Evaluation of Thermocline Depth Bias in the Seychelles-Chagos Thermocline Ridge (SCTR) simulated by the CMIP6 models

Abstract

The Seychelles-Chagos Thermocline Ridge (SCTR) refers to an open ocean upwelling region in the southwestern Indian Ocean (5oS–10oS, 50oE–80oE), a result of the negative wind stress curl between the southeasterly trade wind and equatorial westerlies. Considering the increasing need to understand the sea-air interaction that shapes the variability of the Indian Ocean, the simulation of SCTR is evaluated using outputs from the Coupled Model Intercomparison Project Phase Sixth (CMIP6) and Phase Fifth (CMIP5) models. Compared with observations, by calculating the 20°C isotherm depth (D20), 23 of 27 CMIP6 models tend to simulate considerably deeper thermocline depth – a common bias in CMIP5 models. The thermocline dome, the shallowest D20 in the 5oS–10oS band, is also located in the eastern part of the Indian Ocean in most of the CMIP6 and CMIP5 models, whereas the dome from observation is located at the western part around 60oE. These biases are found to be associated with the easterly wind bias in the equator, which generates markedly weak Ekman pumping velocity. The models with deeper thermocline depth bias showed a relatively warmer sea surface temperature, probably owing to a weaker thermocline feedback mechanism. Quantitative analysis showed that the CMIP6 models simulate better the thermocline depth and dome, compared to the CMIP5 models. This study highlights the problems with the CMIP6 models for reproducing the SCTR region in the Indian Ocean and might assist the improvement of climate models in the future.