Sea Ice Growth Rates Research Articles

The sensitivity of sea-ice brine fraction to the freezing temperature and orientation

Abstract The changing conditions in which sea ice forms and exists are likely to affect the properties of sea ice itself, and potential climate feedbacks need to be identified and understood to improve future projections. Here we perform a set of idealised laboratory experiments that model sea-ice growth under a range of freezing conditions. The results confirm existing theories; sea-ice growth rate is largest for cooler freezing temperatures, fresher ambient salinities and cases with bottom cooling. Our primary metric of interest is the brine fraction (the volume ratio of brine inclusions to the total sea ice), which we quantify and determine its sensitivity with respect to the ambient salinity, freezing temperature and, for the first time, the freezing direction. We find that the brine fraction of our model sea ice is most sensitive to freezing temperature, and increases 2.5% per 1 $^\circ$ C increase of freezing temperature.

Improved Simulation of Antarctic Sea Ice by Parameterized Thickness of New Ice in a Coupled Climate Model

AbstractSea ice formation over open water exerts critical control on polar atmosphere‐ocean‐ice interactions, but is only crudely represented in sea ice models. In this study, a collection depth parameterization of new ice for flux polynya models is modified by including the sea ice concentration and ice growth rate as additional factors. We evaluated it in a climate model BCC‐CSM2‐MR and found that it improves simulation of Antarctic sea ice concentration and thickness in most of Indian and Atlantic sectors. Disagreement between the observed Antarctic sea ice expansion during 1981–2014 and the modeled decline still exists but is mitigated when the modified scheme is implemented. Further analysis indicates that these improvements are associated with the overcoming of premature closure of open water, which enhances the response of ocean to surface wind intensification during 1981–2014, and consequently slowdowns the sea surface temperature increase and the resulting Antarctic sea ice reduction.

Remotely Monitored Buoys for Observing the Growth and Development of Sea Ice In Situ

Abstract This paper describes a remotely monitored buoy that, when deployed in open water prior to freeze up, permits scientists to monitor not only temperature with depth, and hence freeze up and sea ice thickness, but also the progression of sea ice development—e.g., the extent of cover at a given depth as it grows (solid fraction), the brine volume of the ice, and the salinity of the water just below, which is driven by brine expulsion. Microstructure and In situ Salinity and Temperature (MIST) buoys use sensor “ladders” that, in our prototypes, extend to 88 cm below the surface. We collected hourly measurements of surface air temperature and water temperature and electrical impedance every 3 cm to track the seasonal progression of sea ice growth in Elson Lagoon (Utqiaġvik, Alaska) over the 2017/18 ice growth season. The MIST buoy has the potential to collect detailed sea ice microstructural information over time and help scientists monitor all parts of the growth/melt cycle, including not only the freezing process but the effects of meteorological changes, changing snow cover, the interaction of meltwater, and drainage. Significance Statement There is a need to better understand how an increasing influx of freshwater, one part of a changing Arctic climate, will affect the development of sea ice. Current instruments can provide information on the growth rate, extent, and thickness of sea ice, but not direct observations of the structure of the ice during freeze up, something that is tied to salinity and local air and water temperature. A first deployment in Elson Lagoon in Utqiaġvik, Alaska, showed promising results; we observed fluctuations in ice temperatures in response to brief warmings in air temperature that resulted in changes in the conductivity, liquid fraction, and brine volume fraction within the ice.

Spatiotemporal Analysis and Modeling of Sea Ice Growth Rate in Nella Fjord, Antarctica: Based on Observations During the 36th Chinese National Antarctic Research Expedition in 2020

