Mixed-layer Instabilities within the Loop Current Eddy, Poseidon (2016-2017).

The upper layer of Loop Current Eddies (LCEs) in the Gulf of Mexico (GoM) can be perceived as a large bulge of nearly homogeneous water or pycnostat separated by a double thermocline. Mixed-layer depth is shallow (~30m) in summer and deep (~150m) in winter. LCEs are mainly in geostrophic balance and contain different water masses than the Gulf Common Waters. Recent high-resolution numerical simulations show that submesoscale ageostrophic dynamics significantly enhance the vertical fluxes of thermohaline properties. Submesoscale variability arises in the form of filaments/fronts in regions of straining on the LCEs’ rim. This phenomenon has been poorly sampled in-situ due to its small spatial and temporal scales (and persistence). We describe high-resolution glider observations of mixed-layer instabilities during the life of the LCE Poseidon using 7 hydrographic sections of Poseidon performed between August 2016 and 2017. Sections were horizontally and vertically interpolated at 2km and 2m, respectively. Data indicate that mixed symmetric-gravitational instability is often enhanced within the core of the LCE’s presumably driven by wind and surface cooling in winter. Pure symmetric instabilities are occasional and enhanced near the LCEs periphery; while pure gravitational instabilities are enhanced during seasonal transitions from summer to winter, and vice versa. Tracer spectra become shallower at high-wavenumbers (k) and approach to a ~k^-2 slope typical of submesoscale flows. Although this work shows that submesoscale can be addressed using data obtained from a slow moving platform, there is a clear need of increasing the horizontal resolution to better resolve the smaller scales in the mixed-layer. In this sense we have recently performed a mission with two gliders synchronized following interlaced trajectories and only diving from surface down to 200m to increase horizontal resolution up to 1km.