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Fjord-scale circulation forced by rising turbulent plumes of subglacial meltwater has been identified as one possible mechanism of oceanic heat transfer to marine-terminating outlet glaciers. This study uses buoyant plume theory and a nonhydrostatic, three-dimensional ocean–ice model of a typical outlet glacier fjord in west Greenland to investigate the sensitivity of meltwater plume dynamics and fjord-scale circulation to subglacial discharge rates, ambient stratification, turbulent diffusivity, and subglacial conduit geometry. The terminal level of a rising plume depends on the cumulative turbulent entrainment and ambient stratification. Plumes with large vertical velocities penetrate to the free surface near the ice face; however, midcolumn stratification maxima create a barrier that can trap plumes at depth as they flow downstream. Subglacial discharge is varied from 1–750 m 3 s −1 ; large discharges result in plumes with positive temperature and salinity anomalies in the upper water column. For these flows, turbulent entrainment along the ice face acts as a mechanism to vertically transport heat and salt. These results suggest that plumes intruding into stratified outlet glacier fjords do not always retain the cold, fresh signature of meltwater but may appear as warm, salty anomalies. Fjord-scale circulation is sensitive to subglacial conduit geometry; multiple point source and line plumes result in stronger return flows of warm water toward the glacier. Classic plume theory provides a useful estimate of the plume’s outflow depth; however, more complex models are needed to resolve the fjord-scale circulation and melt rates at the ice face.

The daily evolution of temperature, stratification, and turbulence in the diurnal warm layer is described from time series measurements at low to moderate winds and strong insolation in the equatorial Indian Ocean. At 2.0-m depth, turbulence dissipation rates ( ε ) decreased by two orders of magnitude over 1–2 h immediately after sunrise, initiated by stratification caused by penetrating solar radiation prior to the change in sign of net surface heat flux from cooling to warming. Decaying turbulence preceded a period of rapid growth, in which ε increased by two orders of magnitude over a few hours, and following which ε approached a daytime period of near-steady state. Decay and growth rates predicted by a simplified turbulence model are consistent with those observed. During the daytime period of near-steady state, asymmetric temperature ramps were associated with enhanced ε , supporting the interpretation that this period represents a balance between buoyancy and shear production associated with a shear-driven response to trapping of momentum within the diurnal warm layer.

Greenland’s contribution to future sea-level rise remains uncertain and a wide range of upper and lower bounds has been proposed. These predictions depend strongly on how mass loss—which is focused at the termini of marine-terminating outlet glaciers—can penetrate inland to the ice-sheet interior. Previous studies have shown that, at regional scales, Greenland ice sheet mass loss is correlated with atmospheric and oceanic warming. However, mass loss within individual outlet glacier catchments exhibits unexplained heterogeneity, hindering our ability to project ice-sheet response to future environmental forcing. Using digital elevation model differencing, we spatially resolve the dynamic portion of surface elevation change from 1985 to present within 16 outlet glacier catchments in West Greenland, where significant heterogeneity in ice loss exists. We show that the up-glacier extent of thinning and, thus, mass loss, is limited by glacier geometry. We find that 94% of the total dynamic loss occurs between the terminus and the location where the down-glacier advective speed of a kinematic wave of thinning is at least three times larger than its diffusive speed. This empirical threshold enables the identification of glaciers that are not currently thinning but are most susceptible to future thinning in the coming decades. Greenland’s ice loss depends on propagation of mass loss from the marine glacier termini to the interior. An analysis of surface elevation change in 16 glacier catchments shows that the up-glacier extent of thinning is limited by glacier geometry.

