Kenneth Hunkins

Ekman drift currents in the Arctic Ocean

Abstract Current observations from a drifting ice floe in the central Arctic Ocean give clear evidence of a clockwise spiral structure in the upper layers. The data for steady conditions show a boundary layer just beneath the ice and an Ekman spiral layer below it. The depth of frictional influence is 18 m for winds of 4 m/sec. This is apparently the first detailed confirmation of the Ekman spiral in deep waters.

Salt Dispersion in the Hudson Estuary

Abstract The seaward transport of salt by river discharge through an estuary is balanced under steady conditions by landward dispersion effected by various physical mixing processes. Observations of current and salinity in the lower Hudson estuary provide a basis for assessing the relative importance of these different dispersion processes. Wind effects were estimated from two moored current meter records of 1- and 2-months length. There was a significant but weak correlation between wind and currents. Three cross-sectional surveys each lasting 25 h provide estimates of salt dispersion by other processes. Current and salinity data from the surveys were decomposed into various temporal and spatial means and departures from these means. Covariances between the various quantities are interpreted in terms of physical dispersion processes. The largest contributor to salt dispersion in the Hudson is the steady shear of gravitational circulation. Steady shear dispersion varies by a factor of 5 between spring hig...

Subsurface eddies in the Arctic ocean

Abstract In March and April of 1972, four transient undercurrents were associated with eddies at depths between 50 and 300 m in the Arctic Ocean. The velocity profile was parabolic with a maximum of 40 cm s−1 at 150m. Two of the eddies were anticyclonic and two, cyclonic. The eddies persisted at least several days, they were 10–20 km in diameter, and were separated by 20–50 km. The subsurface currents in the eddies were swifter than those in the upper mixed layer, with little correlation between the two levels. The eddies are believed to have their origin in the instability of the basic baroclinic current.

Quaternary sedimentation in the Arctic ocean

Abstract A distinct boundary between sediment types usually occurs at a depth of about 10 cm in bottom cores raised from the Alpha Rise in the Arctic Ocean. The sediment between the tops of the cores and the 10 cm boundary is a dark brown, foraminiferal lutite mixed with ice-rafted sand and pebbles. The sediment between the 10 cm boundary and a depth of about 40 cm is a light brown sand with ice-rafted material but few Foraminifera. The 10 cm boundary apparently represents the most recent change in pelagic deposition in this region and must be connected with climatic changes. Foraminifera from a zone between 7 and 10 cm have been dated by the C14 method as 25,000 ± 3000 and as 30,000 years BP in two different samples. The 10 cm boundary itself has been dated as 70, 000 years BP by a uranium series method. If these dates are accepted, a low sedimentation rate of 1 1 2 to 3 mm/1000 years is indicated for the Alpha Rise and for the Arctic Ocean as a whole if pelagic sedimentation has been uniform over the entire ocean. Cores from the Canada Abyssal Plain differ in character from the Alpha Rise cores consisting primarily of olive-gray lutite without Foraminifera or ice-rafted material. This sediment was probably deposited by turbidity currents. A 3 mm layer of dark brown, foraminiferal lutite occurs at the top of the Canada Abyssal Plain cores. This layer is similar to the upper layer in Alpha Rise cores and apparently represents continued pelagic deposition since the last turbidity current. Foraminifera from this upper 3 mm layer have been dated as 700 ± 100 years BP by the C14 method. The conclusion is that pelagic sedimentation has continued unchanged in the Arctic Ocean from about 70,000 years ago to the present. This implies that the present ice cover has existed for that length of time.

Laboratory simulation of exchange through Fram Strait

Laboratory experiments and theory were conducted to observe the flow patterns and transport in both buoyancy-driven and wind-driven rotating fluids. In “lock-exchange” experiments, water with one density flows into a second basin after a sliding gate is removed. Water of a second density flows back into the first basin. The size and location of the currents for various values of density difference, rotation rate, and assorted sidewall geometries was recorded. Volume flux of the fluid was also measured and compared with a theory for lock-exchange flow of a rotating fluid. In a separate group of experiments with a passive upper layer, easterly winds (like those in the Arctic Ocean) drive the upper level water into the Arctic Ocean and therefore oppose the buoyant exchange. Westerly winds would drive the water out of the Arctic Ocean. This indicates that the exchange between the Arctic Ocean and the Greenland-Norwegian Sea is likely to be driven by buoyancy rather than by driven by wind. Crude estimates of the volumetric and fresh water exchange rate from the lock-exchange formulas are compared with observed ocean fluxes, and approximate agreement is found.