Obtaining field sea ice data is crucial for the validation and improvement of satellite data and numerical models. During the 36th Chinese National Antarctic Research Expedition, seven field observations covering the whole area were conducted in Nella Fjord, and large amounts of in situ sea ice and snow thickness data were acquired. Then, the influences of snow cover, topography, and air temperature on the sea ice growth rate were examined. Based on the relationship between the sea ice growth rate and air temperature, an empirical model of sea ice thickness was developed. The results show that sea ice started to freeze in early March, and the peak growth rate was reached in mid-June, with a mean growth rate of 0.62 cm/d. Between mid-June and mid-October, the mean growth rate was 0.36–0.4 cm/d; between mid-October and mid-November, the mean growth rate decreased to 0.15 cm/d. After mid-November, ice growth stopped, and the ice thickness decreased. The growth rate and thickness of sea ice near the shore were generally greater than those closer to the center of the fjord, with differences of 0.25 cm/d in mean growth rate and 7.16 cm in mean thickness. Because of the large exchanges of heat between the atmosphere and the ocean, the sea ice thickness increased rapidly during the early stage of freeze-up. When the air temperature was between -12 and −15°C, the ice growth rate was between 0.36 and 0.4 cm/d. However, when the ice thickness reached 0.5 m, the sea ice growth rate became less sensitive to air temperature. In addition, we found that the proposed model has high simulation accuracy for the growth rate of sea ice. Based on the model, we estimated that the growth rates become zero at an air temperature of approximately −4°C. Our study highlights the inverse relationship between the sea ice growth rate and the snow thickness and the fact that the sea ice thickness is mainly affected by air temperature.

Effects of the atmospheric forcing resolution on simulated sea ice and polynyas off Adélie Land, East Antarctica

Coastal polynyas of the Southern Ocean play a central role in the ventilation of the deep ocean and affect the stability of ice shelves. It appears crucial to incorporate them into climate models, but it is unclear how to adequately simulate them. In particular, there is no consensus on the atmospheric forcing resolution needed to appropriately model the sea ice in coastal Antarctica. A high resolution might be required to represent the local winds such as katabatic winds which are key drivers of coastal polynyas. To fill in this gap, we have tested the sensitivity of sea ice and air-sea-ice interactions to the resolution of the atmospheric forcing in a high-resolution ocean–sea ice model. A set of regional atmospheric simulations at horizontal resolutions of 20, 10, and 5 km are performed with an atmospheric regional model and used to force three ocean–sea ice simulations in the Adélie Land sector, East Antarctica. Due to the better representation of topography with a refined grid, the offshore component of coastal winds becomes stronger at increased resolution. The wind intensification is particularly strong down valleys channelizing the katabatic flow, with increase in wind speed ranging between 1 and 3 m/s. Under a higher resolution forcing, polynyas open more frequently and are wider. This fosters the growth rate of sea ice in polynyas, while landfast ice and pack ice are weakly affected. In polynyas, the production of sea ice is increased by up to 30% at 5 km resolution compared to 20 km resolution. Polynyas downstream of the katabatic wind pathway are more affected than the ones driven by easterly winds, highlighting the importance of the local wind conditions. Brine rejection associated with these higher sea ice production rates affects the salinity budget of the ocean and enhances both the volume and density of the dense Shelf Water produced off Adélie Land. These results underpin the need to better account for local coastal winds and polynyas in ocean and climate models.

Arctic Sea Ice Growth in Response to Synoptic- and Large-Scale Atmospheric Forcing from CMIP5 Models

AbstractWe explore the response of wintertime Arctic sea ice growth to strong cyclones and to large-scale circulation patterns on the daily scale using Earth system model output in phase 5 of the Coupled Model Intercomparison Project (CMIP5). A combined metrics ranking method selects three CMIP5 models that are successful in reproducing the wintertime Arctic dipole (AD) pattern. A cyclone identification method is applied to select strong cyclones in two subregions in the North Atlantic to examine their different impacts on sea ice growth. The total change of sea ice growth rate (SGR) is split into those respectively driven by the dynamic and thermodynamic atmospheric forcing. Three models reproduce the downward longwave radiation anomalies that generally match thermodynamic SGR anomalies in response to both strong cyclones and large-scale circulation patterns. For large-scale circulation patterns, the negative AD outweighs the positive Arctic Oscillation in thermodynamically inhibiting SGR in both impact area and magnitude. Despite the disagreement on the spatial distribution, the three CMIP5 models agree on the weaker response of dynamic SGR than thermodynamic SGR. As the Arctic warms, the thinner sea ice results in more ice production and smaller spatial heterogeneity of thickness, dampening the SGR response to the dynamic forcing. The higher temperature increases the specific heat of sea ice, thus dampening the SGR response to the thermodynamic forcing. In this way, the atmospheric forcing is projected to contribute less to change daily SGR in the future climate.

