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Abstract The ocean is home to many different submesoscale phenomena, including internal waves, fronts, and gravity currents. Each of these processes entail complex nonlinear dynamics, even in isolation. Here we present shipboard, moored, and remote observations of a submesoscale gravity current front created by a shoaling internal tidal bore in the coastal ocean. The internal bore is observed to flatten as it shoals, leaving behind a gravity current front that propagates significantly slower than the bore. We posit that the generation and separation of the front from the bore is related to particular stratification ahead of the bore, which allows the bore to reach the maximum possible internal wave speed. After the front is calved from the bore, it is observed to propagate as a gravity current for ≈4 hours, with associated elevated turbulent dissipation rates. A strong cross-shore gradient of along-shore velocity creates enhanced vertical vorticity (Rossby number ≈ 40) that remains locked with the front. Lateral shear instabilities develop along the front and may hasten its demise. 

After a relaxation of the regional southward, upwelling‐favorable winds along the central California coast, warm water from the Santa Barbara Channel propagates northward as a buoyant plume. As the plume transits up the coast, it causes abrupt temperature changes and modifies shelf stratification. We use temperature and velocity data from 35 moorings north of Pt. Arguello to track the evolution of a buoyant plume after a wind relaxation event in October 2017. The moorings were deployed September–October 2017 and span a ∼30 km stretch of coastline, including nine cross‐shelf transects that range from 17 to 100 m water depth. The high spatial resolution of the data set enables us to track the spatiotemporal evolution of the plume, including across‐front temperature difference, cross‐shore structure, and propagation velocity. We observe an alongshore current velocity signal that takes ∼10 hr to propagate ∼25 km alongshore (∼0.7 m/s) and a temperature signal that takes ∼34 hr to propagate the same distance (∼0.2 m/s). The plume cools as it transits northward, leading to a decrease in the cross‐front temperature difference and the reduced gravity (g’). The plume’s propagation velocity is nonuniform in space and time, with accelerations and decelerations unexplained by the alongshore reduction ing’or advection by tidal currents. As the plume reaches the northernmost part of the mooring array, its temperature variability is obscured by internal waves, a prominent feature in the region. We focus on one relaxation event but observe five other similar events over the 2 months record.

 

The history of over 100 years of observing the ocean is reviewed. The evolution of particular classes of ocean measurements (e.g., shipboard hydrography, moorings, and drifting floats) are summarized along with some of the discoveries and dynamical understanding they made possible. By the 1970s, isolated and “expedition” observational approaches were evolving into experimental campaigns that covered large ocean areas and addressed multiscale phenomena using diverse instrumental suites and associated modeling and analysis teams. The Mid-Ocean Dynamics Experiment (MODE) addressed mesoscale “eddies” and their interaction with larger-scale currents using new ocean modeling and experiment design techniques and a suite of developing observational methods. Following MODE, new instrument networks were established to study processes that dominated ocean behavior in different regions. The Tropical Ocean Global Atmosphere program gathered multiyear time series in the tropical Pacific to understand, and eventually predict, evolution of coupled ocean–atmosphere phenomena like El Niño–Southern Oscillation (ENSO). The World Ocean Circulation Experiment (WOCE) sought to quantify ocean transport throughout the global ocean using temperature, salinity, and other tracer measurements along with fewer direct velocity measurements with floats and moorings. Western and eastern boundary currents attracted comprehensive measurements, and various coastal regions, each with its unique scientific and societally important phenomena, became home to regional observing systems. Today, the trend toward networked observing arrays of many instrument types continues to be a productive way to understand and predict large-scale ocean phenomena.

 

The Columbia River (CR) is the largest source of freshwater along the U.S. Pacific coast. The resultant plume is often transported southward and offshore forming a large buoyant feature off Oregon and northern California in spring‐summer—the offshore CR plume. Observations from autonomous underwater gliders and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery are used to characterize the optics of the offshore CR plume off Newport, Oregon. Vertical sections, under contrasting river flow conditions, reveal a low‐salinity and warm surface layer of ∼20–25 m (fresher in spring and warmer in summer), high Colored Dissolved Organic Matter (CDOM) concentration, and backscatter, and associated with the base of the plume high chlorophyll fluorescence. Plume characteristics vary in the offshore direction as the warm and fresh surface layer thickens progressively to an average 30–40 m of depth 270–310 km offshore; CDOM, backscatter, and chlorophyll fluorescence decrease in the upper 20 m and increase at subsurface levels (30–50 m depth). MODIS normalized water‐leaving radiance (nLw(λ)) spectra for CR plume cases show enhanced water‐leaving radiance at green bands (as compared to no‐CR plume cases) up to ∼154 km from shore. Farther offshore, the spectral shapes for both cases are very similar, and consequently, a contrasting color signature of low‐salinity plume water is practically imperceptible from ocean color remote sensing. Empirical algorithms based on multivariate regression analyses of nLw(λ) plus SST data produce more accurate results detecting offshore plume waters than previous studies using single visible bands (e.g., a_dg(412) or nLw(555)).