Abstract

Observations and models suggest that upper-ocean density fronts sometimes exhibit higher chlorophyll, more biomass and/or different plankton community composition at and/or below the surface in a narrow region localized to the front. However, the dynamics that lead to biogeochemical anomalies at fronts are not fully understood.

In this talk, I will briefly review observations of anomalous biogeochemistry at fronts in the upper ocean. Then, I will present some results from an ongoing numerical process study of how the unsteady part of the wind stress frequency spectrum can sustain higher upward nutrient fluxes and plankton biomass at geostrophic fronts. In particular, I will use experiments with a wind-forced primitive-equation model of an idealized geostrophic front coupled to a four-component ecosystem model to illustrate a synergistic interaction between stronger low frequency (sub-inertial) and weaker high frequency (near-inertial) parts of the wind stress. In this scenario, the addition of a weak high-frequency stress to a strong low-frequency stress leads to a qualitative change from deep biomass maximum to surface bloom and a large increase in the depth-integrated biomass at the front. I will discuss the physics that lead to this biogeochemical sensitivity and the potential implications for other ocean biogeochemical models with different levels of the high-frequency wind stress variance and/or mesoscale-submesoscale geostrophic kinetic energy.