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Assessment of the evolution of water-column stratification following the hypothetical implementation of widespread controlled upwelling in the North Pacific Subtropical Gyre
Gérard C. Nihous

The concept of widespread controlled upwelling proposed by Karl and Letelier (2008; Mar. Ecol. Prog. Ser. 364:257–268) is examined in a time-domain one-dimensional advection diffusion model of the oceanic water column for conditions representative of the North Pacific Subtropical Gyre (NPSG). Temperature is a proxy for potential density. When unstable stratification occurs in the time-stepping numerical algorithm, local homogenization is allowed to mimic buoyancy- driven mixing. Steady-state as well as time-varying solutions of the temperature field are calculated under various upwelling scenarios. Results suggest that upwelling rates exceeding the model’s background vertical advection rate (≈ 4 m yr-1) would profoundly affect upper ocean stratification. With large upwelling rates, the temporary cooling of the mixed layer would trigger a large compensating heat input from the atmosphere. High downward return advection rates associated with the controlled upwelling process would then transport this heat to deeper layers. In the long run, a net ocean warming would occur as well as a deepening of the mixed layer. This suggests that the potential carbon sequestration benefits from widespread controlled upwelling across the NPSG are unlikely to exceed 0.1 Gt C yr-1. Finally, it is argued that Ocean Thermal Energy Conversion (OTEC) electricity production throughout the NPSG should be considered for comparative purposes since OTEC also would use large amounts of deep seawater. For comparable flow rates, it is shown that OTEC systems would provide a much more effective atmospheric carbon control tool if they replaced fossil-fuel thermal power plants.

Keywords: Artificial upwelling, Mixed layer depth. Ocean stratification, Carbon sequestration

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