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Wind over the ocean generates near‐inertial velocities. In the open ocean, horizontal variability in the inertial frequency and mesoscale vorticity generate internal waves that transport energy laterally and drive diapcynal mixing in remote locations. In the coastal ocean, horizontal variability is produced by the coastline. This study analyzes observations along a straight coastline in Lake Superior, which acts as a "natural laboratory" for the coastal ocean. Depth‐profiles of velocity, temperature, and turbulent miscrostructure were collected during a 96 hr repeat survey from 3 to 20 km offshore in Aug 2018. Wind work was 2 mW m−2 ${\mathrm{m}}^{-2}$ and generated 0.2 m s−1 ${\mathrm{s}}^{-1}$ near‐inertial velocities that were inhibited within two internal Rossby radii (6 km) of the coast. The velocities are interpreted as a superposition of a "forced flow", which is horizontally uniform, and a "wave flow", associated with offshore propagating near‐inertial waves. A 1D momentum equation skillfully predicts r2=0.82 $\left({r}^{2}=0.82\right)$ the horizontally averaged near‐inertial velocities and the TKE shear production, which matches the 1 mW m−2 ${\mathrm{m}}^{-2}$ observed TKE dissipation rate. The offshore propagating wave has an energy flux of 10 W (m‐coastline)−1 and a downward energy flux of 1 mW m−2 ${\mathrm{m}}^{-2}$. These results suggest that most near‐inertial wind work is lost directly to TKE shear production, but some energy is transferred to offshore propagating waves that may help catalyze shear instability away from the coast. Plain Language Summary: Wind blowing over the ocean sets the surface layer in motion. Once in motion, the water continues moving under its inertia, slowly curving to the right (in the Northern Hemisphere) and eventually completing circles due to the Earth's rotation. These motions, known as inertial oscillations, are ubiquitous in the ocean and large lakes, but the mechanisms that eventually cause them to die out are not fully understood. Here, we examine observations of inertial oscillations in Lake Superior. As the currents run up against a coastal wall the incoming or outgoing currents push the base of the warm surface layer down or up, respectively. This pumping generates waves on the interface between the warm surface water and cold deep water, which propagate offshore. We examine observations of currents, temperature profiles, and small‐scale turbulence; and we find that most inertial currents are damped by turbulence, so that relatively little energy is transferred to the offshore propagating sub‐surface waves. Key Points: Observed near‐inertial velocities in Lake Superior were 0.2 m/s and inhibited within two internal Rossby radii (6 km) of the coastMost TKE shear production (1 mW/m2) was driven by direct wind forcing and predictable from a horizontally averaged modelCoastally generated near‐inertial waves with an energy flux of 10 W (m‐coastline)−1 radiated offshore after a wind event [ABSTRACT FROM AUTHOR]

High resolution shipboard observations reveal the complex processes controlling the evolution and subduction of a cold and salty, dense filament in the Bay of Bengal (BoB). The filament, likely formed through coastal upwelling, was advected offshore by the mesoscale velocity field, and was brought adjacent to a shallow, low salinity mixed layer by mesoscale strain. The front that formed on the Eastern edge of the dense filament was observed to undergo both restratification and steepening, creating barrier layers, in response to evolving mesoscale and submesoscale convergence and divergences. Measurements and analyses indicate that the development of both small‐scale instabilities such as symmetric instability (SI) and slightly larger scale ageostrophic secondary circulation (ASC), acted in concert to subduct and stir surface heat into the interior. These mechanisms also drive the generation of temperature inversions from O (1 km) lateral temperature variability at the surface, demonstrating the need for model parameterizations at such spatial scales. Our results highlight the importance of submesoscale dynamics in creating and warming barrier layers, facilitating vertical heat exchanges, and setting sea surface temperature in the Bay of Bengal, a critical input to coupled atmosphere/ocean models of the southwest monsoon. Such complex dynamics should be considered in regions where salinity governs stratification and compensated waters stir. Plain Language Summary: Improving predictions for the monsoon seasons in the Bay of Bengal depends on the accuracy of the ocean's surface temperatures, which involve both one‐dimensional and three‐dimensional processes that impact the vertical structure of the upper ocean. In this study, we observed a cold and salty water mass that upwelled near the coast of India drift into the center of the Bay where it met ambient low salinity water to create surface lateral density gradients. High‐resolution shipboard observations revealed evidence of small‐scale physical processes as well as features typical of regions where the vertical structure is governed by salinity. The observations document the complexity of upper ocean heat content on scales less than 10 km, with impacts for sea surface temperature and couple ocean ‐atmosphere forecasting of the southwest monsoon. Key Points: The subduction of a dense filament in the Bay of Bengal suggests the role of submesoscale processes in setting upper ocean heat contentBarrier layers form at edges of fronts, sequestering heat from the surface and creating shallow mixed layers with lateral scales O (10 km)Small‐scale lateral temperature variability is exchanged vertically through instabilities, encouraging formation of temperature inversions [ABSTRACT FROM AUTHOR]

