Impact of Anticyclonic Eddies Under Stormy Weather on the Mixed Layer Variability in April South of the Kuroshio Extension

Abstract Subtropical mode water (STMW) plays an important role in the North Pacific climate system by recording climate variability and by taking in anthropogenic carbon dioxide. A thick STMW forms in anticyclonic eddies (AEs) with deeper mixed layers during winter and early spring. However, the mixed layer depth (MLD) variability and mode water formation in AEs under synoptic weather events are not well known. In this study, we tracked an AE located south of the Kuroshio Extension for several months by 17 Argo floats with daily sampling. We found that the MLD inside the target AE deepened sharply to more than 150 m during storms in April, while the MLD outside had little change and was only 50 m on average. We further compared the different mixed layer deepening processes inside and outside other AEs over the same region based on historical Argo data from 2004 to 2019. Since the relatively weak stratification in AEs is easily destroyed by storms with strong wind bursts and significant surface cooling in early spring, the mixed layer deepening time period lasts longer (December–April) inside AEs than outside AEs (January–March). The mode water ventilation and formation period are correspondingly extended in AEs, with important implications for North Pacific climate variability and the biogeochemical cycle.