Andreas Münchow

Buoyancy and wind forcing of a coastal current

Local winds and lateral buoyancy fluxes from estuaries constitute two major forcing mechanisms on the inner continental shelf of the Mid Atlantic Bight on the eastern seaboard of the U.S.A. We report observations of the resulting coastal current that suggest a linear superposition of the wind and buoyancy forced motions. This current, which we term the Delaware Coastal Current, has a mean flow of about 10 cm/s in the direction of Kelvin wave phase propagation. It opposes the generally upwelling favorable local winds there. The same winds, however, force important across-shelf flows that agree qualitatively with Ekman dynamics with Ekman numbers that are 0( 1). Velocity fluctuations at current meter mooring are consistent with the above dynamics, and explain the local hydrography well. Trajectories from drifters and derived velocity fields, too, reveal consistent flow patterns. We further find that Lagrangian and Eulerian integral time scales are similar, implying a linear flow field. We estimate dispersion coefficients for this buoyancy driven coastal current to be about 2000 and 200 mz/s in the along- and across-shelf direction, respectively. Our results disagree both qualitatively and quantitatively with those of a recent numerical model of the study area.

Dynamical properties of a buoyancy-driven coastal current

The outflow of buoyant waters from major estuaries affects the dynamics of inner continental shelves profoundly as lateral density gradients force an alongshore current. Often the Coriolis force causes the outflow to remain trapped near the coast. We observed one such current, the Delaware Coastal Current, on the inner shelf near the Delaware Estuary on the eastern seaboard of the United States. The spatial variability along the shelf, however, suggests at least two dynamically distinct regions that we term source and plume regions. In the source region we find fronts, a current whose width scales well with the internal deformation radius, and a ratio of relative to planetary vorticity that reaches unity, that is, the Rossby number is O(1). As nonlinear inertial forces in the across-shelf momentum balance are weak, we suggest that such forces contribute to the along-shelf momentum balance only. Farther downstream in the plume region, we find much reduced lateral density gradients, a current much wider than the deformation radius, and relative vorticities that are much smaller than the planetary vorticity. From our observations we compute nondimensional dynamical parameters, with which we discuss our observations. The Burger, Rossby, and Ekman numbers for the Delaware Coastal Current suggest that most models of buoyancy-driven coastal currents do not apply to this coastal flow.

Ocean circulation and properties in Petermann Fjord, Greenland

The floating ice shelf of Petermann glacier interacts directly with the ocean and isthought to lose at least 80% of its mass through basal melting. Based on three opportunisticocean surveys in Petermann Fjord we describe the basic oceanography: the circulationat the fjord mouth, the hydrographic structure beneath the ice shelf, the oceanic heatdelivered to the under‐ice cavity, and the fate of the resulting melt water. The 1100 m deepfjord is separated from neighboring Hall Basin by a sill between 350 and 450 m deep.Fjord bottom waters are renewed by episodic spillover at the sill of Atlantic water from theArctic. Glacial melt water appears on the northeast side of the fjord at depths between200 m and that of the glacier’s grounding line (about 500 m). The fjord circulation isfundamentally three‐dimensional; satellite imagery and geostrophic calculations suggest acyclonic gyre within the fjord mouth, with outflow on the northeast side. Tidal flowsare similar in magnitude to the geostrophic flow. The oceanic heat flux into the fjordappears more than sufficient to account for the observed rate of basal melting. Cold,low‐salinity water originating in the surface layer of Nares Strait in winter intrudes farunder the ice. This may limit basal melting to the inland half of the shelf. The melt rate andlong‐term stability of Petermann ice shelf may depend on regional sea ice cover andfjord geometry, in addition to the supply of oceanic heat entering the fjord.

Kinematics of Inner Shelf Motions during the Summer Stratified Season off New Jersey

Abstract Subinertial currents on a wide (∼100 km), shallow (∼20 m), but nevertheless vertically stratified shelf off the Atlantic seaboard of the United States are investigated at spatial scales of about 20 km in the alongshore and 10 km in the across-shore direction. During the summer of 1996 the inner shelf off New Jersey was stratified due to both temperature and salinity that varied vertically by more than 12°C and 4 psu, respectively. Upwelling favorable winds and an intermittent buoyancy-driven Hudson coastal current impact this stratification inshore of the 15-m isobath. Waters offshore were always stratified except during the passage of Tropical Storm Bertha. Mean currents are weak because wind-forced upwelling and buoyancy-forced downwelling events occurred about evenly during the observational study period. At monthly to daily timescales currents always veered counterclockwise with depth in a bottom Ekman-layer sense by more than 30° inshore and 50° offshore. Complex empirical orthogonal functio...

