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Andreas Münchow

Abstract

A three-dimensional data interpolation technique is proposed that efficiently removes tidal currents from spatial velocity surveys. The least squares method extends prior two-dimensional detiding methods to three spatial dimensions using biharmonic splines. Biharmonic splines are fitted to velocity data from acoustic Doppler current profiler (ADCP) surveys, moorings, and ocean surface current radar (OSCR). The data are used to predict diurnal and semidiurnal tidal currents on the inner shelf off New Jersey that vary between 1 and 15 cm s−1 at spatial scales of about 20 km. The (tidal) signal to (subtidal) noise is thus O(1) in the study area. Although the main task of this study is to remove tidal variance from the ADCP survey data, an attempt is made to accurately“predict” tidal currents from the data. The latter task is more difficult. Both artificial data with known signal-to-noise properties and actual measurements indicate that the method estimates both diurnal and semidiurnal tidal currents to within about 3.5 cm s−1 rms, or 30% of the true tidal signals. While the biharmonic splines remove tidal currents successfully, the prediction of the vertical structure of tidal currents is only fair. Some experimentation guided by physical intuition and prior knowledge of the tidal fields is necessary in order to obtain an accurate and stable solution. While this ambiguity constitutes the main disadvantage of the method, its simple algebraic expression to predict tidal currents in space and time is its main advantage. Properly weighting velocity data from different sources, such as moorings, surface current radar, and ADCP surveys of different quality, improves the quality of the fit.

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Andreas Münchow

Abstract

Near Point Conception, California, the atmospheric flow separates from the coast and a large wind stress curl results. Direct spatial wind field observations from 20 aircraft overflights in the spring of 1983 suggest that Ekman pumping of on average 4 m day−1 contributes to the local dynamics. During strong and persistent upwelling events the curl-driven Ekman pumping reaches up to 20 m day−1. A single complex empirical orthogonal function explains more than 72% of the spatial and temporal wind stress variance. It reveals large wind stress curl at the center of the western Santa Barbara Channel entrance. This dominant mode correlates strongly (r 2 = 0.79) with the wind stress observed at a moored buoy. The location of largest Ekman pumping in the ocean indicated by this mode coincides with the location of a laterally sheared, cyclonic flow. Cold upwelled waters enter the Santa Barbara Channel along its southern perimeter while warm, and thus buoyant, waters from the Southern California Bight exit the channel along its northern perimeter. Buoy wind stress and lateral current shear at 30-m depth correlate significantly at periods of about 3.5 and 6 days with phase lags of about 0.5 and 2 days, respectively. Hydrographic observations from 1983 do not, however, indicate effects of Ekman pumping on the internal mass field as winds vary in speed and direction at daily timescales. This contrasts with 1984 observations that do indicate strong doming of isopycnals over the center of the channel at its western entrance. Winds prior to and during hydrographic observations in the spring of 1984 were both stronger and more steady than they were in 1983. Cyclonic shears reach 0.4f at the entrance of the Santa Barbara Channel near Point Conception;here f is the planetary vorticity. The thermal wind balance explains the lateral shear of the alongchannel surface flow rather well.

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Andreas Münchow

Abstract

Time series observations of velocity, salinity, pressure, and ice draft provide estimates of advective fluxes in Nares Strait from 2003 to 2009 at daily to interannual time scales. Velocity and salinity are integrated across the 36-km-wide and 350-m-deep channel for two distinct multiyear periods of sea ice cover. These observations indicate multiyear mean fluxes that range from 0.71 ± 0.09 to 1.03 ± 0.11 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1 = 31 536 km3 yr−1) for volume and from 32 ± 5.7 to 54 ± 9.3 mSv (1 mSv ≡ 103 m3 s−1) for oceanic freshwater relative to a salinity of 34.8 for the first (2003–06) and second (2007–09) periods, respectively. Advection of ice adds another 8 ± 2 mSv or 260 ± 70 km3 yr−1 to the freshwater export. Flux values are larger when the sea ice is mobile all year. About 75% of the oceanic volume and freshwater flux variability is correlated at daily to interannual time scales. Flux variability peaks at a 20-day time scale and correlates strongly with along-channel pressure gradients (r 2 = 0.68). The along-channel pressure gradient peaks in early spring when the sea ice is often motionless with higher sea level in the Arctic that drives the generally southward ocean circulation. Local winds contribute only when the sea ice is mobile, when they explain 60% of its variance (r 2 = 0.60). Observed annual to interannual change in the duration of motionless sea ice conditions impacts ocean stratification and freshwater flux, while seasonal variations are small.

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Andreas Münchow and Robert J. Chant

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 function (CEOF) analyses revealed that these veering angles are contained in the first mode that explains 80%–85% of the total variance at individual locations. It also explains 72% of the variance of 44 current time series of an across-shore section. The veering constitutes a robust feature that cannot be rationalized by Ekman dynamics in shallow water alone. The authors hypothesize that the veering represents a frictional response common to both upwelling and downwelling events. The CEOF analysis does not separate wind from buoyancy forcing. The two forcing mechanisms thus appear to be dynamically coupled. Nevertheless, the first two CEOFs suggest distinctly different circulation regimes: For positive and negative temporal amplitudes mode 1 represents a wind-forced upwelling and a buoyancy-forced downwelling circulation while mode 2 represents the lateral shear of the flow field. Synoptic maps of surface currents from ocean surface current radar reveal realizations of these event types.

