Simulated Thermocline Tilt Over the Tropical Indian Ocean and Its Influence on Future Sea Surface Temperature Variability
Guojian Wang, Wenju Cai, Agus Santoso
Published in Geophysical Research Letters, MAR 2021
Most coupled climate models simulate an overly shallow thermocline over the southeastern equatorial Indian Ocean (SEIO). Through the Bjerknes feedback loop, the associated unrealistic thermocline tilt leads to warmer sea surface temperature (SST) over the western equatorial Indian Ocean than the SEIO, which is conducive to equatorial easterly winds. Similar to phase 5 of the Coupled Model Intercomparison Project (CMIP), phase 6 of the CMIP continues to simulate such overly tilted thermocline. On the one hand, this leads to overly strong SST variability, and on the other hand, it provides an inherent limit for the thermocline to incline further under greenhouse warming. Consequently, models with a more realistic tilt in the present-day climate project a greater shoaling of the thermocline under global warming, which facilities upwelling of cool subsurface water, leading to stronger increase in SST variability over the SEIO, compared to models with an overly large thermocline tilt.
Plain Language Summary
Coupled Model Intercomparison Project phase 6 (CMIP6) models continue to simulate an unrealistic mean state over the tropical Indian Ocean, that is, a positive Indian Ocean Dipole-like pattern. Compared to CMIP5, there is no noticeable improvement especially in the simulation of the climatological zonal wind stress and the thermocline tilt. The systematic model biases are linked and provide an inherent limit in the impact of the thermocline tilt, limiting the response to greenhouse warming. The present-day tilt can dynamically impact the projected changes in SST variability over the southeastern equatorial Indian Ocean, the source region of the Indian Ocean Dipole, suggesting that the bias may lead to an underestimation in the projected increase in SST variability.
Wang, G., Cai, W., & Santoso, A. (2021). Simulated thermocline tilt over the tropical Indian Ocean and its influence on future sea surface temperature variability. Geophysical Research Letters, 48, e2020GL091902. https://doi.org/10.1029/2020GL091902