating tropical ocean–atmosphere interactions in a warming climate.
URI
https://link.springer.com/article/10.1007/s00704-025-05887-0http://drs.nio.res.in/drs/handle/2264/9586
Collections
- Scholarly literature [9297]

