Significant progress has been made in the observation, understanding, simulation and prediction of large-scale air-sea interactions in the tropical regions, while that in mesoscale air-sea interactions in the mid-high latitudes are still very limited.

There is a clear lack of theoretical frameworks that can systematically explain the mesoscale air-sea coupling dynamics. Moreover, great uncertainties still exist in representing the mesoscale air-sea coupling processes and simulating corresponding observations using numerical models; the effects of mesoscale air-sea coupling processes on large-scale air-sea interaction and climate are not well known.

A research team led by Prof. ZHANG Rong-Hua at the Institute of Oceanology, Chinese Academy of Sciences (IOCAS), has undertaken studies on mesoscale air-sea interactions and the underlying mechanisms in the mid-latitudes, providing a new insight into the mechanisms of atmospheric responses to mesoscale sea surface temperature perturbations.

The recent study was published in Journal of Advances in Modeling Earth Systems (https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021MS002822).

High-resolution satellite measurements indicate a strong positive correlation between mesoscale sea surface temperature (SST) and surface wind stress perturbations in the extratropical oceans, but low-resolution coupled ocean-atmosphere models have difficulty in accurately simulating the corresponding spatial patterns and coupled intensity.

Simulated products from a high-resolution Community Earth System Model (CESM-HR) are used to assess the extent to which the mesoscale coupling is captured in this study, focusing on two regions respectively, the Kuroshio Extensions (KE) and Agulhas Return Current (ARC).

It is found that compared with satellite observations, the CESM-HR can depict the characteristics of the mesoscale air-sea coupling well, with its strength being comparable to what is observed; but in the KE region, the simulated strength during summer is only half of the observed.

Researchers analyzed mechanisms for the atmospheric responses to mesoscale SST perturbations through the pressure adjustment (PA) and the downward momentum transport (DMT). The results highlight different mechanisms for controlling the atmospheric responses over the KE and ARC regions, which are regionally and seasonally dependent.

In the ARC region, pronounced dipole patterns of sea surface pressure (SLP) and vertical velocity perturbation responses indicate that the DMT exerts a dominant effect in both winter and summer. In the KE region, monopole patterns show the main role played by the PA mechanism for the atmospheric adjustment in summer, while dipole patterns indicate the main role played by the DMT mechanism in winter.

Because the PA mechanism is mainly manifested as vertical heat transport which acts to adjust sea surface air pressure, the weak coupling strength indicates that the parameterization of vertical heat mixing in the atmospheric model is not appropriately represented and thus needs to be improved.

This finding has important implications for improving the parameterizations of the CESM-HR for enhancing high-resolution and long-time prediction capability.

This study was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences, the National Natural Science Foundation of China, the National Key Research and Development Program of China, etc.

A schematic showing the atmospheric response patterns to mesoscale SST perturbations in the atmospheric planetary boundary layer in association with (a) the PA mechanism and (b) the DMT mechanism, respectively.

Zhijia Tang, Rong-Hua Zhang*, Hongna Wang, Shaoqing Zhang, Hong Wang. (2021). Mesoscale surface wind-SST coupling in a high-resolution CESM over the KE and ARC regions. Journal of Advances in Modeling Earth Systems, 13, e2021MS002822. https://doi.org/10.1029/2021MS002822 

(Editor: ZHANG Yiyi)