Regional Energy Exchanges, Turbulent Cascades

Ocean scientists from six U.S. and two Taiwanese institutions aboard R/V Thomas G. Thompson, having departed Guam at the end of January 2025, spent a part of their one-month research cruise following a small vortex, crossing back and forth with the ship through the swirling currents. The 25-day cruise was one of the final observational campaigns of the Office of Naval Research Departmental Research Initiative called ARCTERX — Island ARC Turbulent Eddy Regional eXchange.

Research efforts targeted the Philippine Sea at the end of winter, when the currents and thick surface ocean mixed layers interact, spinning off energetic eddies and meanders. These, in turn, can generate smaller, ephemeral instabilities – fronts and filaments – along their edges, breaking down to smaller and smaller eddies leading to turbulence or, counterintuitively, can merge and lead to a transfer of energy to larger scales. The initiative’s broad goal is to characterize the strength and properties of the cascade of kinetic energy at the submesoscale, that is, ocean features that are 100 m to 10 km wide, 10 to 100 m deep, and last hours or days. Though ubiquitous throughout the world’s oceans, submesoscale features, unlike major currents, are inherently chaotic and evolve quickly, limiting their predictability.

Researchers, with aid from the ship's crew, deployed autonomous instruments – 62 floats, 10 undersea gliders, a Wave Glider, and 135 surface drifters – and completed five surveys with the Triaxus towbody that profiled the ocean's upper 150 m 4635 times while covering 2000 km. Arrays of floats, programmed to profile the upper 175 m of the ocean to measure quantities such as temperature, salinity, and velocity, were deployed near observed instabilities developing along a temperature front. "We chose to deploy the floats in two arrays, where a smaller array was nested inside a larger array to capture the range of scales important for energy exchange in the surface mixed layer," explains Principal Oceanographer Caitlin Whalen. The profiling float arrays were placed on the edge of a cyclonic eddy so that they might remain caught up in the evolving and unstable region for the longest period, instead of being swept away by the larger regional currents.

As the ocean was seeded with floats, two Seagliders were deployed to spend the duration of the cruise sampling across features in coordination with a Wave Glider on the surface. The evolving arrays of floats provided a coordinate frame for the continuous ship surveys. Triaxus was towed across temperature fronts and along the edges of filaments, capturing the horizontal and vertical structure of submesoscale processes. 

Several of the floats, the two Seagliders, and a Slocum glider were equipped with sensors to sample the smallest scales in the ocean — centimeter-scale fluctuations of velocity and temperature used to estimate turbulence. Combined with vertical microstructure profiler sections conducted from R/V Thompson, the team collected thousands of microstructure measurements to study the smallest extent of the submesoscale range, where instabilities on the fronts and filaments drive turbulence, dissipating energy. 

Each of the tools used has one way that it samples the ocean, and none is perfect, but together they make a powerful observing system. Ongoing analyses are exciting. Senior Principal Oceanographer Luc Rainville, the cruise Chief Scientist, explains: "With so many instruments and ways to measure the ocean, we are now discovering how what is measured at one spot can relate to everything else."

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