Sea ice thickness in the Eastern Canadian Arctic: Hudson Bay Complex & Baffin Bay

Past observations of sea ice thickness in the Eastern Canadian Arctic (ECA) have generally been restricted to drill-hole measurements at a few local sites on landfast ice. Here we use data from the laser altimeter ICESat and the radar altimeter Cryosat-2 to present a 14-year record (2003–2016) of high-resolution and spatially extensive ice thickness observations for the ECA and identify 12 sub-regions with distinct patterns. The mean sea ice growth rate within the seasonally ice-covered ECA from November to April is 23cmmo−1 (565km3mo−1), with the fastest increase in thickness occurring through strong ice convergence and deformation in eastern Hudson Bay and Foxe Basin. Our results demonstrate characteristically asymmetrical distributions of sea ice thickness in both Hudson Bay and Baffin Bay, but in opposing directions. In Hudson Bay the spring ice cover is 40cm thicker in the eastern region compared to the northwestern region, whereas in Baffin Bay the ice is 20cm thicker in the western half of the bay compared to the eastern half. In Hudson Bay we find that years with strong and positive ice drift vorticity (i.e. cyclonic and convergent conditions) correlate with increasingly asymmetrical sea ice covers, with the level of west-east asymmetry varying from 2 to 11cm per 100km. However, in Baffin Bay the ice drift vorticity is typically negative (i.e. anticyclonic and divergent) with no obvious link to the asymmetry of the spring ice cover. Finally, we estimate that large interannual variations in spring sea ice volume within the ECA lead to ±15% variations in the volume of freshwater available at the ocean surface during summer.

The evolution of water property in the Mackenzie Bay polynya during Antarctic winter

Temperature and salinity profile data, collected by southern elephant seals equipped with autonomous CTD-Satellite Relay Data Loggers (CTD-SRDLs) during the Antarctic wintertime in 2011 and 2012, were used to study the evolution of water property and the resultant formation of the high density water in the Mackenzie Bay polynya (MBP) in front of the Amery Ice Shelf (AIS). In late March the upper 100–200 m layer is characterized by strong halocline and inversion thermocline. The mixed layer keeps deepening up to 250 m by mid-April with potential temperature remaining nearly the surface freezing point and sea surface salinity increasing from 34.00 to 34.21. From then on until mid-May, the whole water column stays isothermally at about −1.90℃ while the surface salinity increases by a further 0.23. Hereafter the temperature increases while salinity decreases along with the increasing depth both by 0.1 order of magnitude vertically. The upper ocean heat content ranging from 120.5 to 2.9 MJ m−2, heat flux with the values of 9.8–287.0 W m−2 loss and the sea ice growth rates of 4.3–11.7 cm d−1 were estimated by using simple 1-D heat and salt budget methods. The MBP exists throughout the whole Antarctic winter (March to October) due to the air-sea-ice interaction, with an average size of about 5.0×103 km2. It can be speculated that the decrease of the salinity of the upper ocean may occur after October each year. The recurring sea-ice production and the associated brine rejection process increase the salinity of the water column in the MBP progressively, resulting in, eventually, the formation of a large body of high density water.

Winter to summer oceanographic observations in the Arctic Ocean north of Svalbard

AbstractOceanographic observations from the Eurasian Basin north of Svalbard collected between January and June 2015 from the N‐ICE2015 drifting expedition are presented. The unique winter observations are a key contribution to existing climatologies of the Arctic Ocean, and show a ∼100 m deep winter mixed layer likely due to high sea ice growth rates in local leads. Current observations for the upper ∼200 m show mostly a barotropic flow, enhanced over the shallow Yermak Plateau. The two branches of inflowing Atlantic Water are partly captured, confirming that the outer Yermak Branch follows the perimeter of the plateau, and the inner Svalbard Branch the coast. Atlantic Water observed to be warmer and shallower than in the climatology, is found directly below the mixed layer down to 800 m depth, and is warmest along the slope, while its properties inside the basin are quite homogeneous. From late May onwards, the drift was continually close to the ice edge and a thinner surface mixed layer and shallower Atlantic Water coincided with significant sea ice melt being observed.