Packets of nonlinear internal waves (NLIWs) in a small area of the Mid-Atlantic Bight were 10 times more energetic during a local neap tide than during the preceding spring tide. This counterintuitive result cannot be explained if the waves are generated near the shelf break by the local barotropic tide since changes in shelfbreak stratification explain only a small fraction of the variability in barotropic to baroclinic conversion. Instead, this study suggests that the occurrence of strong NLIWs was caused by the shoaling of distantly generated internal tides with amplitudes that are uncorrelated with the local spring-neap cycle. An extensive set of moored observations show that NLIWs are correlated with the internal tide but uncorrelated with barotropic tide. Using harmonic analysis of a 40-day record, this study associates steady-phase motions at the shelf break with waves generated by the local barotropic tide and variable-phase motions with the shoaling of distantly generated internal tides. The dual sources of internal tide energy (local or remote) mean that shelf internal tides and NLIWs will be predictable with a local model only if the locally generated internal tides are significantly stronger than shoaling internal tides. Since the depth-integrated internal tide energy in the open ocean can greatly exceed that on the shelf, it is likely that shoaling internal tides control the energetics on shelves that are directly exposed to the open ocean.

Neighboring tidewater glaciers often exhibit asynchronous dynamic behavior, despite relatively uniform regional atmospheric and oceanic forcings. This variability may be controlled by a combination of local factors, including glacier and fjord geometry, fjord heat content and circulation, and glacier surface melt. In order to characterize and understand contrasts in adjacent tidewater glacier and fjord dynamics, we made coincident ice-ocean-atmosphere observations at high temporal resolution (minutes to weeks) within a 10 000 km 2 area near Uummannaq, Greenland. Water column velocity, temperature and salinity measurements reveal systematic differences in neighboring fjords that imply contrasting circulation patterns. The observed ocean velocity and hydrography, combined with numerical modeling, suggest that subglacial discharge plays a major role in setting fjord conditions. In addition, satellite remote sensing of seasonal ice flow speed and terminus position reveal both speedup and slow-down in response to melt, as well as differences in calving style among the neighboring glaciers. Glacier force budgets and modeling also point toward subglacial discharge as a key factor in glacier behavior. For the studied region, individual glacier and fjord geometry modulate subglacial discharge, which leads to contrasts in both fjord and glacier dynamics.

The daily formation of near-surface ocean stratification caused by penetrating solar radiation modifies heat fluxes through the air–sea interface, turbulence dissipation in the mixed layer, and the vertical profile of lateral transport. The transport is altered because momentum from wind is trapped in a thin near-surface layer, the diurnal warm layer. We investigate the dynamics of this layer, with particular attention to the vertical shear of horizontal velocity. We first develop a quantitative link between the near-surface shear components that relates the crosswind component to the inertial turning of the along-wind component. Three days of high-resolution velocity observations confirm this relation. Clear colocation of shear and stratification with Richardson numbers near 0.25 indicate marginal instability. Idealized numerical modeling is then invoked to extrapolate below the observed wind speeds. This modeling, together with a simple energetic scaling analysis, provides a rule of thumb that the diurnal shear evolves differently above and below a 2 m s −1 wind speed, with limited sensitivity of this threshold to latitude and mean net surface heat flux. Only above this wind speed is the energy input sufficient to overcome the stabilizing buoyancy flux and thereby induce marginal instability. The differing shear regimes explain differences in the timing and magnitude of diurnal sea surface temperature anomalies.

Penetration of solar radiation in the upper few meters of the ocean creates a near-surface, stratified diurnal warm layer. Wind stress accelerates a diurnal jet in this layer. Turbulence generated at the diurnal thermocline, where the shear of the diurnal jet is concentrated, redistributes heat downward via mixing. New measurements of temperature and turbulence from fast thermistors on a surface-following platform depict the details of this sequence in both time and depth. Temporally, the sequence at a fixed depth follows a counterclockwise path in log ϵ –log N parameter space. This path also captures the evolution of buoyancy Reynolds number (a proxy for the anisotropy of the turbulence) and Ozmidov scale (a proxy for the outer vertical length scale of turbulence in the absence of the free surface). Vertically, the solar heat flux always leads to heating of fluid parcels in the upper few meters, whereas the turbulent heat flux divergence changes sign across the depth of maximum vertical temperature gradient, cooling fluid parcels above and heating fluid parcels below. In general, our measurements of fluid parcel heating or cooling rates of order 0.1°C h −1 are consistent with our estimates of heat flux divergence. In weak winds (<2 m s −1 ), sea surface temperature (SST) is controlled by the depth-dependent absorption of solar radiation. In stronger winds, turbulent mixing controls SST.

