Seasonal Reversal of the Near‐Surface Chlorophyll Response to the Presence of Mesoscale Eddies in the South Pacific Subtropical Countercurrent

Abstract

Abstract Mesoscale eddies have been shown to have significant effects on biogeochemical cycles, as observed in local levels of near‐surface chlorophyll. There are many regional differences in the expected response of near‐surface chlorophyll to the presence of eddies, caused by differences in the driving mechanisms. However, in many high eddy activity areas, previous studies found low correlation and an inconsistent chlorophyll anomaly response to the presence of eddies. One such area is in the South Pacific Subtropical Countercurrent. Using GlobColour ocean color data and Aviso altimetry data, an investigation of the area found that a seasonal reversal occurs in the character of the chlorophyll anomaly within eddies (reversal from positive to negative and vice versa). The cause of this reversal is inferred to be a seasonally changing limiting factor within the region. Argo float profiles colocated inside and outside of eddies are used to show the coincidence of chlorophyll anomalies with seasonally changing mixed layer depths and the ability of the eddies to access deep nutrient pools. Observations of other mechanisms, such as eddy stirring or eddy‐Ekman pumping, are found to be seasonally less important than the mixed layer depth change‐induced nutrient flux . Additionally, metrics are developed to globally identify oceanic regions in which such seasonal reversals in chlorophyll anomalies could occur.