Stanford School of Earth Sciences

Leif Thomas

Current research projects

Lateral mixing in the ocean by submesoscale flows

Lateral stirring is one of the most basic oceanographic phenomena affecting the distribution of physical and biological fields throughout the ocean. Yet, it is poorly understood. Presently, there are no parameterizations for numerical models that handle both biological and physical dynamics at the submesoscale (100m-10km). A collaborative research initiative funded by the Office of Naval Research is currently underway to characterize, measure, and model lateral mixing at scales of 100 m - 10 km. This will involve numerical modeling, theory, and field observations taken off of Cape Hatteras where the confluence of the Gulf Stream and Slope Sea waters leads to an explosion of submesoscale activity. My role in the project is to study the fundamental physics of wind-driven submesoscale flows and to characterize their along-isopycnal transport and mixing of tracers. As part of this project, my student Dan Whitt and I are also investigating the interaction of near-inertial waves with the Gulf Stream front.

Mode water formation in the Gulf Stream

CLIMODE observations suggest that a significant fraction of the formation of Eighteen Degree Water (EDW), the subtropical mode water of the Atlantic, occurs within the eastward-flowing, separated Gulf Stream. Estimates that 50%-90% of the needed amount of new EDW is formed within this frontal region indicate that a new paradigm of EDW formation may be needed: one that departs significantly from the quasi-one dimensional ideas of purely cooling-driven formation in the Northern Sargasso Sea. This project aims to examine the robustness of these results through innovative analyses of the observations available from CLIMODE and high-resolution numerical simulations, including evidence for cooling and wind-driven production of EDW within the Gulf Stream frontal region, vigorous cross-frontal mixing associated with submesoscale instabilities and inertial shear.

Frontal dynamics and lateral mixing at the Equator

OGCM simulations of the equatorial ocean, and specifically the "cold tongue" region in the eastern Pacific, are particularly sensitive to parameterizations for lateral mixing. In the midlatitudes, frontal processes and submesoscale instabilities are thought to play an important role in the transport and mixing of tracers. In the low-latitudes currents are characterized by high Rossby numbers (because of the small Coriolis parameter) and thus their dynamics are in some ways analogous to mid-latitude submesoscale flows. Using theory adapted from studies of mid-latitude submesoscale processes and high-resolution, nested simulations of the Eastern Equatorial Pacific, my student Ryan Holmes and I are collaborating with Luanne Thompson and David Darr at the University of Washington in a NOAA-funded project to investigate the physics of subduction at the Equatorial front, tropical instability waves/vortices, and wind-forced symmetric instability, with the ultimate goal of characterizing and parameterizing the lateral mixing that they induce.

Past Research Projects

Mixing in sloping bottom boundary layers

My student Jessica Benthuysen, from the WHOI/MIT Joint Program, and I have been studying the formation of bottom mixed layers (BML), the modification of the PV, and the generation of secondary circulations during the spin-down of geostrophic currents over a sloping bottom. The BMLs generated during spin-down naturally develop horizontal buoyancy gradients, and thus are in some ways analogous to surface mixed layers in frontal zones.


Phytoplankton bloom in the Ross Sea

Role of fronts on the carbon uptake in the Ross Sea

The presence of submesoscale flows in the upper ocean confounds the traditional one-dimensional paradigm for the evolution of the mixed layer (ML). In frontal regions submesoscale processes can modify the stratification at rates that can easily exceed those associated with air-sea fluxes. This has important implications for carbon uptake in high-latitude, nutrient-rich waters such as the Ross Sea. In these regions phytoplankton growth can be light-limited, and thus submesoscale processes that affect the shoaling and deepening of the ML can modulate the strength and variability of primary productivity. Observations from the Ross Sea suggest that this effect was particularly active at the start of the spring bloom. Numerical simulations of fronts representative of the region and forced by observed air-sea fluxes performed by former EESS graduate student Matthew Long suggest that submesoscale processes play an important role in setting stratification, productivity, and CO2 fluxes in the Ross Sea.