As the largest ventilation area in the world, the Southern Ocean (SO) plays an important role in the global climate system. Numerical model simulation of the SO is indispensable for us to obtain a reasonable understanding of the SO as well as its global impacts due to a lack of observations. However, the warm sea surface temperature (SST) bias in the SO has persisted for several generations of climate models participating in the Coupled Model Inter-comparison Project (CMIP), which lowers the credibility of model simulations for not only the current climate but also the future climate change. Thus, exploring the origins of the SO warm SST bias is always a hot and critical topic in the climate community, yet the results are still controversial and unclear. Recently, a study by the Innovation Team for Ocean-Land-Atmosphere Interaction and Global Effect of the Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai) gives a new idea for the origins of the common SO warm SST bias.

“We find that the origins of the SO warm SST bias in the previous CMIP5 models revealed by different researchers are inconsistent, one of the studies showed that such warm bias should be attributed to the overly weaker Atlantic meridional overturning circulation (AMOC), whereas another study claimed that the overestimation of the cloud-related shortwave radiation should be responsible. These two contradictory results prevent a clear judgment of the true source of the SO warm SST bias, which motivate us to further explore the persistent SO warm SST bias in the latest CMIP6 models”, says the corresponding author Dr. Jun Ying.

“We find both the biases of surface net heat flux and the AMOC strength contribute to the SO warm SST bias only in a few individual CMIP6 models”, says Dr. Fengyun Luo, the first author of the paper. “Instead, the robust common warm bias in the Northern Atlantic deep ocean can lead to the common warm SST bias in the SO through the AMOC transport as an adiabatic process, which has been verified by a series of idealized model experiments with passive-tracer”, she goes on to say.

“From the perspective of ocean dynamics, there has been such controversy for a long time—whether the SO warm SST bias comes from dynamic anomaly or thermodynamic anomaly transported by dynamics,” says co-corresponding author Prof. Dake Chen, the director of Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai). “Our study indicates that it should be the latter. Nevertheless, some local factors, such as the too-coarse topography and the insufficient representation of ocean eddy fluxes in the SO that are not considered within the current large-scale framework, could be also important for such warm SST bias”, he continues to say.

“Our findings highlight that some remote oceanic biases that are dynamically connected to the SO should be taken into account, which bring new opportunities and challenges to the development of climate models”, says Dr. Jun Ying.

Fig. 1 | Inter-model relationships. a) Inter-model relationship between the AMOC strength biases (units: Sv) and SST biases in the SO. b) Inter-model relationship between Northern Atlantic deep ocean temperature biases and SST biases in the SO for models with reasonable AMOC strength. c) Inter-model relationship between AMOC strength biases and Northern Atlantic deep ocean warm biases. The solid black line denotes the linear regression, and the correlation coefficient and the p value determined by a two-side Student’s t-test are also shown in each panel.

This study entitled “Origins of Southern Ocean warm sea surface temperature bias in CMIP6 models” has been published in npj Climate and Atmospheric Science in August 2023. Dr. Jun Ying and Prof. Dake Chen, from the Second Institute of Oceanography, Ministry of Natural Resources and the Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), are the corresponding authors, and their Ph.D. student Fengyun luo, from Zhejiang University, is the first author.

This study was jointly supported by the National Natural Science Foundation of China (Grants 42227901 and 42106008), the Scientific Research Fund of the Second Institute of Oceanography, Ministry of Natural Resources (Grand QNYC2001), and the Innovation Group Project of the Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai) (Grant 311022001)

Paper link:https://www.nature.com/articles/s41612-023-00456-6