In low winds (≲2 m s −1 ), diurnal warm layers form but shear in the near-surface jet is too weak to generate shear instability and mixing. In high winds (≳8ms −1 ), surface heat is rapidly mixed downward and diurnal warm layers do not form. Under moderate winds of 3–5 m s −1 , the jet persists for several hours in a state that is susceptible to shear instability. We observe low Richardson numbers of Ri ≈ 0.1 in the top 2 m between 10:00 and 16:00 local time (from 4 h after sunrise to 2 h before sunset). Despite Ri being well below the Ri = 1/4 threshold, instabilities do not grow quickly, nor do they overturn. The stabilizing influence of the sea surface limits growth, a result demonstrated by both linear stability analysis and two-dimensional simulations initialized from observed profiles. In some cases, growth rates are sufficiently small (≪1 h −1 ) that mixing is not expected even though Ri < 1/4. This changes around 16:00–17:00. Thereafter, convective cooling causes the region of unstable flow to move downward, away from the surface. This allows shear instabilities to grow an order of magnitude faster and mix effectively. We corroborate the overall observed diurnal cycle of instability with a freely evolving, two-dimensional simulation that is initialized from rest before sunrise.

The strong salinity stratification in the upper 50–80 m of the Bay of Bengal affects the response of the upper ocean to surface heat fluxes. Using observations from November to December 2013, we examine the effect of surface cooling on the temperature structure of the ocean in a one-dimensional framework. The presence of freshwater adds gravitational stability to the density stratification and prevents convective overturning, even when the surface becomes cooler than the subsurface. This stable salinity stratification traps heat within subsurface layers. The ocean's reluctance to release the heat trapped within these subsurface warm layers can contribute to delayed rise in surface temperature and heat loss from the ocean as winter progresses. Understanding the dispersal of freshwater throughout the bay can help scientists assess its potential for generating the anomalous temperature response. We use the Aquarius along-track surface salinity and satellite-derived surface velocities to trace the evolution and modification of salinity in the Lagrangian frame of water parcels as they move through the bay with the mesoscale circulation. This advective tracking of surface salinities provides a Lagrangian interpolation of the monthly salinity fields in 2013 and shows the evolution of the freshwater distribution. The along-trajectory rate of salinification of water as it leaves the northern bay is estimated and interpreted to result from mixing processes that are likely related to the host of submesoscale signatures observed during our field campaigns.

The ocean surface boundary layer is a gateway of energy transfer into the ocean. Wind-driven shear and meteorologically forced convection inject turbulent kinetic energy into the surface boundary layer, mixing the upper ocean and transforming its density structure. In the absence of direct observations or the capability to resolve subgrid-scale 3D turbulence in operational ocean models, the oceanography community relies on surface boundary layer similarity scalings (BLS) of shear and convective turbulence to represent this mixing. Despite their importance, near-surface mixing processes (and ubiquitous BLS representations of these processes) have been undersampled in high-energy forcing regimes such as the Southern Ocean. With the maturing of autonomous sampling platforms, there is now an opportunity to collect high-resolution spatial and temporal measurements in the full range of forcing conditions. Here, we characterize near-surface turbulence under strong wind forcing using the first long-duration glider microstructure survey of the Southern Ocean. We leverage these data to show that the measured turbulence is significantly higher than standard shear-convective BLS in the shallower parts of the surface boundary layer and lower than standard shear-convective BLS in the deeper parts of the surface boundary layer; the latter of which is not easily explained by present wave-effect literature. Consistent with the CBLAST (Coupled Boundary Layers and Air Sea Transfer) low winds experiment, this bias has the largest magnitude and spread in the shallowest 10% of the actively mixing layer under low-wind and breaking wave conditions, when relatively low levels of turbulent kinetic energy (TKE) in surface regime are easily biased by wave events.