Miles G. McPhee

Surface Heat Budget of the Arctic Ocean

A summary is presented of the Surface Heat Budget of the Arctic Ocean (SHEBA) project, with a focus on the field experiment that was conducted from October 1997 to October 1998. The primary objective of the field work was to collect ocean, ice, and atmospheric datasets over a full annual cycle that could be used to understand the processes controlling surface heat exchanges—in particular, the ice–albedo feedback and cloud–radiation feedback. This information is being used to improve formulations of arctic ice–ocean–atmosphere processes in climate models and thereby improve simulations of present and future arctic climate. The experiment was deployed from an ice breaker that was frozen into the ice pack and allowed to drift for the duration of the experiment. This research platform allowed the use of an extensive suite of instruments that directly measured ocean, atmosphere, and ice properties from both the ship and the ice pack in the immediate vicinity of the ship. This summary describes the project goal...

Turbulent heat flux in the upper ocean under sea ice

Turbulence data from three Arctic drift station experiments demonstrate features of turbulent heat transfer in the oceanic boundary layer. Time series analysis of several w′T′ records shows that heat and momentum flux occur at nearly the same scales, typically by turbulent eddies of the order of 10–20 m in horizontal extent and a few meters in vertical extent. Probability distribution functions of w′T′ have large skewness and kurtosis, where the latter confirms that most of the flux occurs in intermittent “events” with positive and negative excursions an order of magnitude larger than the mean value. An estimate of the eddy heat diffusivity in the outer (Ekman) part of the boundary layer, based on measured heat flux and temperature gradient during a diurnal tidal cycle over the Yermak Plateau slope north of Fram Strait, agrees reasonably well with the eddy viscosity, with values as high as 0.15 m2 s−1. An analysis of measurements made near the ice-ocean interface at the three stations shows that heat flux increases with both temperature elevation above freezing and with friction velocity at the interface. It also reveals a surprising uniformity in parameters describing the heat and mass transfer: e.g., the thickness of the “transition sublayer” (from a modified version of the Yaglom-Kader theory) is about 10 cm at all three sites, despite nearly a fivefold difference in the under-ice roughness z0, which ranges from approximately 2 to 9 cm. A much simplified model for heat and mass transfer at the ice-ocean interface, suggested by the relative uniformity of the heat transfer coefficients at the three sites, is outlined.

Solar heating of the Arctic mixed layer

Data from the 1975 Arctic Ice Dynamics Joint Experiment (AIDJEX) are used to examine energy exchange between the Arctic mixed layer and the ice pack. Conductivity-temperature-depth profiles from four drifting stations reveal significant heat storage in the upper 50 m of the water column during summer, with mixed layer temperature elevation above freezing δT reaching as high as 0.4°C. Combining δT with turbulent friction velocity obtained from local ice motion provides an estimate of heat flux from the ocean to the ice Fw which was found to be strongly seasonal, with maximum values reaching 40–60 W m−2 in August. The annual average value of Fw was 5.1 W m−2, about half again as large as oceanic heat flux inferred from bottom ablation measurements in undeformed ice at the central station. Solar heat input to the upper ocean through open leads and thin ice, estimated using an ice thickness distribution model, totaled about 150 MJ m−2, in general agreement with integrated values of Fw. Results indicate that oceanic heat flux to the ice in the central Arctic is derived mainly from shortwave radiation entering the ocean through the ice pack, rather than from diffusion of warm water from below. Indeed, during the AIDJEX project the mixed layer appears to have contributed 15–20 MJ m−2 of heat to the upper pycnocline. During the summer, Fw was found to vary by as much as 10–30 W m−2 over separations of 100 to 200 km and thus represents an important term in the surface heat budget not controlled by purely local deformation and thermodynamics.

Freshening of the upper ocean in the Arctic: Is perennial sea ice disappearing?

During the Surface Heat Budget of the Arctic (SHEBA) deployment in October, 1997, multiyear ice near the center of the Beaufort Gyre was anomalously thin. The upper ocean was both warmer and less saline than in previous years. The salinity deficit in the upper 100 m, compared with the same region during the Arctic Ice Dynamics Joint Experiment (AIDJEX) in 1975, is equivalent to surface input of about 2.4 m of fresh water. Heat content has increased by 67 MJ m−2. During AIDJEX the change in salinity over the melt season implied melt equivalent to about 0.8 m of fresh water. As much as 2 m of freshwater input may have occurred during the 1997 summer, possibly resulting from decreased ice concentration from changes in atmospheric circulation early in the summer , in the classic albedo-feedback scenario. Unchecked, the pattern could lead to a significantly different sea-ice regime in the central Arctic.

Ocean Heat Flux in the Central Weddell Sea during Winter

Abstract Seasonal sea ice, which plays a pivotal role in air–sea interaction in the Weddell Sea (a region of large deep-water formation with potential impact on climate), depends critically on heat flux from the deep ocean. During the austral winter of 1994, an intensive process-oriented field program named the Antarctic Zone Flux Experiment measured upper-ocean turbulent fluxes during two short manned ice-drift station experiments near the Maud Rise seamount region of the Weddell Sea. Unmanned data buoys left at the site of the first manned drift provided a season-long time series of ice motion, mixed layer temperature and salinity, plus a (truncated) high-resolution record of temperature within the ice column. Direct turbulence flux measurements made in the ocean boundary layer during the manned drift stations were extended to the ice–ocean interface with a “mixing length” model and were used to evaluate parameters in bulk expressions for interfacial stress (a “Rossby similarity” drag law) and ocean-to-...