Buoyancy forced interaction between estuary and inner shelf: observation

Abstract In May and June of 1990 we explored the hydrographic variability of the Delaware Estuary and the adjacent inner shelf with shipboard instruments. We found significant three-dimensional density variability both within the estuary and on the shelf. We found weak vertical stratification but strong transverse variability within the estuary, with denser water concentrating in the center of the estuary and two branches of lighter water near both shores. On the shelf, the buoyant estuarine water forms a southward flowing coastal current in the direction of Kelvin wave phase propagation (downstream). ADCP observations and thermal wind calculations indicate a flow of 10–20 cm s −1 downstream. Both the width of the coastal current and the magnitude of horizontal density gradients undergo substantial variations along the shelf.

Ocean current observations from Nares Strait to the west of Greenland: Interannual to tidal variability and forcing

During 2003–06, as part of the Arctic Sub-Arctic Ocean Flux (ASOF) experiment, an array of oceansensing instruments was deployed at 80.5N latitude to investigate the flux of seawater from the Arctic Ocean via Nares Strait, the pathway to the west of Greenland. Three-year measurements of current from this experiment provide, for the first time at periods longer than a single season, the seawater flux and its variability via this important pathway. Below 30-m depth the average flux of volume 2003–06 was 0.57±0.09 Sv (1 Sv=106 m3 s−1 ) southward over a 38-km wide section reaching 360 m in depth. A linear trend, statistically significant at the 95% confidence level, indicates an increase in the sectionally averaged flow below 30-m depth of 20 ± 10% between 2003 and 2006. The flow is dominated by mixed diurnal and semi-diurnal tidal currents with kinetic energy an order of magnitude larger than that of the subtidal flow. The range of seasonal variation is 30–50% of the long-term mean flow. Variations in flow of daily to monthly period are comparable in magnitude to the average flow. The flow through the cross-section is the net result of a larger southward flux in the deep western two thirds of the strait and a small northward flux within about 5 km of Greenland. The latter is about 5% of the former. Spectral analyses indicates that the cross-channel pressure gradient is highly correlated with the sectionally averaged flow consistent with geostrophy. Along-channel pressure gradient explains 70% of the variance at a 33-day period with a phase lag consistent with a frictional response; at 3–7 day period the response is weaker (<30%) with a phase relation suggestive of contributions by both friction and local acceleration.

An Observational Estimate of Volume and Freshwater Flux Leaving the Arctic Ocean through Nares Strait

Abstract The Arctic Ocean is an important link in the global hydrological cycle, storing freshwater and releasing it to the North Atlantic Ocean in a variable fashion as pack ice and freshened seawater. An unknown fraction of this return flow passes through Nares Strait between northern Canada and Greenland. Surveys of ocean current and salinity in Nares Strait were completed in the summer of 2003. High-resolution data acquired by ship-based acoustic Doppler current profiler and via hydrographic casts revealed subtidal volume and freshwater fluxes of 0.8 ± 0.3 Sv and –25 ± 12 mSv (Sv = 103 mSv = 106 m3 s−1), respectively. The observations resolved the dominant spatial scale of variability, the internal Rossby radius of deformation (LD ∼9 km), and revealed a complex, yet coherent along-channel flow with a Rossby number of about 0.13, close to geostrophic balance. Approximately one-third of the total volume flux was associated with across-channel slope of the sea surface and two-thirds (68%) with across-cha...

