Atmosphere Ocean Science Colloquium

From eddies to climate: Pathways and ecology of ocean biogeochemical tracer transport

Speaker: Mara Frielich, UCSD/Scripps

Location: Online

Date: Wednesday, February 2, 2022, 3:30 p.m.


Vertical transport in the ocean plays an essential role in interactions between the ocean boundary layer, where heating, wind-driven momentum input, and gas exchange occurs, and the ocean interior. The narrow ocean surface layer is also where there is enough light for biological productivity. Vertical transport is therefore an important driver of biogeochemical dynamics and influences the oceans' role in the climate system. This talk discusses recent work using process study models, theory, and field observations of biogeochemical tracers to improve understanding of mesoscale and submesoscale vertical tracer fluxes and their influence on biogeochemical cycles.

I will demonstrate that vertical motion along sloping isopycnal surfaces is particularly important submesoscales (1-10 km spatial scales), as compared with uplift and relaxation of isopycnal surfaces. Motion tangential to isopycnal surfaces is particularly important for tracer transport. I derive a scaling relationship for the importance of the component of vertical motion tangential to isopycnal surfaces and validate it using models of a mid-latitude eddy field across a wide parameter regime including large-scale density structure and model resolution (1 km and 4 km). 

Vertical biogeochemical tracer flux depends both on the spatial and temporal characteristics of the vertical motion and on the rate of uptake by the biological community. Using Lagrangian analysis and analytic theory, I demonstrate that flux is maximized when the growth rate matches the inverse of the decorrelation timescale of vertical motion. As a result, there is a growth-transport feedback where variability in the frequency of vertical motion across different physical features of the flow favors phytoplankton production with different growth rates. In the mid-latitude oceans, phytoplankton productivity is maximized when the growth timescale is on the order of days -- the timescale of submesoscale dynamics -- and net productivity is enhanced in the presence of community diversity.

The results are used to interpret observations of microbial ecosystems and biogeochemical transport in the subtropical oceans. This fundamental understanding of the nature of mesoscale and submesoscale vertical velocity and biophysical interactions provides a framework for improving ecosystem and carbon cycle parameterizations in global and regional models with the goal of projecting changes in the rate and distribution of ocean carbon storage.