Collaborative Research: Isopycnal Spectra and Stirring on the Submesoscale and Finescale in the Upper Ocean

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Lateral stirring is among the primary processes, along with advection and diapycnal mixing, that determines the distribution and fate of water-mass properties, nutrients and dissolved gases in the upper ocean. Ocean general circulation models resolve stirring on scales larger than thirty kilometers, and regional models on scales close to one kilometer, but isopycnal stirring by unresolved submesoscale processes must still be parameterized. Novel high-resolution observations in the sub-tropical Pacific Ocean will provide insight into the underlying physics and how these processes operate in the upper ocean in order to improve such parameterizations for eddy-resolving ocean circulation models. Results should be applicable to the submesoscale in other rotating stratified fluids such as the atmosphere of Earth, stars and gas giants that occupy the same parameter space. A post-doctoral fellow will be trained in interpretation of data in the rapidly emerging field of oceanic submesoscales. Two graduate students will have an opportunity to participate in sea-going research.Order one Rossby number and gradient Richardson number sub-inertial flows have emerged as an area of ocean parameter space that is incompletely understood but becoming accessible to observations, modelling and theory. The kinematics and dynamics of isopycnal stirring on length scales shorter than the Rossby deformation length of order 100 kilometer and longer than outer turbulence scales of order one meter do not have a well-established theoretical underpinning because of the paucity of observational constraints. At scales shorter than the Rossby length scale, quasigeostrophic (QG) theory predicts that tracers should have a spectrum of slope of minus one [corresponding to tracer-gradient spectra with slope plus one]. This contrasts with numerous observations spanning the horizontal horizontal wavelengths 30 m to 500 km which report significantly redder and often almost flat tracer-gradient spectra over this wavenumber band. Similar dynamical comparisons are in short supply. High resolution sections of the upper 150 meter of the ocean will be sampled using a towed chain with tightly spaced temperature, conductivity and pressure (CTD) sensors and the shipboard Acoustic Doppler Current Profiler (ADCP). This project will characterize the horizontal wavenumber spectra of passive tracers (water-mass, spice, salinity anomalies) on isopycnals to higher horizontal wavenumbers than previously accomplished, and resolve the sub-inertial O(1 km) horizontal strain tensor thought responsible for stirring on these scales, as well as determine strain timescales. These measurements will provide key observational constraints on order one Rossby number and Richardson number flows.