Internal Wave Dispersion Calculated Using the Thomson-Haskell Method

Abstract The dispersion and amplitude characteristics of internal wave motion are determined by a matrix method which lends itself readily to computer analysis. A layered density structure may be chosen to fit actual oceanic conditions. The method is shown to have good agreement with a simple analytical solution. Dispersion and amplitude characteristics have been determined for two typical oceanic sites, one in the Arctic Ocean and one in the Atlantic.

Arctic ocean geophysical studies: Chukchi Cap and Chukchi abyssal plain

Abstract A bathymetric chart of the Chukchi Cap region was compiled with soundings obtained from Fletchers Ice Island (T-3), as well as from other ice stations and from U.S. Navy icebreakers. New details of the Chukchi Cap are shown, including two submarine troughs on the southwest side. West of the Chukchi Cap, a small abyssal plain was found with a depth of 2230 m. This abyssal plain is connected through an abyssal gap with the deeper Canada Abyssal Plain. The prominent magnetic anomaly discovered during the drift of Station Charlie was crossed more recently by T-3 and by aeromagnetic flights. The continuity of the anomaly along the western and northern sides of the Chukchi Cap was further established by the new measurements. An interpretation was made of the anomaly as an expression of induced magnetization in basement rocks. The interpretation shows a basement ridge beneath the anomaly maximum at the edge of the Chukchi Cap. The Cap itself is interpreted as being underlaid by a 12 km thickness of sediments. Both magnetic and gravity data were used for an interpretation of total crustal thickness along the same section. Crustal thickness ranges from 18− 1 2 km beneath the Chukchi Cap to 32 km beneath the large basement ridge.

Numerical studies of the 4-day oscillation in Lake Champlain

The summer thermocline of Lake Champlain, which is found at depths of 20-30 m, oscillates with typical vertical amplitudes of 20-40 m and periods of ∼4 days. Fluctuations at the ends of the lake are opposite in phase and accompanied in the central lake by strong shears across the thermocline. Thcsc arc basin-wide baroclinic disturbances which are forced by wind. A numerical, one-dimensional, two-laycr, shallow-water model incorporating nonlinear and frictional effects in a rectangular basin forced by wind was first tested with idealized wind impulses. The results do not resemble the observed thermocline motion. However, when this simple model is forced with wind data from a nearby shore site, there is rcasonable agreement between the model results and observed long-period thermocline motions in Lake Champlain. Dispersion effects appear to be negligible hcre. This contrasts with othcr long, narrow lakes whcre dispersion effects are important and internal surges are followed by wave trains resembling the soliton solutions of the Korteweg-deVries equation. A possiblc explanation for the different regime in Lake Champlain may be found in its unique bathymetry with sloping bottom at the ends and numerous embayments on the sides that provide traps to collect wind-driven warm water and then release it slowly during recovery of cquilibrium, prevcnting the formation of steep fronts and soliton wave trains.

A REVIEW OF THE PHYSICAL OCEANOGRAPHY OF FRAM STRAIT

Fram Strait is the broad and deep gap (width, 450 km; sill depth, 2700 m) separating Greenland and Spitsbergen. There is an exchange through it of cold, fresh Arctic waters and warm, saline Atlantic waters. Narrow coastal boundary currents on either side flowing in opposite directions are important elements of the exchange. Mesoscale eddies are abundant in this region but their part in transport through the strait is uncertain. This appears to be primarily a convective circulation driven by density differences between the Arctic Ocean and Greenland Sea with wind forcing playing a minor part. It is possible that atmospheric pressure gradients and tidal rectification also contribute to the exchange although their importance has not yet been demonstrated. There has been considerable success in describing the ice exchange with numerical models which incorporate a restricted ocean model. Laboratory experiments provide an alternative method which has proved useful in giving insight in to the relative roles of boundary currents and eddies in transport through wide straits.

Shallow‐Water Propagation in the Arctic Ocean

Dispersion characteristics of underwater sound on the Arctic continental shelf north of Alaska were investigated at ranges between 2 and 250 km and for frequencies between 3 and 250 cps. Explosive charges were used as sources, and geophones were used as detectors. Observations were interpreted in terms of normal‐mode theory, and good agreement between theory and experiment was found for both phase and group velocity. Portions of the first and second modes were recognized at all ranges, and, at short ranges, “leaking modes,” associated with the ice layer, were also noted. For long ranges, the water wave amplitude varied as the −1.85 power of range.