Wang¹, Xiujun, Jim R. Christian^{1, 2}, Ragu Murtugudde¹ and Antonio J. Busalacchi¹

¹Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20742 USA, Tel: 301-405-1532, Fax: 301-405-8468, E-mail: wwang@essic.umd.edu and ²Canadian Centre for Climate Modelling and Analysis, University of Victoria, Victoria, BC, V8W 2Y2 Canada

Ecosystem dynamics and air-sea CO₂ flux in the equatorial Pacific: A model study

A carbon model is implemented into a coupled physical-ecosystem model to simulate carbon cycle in the equatorial Pacific. The ecosystem model consists of ten components, two groups (large and small) of phytoplankton, zooplankton and detritus, and carbon and three nutrients: ammonium, nitrate, and iron. The model is forced by climatologically monthly data of solar radiation, air temperature, air humidity, and wind stresses. Biological parameters are tuned so that the model can reproduce observed chlorophyll concentration. The model is capable of reproducing spatial and temporal variations in carbon and nutrient fields on basin-scale. In the Pacific Equatorial Divergence (PEQD), high surface nutrient concentration and pCO₂ are the consequence of upwelling. Primary production, which is equally contributed by both the small and large phytoplankton, is not limited by nutrients. This region shows high biomass and export production, but it is also a strong out-gassing region. Conversely, surface waters of the warm pool in the west are low in biomass, nitrate and iron concentrations, and pCO₂. Primary production is mainly limited by nitrate, and small phytoplankton is dominant in the ecosystem. Our modeled primary production is <30 mmol C/m²/d in the warm pool and >60 mmol C/m²/d in the PEQD, which agree with observations. Modeled oceanic pCO₂ is close to or slightly higher than atmospheric pCO₂ in the warm pool, but consistently higher than atmospheric pCO₂ in the PEQD. The high oceanic pCO₂ in the PEQD results in a strong source of CO₂ that air-sea flux is greater than 10 mmol C/m²/d.