Wind-induced cross-shelf flux of water masses and organic matter at the Gulf of Tehuantepec

The purpose of this study is to quantify the cross-shelf transport of water masses and organic carbon at the Gulf of Tehuantepec and relate this to the physical processes in the area. A medium-complexity, nitrogen-based ecosystem model is developed in order to simulate the ecosystem in the northeast tropical Pacific. The ecosystem model is run “offline” using a hindcast from a realistic physical ocean model as input. The physical model is forced with daily heat and momentum fluxes and therefore captures the intense, but short-lived, wind events at the Gulf of Tehuantepec. Assimilation of altimeter data facilitates a realistic representation of eddies and meso-scale variability. The ecosystem model includes two nutrients (nitrate and ammonium), two size classes of phytoplankton, two size classes of zooplankton, and detritus. The model is validated using SeaWiFS ocean color data and ship-based observations. Cross-shelf fluxes of water, nutrients, and plankton are calculated from the model and related to physical processes. The results show that at the Gulf of Tehuantepec there is substantial offshore export of organic material, particularly during wind events that generate an eddy (on the order of 10 Gg C/day). During these events, there is an even larger onshore carbon flux to the south of the gulf. The model-estimated transport of carbon away from the coast at the Gulf of Tehuantepec is 0.32 Tg C/year, and the onshore transport to the south of the gulf is 2.23 Tg C/year. The volume flux in the upper 150 m is comparable in magnitude, but of opposite sign, at the gulf and to the south of the gulf. The reasons why the carbon flux is seven times larger to the south of the gulf are (1) because of the high organic carbon content of the water advected onshore and (2) because the onshore flux persists throughout the year. At the gulf, the majority of nutrients upwelled by the wind events are upwelled offshore rather than on the shelf; hence the water that is transported offshore is not as rich in organic carbon. In addition, while the transport at the gulf is small or onshore during summer, it remains onshore to the south of the gulf. The total offshore flux of carbon at the Gulf of Tehuantepec could be underestimated since the model underestimates the amount of organic material close to the coast.