Sea ice growth rates from tide‐driven visible banding

AbstractIn this paper, periodic tide‐current‐driven banding in a sea‐ice core is demonstrated as a measure of the growth rate of first‐year sea ice at congelation‐ice depths. The study was performed on a core from the eastern McMurdo Sound, exploiting the well‐characterized tidal pattern at the site. It points the way to a technique for determining early‐season ice growth rates from late‐season cores, in areas where under ice currents are known to be tidally dominated and the ice is landfast, thus providing data for a time of year when thin ice prevents direct thickness (and therefore growth rate) measurements. The measured results were compared to the growth‐versus‐depth predicted by a thermodynamic model.

Impact of surface wind biases on the Antarctic sea ice concentration budget in climate models

We derive the terms in the Antarctic sea ice concentration budget from the output of three models, and compare them to observations of the same terms. Those models include two climate models from the 5th Coupled Model Intercomparison Project (CMIP5) and one ocean–sea ice coupled model with prescribed atmospheric forcing. Sea ice drift and wind fields from those models, in average over April-October 1992-2005, all exhibit large differences with the available observational or reanalysis datasets. However, the discrepancies between the two distinct ice drift products or the two wind reanalyses used here are sometimes even greater than those differences. Two major findings stand out from the analysis. Firstly, large biases in sea ice drift speed and direction in exterior sectors of the sea ice covered region tend to be systematic and consistent with those in winds. This suggests that sea ice errors in these areas are most likely wind-driven, so as errors in the simulated ice motion vectors. The systematic nature of these biases is less prominent in interior sectors, nearer the coast, where sea ice is mechanically constrained and its motion in response to the wind forcing more depending on the model rheology. Second, the intimate relationship between winds, sea ice drift and the sea ice concentration budget gives insight on ways to categorize models with regard to errors in their ice dynamics. In exterior regions, models with seemingly too weak winds and slow ice drift consistently yield a lack of ice velocity divergence and hence a wrong wintertime sea ice growth rate. In interior sectors, too slow ice drift, presumably originating from issues in the physical representation of sea ice dynamics as much as from errors in surface winds, leads to wrong timing of the late winter ice retreat. Those results illustrate that the applied methodology provides a valuable tool for prioritizing model improvements based on the ice concentration budget–ice drift biases–wind biases relationship prevailing in the simulation of Antarctic sea ice over the last decades.

Surface water mass composition changes captured by cores of Arctic land-fast sea ice

In the Arctic, land-fast sea ice growth can be influenced by fresher water from rivers and residual summer melt. This paper examines a method to reconstruct changes in water masses using oxygen isotope measurements of sea ice cores. To determine changes in sea water isotope composition over the course of the ice growth period, the output of a sea ice thermodynamic model (driven with reanalysis data, observations of snow depth, and freeze-up dates) is used along with sea ice oxygen isotope measurements and an isotopic fractionation model. Direct measurements of sea ice growth rates are used to validate the output of the sea ice growth model. It is shown that for sea ice formed during the 2011/2012 ice growth season at Barrow, Alaska, large changes in isotopic composition of the ocean waters were captured by the sea ice isotopic composition. Salinity anomalies in the ocean were also tracked by moored instruments. These data indicate episodic advection of meteoric water, having both lower salinity and lower oxygen isotopic composition, during the winter sea ice growth season. Such advection of meteoric water during winter is surprising, as no surface meltwater and no local river discharge should be occurring at this time of year in that area. How accurately changes in water masses as indicated by oxygen isotope composition can be reconstructed using oxygen isotope analysis of sea ice cores is addressed, along with methods/strategies that could be used to further optimize the results. The method described will be useful for winter detection of meteoric water presence in Arctic fast ice regions, which is important for climate studies in a rapidly changing Arctic. Land-fast sea ice effective fractionation coefficients were derived, with a range of +1.82‰ to +2.52‰. Those derived effective fractionation coefficients will be useful for future water mass component proportion calculations. In particular, the equations given can be used to inform choices made when engaging in end member determination for working out the component proportions of water masses.

