Inga J. Smith

Platelet ice and the land-fast sea ice of McMurdo Sound, Antarctica

Abstract Dendritic crystals of platelet ice appear beneath the columnar land-fast sea ice of McMurdo Sound, Antarctica. These leaf-like crystals are frozen into place by the advancing columnar growth. The platelets most probably begin to appear during July although in some parts of the Sound they may not appear at all. In addition, the amount and extent of platelet ice within the Sound varies from year to year. Previous authors have suggested that the formation of platelet ice is linked to the presence of the nearby ice shelf. It is a matter of debate whether these platelets form at depth and then float upwards or whether they grow in slightly supercooled water at the ice/water interface. The phenomenon is similar to that observed in the Weddell Sea region, but previous authors have suggested the two regions may experience different processes. This paper presents the results of field-work conducted in McMurdo Sound in 1999. Ice-structure analysis, isotopic analysis and salinity and temperature measurements near the ice/water interface are presented. Freezing points are calculated, and the possible existence of supercooling is discussed in relation to existing conjectures about the origin of platelets.

Heat transport in McMurdo Sound first‐year fast ice

We have monitored the temperature field within first-year sea ice in McMurdo Sound 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 °C/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.

The Response of the Southern Ocean and Antarctic Sea Ice to Freshwater from Ice Shelves in an Earth System Model

ABSTRACTThe possibility that recent Antarctic sea ice expansion resulted from an increase in freshwater reaching the Southern Ocean is investigated here. The freshwater flux from ice sheet and ice shelf mass imbalance is largely missing in models that participated in phase 5 of the Coupled Model Intercomparison Project (CMIP5). However, on average, precipitation minus evaporation (P − E) reaching the Southern Ocean has increased in CMIP5 models to a present value that is about greater than preindustrial times and 5–22 times larger than estimates of the mass imbalance of Antarctic ice sheets and shelves (119–544 ). Two sets of experiments were conducted from 1980 to 2013 in CESM1(CAM5), one of the CMIP5 models, artificially distributing freshwater either at the ocean surface to mimic iceberg melt or at the ice shelf fronts at depth. An anomalous reduction in vertical advection of heat into the surface mixed layer resulted in sea surface cooling at high southern latitudes and an associated increase in sea i...

Observed platelet ice distributions in Antarctic sea ice: An index for ocean‐ice shelf heat flux

Antarctic sea ice that has been affected by supercooled Ice Shelf Water (ISW) has a unique crystallographic structure and is called platelet ice. In this paper we synthesize platelet ice observations to construct a continent-wide map of the winter presence of ISW at the ocean surface. The observations demonstrate that, in some regions of coastal Antarctica, supercooled ISW drives a negative oceanic heat flux of −30 Wm−2 that persists for several months during winter, significantly affecting sea ice thickness. In other regions, particularly where the thinning of ice shelves is believed to be greatest, platelet ice is not observed. Our new data set includes the longest ice-ocean record for Antarctica, which dates back to 1902 near the McMurdo Ice Shelf. These historical data indicate that, over the past 100 years, any change in the volume of very cold surface outflow from this ice shelf is less than the uncertainties in the measurements.

Anchor ice in polar oceans

One feature of high-latitude areas is the formation of ice clusters attached to the beds of rivers, lakes and the sea. This anchor ice, as it is widely known, plays an important role in mobilizing bed sediments, as well as ecological roles as a food source, habitat and potentially fatal environment. Much work has been devoted to fluvial anchor ice in the Northern Hemisphere, yet comparatively little work has described anchor ice in polar marine environments, despite its description by Antarctic expedition scientists over a century ago. In this paper, we review the current understanding of anchor ice formation in polar marine environments. Supercooled water is a necessity for anchor ice to form and frazil adhesion is the most likely common mechanism for initial anchor ice growth. Strong biological zonation has led some authors to suggest that anchor ice does not form to depths of greater than 33 m, yet in Antarctica there appear to be no physical reasons for such a limit given the production of supercooled water to substantial depths associated with ice shelves. Future work should focus on the potential extent of anchor ice production and identify the key oceanographic, glaciological and meteorological conditions conducive to its formation.

