A submesoscale cyclonic eddy observed with autonomous vehicles in synergy with ship and satellite measurements

We report on measurements conducted during the Schmidt Ocean Institute Exploring Fronts with Multiple Robots cruise on board RV Falkor, at the Northern Pacific Subtropical Front (STF), June 2018. Observations were conducted synergistically by the ship and multiple robots. Data from a vertical section across the STF performed with a towed CTD and ADCP are analysed in detail. Strain and relative vorticity reveal strong horizontal convergence in the vicinity of outcropping isopycnals associated with the front and coherent patches of strong O(1) cyclonic relative vorticity. The horizontal structure of the front at the surface is also distinctively observed in an SST image, which provides evidence of instability associated with a plume-like structure detaching from the main front to the South. A PDF retrieved from ADCP shows that the relative vorticity along the transect is strongly biased towards positive values (cyclonic vorticity), with a significant portion of O(1) values, corroborating intense submesoscale dynamics. Stratification and Potential Vorticity are also analysed, showing evidence of low PV patches beneath the outcropping isopycnals in the front’s vicinity. These are consistent with subduction of water from near the surface. The subduction occurs near locations associated with horizontal convergence. The zones of convergence exhibit O(f) cyclonic relative vorticity, consistent with the frontogenesis mechanism described by Hoskins & Bretherton (1972). We used Finite-Size Lyapunov Exponents (FSLEs) altimetry based products to study the 2D horizontal dynamics associated with the frontal features, which permits the identification of submesoscale structures. FSLEs were consistent with the available SST and in situ data. In fact, they reveal that the frontal instability is part of a spiralling counter-clockwise arm of a submesoscale eddy. It is suggested that FSLE maps can be useful in guidance and planning of fieldwork at sea, including submesoscale dynamics.