Abstract
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.
Abstract
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.
Abstract
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.
Heat Transport through Diurnal Warm Layers Hughes, Kenneth G.; Moum, James N.; Shroyer, Emily L.
Journal of physical oceanography,
10/2020, Letnik:
50, Številka:
10
Journal Article
Recenzirano
Odprti dostop
Abstract
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.
Freshwater in the Bay of Bengal Mahadevan, Amala; Jaeger, Gualtiero Spiro; Freilich, Mara ...
Oceanography,
06/2016, Letnik:
29, Številka:
2
Journal Article
Recenzirano
Odprti dostop
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.
Observations from a ship-based campaign in July-August 2009, combined with idealized numerical simulations, are used to investigate the seasonal delivery of Pacific Winter Water to Barrow Canyon and ...the subsequent adjustment of the flow down the canyon. As the current advects dense water, it transitions from a nearly barotropic structure near the canyon head to a strongly baroclinic flow with a subsurface maximum near the canyon mouth. Both the data and model indicate that the transit times along the three Chukchi Shelf pathways feeding Barrow Canyon – a coastal pathway, a southern Hanna Shoal pathway, and a northern Hanna Shoal pathway – modulate the mode of winter water that occupies the canyon at a given time. In particular, remnant Pacific Winter Water carried along the rapid coastal pathway can precede the arrival of newly ventilated Pacific Winter Water carried along the two interior pathways. The observations and model indicate that the transition between water types draining from the canyon can occur rapidly over time scales of days to weeks. We also demonstrate that mixing along the path of the current is unlikely to result in the observed down-canyon transition from newly ventilated Pacific Winter Water to remnant winter water, further supporting the dominant role of advection. While the focus here is on the transition of winter water modes, the implication that seasonality within Barrow Canyon is tied to seasonality of the Bering Strait inflow, together with the relative transit times along advective pathways, should hold for other water types as well.
The Bay of Bengal (BoB) is strongly density stratified due to large freshwater input from various rivers and heavy precipitation. This strong vertical stratification, along with physical processes, ...regulates the transport and vertical exchange of surface and subsurface water, concentrating nutrients and intensifying the oxygen minimum zone (OMZ). Here, we use basinwide measurements to describe the spatial distributions of nutrients, oxygen, and phytoplankton within the BoB during the 2013 northeast monsoon (November–December). By the time riverine water reaches the interior bay, it is depleted in the nutrients nitrate and phosphate, but not silicate. Layering of freshwater in the northern BoB depresses isopycnals, leading to a deepening of the nutricline and oxycline. Oxygen concentrations in the OMZ are lowest in the north (<5 μM). Weak along-isopycnal nutrient gradients reflect along-isopycnal stirring between ventilated surface water and deep nutrient-replenished water. Picoplankton dominate the phytoplankton population in the north, presumably outcompeting larger phytoplankton species due to their low nutrient requirements. Micro- and nanoplankton numbers are enhanced in regions with deeper mixed layers and weaker stratification, where nutrient replenishment from subsurface waters is more feasible. These are also the regions where marine mammals were sighted. Physical processes and the temperature-salinity structure in the BoB directly influence the OMZ and the depth of the oxycline and nutricline, thereby affecting the phytoplankton and marine mammal communities.
Abstract
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.
Bay of Bengal Gordon, Arnold L.; Shroyer, Emily L.; Mahadevan, Amala ...
Oceanography,
06/2016, Letnik:
29, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The upper 200 m of the two northern Indian Ocean embayments, the Bay of Bengal (BoB) and the Arabian Sea (AS), differ sharply in their salinity stratification, as the Asian monsoon injects massive ...amounts of freshwater into the BoB while removing freshwater via evaporation from the AS. The ocean circulation transfers salt from the AS to the BoB and exports freshwater from the BoB to mitigate the salinity difference and reach a quasi-steady state, albeit with strong seasonality. An energetic field of mesoscale features and an intrathermocline eddy was observed within the BoB during the R/VRevelleNovember and December 2013 Air-Sea Interactions Regional Initiative cruises, marking the early northeast monsoon phase. Mesoscale features, which display a surprisingly large thermohaline range within their confines, obscure the regional surface water and thermohaline stratification patterns, as observed by satellite and Argo profilers. Ocean processes blend the fresh and salty features along and across density surfaces, influencing sea surface temperature and air-sea flux. Comparing theRevelleobservations to the Argo data reveals a general westward migration of mesoscale features across the BoB.
PDF
