What are Submesoscales?
It is perhaps common knowledge that water at the poles is colder than that found at the equator. What is perhaps less well understood is that the water does not get progressively colder as one moves away from the equator but instead tends to exhibit sudden ‘jumps’ in temperature at specific locations. These ‘jumps’, across which temperature may change by several degrees in just a few kilometres, are called fronts and represent the boundaries between different water masses that have markedly different properties, most notably temperature and salinity. These dynamic boundaries reflect the large-scale circulation throughout the ocean and in particular the role played by winds and convection in driving some of the most well known ocean currents such as the Gulf Stream, the Kurushio and the Antarctic Circumpolar Current (ACC). Considering the example of the ACC as it passes through Drakes Passage between South America and the Antarctic Peninsula, we can see that the fronts found within these major currents are not straight lines but exhibit a series of meanders. These are due to instabilities of the fronts are referred to as ‘mesoscale’ instabilities due to the predominant influence of the earths rotation in their evolution. They have lengthscales of approximately 100 km and evolve over timescales of weeks and months. The animation below shows how these meanders pass through the region over the course of a year.
As one looks closer, however, we notice that rather than these fronts being a single, abrupt ‘jump’ in water properties from one water mass to another, the transition in fact occurs in the form of a multitude of swirls and streaks, within which one can see numerous much smaller, weaker fronts. We can see how these smaller fronts emerge as the horizontal resolution of a numerical model is increased from 20km to 500m; see the figure below. In the top panel you see the sea surface temperature predicted by a coarse-scale simulation that does not resolve submesoscales; it performs calculations at grid points that are around 20 km apart. In the bottom panel you can see the same calculations being made on a grid that has points separated by only 500m and which is therefore able to recreate the small-scale fronts that we are focussing on in SMILES.
The length scale of these submesoscale fronts is typically 10 km or less and they are found throughout the surface mixed layer, which is the top layer of the ocean that is being continuously mixed by the wind, surface waves and cooling. They also evolve over a timescale of hours to days, much more quickly than the larger, mesoscale features within which they are embedded. Rather than being overwhelmingly influenced by the earth’s rotation as in the case of mesoscales, submesoscales are also strongly influenced by gravity. As a result they generate three-dimensional circulations with a significant vertical component compared to the largely horizontal component associated with mesoscales.