The Gulf Stream carries warm waters from the tropics to higher latitudes as it flows along the US East Coast, is a major source of the kinetic energy that stirs the ocean across a range of scales, and exhibits substantial variability in strength and position along the US East Coast at timescales from several weeks to years. While the significance of the Gulf Stream to the global climate system has long been recognized, subsurface observations that resolve its variability and dynamics are limited. Our group is using Spray gliders to fill the observational 1500-km-long gap between established, long-term sampling in the Florida Strait (NOAA’s Western Boundary Time Series) and downstream of Cape Hatteras (e.g., the Oleander program).
Our current Gulf Stream sampling plan sees one glider deployed just offshore of Miami, FL every two months. Each glider is commanded to steer perpendicular to measured currents when in the Gulf Stream. The resulting glider trajectories cross the Gulf Stream obliquely because the gliders are carried downstream while crossing the Gulf Stream. During missions lasting 110 days each, gliders complete about 10 Gulf Stream crossing each while moving downstream from Florida to New England. To date, more than 100 cross-Gulf Stream transects have been collected by our gliders.
Analysis of the observations from this ongoing program have yielded a number of new findings about the Gulf Stream. Among those are:
Potential Vorticity Structure and Stability: Potential vorticity is an important dynamical quantity in western boundary currents and can be used to diagnose the susceptibility of the current to various instabilities. Since gliders provide high-resolution measurements of both hydrography and velocity, we are able to estimate the full Ertel potential vorticity from the observations. Todd et al. (2016, J. Phys. Oceanogr.) combined early Gulf Stream glider measurements with similar measurements in the Loop Current to characterize potential vorticity structure in two segments of the North Atlantic western boundary current and to evaluate classic criteria for barotropic, baroclinic, centrifugal, and symmetric instabilities. As part of her thesis research supported under a previous grant, Joleen Heiderich is expanding upon this analysis of the Gulf Stream’s stability.
Topographic Effects: Over the relatively shallow Blake Plateau, strong flow associated with the Gulf Stream reaches to the seafloor. Todd (2017, Geophys. Res. Lett.) identified two mechanisms by which energy is extracted from the Gulf Stream as it flows over this shallow topography. Large-amplitude internal lee waves with frequencies near the local buoyancy frequency are generated near the Charleston Bump, a prominent topographic feature, and their vertical motion can be inferred from perturbations in the flight of gliders. In the same region, O(100)-m-thick bottom mixed layers are generated in the lee of topography, likely due to enhanced turbulence generated by O(1) m s-1 near-bottom flows.
Impact on Numerical Simulations: Temperature and salinity observations from Spray gliders operating in the Gulf Stream are routinely distributed in near-real time for operational usage and are made publicly available and citable after post-processing (Todd and Owens, 2016; doi:10.21238/S8SPRAY2675). Using the US Naval Oceanographic Office’s operational NCOM US East simulation as an example, Todd and Locke-Wynn (2017, Oceanography) showed how high-resolution, subsurface measurements of the strong, narrow front associated with the Gulf Stream can improve the model’s representation of the Gulf Stream’s structure. Similar impacts in other operational and reanalysis simulations that assimilate the observations are expected.
Glider-based Doppler Current Measurements: Todd et al. (2017, J. Atmos. Oceanic Tech.) used measurements from Spray gliders flying in the Gulf Stream as well as in the equatorial Pacific and California Current System to advance the techniques for estimate absolute velocity using gliders equipped with small Doppler current profilers. In particular, the Gulf Stream proved to be a region where surface current estimates from gliders drifting during surface communications was a useful additional constraint on the absolute velocity profile.