The Physical-Biogeochemical Responses to a Subsurface Anticyclonic Eddy in the Northwest Pacific

Ya-Nan Ding,Feng Nan,Ying Tang,Ran Wang,Fei Yu,Yansong Liu,Qiang Ren

doi:10.3389/fmars.2021.766544

Abstract

Due to the unique physical processes of mesoscale eddies, the physical and biogeochemical properties within the subsurface anticyclonic eddy (SSAE) and in the surrounding water are distinct. Analyses using satellite and model data have revealed distinct seasonal variations in the central potential density structure of a long-standing SSAE south of Japan; this SSAE exhibits a normal concave isopycnals structure from January to April and a convex lens isopycnals structure from May to December, and these variations may be related to the subduction of low-potential vorticity (PV) mode water. In contrast to the idea of the self-sustained oscillation mechanism, the strength of the SSAE was enhanced due to the eddy kinetic energy provided by dramatic increasing of the positive baroclinic conversion rate during the Kuroshio path transition period from the non-large meander (NLM) path to the large meander (LM) path. Twofold to threefold enhancement of chlorophyll (CHL) was detected in the subsurface CHL maximum layers at the core of the SSAE, and this enhancement was related to the injection of nutrients into the euphotic layer due to winter mixing and the convex of isopycnals. During the period from May to December, elevated CHL and dissolved oxygen (DO) levels and reduced nitrate levels were observed along the periphery of the eddy below the maximum subsurface CHL anomaly depth. The combined result of these two processes: (1) the central downward displaced isopycnals caused by intensified SSAE, and (2) winter mixing deepened to the nutricline due to the thickened mixed layer depth (MLD) and weakened stratification in winter 2017 (during the NLM period) may have led to numerous nutrients and CHL enrichments throughout the mixed layer, thus generating a CHL bloom in the following April. The SSAE intensified in winter 2018 (during the LM period), whereas the shallower MLD and stronger stratification limited the depth of CHL downward displacement.

Highlights

Mesoscale eddies are common physical processes in oceans characterized by energetic, swirling, time-dependent circulations that are approximately 100 km wide and include both warm-core and cold-core eddies (Rhines, 2001)
With the development of the large meander (LM) path, the radius, amplitude, and intensity of the anticyclonic eddy gradually increased as its position shifted to the southwest
The composite anomaly structure suggested that this eddy was a subsurface anticyclonic eddy (SSAE), characterized by having the maximum temperature and salinity anomalies core as well as maximum velocity in the subsurface

Summary

INTRODUCTION

Mesoscale eddies are common physical processes in oceans characterized by energetic, swirling, time-dependent circulations that are approximately 100 km wide and include both warm-core and cold-core eddies (Rhines, 2001). Biogeochemistry Data The CHL (units: mg m−3) concentration data were downloaded by file transfer protocol (FTP) from the CMEMS2 global ocean biogeochemistry analysis and forecast product (001-028), which was produced at Mercator-Ocean (Toulouse, France) This product is displayed on a regular grid at 1/4 degree resolution, with 50 vertical levels ranging from 0 to 5,500 m, on the global ocean, and includes daily and monthly mean fields of the 10 following biogeochemical variables over the period from January 2017 to the present: CHL, nitrate, phosphate, silicate, dissolved oxygen, dissolved iron, primary production, phytoplankton, PH, and surface partial pressure of carbon dioxide.

RESULTS

SUMMARY AND DISCUSSION

DATA AVAILABILITY STATEMENT