Mixed layer depth dominates over upwelling in regulating the seasonality of ecosystem functioning in the Peruvian upwelling system

Abstract. The Peruvian upwelling system hosts a marine ecosystem with extremely high productivity. Observations show that the is only eastern boundary (EBUS) an out-of-phase relationship between seasonal surface chlorophyll concentrations and intensity. This “seasonal paradox” triggers following questions: (1) what are unique characteristics of system, compared other EBUSs, lead to relationship, (2) how does paradox influence functioning? Using observational climatologies for four we diagnose one reveal intense coincides deep mixed layers. We then apply coupled regional ocean circulation biogeochemical model (CROCO–BioEBUS) assess interplay layers regulates seasonality in system. Our reproduces within 200 km off coast. confirm previous findings regarding main contribution layer depth chlorophyll, relative upwelling. Deep austral winter cause vertical dilution phytoplankton strong light limitation, impacting growth. effect advection, though second-order, consistent enhanced offshore export opposing coastal build-up biomass. In addition, find relatively colder temperatures upwelled waters slightly dampen productivity further slow impact from combination propagates through ecosystem, primary production efficiency. emphasize crucial role suggest may increase, along weakened paradox, response shoaling under climate change.