Evaluation of CryoSat-2 derived sea-ice freeboard over fast ice in McMurdo Sound, Antarctica

AbstractUsing in situ data from 2011 and 2013, we evaluate the ability of CryoSat-2 (CS-2) to retrieve sea-ice freeboard over fast ice in McMurdo Sound. This provides the first systematic validation of CS-2 in the coastal Antarctic and offers insight into the assumptions currently used to process CS-2 data. European Space Agency Level 2 (ESAL2) data are compared with results of a Waveform Fitting (WfF) procedure and a Threshold-First-Maximum-Retracker-Algorithm employed at 40% (TFMRA40). A supervised freeboard retrieval procedure is used to reduce errors associated with sea surface height identification and radar velocity in snow. We find ESAL2 freeboards located between the ice and snow freeboard rather than the frequently assumed snow/ice interface. WfF is within 0.04 m of the ice freeboard but is influenced by variable snow conditions causing increased radar backscatter from the air/snow interface. Given such snow conditions and additional uncertainties in sea surface height identification, a positive bias of 0.14 m away from the ice freeboard is observed. TFMRA40 freeboards are within 0.03 m of the snow freeboard. The separation of freeboard estimates is primarily driven by the different assumptions of each retracker, although waveform alteration by variations in snow properties and surface roughness is evident. Techniques are amended where necessary, and automatic freeboard retrieval procedures for ESAL2, WfF and TFMRA40 are presented. CS-2 detects annual fast-ice freeboard trends using all three automatic procedures that are in line with known sea-ice growth rates in the region.

Insights into brine dynamics and sea ice desalination from a 1-D model study of gravity drainage

[1] We study gravity drainage using a new 1-D, multiphase sea ice model. A parametrization of gravity drainage based on the convective nature of gravity drainage is introduced, whose free parameters are determined by optimizing model output against laboratory measurements of sea ice salinity evolution. Optimal estimates of the free parameters as well as the parametrization performance remain stable for vertical grid resolutions from 1 to 30 mm. We find a strong link between sea ice growth rate and bulk salinity for constant boundary conditions but only a weak link for more realistic boundary conditions. We also demonstrate that surface warming can trigger brine convection over the whole ice layer. Over a growth season, replacing the convective parametrization with constant initial salinities leads to an overall 3% discrepancy of stored energy, thermal resistance, and salt release. We also derive from our convective parametrization a simplified, numerically cheap and stable gravity-drainage parametrization. This parametrization results in an approximately 1% discrepancy of stored energy, thermal resistance, and salt release compared to the convective parametrization. A similarly low discrepancy to our complex parametrization can be reached by simply prescribing a depth-dependent salinity profile.

Sea ice growth rates near ice shelves

Abstract Sea ice growth rates near ice shelves are influenced by ocean-ice shelf interactions. Sea ice growth rates and ocean observations from McMurdo Sound, Antarctica in 1999 and 2000 are presented in this paper. Growth rate measurements were made for an individual platelet crystal through video camera observations. It was found that the crystal grew in discontinuous, episodic bursts at rates of the order of 10 − 6 m s − 1 . Sea water 0.15 m beneath the lower ice surface was measured to be supercooled by 0.01 K. Indications are that supercooling was continuous over the period of episodic platelet ice crystal growth and the growth bursts are attributed to the influence of variable currents. Growth rates for bulk sea ice (i.e., columnar and incorporated platelet ice) and heat fluxes were derived from ice temperature measurements. The growth rates for bulk sea ice were found to be of the order of 10 − 7 m s − 1 , an order of magnitude less than the rates for the individual platelet ice crystal. The residual of the energy balance suggested that a negative oceanic heat flux (i.e., heat transport down into the ocean) occurred, in addition to conduction of heat up into the atmosphere. Both salinity-based growth rate models and an oxygen isotope-based growth rate model (Eicken, 1998) were found to under-predict growth rates compared to those derived from ice temperature measurements. In addition, inverting the growth rates predicted by the models and integrating over the depth of the core failed to accurately predict the date of initial sea ice formation. Modifications are proposed to the models for sea ice formation occurring near ice shelves, where platelet ice formation is likely. Differences between bulk and individual platelet ice crystal growth rates are discussed with reference to heat fluxes, oceanic flows and the Eicken (1998) model.

