Science Background

The importance of ocean variability on small horizontal scales ("sub-mesoscales") has become increasingly emphasized in recent years as numerical models are able to simulate a wider range of dynamics from global and ocean basin scales to mesoscale eddies (typically 100–200 km in diameter—the dominant scale that results when large deep-ocean currents go unstable), but it is not yet clear how to handle the decay of eddy energy due to smaller scale processes that the models can't yet resolve. Lateral eddies generated by baroclinic instability within the mixed layer during restratification are one candidate process that has been described theoretically, but others (including vertical mixing by inertial oscillations and coupling between the atmospheric and oceanic boundary layers) could be dominant or acting simultaneously.

The new project (which we've labeled SMILE, for the "Sub-mesoscale MIxed-Layer Eddies" experiment) is funded by the National Science Foundation to study upper-ocean turbulence and mixed-layer restratification by baroclinic eddies. Field work on the R/V Sikuliaq in March 2017 will use a combination of a ship-towed profiler (SWIMS) and clusters of autonomous profiling floats (EM-APEX, which measure turbulence through temperature microstructure and horizontal currents through electromagnetic motional induction, in addition to the standard T and S measurements present on Argo floats). This will be an observational study of the theoretical process described by Fox Kemper et al 2008 in the oceanic environment described by Hosegood et al 2008.

Scientific leads ("Principal Investigators") on the project include Eric Kunze (Northwest Research Associates), Tom Farrar (Woods Hole Oceanographic Institution), and John Mickett and James Girton (University of Washington Applied Physics Lab).

Figure 1: Meso- and submesoscale structures in the North Pacific Subtropical Front zone (location marked in lower center panel), as seen in satellite sea-surface temperature (MODIS 1 km data) and height (OSCAR 1/3° currents—mainly from altimetry) fields. Two examples of the types of features that could be targeted by our autonomous arrays and ship-based surveys are shown in the lower left and right panels.