Sea ice formation in the Mertz Glacier polynya
Antarctic coastal polynyas – areas of open water within pack ice – are thought to be winter-time sources of 'Antarctic bottom water'. Cold and windy conditions encourage rapid ice formation, while persistent winds remove the ice cover. Previous estimates of polynya ice production rates have largely been based on satellite remote sensing data and meteorological data from numerical models. Here, we report on measurements collected in August 1999 in the Mertz Glacier polynya (MGP) on the Antarctic coast about 145°E 67°S, west of the Mertz Glacier tongue. We used the data to determine directly the rate of ice growth in winter and the processes occurring as the ice forms.
Air temperature, air pressure, windspeed and wind direction were measured continually from the ship during the voyage and from an automatic weather station on Eder Island to the west of the polynya. For most of the experiment, air temperatures ranged from about minus 15°C to minus 23°C, although they dropped as low as minus 28°C for a short period. The exception to this was from 11 to 15 August, when a low pressure system brought a brief period of northeasterly winds during which temperatures reached as high 0°C. These warm northerlies occur often around Antarctica, even in the middle of winter. Apart from this warm spell, winds were generally south or southeasterly with speeds frequently exceeding 70 km/h.
The photograph above shows the early stages of sea ice formation and consolidation. Snow can be seen blowing off the glacier cliffs at bottom left. Wind speeds were frequently high enough for blowing snow; these same strong winds also increased the heat transfer from the open water of the polynya, and increased the ice formation rate. Under these turbulent conditions, the ice initially forms as frazil ice, or individual ice crystals, seen as white streaks. As these ice crystals form they reject salt into the water column. The ice is then blow away from the coast by the persistent winds.
We observed frazil ice and grease streamers forming near the upwind end of the polynya, the frazil being thicker in the middle of the long streams of grease ice and thinning towards the edges. The ice drifted generally north-northwest, often forming small pancakes or thin sheets of nilas before consolidating into a solid sheet of ice. Unconsolidated frazil ice was often observed beneath consolidated ice while travelling through the ice, with any new cracks or openings created as the ship broke through the ice rapidly filling with frazil.
As this frazil ice accumulates, it eventually forms a continuous cover of ice crystals, called 'grease ice' because of its matt texture and the resemblance to oil floating on top of the ocean. This ice could then take several days to consolidate into a solid ice cover – during a period of warmer weather this took over four days. Even after the surface was relatively solid, frazil was still abundant under the surface, rapidly filling any newly-drilled holes or recently-opened leads.
During the experiment, 15 buoys in three different arrays were deployed in open water or newly forming frazil ice along the Mertz Glacier tongue or near polynya's southeast boundary in Buchanan Bay. These buoys transmitted their position via satellite, and were used to track the ice as it drifted through the polynya. While travelling between each of the buoys we collected detailed, standardised ice observations from the ship based on the method developed by Allison and Worby in 1994. These observations recorded ice and snow thickness, ridging statistics, and ice concentration every half hour. During the experiment the ship travelled between the buoys three different times. Using these data, we have calculated the average ice thickness between buoy locations, and ice growth rates. The estimates provide an area-averaged value for ice thickness, including any open water areas.
The undeformed new ice growth rate averaged about 4 cm a day for the first five days of formation. Ridging and rafting doubled the total growth rate to an average of 8 cm a day. Blowing and falling snow was incorporated into the surface of the newly forming ice, with 16 of 22 ice cores having some snow in the top few centimetres.
Evidence of snow incorporation into the upper layer of ice was found in 16 of the 22 ice samples, or 13 percent of the total ice thickness sampled. Of the 42 days in the ice, snowfall was recorded on 23 days, and drifting snow on 13 days. In addition, observations from helicopters flying near the coast regularly noted considerable snow blowing off the coast into the open water. Four of the six cores with no snow were from ice which had quickly consolidated under cold conditions, with little or no snowfall recorded. Therefore, although there is considerable blowing snow, it is possible for ice to form with no snow incorporation when the initial consolidation occurs rapidly when there is no falling or drifting snow.
This is a novel approach to estimating ice production rates within the polynya. These growth rate estimates are based on direct measurements of ice thickness and do not rely on any heat flux estimates or parameterisation which is often used to estimate ice growth and hence salt flux. The average ice growth rates reported here are for the first few days of ice formation. They are averages from the time of initial frazil formation through ice consolidation, and are also a sort of spatial integration as ice drifts north-northwest through the polynya. Higher ice formation rates (12 and 25 cm a day) such as those reported in Roberts and others are applicable over a smaller area of open water.
About half of the ice thickness results from dynamic processes including both rafting and ridging, while the other half is thermodynamic in origin. Consequently, any model estimates need to consider both processes in estimating ice growth. Midwinter air temperatures can reach 0°C, reducing ice formation rates. This reduces or delays consolidation of frazil ice, affecting timing and location of salt rejection into the upper ocean, as the ice may drift farther before complete consolidation occurs. The timing and frequency of these warm storms could have a large influence on the annual ice growth rate and hence the ocean salt flux. Although it is difficult to assess the relative importance of snow in the overall ice formation with these data, it is clear that snow is frequently incorporated into the ice, probably as blowing snow or snowfall during the initial frazil formation and before the ice is consolidated.
Antarctic CRC and AAD