Numerical investigation and validation of stratified up-side down Langmuir turbulence

This project will investigate wind-aligned circulation features in the ocean upper layer during conditions of low winds and strong (typically rain-induced) stratification. Under these conditions, such features do not conform to expectations for either typical Langmuir turbulence or wind-driven viscous sheer induced structures. The hypothesis is that under appropriate forcing conditions, stratified shear flow at the base of a surface-trapped jet becomes unstable to convergence and upward displacement in the presence of surface-intensified Stokes shear.

This drives upwelling of slower, denser water towards the surface, and recirculation. Numerical modeling and further analysis of existing data will be used to test the hypothesis that the observed streaks in question arise from this process of ‘stratified upside-down Langmuir turbulence’ (SUDL-T). The proposed activity will lead to better parameterizations of mixing at the ocean surface, with broad relevance for air-sea fluxes of heat, gas, and momentum.

The project will support a PhD graduate student, contribute to local public outreach and educational programs, and have broad dissemination through presentations at conferences, publication in peer-reviewed journals, and public availability of validated numerical simulations of near-surface mixing.

Infrared imagery of the sea surface during periods of modest winds and strong near-surface salinity stratification from rain shows wind streaks at spacings that suggest they are generated by a different mechanism than classic Langmuir circulations or wind-driven surface stress. The project objectives are to investigate this potentially newly observed mechanism by: 1) characterizing the vertical scale, kinematics, and energetics of the secondary circulation associated with the wind streaks as a function of environmental conditions; 2) determining the mechanism responsible for the observed elevated crosswind length scales at low wind speeds and the balance of forcing associated with transition to smaller scales and Langmuir turbulence in moderate wind; and 3) evaluating the impact of the secondary circulation at low winds on near-surface vertical mixing.

To do this the project will use a combination idealized and realistic Large Eddy Simulation modeling, coupled to further data analysis and direct model-data comparison. Modeling will include suites of runs from spin-up, or increasing forcing from low to moderate ranges, and spin-down, the relaxation of forcing through similar forcing levels. Reanalysis of the observations will focus on determining the surface forcing and subsurface hydrography and shear during the observations, in order to form a more stable basis for validating the project hypothesis with a more realistic modeling suite and direct model-data comparison.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.