Heat transport in McMurdo Sound first‐year fast ice

Abstract

Abstract. We have monitored the temperature field within first-year sea ice in McMurdoSound over two winter seasons, with sufficient resolution to determine the thermal conductivity from the thermal waves propagating down through the ice. Data reduction has been accomplished by direct reference to energy conservation, relating the rate of change of the internal energy density to the divergence of the heat current density. Use of this procedure, rather than the wave attenuation predicted by the thermal diffusion equation, avoids difficulties arising from a strongly temperature dependent thermal diffusivity. The thermal conductivity is an input parameter for ice growth and climate models, and the values commonly used in the models are predicted to depend on temperature, salinity, and the volume fraction of air. The present measurements were performed at depths in the ice where the air volume is small and the salinity is nearly constant, and they permit the determination of the absolute magnitude of the thermal conductivity and its temperature dependence. The weak temperature dependence is similar to that predicted by the models in the literature, but the magnitude is smaller by • 10% than the predicted value most commonly used in climate and sea ice models. In the first season we find an additional scatter in the results at driving temperature gradients larger than •10-15 øC/m. We suggest that the scatter arises from a nonlinear contribution to the heat current, possibly associated with the onset of convective motion in brine inclusions. Episodic convective events are also observed. We have further determined the growth rate of the ice and compared it with the rate explained by the heat flux from the ice-water interface. The data show a sudden rise of growth rate, without a rise in heat flux through the ice, which coincides in time and depth with the appearance of platelet ice. Finally, we discuss the observation of radiative solar heating at depth in the ice and demonstrate that the absorption exceeds that in the ice alone; dust or algae must contribute to the absorption.

Direct measurement of sea ice thermal conductivity: No surface reduction

“…From a conservation of energy analysis (see below) these constitute the most comprehensive measurements of k si to date. These results showed departures from the behavior predicted by effective medium models in that the conductivity of congelation ice ( z > 30 cm) was about 10% lower than predicted [ Trodahl et al , 2000, 2001] and there was a systematic near‐surface conductivity reduction of up to ∼25% over the top ∼50 cm which was most pronounced closest to the surface [ Trodahl et al , 2001].…”

Depth and seasonal variations in the thermal properties of Antarctic Dry Valley permafrost from temperature time series analysis

“…We use a graphical finite difference scheme, which we have previously applied to sea ice measurements [ McGuinness et al , 1998; Trodahl et al , 2000, 2001], that does not require harmonic forcing and is insensitive to interthermistor calibration offsets. As for the other finite difference methods discussed in the preceding paragraph, we make the assumption of locally constant thermal properties for the space between adjacent thermistors.…”

“…The thermistor arrays are adapted from units custom‐built for measuring heat flow in first‐year sea ice [ McGuinness et al , 1998; Trodahl et al , 2000, 2001]. The body of the array is a 2‐m‐long, 0.25‐inch (6.35 mm) diameter, thin‐walled (0.2 mm) stainless steel tube.…”

The sub-ice platelet layer and its influence on freeboard to thickness conversion of Antarctic sea ice

“…At the large scale, the interaction of water sourced from ice shelf basal melting, which freshens the surface ocean, has been suggested as a potential contributor to increasing sea ice extent in the Southern Ocean (Bintanja et al, 2013). Of further interest, it is well known that the outflow of supercooled water from the ice shelf cavity creates an additional heat sink to the ocean promoting sea ice growth (Trodahl et al, 2000;Hellmer, 2004;Purdie et al, 2006;Gough et al, 2012), which increases sea ice thickness in close proximity to ice shelves (Hellmer, 2004;Hughes et al, 2014). This additional ice that forms as a direct result of oceanic heat flux driven by the availability of supercooled water can be split into three components: platelet (or frazil) crystals suspended in the water column, an unconsolidated porous layer of sub-ice platelets directly beneath the sea ice and a layer of consolidated platelet ice incorporated into the sea ice (Dempsey et al, 2010).…”