The ability to forecast the biological productivity of the coastal ocean relies on the quantification of the physical processes that deliver nutrients to the euphotic zone. Here we explore these pathways using observations of the coupled biological and physical variability of waters offshore of the east coast of Tasmania in the summertime. The observations include an array of moored autonomous profilers deployed over an 18‐d period—providing continuous, full‐depth measurements of turbulent microstructure, temperature, velocity, and chlorophyll a (Chl a) fluorescence, complemented by shipboard nutrient measurements. Local upwelling was driven by the encroaching East Australian Current (EAC) extension onto the shelf and to a lesser extent the local winds. The interaction of the local winds and the encroaching boundary current was reflected in the shelf nutrient budget and led to a rapid increase in subsurface Chl a. Diffusive vertical fluxes had minimal impact on subsurface Chl a in the mid‐shelf and outer‐shelf. Upwelling‐favorable winds were too weak to drive significant vertical mixing, and mixing associated with the current‐driven Ekman transport was too deep compared to the euphotic zone depth. The observed subsurface Chl a did not reflect the satellite estimates of productivity. Since the EAC extension transports warm, low‐nutrient surface waters from the subtropics, satellite chlorophyll measurements decreased during the same period the depth‐averaged Chl a increased. This seeming paradox illustrated how long duration, full water column sampling can elucidate the coupled biological and physical processes that aid our ongoing effort to forecast the biological state of the coastal ocean. [ABSTRACT FROM AUTHOR]

Cross-shore velocities in the coastal ocean typically vary with depth. The direction and magnitude of transport experienced by meroplanktonic larvae will therefore be influenced by their vertical position. To quantify how swimming behavior and vertical position in internal waves influence larval cross-shore transport in the shallow (~ 20 m), stratified coastal waters off Southern California, we deployed swarms of novel, subsurface larval mimics, the Mini-Autonomous Underwater Explorers (M-AUEs). The M-AUEs were programmed to maintain a specified depth, and were deployed near a mooring. Transport of the M-AUEs was predominantly onshore, with average velocities up to 14 cm s-1. To put the M-AUE deployments into a broader context, we simulated > 500 individual high-frequency internal waves observed at the mooring over a 14-d deployment; in each internal wave, we released both depth-keeping and passive virtual larvae every meter in the vertical. After the waves' passage, depth-keeping virtual larvae were usually found closer to shore than passive larvae released at the same depth. Near the top of the water column (3-5-m depth), ~ 20% of internal waves enhanced onshore transport of depth-keeping virtual larvae by ≥ 50 m, whereas only 1% of waves gave similar enhancements to passive larvae. Our observations and simulations showed that depth-keeping behavior in high-frequency internal waves resulted in enhanced onshore transport at the top of the water column, and reduced offshore dispersal at the bottom, compared to being passive. Thus, even weak depth-keeping may allow larvae to reach nearshore adult habitats more reliably than drifting passively. [ABSTRACT FROM AUTHOR]

