Mesoscale eddies are the primary source of mesoscale sea surface temperature (SST) variability, triggering mesoscale air-sea interaction and leaving rectification effects on large-scale ocean circulations and climate systems. 

However, how eddies drive structure of sea surface temperature anomalies (SSTAs) remains an open question.

Recently, a research team led by Prof. WANG Fan from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) provided new insights into the formation of structure of sea surface temperature anomalies induced by mesoscale eddies in the North Pacific.

The study was published in Journal of Geophysical Research: Oceans on Feb. 26.

The researchers used satellite data to analyze three regions of the North Pacific: the Kuroshio extension (KET), the subtropical countercurrent (STCC), and the North Equatorial Countercurrent (NECC). They found that eddy SSTAs showed a monopole structure in KET and NECC regions, and a dipole structure in STCC region.

They suggested that lateral stirring acted as the major driver for eddy SSTA. Further, they independently constructed a simple model and isolated the effects of four processes: Stirring, Rotation, Transport and Translation. 

"Stirring process produced strong dipole SSTA, while Rotation process rotated this dipole SSTA about 45 degrees and weakened its amplitude. Transport and Translation contributed to the formation of the monopole SSTA and further reduced the SSTA created by stirring," said Prof. WANG.

By this model, the study found that larger background SST gradient and eddy rotation speed favored large amplitude eddy SSTA. However, rapid mean current and eddy translation weakened the strength of eddy SSTA.

Furthermore, mean current (U_C) and eddy movement (U_T) can modify the SSTA structure when only the lateral stirring regime operates.

"The monopole SSTA in the NECC region results from both the fast westward U_T and the strong eastward U_C. By contrast, since both U_C and U_T are small in the STCC region, we observe dipole-like eddy SSTAs," said LV Mingkun, first author of the study.

"Many aspects of mesoscale eddies are poorly simulated in present climate models, leading to notable model biases. With this regard, this work provides a valuable benchmark for the evaluation of model capability in simulating the mesoscale SST variability for better predicting the future climate," said Prof. WANG.

"Since mesoscale eddies are ubiquitous in the world's ocean, the generation of monopole SSTA maybe universal in other regions with a large U_C or U_T. Dipole and monopole SSTAs may cause different flavors of surface turbulent heat flux anomalies, leading to diverse atmosphere-eddy feedbacks, and possibly further feedback on our climate," said Dr. LI Yuanlong, the corresponding author.

The research was supported by the Strategic Priority Research Program of Chinese Academy of Sciences, National Key R&D Program of China, National Natural Science Foundation of China, Shandong Provincial Natural Science Foundation, and the Key Deployment Project of CAS Centre for Ocean Mega-Science.