Synoptic Flow and Density Observations near an Arctic Shelf Break

Analyses of data from three shipborne surveys describe the quasi-synoptic density and velocity fields near Barrow Canyon, Alaska. The canyon parallels the northwestern coast of Alaska and contains three different water masses. These are 1) warm and fresh Alaskan coastal waters that originate from the Bering Strait; 2) cold and moderately salty waters that originate from the Chukchi shelf; and 3) warm and salty waters that originate from the Atlantic layer of the Arctic Ocean. A halocline separates the Chukchi shelf and Atlantic layer waters. The halocline slopes upward into the canyon where it is then twisted to slope across the wide canyon. An intensification of the Beaufort gyre near the shelf break just seaward of Barrow Canyon raises the halocline more than 100 m toward the surface. Locally upwelling favorable winds raise the Arctic halocline, which thus is ventilated within Barrow Canyon adjacent to the coast. In the absence of winds the halocline slopes acrosscanyon in the thermal wind sense due to a northward flowing coastal current. Velocity measurements from a towed acoustic Doppler current profiler reveal a northward flowing jet that transports about 0.3 Sv (Sv [ 106 kg m23) of Bering Sea summer water into the Arctic Ocean at speeds that exceed 0.7 m s21. Total northward transports through the canyon exceed 1.0 Sv. The warm waters of this coastal current supply more than 100 W m22 of heat to the atmosphere. The jet separates both from the bottom and from the coast. Hence, a laterally and vertically sheared jet forms, which breaks into three branches at about 71.88N latitude.

Spatial continuity of measured seawater and tracer fluxes through Nares Strait, a dynamically wide channel bordering the Canadian Archipelago

Freshwater delivered as precipitation and runoff to the North Pacific and Arctic oceans returns to the Atlantic principally via the Canadian polar shelf and Fram Strait. It is conveyed as ice or freshened seawater. Here we use detailed ship-based measurements to calculate a snap-shot of volume, freshwater, and tracer fluxes through Nares Strait, a 500-km long waterway separating Greenland and Ellesmere Island. We use quasi-synoptic observations of current by ship-mounted acoustic Doppler current profiler (ADCP), of salinity and temperature by CTD probe and of dissolved nutrients by rosette bottle sampler on four cross-sections between 82 and 78N latitude. Data were collected during the first half of August 2003. We partition the fluxes into components derived from Pacific and Atlantic inflows into the Arctic Ocean. During the time of the survey, there was a net southward 0.91 0.10 Sv (10 m s) flux of volume and a net southward 31 4 10 Sv (977 127 km y) flux of freshwater relative to a salinity of 34.8. Much of the volume flux was carried within a strong (40 cm s), narrow (10 km) subsurface jet hugging the western (Ellesmere Island) side of the strait. The presence of this jet in four sections spanning the 500-km length of the strait is evidence of a buoyant boundary current through the strait. The jet was coincident with elevated concentrations of phosphate (1.0 mmol m) and silicate (11 mmol m) which both indicate a Pacific Ocean source. We interpreted the ratio of dissolved total inorganic nitrogen to phosphate in terms of fractional dilution of Atlantic by Pacific waters. About 0.43 0.10 Sv (39%) of the southward flow was of Pacific origin. These results are a snapshot during the summer of 2003 following a prolonged period of northward directed wind stress when ice cover was mobile. Although long-term mean values are likely different, we determined that the major fraction of the through-flow is carried by a jet of scale determined by the internal Rossby radius (5-10 km).

The Summer Hydrography and Surface Circulation of the East Siberian Shelf Sea

During the ice-free summer season in 1995 the authors deployed and subsequently tracked 39 surface drifters to test the hypothesis that the discharge from the Kolyma River forces a buoyancy-driven coastal current from the East Siberian Sea toward Bering Strait. The observed mean flow is statistically significant at the 95% level of confidence, but its direction contradicts their initial hypothesis. Instead of a coastally trapped eastward flow, the authors find a laterally sheared westward flow with maximum velocities offshore that correlate only weakly with the local winds. At a daily, wind-dominated timescale the drifter data reveal spatially coherent flows of up to 0.5 m s21. The Lagrangian autocorrelation scale is about 3 days and the Lagrangian eddy length scale reaches 40 km. This spatial scale exceeds the nearshore internal deformation radius by a factor of 3; however, it more closely corresponds to the internal deformation radius associated with the offshore ice edge. Bulk estimates of the horizontal mixing coefficient resemble typical values of isotropic open ocean dispersion at midlatitudes. Hydrographic observations and oxygen isotope ratios of seawater indicate a low proportion of riverine freshwater relative to sea ice melt in most areas of the East Siberian Sea except close to the Kolyma Delta. The observations require a reevaluation of the conceptual view of the summer surface circulation of the East Siberian Sea. Eastward buoyancy-driven coastal currents do not always form on this shelf despite large river discharge. Instead, ice melt waters of a retreating ice edge act as a line source of buoyancy that in 1995 forced a westward surface flow in the East Siberian Sea.