Seasonality and spatial dependence of meso- and submesoscale ocean currents from along-track altimetry data

Submesoscale flows have been seen in simulations and shipboard data to exhibit qualitative seasonal changes, consistent with submesoscale baroclinic instabilities being activated in deep winter mixed layers. To further test and understand this, we use Jason-2/OSTM data to compute along-track sea surface height (SSH) spectra across the global ocean between 60°S and 60°N, for each calendar month. In each location and month we fit the spectra to a model that takes into account meso- and submesoscale balanced flow, altimeter noise, and where they are significant, the mode-1 and 2 internal tides. The balanced flow is modeled by a low-wavenumber plateau transitioning to a submesoscale power-law falloff.

When tides are weak, we find statistically significant variation with location and season in the parameters describing balanced motion. The power law exponent (spectral slope) of the balanced motion decreases in the winter and increases in the summer, and there is evidence of kinetic energy moving upwards to larger scales in the months following the month of maximum mixed layer depth. This correlates with the deepening and shoaling of the mixed layer (computed from climatology) and is consistent with shipboard ADCP data. We discuss the reliability of estimating the spectral slope when the internal tides are significant.