Asian summer monsoon has a planetary‐scale, westward propagating "quasi‐biweekly" mode of variability with a 10–25 day period. Six years of moored observations at 18°N, 89.5°E in the north Bay of Bengal (BoB) reveal distinct quasi‐biweekly variability in sea surface salinity (SSS) during summer and autumn, with peak‐to‐peak amplitude of 3–8 psu. This large‐amplitude SSS variability is not due to variations of surface freshwater flux or river runoff. We show from the moored data, satellite SSS, and reanalyses that surface winds associated with the quasi‐biweekly monsoon mode and embedded weather‐scale systems, drive SSS and coastal sea level variability in 2015 summer monsoon. When winds are calm, geostrophic currents associated with mesoscale ocean eddies transport Ganga‐Brahmaputra‐Meghna river water southward to the mooring, salinity falls, and the ocean mixed layer shallows to 1–10 m. During active (cloudy, windy) spells of quasi‐biweekly monsoon mode, directly wind‐forced surface currents carry river water away to the east and north, leading to increased salinity at the moorings, and rise of sea level by 0.1–0.5 m along the eastern and northern boundary of the bay. During July–August 2015, a shallow pool of low‐salinity river water lies in the northeastern bay. The amplitude of a 20‐day oscillation of sea surface temperature (SST) is two times larger within the fresh pool than in the saltier ocean to the west, although surface heat flux is nearly identical in the two regions. This is direct evidence that spatial‐temporal variations of BoB salinity influences sub‐seasonal SST variations, and possibly SST‐mediated monsoon air‐sea interaction. Plain Language Summary: The north Bay of Bengal (BoB) is characterized by 1–10 m deep layer of river water, very stable density stratification, and deep isothermal layer warmed by penetration of sunlight below the thin mixed layer. Thermodynamic structure of the upper ocean influences intraseasonal active‐break cycles of the summer monsoon and promotes intensification of postmonsoon tropical cyclones by inhibiting storm‐induced cooling of sea surface temperature. Hence, it is important to understand the space‐time variability of surface salinity in this basin. The quasi‐biweekly (10–25 day) oscillation is a prominent mode of the Asian summer monsoon, seen in winds, cloudiness, rainfall and surface heat flux. Six years of mooring observations at 18°N in the north BoB show large amplitude (2–8 psu) changes in surface salinity on quasi‐biweekly timescales in summer and autumn. Using moored observations, satellite data and reanalyses, we show that changes in surface winds associated with quasi‐biweekly monsoon mode and its embedded low‐pressure systems drive large changes in surface salinity and coastal sea level. We show that the response of SST to subseasonal variations of surface heat flux is enhanced in the presence of a thin layer of river water. These observations have important implications for regional air‐sea interaction on subseasonal timescales. Key Points: Moored observations show large amplitude quasi‐biweekly variability of surface salinity in the north Bay of BengalMesoscale eddies and shallow wind‐driven monsoon currents lead to lateral dispersal of river waterShallow, fresh layer enhances sea surface temperature response to surface heat flux on subseasonal timescales [ABSTRACT FROM AUTHOR]

