Patchy productivity in the open ocean

Localized upwelling is a strong driver of primary production in many regions of the open ocean. This is achieved through providing an increased vertical flux of nutrients to otherwise starved surface waters. The impact of such upwelling hot spots on regional production is investigated with particular emphasis on three parameters: the fraction of the region experiencing upwelling, the increase in nutrient flux within upwelling regions with respect to ambient waters, and the rate of horizontal mixing between upwelled and ambient waters. It is demonstrated that although independently increasing one of these parameters has a limited effect on the production of the region, increasing them simultaneously can dramatically increase production. Furthermore, it is shown that the spatial distribution of upwelling regions can have a strong influence on productivity. Numerous small upwelling regions increase production significantly more than one large upwelling hot spot even if the total rate of upwelling is constant for the two cases. The magnitude of this discrepancy is shown to be influenced by the presence of coherent structures such as eddies in the surface ocean. The ocean components of global carbon cycle models are still far from being able to resolve mesoscale features fully. These results imply that such models may be greatly underestimating primary production in large parts of the ocean as a consequence.