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Andreas Münchow and Eddy C. Carmack

Abstract

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 across-canyon 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 m−3) of Bering Sea summer water into the Arctic Ocean at speeds that exceed 0.7 m s−1. Total northward transports through the canyon exceed 1.0 Sv. The warm waters of this coastal current supply more than 100 W m−2 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 71.8°N latitude.

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Andreas Münchow, Janin Schaffer, and Torsten Kanzow

Abstract

From 2014 through 2016 we instrumented the ~80-km-wide Norske Trough near 78°N latitude that cuts across the 250-km-wide shelf from Fram Strait to the coast. Our measurements resolve a ~10-km-wide bottom-intensified jet that carries 0.27 ± 0.06 Sv (1 Sv ≡ 106 m3 s−1) of warm Atlantic water from Fram Strait toward the glaciers off northeast Greenland. Mean shoreward flows along the steep canyon walls reach 0.1 m s−1 about 50 m above the bottom in 400-m-deep water. The same bottom-intensified vertical structure emerges as the first dominant empirical orthogonal function that explains about 70%–80% of the variance at individual mooring locations. We interpret the current variability as remotely forced wave motions that arrive at our sensor array with periodicities longer than 6 days. Coherent motions with a period near 20 days emerge in our array as a dispersive topographic Rossby wave that propagates its energy along the sloping canyon toward the coast with a group speed of about 63 km day−1. Amplitudes of wave currents reach 0.1 m s−1 in the winter of 2015/16. The wave is likely generated by Ekman pumping over the shelfbreak where sea ice is always mobile. More than 40% of the along-slope ocean current variance near the bottom of the canyon correlates with vertical Ekman pumping velocities 180 km away. In contrast, the impact of local winds on the observed current fluctuations is negligible. Dynamics appear linear and Rossby wave motions merely modulate the mean flow.

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Andreas Münchow, Humfrey Melling, and Kelly K. Falkner

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-channel slope of isopycnal surfaces. During the period of observation, sustained wind from the southwest weakened the average down-channel flow at the surface. The speed of tidal currents exceeded subtidal components by a factor of 2. Tidal signals were resolved and removed from the observations here using two independent methods resolving horizontal and vertical variability of tidal properties, respectively. Tidal current predictions from a barotropic model agreed well with depth-averaged observations in both amplitude and phase. However, because estimates of freshwater flux require accurate surface currents (and salinity), a least squares fitting procedure using velocity data was judged more reliable, since it permits quantification of vertical tidal current variations.

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Andreas Münchow, Thomas J. Weingartner, and Lee W. Cooper

Abstract

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 s−1. 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.

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Céline Heuzé, Anna Wåhlin, Helen L. Johnson, and Andreas Münchow

Abstract

Intrusions of Atlantic Water cause basal melting of Greenland’s marine-terminating glaciers and ice shelves, such as that of Petermann Glacier, in northwest Greenland. The fate of the resulting glacial meltwater is largely unknown. It is investigated here, using hydrographic observations collected during a research cruise in Petermann Fjord and adjacent Nares Strait onboard icebreaker (I/B) Oden in August 2015. A three end-member mixing method provides the concentration of Petermann ice shelf meltwater. Meltwater from Petermann is found in all of the casts in adjacent Nares Strait, with the highest concentration along the Greenland coast in the direction of Kelvin wave phase propagation. The meltwater from Petermann mostly flows out on the northeast side of the fjord as a baroclinic boundary current, with the depth of maximum meltwater concentrations approximately 150 m and shoaling along its pathway. At the outer sill, which separates the fjord from the ambient ocean, approximately 0.3 mSv (1 Sv ≡ 106 m3 s−1) of basal meltwater leaves the fjord at depths between 100 and 300 m. The total geostrophic heat and freshwater fluxes close to the glacier’s terminus in August 2015 were similar to those estimated in August 2009, before the two major calving events that reduced the length of Petermann’s ice tongue by nearly a third and despite warmer inflowing Atlantic Water. These results provide a baseline but also highlight what is needed to assess properly the impact on ocean circulation and sea level of Greenland’s mass loss as the Atlantic Water warms up.

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Alexander E. Yankovsky, Richard W. Garvine, and Andreas Münchow

Abstract

Shipboard hydrographic and acoustic Doppler current profiler surveys conducted in August 1996 on the New Jersey inner shelf revealed a buoyant intrusion advancing southward along the coast. This buoyant intrusion originated from the Hudson estuary more than 100 km upshelf and appeared as a bulge of less saline water with a sharp across-shelf frontal zone at its leading edge. During this time, the study area was also forced by a brief upwelling-favorable wind event opposing the direction of buoyant flow propagation.

The interaction of buoyancy and wind forcing generated a spatially variable velocity field. In particular, across-shelf currents were comparable to their alongshelf counterparts. Variability in the alongshelf direction occurred on the scales of the order of the baroclinic Rossby radius. Intensive across-shelf currents reached speeds of 20– 40 cm s−1 and appeared as spatially localized mesoscale flows with a width of O(10 km). They were generated at the leading edge of the buoyant intrusion and persisted over the period of observations, slowly propagating southward along with the buoyant flow. They were essentially baroclinic with strong vertical shear and were further amplified by the wind forcing.

The upwelling-favorable wind event also generated cyclonic circulation within the buoyant intrusion, which has not been observed before. Interaction of the opposing wind and buoyancy forcings deformed the pycnocline into a dome. This dome was effectively isolated from wind-induced turbulent mixing by overlying buoyant water. The adjustment of the velocity field to this density disturbance occurred geostrophically, even though the water depth was only 20–30 m and friction was important. Relative vorticity associated with this cyclonic flow was at least 0.3f.

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