Measurements of the Turbulent Boundary Layer under Pack Ice

Abstract The mean and turbulent flow structure under pack ice was measured during the 1972 AIDJEK pilot study with small mechanical current meter triplets at eight levels in the planetary boundary layer. CTD profiles showed a well-mixed layer of nearly neutral stability to about 35 m, bounded below by a strong pycnocline. The skin friction velocity u* was determined by measuring the Reynolds stress at 2 and 4 m below the ice (beyond the surface layer) and from consideration of other terms in the mean momentum equation. Local pressure gradients and advective acceleration due to topography could not be ignored; when an estimate of the effect was included, u* was 1.0±0.1 cm s−1 when the ice velocity relative to the ocean was 24 cm s−1. With the proper coordinate transformation, the planetary boundary layer of the ocean resembles that of the atmosphere. Composite averages of non-dimensional Reynolds stress and mean flow in the ocean, when compared with recent models of a neutrally buoyant, horizontally homoge...

Ice-Seawater Turbulent Boundary Layer Interaction with Melting or Freezing

Abstract A second-moment, turbulence closure model is applied to the problem of the dynamic and thermodynamic interaction of sea ice and the ocean surface mixed layer. In the case of ice moving over a warm, ocean surface layer, melting is intrinsically a transient process; that is, melting is rapid when warm surface water initially contacts the ice. Then the process slows when surface water is insulated from deeper water due to the stabilizing effect of the melt water, and the thermal energy stored in the surface layer is depleted. Effectively, the same process prevails when ocean surface water flows under stationary ice in which case, after an initial rapid increase, the melting process decreases with downstream distance. Accompanying the stabilizing effect of the melt water is a reduction in the ice-seawater interfacial shear stress. This process and model simulators are used to explain field observations wherein ice near the marginal ice zone diverges from the main pack. When the surface ice layer is m...

Turbulent Mixing Under Drifting Pack Ice in the Weddell Sea

By providing cold, dense water that sinks and mixes to fill the abyssal world ocean, high-latitude air-sea-ice interaction is the main conduit through which the deep ocean communicates with the rest of the climate system. A key element in modeling and predicting oceanic impact on climate is understanding the processes that control the near surface exchange of heat, salt, and momentum. In 1992, the United States—Russian Ice Station Weddell-1 traversed the western Weddell Sea during the onset of winter, providing a platform for direct measurement of turbulent heat flux and Reynolds stress in the upper ocean. Data from a storm early in the drift indicated (i) well-formed Ekman spirals (in both velocity and turbulent stress); (ii) high correlation between mixed layer heat flux and temperature gradients; (iii) that eddy viscosity and eddy thermal diffusivity were similar, about 0.02 square meters per second; and (iv) that the significant turbulent length scale (2 to 3 meters through most of the boundary layer) was proportional to the wavelength at the peak in the weighted vertical velocity spectrum. The measurements were consistent with a simple model in which the bulk eddy viscosity in the neutrally buoyant mixed layer is proportional to kinematic boundary stress divided by the Coriolis parameter.

The Effect of the Oceanic Boundary Layer on the Mean Drift of Pack Ice: Application of a Simple Model

Abstract Smoothed records of ice drift, surface wind and upper ocean currents at four manned stations of the 1975–76 AIDJEX experiment in the central Arctic have been analyzed to provide a statistical relationship between stress at the ice-ocean interface and ice-drift velocity during a 60-day period when the ice, was too weak to support internal forces. Using interfacial stress calculated from a balance with air stress and Coriolis force on the ice column for times longer than the inertial period, logarithmic linear regression of the stress-velocity samples provided the relation τ = 0.010V1.78, where τ is the magnitude of interfacial stress and V the ice speed relative to the geostrophic current in the ocean. This result is statistically indistinguishable from predictions of a numerical model adapted from Businger and Arya (1974) with surface roughness Z0 = 10 cm. Essential features of the model are dynamic scaling by u*, u*2 and u*/f for velocity, kinematic stress and length, with exponential attenuatio...

The Antarctic Zone Flux Experiment

In winter the eastern Weddell Sea in the Atlantic sector of the Southern Ocean hosts some of the most dynamic air–ice–sea interactions found on earth. Sea ice in the region is kept relatively thin by heat flux from below, maintained by upper-ocean stirring associated with the passage of intense, fast-moving cyclones. Ocean stratification is so weak that the possibility of deep convection exists, and indeed, satellite imagery from the Weddell Sea in the 1970s shows a large expanse of open water (the Weddell Polynya) that persisted through several seasons and may have significantly altered global deep-water production. Understanding what environmental conditions could again trigger widespread oceanic overturn may thus be an important key in determining the role of high latitudes in deep-ocean ventilation and global atmospheric warming. During the Antarctic Zone Flux Experiment in July and August 1994, response of the upper ocean and its ice cover to a series of storms was measured at two drifting stations s...