Decades ago, a transformative moment in oceanography was the full realization that, rather than being comprised of a series of slow elegant currents, ocean circulation was instead alive with swirly “eddies”. The full complexity of these motions, which typically have scales of tens to hundreds of kilometers, can be see in this compelling simulation from NASA / SVS.
Now, the research frontier is moving to smaller scales. One of the hot research topics we are investigating is the so-called “sub-mesoscale”, the often nonlinear, energetic, sometimes chaotic motions at the edges of eddies. These motions diffuse and dissipate the mesoscale eddy energy, and can greatly enhance vertical transport of biologically essential nutrients, particularly in the upper ocean.
Submesoscale instabilities often are visible as small-scale, cuspy wiggles in high resolution sea-surface temperature (SST) images. Satellite observations give us a beautiful snapshot of the surface view. However, the instabilities riding on these eddies are often rapidly evolving, with a complex three dimensional vertical structure.
Alternately, submesoscale structures can be seen using satellite observations of biological tracers, such as the "ocean color" of phytoplankton. These complex, large-scale structures are quite challenging to observe with traditional oceanographic tools and sampling techniques.
The majority of our MOD sub-mesoscale work has been focused in the Bay of Bengal, as part of the ASIRI and MISO-BoB projects, as well as the in the Arctic.
CASE: OBSERVING sub-mesoscale instability in the Arctic ocean
Here we show an example of what is likely a sub-mesoscale instability observed in the Arctic ocean, highlighting the intrinsic interconnectedness of many of our research themes.
The upper left panel is a snapshot of simulated Arctic near-surface temperature (courtesy of the Naval Research Lab) during the time of our Arcticmix experiment in late summer 2015.
Near the edge of a strong front, we observed regular, very small-scale (< 1km) features riding on the interface between warm surface water and the cooler water below. The right panel shows ‘cuspy’ features of convergent sea ice, which may be created by the surface manifestation of related sub-mesoscale instabilities.
These surface features are clearly seen in the distribution of floating ice during our experiment.