Simulating Carbon to Chlorophyll Ratio and Deep Chlorophyll Maximum in the Equatorial Pacific: Diurnal, Seasonal and Interannual Variations

Deep chlorophyll maximum (DCM) layers are commonly observed in the euphotic zone of the equatorial Pacific. There has been evidence for significant spatial and temporal variability in DCM layers (e.g., chlorophyll concentration and DCM depth) at diurnal to seasonal and longer time scales. However, the mechanisms underlying DCM formation and contributing to DCM variability are not well understood or documented. DCM fluctuations may reflect a combination of several physical and biogeochemical factors, including mixed layer and nutricline depths, ecosystem structure, a variable carbon to chlorophyll (C:Chl) ratio in phytoplankton, nutrient limitation and light conditions in the euphotic zone. Among these, the C:Chl ratio plays a large role in DCM formation and variability, which is not yet well resolved in most regional and global biogeochemical models. In this study, we apply a fully coupled three dimensional physical-biogeochemical model which has realistic vertical mixing and food-web dynamics. Model forcing includes six-day winds, and seasonal incident light with diurnal cycle. Chlorophyll is computed using a variable C:Chl ratio which is a function of light, temperature and nutrient concentration. The model reproduces observed DCM and C:Chla ratio for the tropical Pacific. Our study indicates that the DCMs are permanent but not stable features, revealing pronounced fluctuations at both seasonal and interannual time scales. Our analyses focus on the temporal variations of DCM and C:Chl ratio and the impacts of physical, biological and chemical processes. Our study also provides comparisons between the oligotrophic waters in the equatorial Pacific western warm pool and the eastern and central upwelling region.