Towards a process model for predicting potential anchor ice formation sites in coastal Antarctic waters

Anchor ice describes clusters of ice attached to the beds of rivers, lakes or seas. In Antarctica, ice shelves are considered to be a main driver of anchor ice formation through a process commonly referred to as an ‘ice pump’. These pumps melt the base of an ice shelf at depth and produce a buoyancy-driven plume of meltwater that rises along the basal plane, becoming potentially supercooled in the process. Anchor ice growth may be initiated in regions where plumes intersect the local seafloor. A simple process model is proposed to predict these growth sites in coastal Antarctic waters. A comparison with model output and anchor ice observations in McMurdo Sound reveals that model-predicted formation sites are consistent with these observations. Knowledge of ice shelf draft, basal slope and cavity circulation is necessary to extend the model beyond the confines of McMurdo Sound.

Estimation of Antarctic Land‐Fast Sea Ice Algal Biomass and Snow Thickness From Under‐Ice Radiance Spectra in Two Contrasting Areas

Fast ice is an important component of Antarctic coastal marine ecosystems, providing a prolific habitat for ice algal communities. This work examines the relationships between normalized difference indices (NDI) calculated from under‐ice radiance measurements and sea ice algal biomass and snow thickness for Antarctic fast ice. While this technique has been calibrated to assess biomass in Arctic fast ice and pack ice, as well as Antarctic pack ice, relationships are currently lacking for Antarctic fast ice characterized by bottom ice algae communities with high algal biomass. We analyze measurements along transects at two contrasting Antarctic fast ice sites in terms of platelet ice presence: near and distant from an ice shelf, i.e., in McMurdo Sound and off Davis Station, respectively. Snow and ice thickness, and ice salinity and temperature measurements support our paired in situ optical and biological measurements. Analyses show that NDI wavelength pairs near the first chlorophyll a (chl a) absorption peak (≈440 nm) explain up to 70% of the total variability in algal biomass. Eighty‐eight percent of snow thickness variability is explained using an NDI with a wavelength pair of 648 and 567 nm. Accounting for pigment packaging effects by including the ratio of chl a‐specific absorption coefficients improved the NDI‐based algal biomass estimation only slightly. Our new observation‐based algorithms can be used to estimate Antarctic fast ice algal biomass and snow thickness noninvasively, for example, by using moored sensors (time series) or mapping their spatial distributions using underwater vehicles.

Measurements of Ice Shelf Water beneath the front of the Ross Ice Shelf using gliders

ABSTRACT Measurements made by an underwater glider deployed near the Ross Ice Shelf were used to identify the presence of Ice Shelf Water (ISW), which is defined as seawater with its potential temperature lower than its surface freezing point temperature. Properties logged by the glider included in situ temperature, electrical conductivity, pressure, GPS location at surfacings and time. For most of the first 30 recorded dives of its deployment, evidence suggests the glider was prevented from surfacing due to being under the ice shelf. For dives under the ice shelf, farthest from the ice shelf front, ISW layers of varying thicknesses and depth locations were observed; between 2 m thick (centred at 231 m depth) to >93 m thick (centred at >360 m). For dives under the ice shelf, close to the ice shelf front, either no ISW was observed or ISW layers were centred at shallower depths (116–127 m). Thicker ISW layers (e.g. up to 250 m thickness centred at 421 m) were observed for some glider dives in open water in front of the Ross Ice Shelf. No in situ supercooling (water colder than the pressure-dependent freezing point temperature) was observed.

A framework for estimating anchor ice extent at potential formation sites in McMurdo Sound, Antarctica

Abstract A distinctive feature of polar regions is the formation of ice clusters attached to the seabed, known as ‘anchor ice’. Anchor ice plays an important role in mobilizing bed sediments, and serves ecological roles providing habitats, or as an agent of disturbance creating potentially fatal environments to benthic fauna. The sublittoral zone associated with the landward margin represents the most likely environment for anchor ice formation, where conditions conducive to the advection of supercooled water from sub-ice-shelf cavities are favourable. We develop a framework to estimate the areal extent of anchor ice formation assuming a northerly flow of 75m deep supercooled water plumes from the Ross and McMurdo Ice Shelf cavities, Antarctica. In McMurdo Sound our results indicate that regions beneath the McMurdo Ice Shelf, extending along Brown Peninsula and White and Black Islands, are likely conducive to anchor ice formation. Anchor ice may also form along the Hut Point Peninsula and around Ross Island, and in pockets along the southern Victoria Land coast. The limitations of our approach include an imposed northerly flow of Ice Shelf Water, poorly constrained sub-ice-shelf bathymetry, and temporal variability in supercooled water depth production, particularly in the eastern Sound.