Upper ocean stratification and sea ice growth rates during the summer-fall transition, as revealed by Elephant seal foraging in the Adélie Depression, East Antarctica

Abstract. Southern elephant seals (Mirounga leonina), fitted with Conductivity-Temperature-Depth sensors at Macquarie Island in January 2005 and 2010, collected unique oceanographic observations of the Adélie and George V Land continental shelf (140–148° E) during the summer-fall transition (late February through April). This is a key region of dense shelf water formation from enhanced sea ice growth/brine rejection in the local coastal polynyas. In 2005, two seals occupied the continental shelf break near the grounded icebergs at the northern end of the Mertz Glacier Tongue for several weeks from the end of February. One of the seals migrated west to the Dibble Ice Tongue, apparently utilising the Antarctic Slope Front current near the continental shelf break. In 2010, immediately after that year's calving of the Mertz Glacier Tongue, two seals migrated to the same region but penetrated much further southwest across the Adélie Depression and sampled the Commonwealth Bay polynya from March through April. Here we present observations of the regional oceanography during the summer-fall transition, in particular (i) the zonal distribution of modified Circumpolar Deep Water exchange across the shelf break, (ii) the upper ocean stratification across the Adélie Depression, including alongside iceberg C-28 that calved from the Mertz Glacier and (iii) the convective overturning of the deep remnant seasonal mixed layer in Commonwealth Bay from sea ice growth. Heat and freshwater budgets to 200–300 m are used to estimate the ocean heat content (400→50 MJ m−2), flux (50–200 W m−2 loss) and sea ice growth rates (maximum of 7.5–12.5 cm day−1). Mean seal-derived sea ice growth rates were within the range of satellite-derived estimates from 1992–2007 using ERA-Interim data. We speculate that the continuous foraging by the seals within Commonwealth Bay during the summer/fall transition was due to favorable feeding conditions resulting from the convective overturning of the deep seasonal mixed layer and chlorophyll maximum that is a reported feature of this location.

Stable isotope clue to episodic sea ice formation in the glacial North Atlantic

Sea ice is a determinant parameter of the climate system, which acts as amplifier through positive ice-albedo feedbacks and induces deep-water formation through brine production. It is also one of the most elusive parameters in the paleoclimate record. Most proxies of sea ice provide information on its presence and extent of (e.g., diatoms or dinocysts), but do not permit assessment on its rate of formation. However, seemingly off-equilibrium, low δ18O values in mesopelagic planktic foraminifer species, as observed today in the Arctic Ocean, are thought to relate to the production of isotopically light brines during sea ice formation, and might thus provide a clue on the rate of sea ice growth. With reference to the isotopic properties of modern planktic foraminifers in the Arctic Ocean and using multi-proxy data sets from the last glacial stage in the northwest North Atlantic, we re-examined some of the current interpretations of planktic isotope records in the glacial NW North Atlantic. The large amplitude light isotopic excursions recorded in Neogloboquadrina pachyderma left coiling during Heinrich events 1, for example, correspond to extensive sea ice cover as reconstructed from dinocysts, and do not seem unequivocally linked to low-salinity pulses. Such light isotopic excursion more likely responded to enhanced rates of sea ice formation resulting in the production and sinking of isotopically light brines. On the contrary, the isotopically heavy planktic foraminifers of the last glacial maximum (LGM) interval stricto sensu would rather suggest relatively low rates of brine production, thus low sea ice growth rates in the area. This would imply that the LGM distribution of sea ice in the North Atlantic was primarily linked to spreading and drifting from marginal and Nordic source areas and not from enhanced in situ production. At the scale of glacial–interglacial cycles, isotopic distillation processes relating to sea ice production at high latitudes could be one of the factors controlling the 18O–salinity relationship in deep ocean water masses that might deserve closer examination, as it would be independent of continental ice volume changes.

