Frontogenesis Elevates the Maximum Chlorophyll a Concentration at the Subsurface Near the Kuroshio During Well‐Stratified Seasons

Abstract

AbstractThe contribution that fronts and frontogenesis make to subsurface chlorophyll a (CHL) concentration was investigated near the Kuroshio using data from multiple surveys across the Kuroshio during summer and fall and satellite sea surface height (SSH) data alongside high‐resolution reanalysis data set. An intensive survey was conducted during a period of frontogenesis as seen from the in‐situ and satellite‐derived velocity data as well as the analysis of the reanalysis data set. Physical and biological observation showed shoaling of the nitracline and a subsurface CHL maximum (SCM) near the evolving front. The CHL concentration at the SCM (CHLSCM) was large near the front compared with the concentration in ambient water. Sections of 10 repeat seasonal surveys also captured the frontal structure, the shoaling of the nitracline, and the increase in CHLSCM as well as their interrelationship. The CHLSCM was large, and the depth of the SCM (ZSCM) was shallow above a large horizontal density gradient (i.e., marked mesoscale frontal structure) in the ocean interior. In addition, we demonstrate that the lateral strain rate, which is a measure of frontogenesis obtained from the SSH, can be used to estimate ZSCM and CHLSCM. Frontogenesis, secondary circulation, symmetric instability, and vertical diffusion were considered to cause the nutrient supply to the subsurface layer. The surveys and analysis indicate that fronts and frontogenesis increase CHLSCM, and the physical data obtained from both in‐situ and satellite observations can be used to estimate CHL distribution in the ocean interior.