Structure of Sea Surface Temperature Anomaly Induced by Mesoscale Eddies in the North Pacific Ocean

Sea surface temperature anomalies (SSTAs) induced by oceanic mesoscale eddies trigger mesoscale air-sea interactions and modulate large-scale climate systems. Yet, how eddies drive SSTAs has not been firmly established; particularly, the relative importance of lateral stirring and vertical pumping remains a debated issue. This study investigates characteristics and generation mechanisms of mesoscale eddy SSTAs in three eddy-enriched domains of the North Pacific Ocean: the Kuroshio Extension (KET), the Subtropical Countercurrent (STCC), and the North Equatorial Countercurrent (NECC). Analysis of satellite observational data reveals quasi-monopole eddy SSTAs in the KET and NECC and dipole-like eddy SSTAs in the STCC. By investigating spatial and seasonal variations and performing sensitivity experiments using an idealized model, we demonstrate that lateral stirring plays a more important role than vertical pumping in causing mesoscale eddy SSTAs. The eastward transport by the strong background current U-C (similar to 0.6 and similar to 0.4 m s(-1) in the KET and the NECC, respectively) and the westward eddy translation U-C (similar to-0.2 m s(-1) in the NECC) can dramatically modify the structure of SSTA. The pure stirring effect of eddy rotation velocities generates strong dipole-like SSTAs. Owing to the eastward U-C and the westward U-T, the western SSTA pole tends to approach the eddy center, while the eastern SSTA pole departs from the eddy and scatters along the eddy trajectory. These effects reduce the eddy SSTA amplitude and favor the emergence of quasi-monopole structure. This work provides a useful benchmark for model simulations of mesoscale SST variability and air-sea interaction. Plain Language Summary Mesoscale sea surface temperature anomalies (SSTAs) of tens to hundreds of kilometers induced by oceanic eddies are one of the most striking features of satellite SST images. Yet, we still have no firm answer to the question as to how eddies drive SSTAs. Particularly, whether the eddy SSTAs are induced by lateral stirring or vertical pumping is unknown. Here, by analyzing satellite data, we identify both monopole-like (a single warming/cooling core within the eddy) and dipole-like (a pair of opposite-sign anomaly patches) eddy SSTAs in the North Pacific, and they are both intimately associated with the lateral stirring of eddies. We further demonstrate that the existence of eastward background current and westward movement of eddies are favorable for the generation of such quasi-monopole SSTAs. While the pure stirring effect of the eddy rotation produces dipole-like SSTAs, the eastward U c and the westward U T gradually push the western SSTA pole toward the eddy center and finally give rise to quasi-monopole SSTA structure. In addition to insights into the mechanisms, our work provides useful knowledge for the simulation of mesoscale SST variability and air-sea interaction by climate models.

Automated summary

This study investigates the characteristics and generation mechanisms of mesoscale sea surface temperature anomalies induced by oceanic mesoscale eddies and demonstrates that lateral stirring plays a more important role than vertical pumping in causing these anomalies.