Frontal Coupling Regulated by Mesoscale Eddies in the Subarctic Frontal Zone

Abstract. Frontal coupling and its interaction with mesoscale eddies plays a critical role in water mass exchange and the ecodynamic balance in the Subarctic Frontal Zone of the northwestern Pacific. Using ten years (2014–2023) of daily satellite observations, this study systematically investigates the coupling characteristics of temperature and chlorophyll fronts under eddy modulation, along with the spatiotemporal responses of frontal coupling to eddy evolution by eddy partitioning, normalization, and lagged cross-correlation and mediation analyses. The eddy modulation on temperature and chlorophyll-a fronts pronouncedly depend on eddy polarity and developmental stage. Cyclonic eddies enhance chlorophyll-a fronts in the eddy core and concentrate temperature fronts at their periphery, whereas anticyclonic eddies suppress both frontal types in the core with recovery in the exterior. Under cyclonic eddy perturbation, the probability of frontal coupling increases with eddy intensity. The optimal lag time is 3–8 days, and the coupling probability in the eddy core is much higher than that of exterior. Anticyclonic eddies show a weaker promoting effect that is inversely related to eddy intensity, with a lag time of longer than 10 days. The responses of the two frontal types to eddy forcing are asynchronous. There is a time lag of several to over ten days between the physical-biological frontal coupling and eddy generation. While temperature fronts play a significant mediating role in this process, the mediation mechanisms differ according to eddy dynamic parameters and lag time. These findings demonstrate that eddy polarity and intensity jointly determine the direction, magnitude and duration of frontal coupling. That will provide a better understanding for the coupled mechanisms between physical and ecological fronts, as well as the oceanic dynamical and biological responses to eddy modulation.