The land‐sea breeze is resonant with the inertial response of the ocean at the critical latitude of 30°N/S. 1‐D vertical numerical experiments were undertaken to study the key drivers of enhanced diapycnal mixing in coastal upwelling systems driven by diurnal‐inertial resonance near the critical latitude. The effect of the land boundary was implicitly included in the model through the "Craig approximation" for first‐order cross‐shore surface elevation gradient response. The model indicates that for shallow water depths (<∼100 m), bottom shear stresses must be accounted for in the formulation of the "Craig approximation," as they serve to enhance the cross‐shore surface elevation gradient response, while reducing shear and mixing at the thermocline. The model was able to predict the observed temperature and current features during an upwelling/mixing event in 60 m water depth in St Helena Bay (∼32.5°S, southern Benguela), indicating that the locally forced response to the land‐sea breeze is a key driver of diapycnal mixing over the event. Alignment of the subinertial Ekman transport with the surface inertial oscillation produces shear spikes at the diurnal‐inertial frequency; however their impact on mixing is secondary when compared with the diurnal‐inertial resonance phenomenon. The amplitude of the diurnal anticyclonic rotary component of the wind stress represents a good diagnostic for the prediction of diapycnal mixing due to diurnal‐inertial resonance. The local enhancement of this quantity over St Helena Bay provides strong evidence for the importance of the land‐sea breeze in contributing to primary production in this region through nutrient enrichment of the surface layer. Plain Language Summary: Winds near the coast often have a daily cycle known as the land‐sea breeze. Near latitudes of 30°N/S ubiquitous rotating ocean currents also have a daily frequency and therefore become enhanced by daily winds at these latitudes. The ocean currents result in vertical mixing of subsurface and surface water layers, bringing subsurface nutrients to the surface where they stimulate phytoplankton growth. In this study we use a simple model of the ocean (composed of the vertical dimension only) to study the key drivers of vertical mixing due to the land‐sea breeze. We show how vertical mixing is reduced in shallow water (<∼100 m) near the coast, where currents are slowed down by friction at the seabed. We find that vertical mixing can be predicted by a parameter computed from wind speed and direction over time. This parameter is shown to be enhanced over St Helena Bay on the west coast of South Africa, where phytoplankton blooms are known to be particularly prevalent. The results suggest that the land‐sea breeze is likely to be an important contributor to phytoplankton bloom development in this region. Similar processes are likely to be at play in other coastal regions. Key Points: Land‐sea breeze driven vertical mixing is studied using a 1‐D model including the land boundary effectThe land boundary effect dampens vertical mixing, particularly when bottom friction is nonnegligibleThe diurnal anticyclonic rotary component of the wind stress provides a diagnostic for diapycnal mixing [ABSTRACT FROM AUTHOR]

Coastal physical processes are essential for the cross‐shore transport of meroplanktonic larvae to their benthic adult habitats. To investigate these processes, we released a swarm of novel, trackable, subsurface vehicles, the Mini‐Autonomous Underwater Explorers (M‐AUEs), which we programmed to mimic larval depth‐keeping behavior. The M‐AUE swarm measured a sudden net onshore transport of 30–70 m over 15–20 min, which we investigated in detail. Here, we describe a novel transport mechanism of depth‐keeping plankton revealed by these observations. In situ measurements and models showed that, as a weakly nonlinear internal wave propagated through the swarm, it deformed surface‐intensified, along‐isopycnal background velocities downward, accelerating depth‐keeping organisms onshore. These higher velocities increased both the depth‐keepers' residence time in the wave and total cross‐shore displacement, leading to wave‐induced transports twice those of fully Lagrangian organisms and four times those associated with the unperturbed background currents. Our analyses also show that integrating velocity time series from virtual larvae or mimics moving with the flow yields both larger and more accurate transport estimates than integrating velocity time series obtained at a point (Eulerian). The increased cross‐shore transport of organisms capable of vertical swimming in this wave/background‐current system is mathematically analogous to the increase in onshore transport associated with horizontal swimming in highly nonlinear internal waves. However, the mechanism described here requires much weaker swimming speeds (mm s−1 vs. cm s−1) to achieve significant onshore transports, and meroplanktonic larvae only need to orient themselves vertically, not horizontally. [ABSTRACT FROM AUTHOR]

The meroplanktonic larvae of many invertebrate and vertebrate species rely on physical transport to move them across the shelf to their adult habitats. One potential mechanism for cross‐shore larval transport is Stokes drift in internal waves. Here, we develop theory to quantify the Stokes velocities of neutrally buoyant and depth‐keeping organisms in linear internal waves in shallow water. We apply the analyses to theoretical and measured internal wave fields, and compare results with a numerical model. Near the surface and bottom boundaries, both neutrally buoyant and depth‐keeping organisms were transported in the direction of the wave's phase propagation. However, neutrally buoyant organisms were transported in the opposite direction of the wave's phase at mid depths, while depth‐keeping organisms had zero net transport there. Weakly depth‐keeping organisms had Stokes drifts between the perfectly depth‐keeping and neutrally buoyant organisms. For reasonable wave amplitudes and phase speeds, organisms would experience horizontal Stokes speeds of several centimeters per second—or a few kilometers per day in a constant wave field. With onshore‐polarized internal waves, Stokes drift in internal waves presents a predictable mechanism for onshore transport of meroplanktonic larvae and other organisms near the surface, and offshore transport at mid depths. [ABSTRACT FROM AUTHOR]