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Seasonality in surface (sub)mesoscale turbulence and its impact on iron transport and primary production

Uchida, Takaya

Mesoscale turbulence is ubiquitous in the surface ocean and has significant impact on the large-scale ocean circulation and its interaction with the climate. Ocean currents are most energetic in the mesoscale range on the scales of 20-200 km and recent studies have shown that the surface kinetic energy associated with the mesoscale undergo a large seasonal modulation. At scales below the mesoscale where geostrophic approximation breaks down lies the submesoscale (1-20 km). It is at this scale that baroclinic instabilities feed off the available potential energy stored in the deep wintertime mixed layers, known as mixed-layer instability, and in return energize the mesoscale via inverse energy cascade under the constraint of stratification and rotation. Mixed-layer instability (MLI) is inherently submesoscale due to the depth scale associated with it. We show the robustness of MLI on global scale in modulating seasonality in surface mesoscale turbulence by analyzing outputs from a Community Earth System Model fully ocean-atmosphere coupled run with eddying resolution.

Due to the rigorous vertical velocities associated with mesoscale turbulence, in the context of climate, they have been shown to make major contributions to the transport of heat and tracers including carbon. More recently, it has been argued that submesoscale heat transport may dominate over the mesoscale. We ask the same question for tracers: What is the relative contribution of submesoscale transport (local effect) over the energized mesoscale via inverse energy cascade (remote effect)? In order to investigate their impact on the dynamics and tracer transport, we run our own seasonally resolving submesoscale permitting channel model configured to represent the zonal-mean view of the Southern Ocean coupled to a full biogeochemical model.

The Southern Ocean is unique in that, apart from it being the only zonally re-entrant basin on Earth, it is one of the high-nutrient low-Chlorophyll oceans and iron is predominantly the limiting nutrient for primary production within the open-ocean region. As the basin responsible for generating the densest water mass properties, i.e. Antarctic Bottom Water, and outcropping isopycnals, primary production and the associated biological carbon pump have been of long interest to the biogeochemical and climate community. We provide an independent estimate from satellite observations of the seasonal cycle in phytoplankton biomass by taking advantage of the biogeochemical Argo floats, in which we show that the biomass reaches its maximum around December in the open-ocean region. Our modelled ecosystem reaches its maximum in November, roughly a month earlier, likely due to the lack of aeolian dust input at the surface, and glacial and bathymetric sources from the south in our model.

Utilizing spectral analysis and the generalized Omega equation, we decompose the eddy transport of heat and iron to its submesoscale (local) and mesoscale (remote) contributions. With the exception near the surface where mixed-layer instability is active, our results indicate that mesoscale vertical transport is of first-order significance in calculating the budgets and supplying iron across the mixed-layer base to the surface where phytoplankton can effectively photosynthesize.

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More About This Work

Academic Units
Earth and Environmental Sciences
Thesis Advisors
Abernathey, Ryan Patrick
Degree
Ph.D., Columbia University
Published Here
October 21, 2019