Arctic sea ice response to wind stress variations

Timescale of sea ice response to wind stress variations and the mechanisms controlling the timescale are investigated by a coupled sea ice–ocean model. Wind stress variations account for a significant part of the changes of sea ice volume in the Arctic Ocean over the last several decades. The changes of sea ice volume associated with wind stress are mainly induced by changes of sea ice outflow from the Arctic Ocean. While outflow immediately responds to wind stress variations, responses of sea ice volume lag behind the changes of the outflow by several years. For example, when the model is driven by interannually varying wind stress with the other atmospheric forcing components given by climatology, sea ice outflow abruptly decreases in 1996 and remains almost constant for the following several years. However, the responding increase of sea ice volume is gradual and continues for several years. In order to clarify the timescale of sea ice response to an abrupt change of wind stress and the mechanisms controlling the timescale, the model is forced by two typical wind stress fields, which cause small and large outflow. Equilibrium annual mean ice thickness averaged over the Arctic Ocean is different by about 50 cm between these two wind‐forcing fields. Starting from the equilibrium state obtained under one of these two fields, wind‐forcing is switched to the other. Adjustment time of ice thickness is then defined as the year when annual mean ice thickness is adjusted to new equilibrium by 90% of the difference between the two equilibria. The timescale of the sea ice response and the mechanisms controlling the timescale are found to be different depending on whether the outflow increases or decreases. When the outflow increases, the timescale depends on the advection time of the sea ice flowing toward the exits. When the outflow decreases, the timescale depends on the thermodynamic growth rate of sea ice. The change in the late 1990s corresponds to the latter case.

Coincident buoy‐ and SAR‐derived surface fluxes in the western Weddell Sea during Ice Station Weddell 1992

We examine sea ice kinematics relevant to surface fluxes using ERS‐1 synthetic aperture radar (SAR) images coincident with buoys in the western Weddell Sea in austral autumn of 1992. Careful matching of temporal and spatial scales shows that buoy‐ and SAR‐derived velocities differ in root‐mean‐square error (RMSE) by 0.6 cm s−1 and 7.80° in magnitude and direction, respectively. These values represent agreements of 91.3% and 92.7%, respectively, and correspond to instrument uncertainties. Scaling analysis shows that shear matching is best at the smallest scales (≤5 km), while divergence is better represented at scales of 40 km and larger. Sensitivity to error propagation shows lower agreement for divergence (47.4%; RMSE = 7.46 × 10−8 s−1), but we find these results sufficient for integrated surface flux comparisons. Using a toy model, we test the effects of aliasing in surface flux determination. The results show that variability associated with storms, ocean tides, inertial oscillations, and other high‐frequency forcing affects integrated sea ice growth rates along this continental slope location. Integrated salt and new ice production rates computed from buoys are found to be two times larger than those computed from ERS‐1 SAR motion products. We show that these differences in salt and ice production rates result primarily from inadequate temporal resolution of heat flux variability and sea ice divergence. Comparison with other studies shows that the problem is widespread, thereby impacting the modeling of sea ice mass balance and variability. The small‐scale processes cited here have significant ramifications for larger scales and the global thermohaline circulation.

An in situ experimental study of young sea ice formation on an Antarctic lead

Three series of experimental results were obtained in situ during the U.S.‐Russian Ice Station Weddell 1 Expedition 1992 in the western Weddell Sea. Changes in salinity, silicate, and chlorophyll a concentrations were examined over sampling scales of hours, days, and months as microalgal populations grew in young sea ice. Sea ice growth rates were 0.38 cm h−1 for ice up to 9 cm (May 19–20), 0.13 cm h−1 for ice growth to 28 cm over the next 8 days, and 0.03 cm h−1 during 81 days of observations on ice 42–97 cm thick (March 18–June 7). It was shown that at the initial stage of ice formation, salt and nutrient accumulation occurred, then ice desalination intensified with increasing ice thickness. Brine within the ice showed a 12‐hour period of oscillatory motion during the first day of ice growth and a 1.5‐ to 2‐hour oscillation in the skeletal layer of 28‐cm ice. Small numbers of diatoms were entrapped from seawater during the initial ice formation. Their reproduction (in terms of chlorophyll a concentration) markedly increased after the third day of ice formation. The highest concentrations of chlorophyll a (100 to 1000 times higher than the underlying seawater) were recorded within the bottom, brown‐colored layer of all young ice cores studied during the 81‐day experiment. The species composition of ice algal populations (99 species) was more diverse and rich than observed for phytoplankton (18 species) in surface seawater. The temporal and spatial distribution of all parameters studied were controlled by meteorological factors and brine drainage mechanisms.