Robert Drucker

The production of ice and dense shelf water in the Okhotsk Sea polynyas

This paper examines the ice and dense water production in the Okhotsk Sea coastal polynyas for the 1990–1995 winters. The dominant polynyas occur on the northwest and northern shelves and in Shelikhov Bay. We use an algorithm developed for the special sensor microwave/imager (SSM/I) to derive for each polynya the area and composition of thin ice and open water and a heat flux algorithm to derive the ice and brine production. Historic oceanographic observations show that the northwest shelf is the only North Pacific region where the σϑ = 26.8 potential density surface outcrops to the surface and is also that part of the Okhotsk shelf where the densest water is observed to occur. In support of these observations, we find that the northwest shelf polynya is the dominant ice and brine producer, contributing on average about 55% of the total production. Shelikhov Bay is the second largest producer with about 25% of the total; this region has been previously neglected by both oceanographic and remote sensing studies. Using a combination of two dense water production models, we find that the 6 year average dense water production lies between 0.2–0.4 Sv. The ice and brine production for the dominant northwest shelf vary interannually by a factor of 3, while the production from all the northern polynyas varies by factor of 2. The source of the variability for the northwest shelf comes from the fact that the southwest-to-northeast trend of the coastline and the mean winter geostrophic wind velocities are roughly parallel, which means that small variations in the wind direction yield large changes in the ice production.

Estimation of the thin ice thickness and heat flux for the Chukchi Sea Alaskan coast polynya from Special Sensor Microwave/Imager data, 1990-2001

[1] One of the largest Arctic polynyas occurs along the Alaskan coast of the Chukchi Sea between Cape Lisburne and Point Barrow. For this polynya, a new thin ice thickness algorithm is described that uses the ratio of the vertically and horizontally polarized Special Sensor Microwave/Imager (SSM/I) 37-GHz channels to retrieve the distribution of thicknesses and heat fluxes at a 25-km resolution. Comparison with clear-sky advanced very high resolution radiometer data shows that the SSM/I thicknesses and heat fluxes are valid for ice thicknesses less than 10–20 cm, and comparison with several synthetic aperture radar (SAR) images shows that the 10-cm ice SSM/I ice thickness contour approximately follows the SAR polynya edge. For the twelve winters of 1990–2001, the ice thicknesses and heat fluxes within the polynya are estimated from daily SSM/I data, then compared with field data and with estimates from other investigations. The results show the following: First, our calculated heat losses are consistent with 2 years of over-winter salinity and temperature field data. Second, comparison with other numerical and satellite estimates of the ice production shows that although our ice production per unit area is smaller, our polynya areas are larger, so that our ice production estimates are of the same order. Because our salinity forcing occurs over a larger area than in the other models, the oceanic response associated with our forcing will be modified. INDEX TERMS: 4540 Oceanography: Physical: Ice mechanics and air/sea/ice exchange processes; 3360 Meteorology and Atmospheric Dynamics: Remote sensing; 4504 Oceanography: Physical: Air/sea interactions (0312); 4572 Oceanography: Physical: Upper ocean processes; 4207 Oceanography: General: Arctic and Antarctic oceanography; KEYWORDS: Chukchi Sea, coastal polynya, remote sensing

A laboratory study of frost flower growth on the surface of young sea ice

Frost flowers are fragile ice crystals containing salt which grow to a height of 10–30 mm on the surface of young sea ice. Such flowers are observed all over the Arctic. The importance of the flowers and their accompanying slush layer is that they provide a rapid way to change the surface albedo and increase the surface roughness of young sea ice. This paper describes a laboratory technique for growing frost flowers and the physical processes which accompany the growth. The study was carried out in a saltwater tank located in a cold room. To grow frost flowers, we alternately cool the surface of the growing sea ice with a fan, then supply it with water vapor from a vaporizer. For these conditions and a room temperature of −22°C, the frost flowers begin to grow when the ice thickness reaches 5–8 mm. The flowers form at random locations on the ice and grow vertically to a height of 10–15 mm while spreading laterally from their original sites. Beneath the flowers, the surface is initially dry; then as the flowers spread laterally, a high-salinity slush layer forms beneath them. This layer, which forms only under the flowers, grows to a thickness of 5 mm in 48 hours and has a characteristic lateral scale of 100–200 mm. The salinity of the slush layer is about 80 psu, compared with a frost flower salinity of 100 psu. Within 24 hours of their appearance, the flowers grow to cover 75–90% of the surface. A surface water budget for the flowers and slush layer shows that most of the water in the flowers and slush layer comes from the ice interior, not from the vaporizer. This implies that an external vapor source may be important in determining the initial growth of the flowers but not in their subsequent development.

The temperature dependence of frost flower growth on laboratory sea ice and the effect of the flowers on infrared observations of the surface

This paper describes a laboratory study of frost flower growth on young sea ice at different temperatures and the effect of these flowers on the surface temperature observed with an infrared radiometer. The flowers grew on sea ice which formed in a salt water tank at room temperatures of −20, −24, and −30°C, with an additional experiment at −16°C, where no flowers appeared. The growth habit and height of the observed crystals depended on the existence of a region of supersaturated vapor adjacent to the surface and on the range of temperatures in the surface boundary layer. The source of the surface brine from which the flowers grew was probably brine transport within the ice toward the cold upper surface driven by the thermomolecular pressure gradient. The evaporation of vapor from this liquid into the atmospheric boundary layer provided the supersaturated region adjacent to the ice surface. Two kinds of flowers were observed; at −20 and −24°C, dendritic crystals grew approximately between the −12 and −16°C isotherms, and, at −30°C, rod-like flowers appeared between −16 and −25°C. These limits correspond to earlier work on crystal growth from the vapor. In each case, the maximum flower height approximately equaled the height of the isotherm corresponding to the colder temperature limit for each crystal type, −16°C for the dendrites and −25°C for the rods. The effect of the flowers on the radiometer surface temperature was as follows: because the flowers protrude 10–20 mm above the surface into the boundary layer, the infrared temperature of the flower-covered ice was about 4–6°C colder than that of the same ice cleared of flowers. We also found that the insulating effect of the flowers caused the ice surface temperature beneath the flowers to be 1–2°C warmer than the surrounding bare ice. The importance of the flower growth is that infrared satellite observations of thin ice in winter will be colder than the actual surface temperature, which may account for the absence of warm young ice in infrared satellite images.

Improvements in the estimates of ice thickness and production in the Chukchi Sea polynyas derived from AMSR‐E

For January-March 2003, we use 12.5-km resolution Advanced Microwave Scanning Radiometer (AMSR) data for the first time in a comparison with Synthetic Aperture Radar (SAR) and Special Sensor Microwave/Imager (SSM/I) data to study two Chukchi coast polynyas, one consisting of many, the other of only a few 25-km SSM/I pixels. Within these polynyas, the ice thicknesses are derived separately from the SMM/I 37-GHz and AMSR 36-GHz channels; the heat fluxes are derived by combining thicknesses with meteorological data. Comparison with ScanSAR data shows that for the large polynya, because AMSR provides better resolution of the surrounding coastline and first-year ice, the AMSR heat losses are greater than the SSM/I; for the small polynya, AMSR measures its variability even when its area is order of a single SSM/I pixel. This means that AMSR permits more accurate calculation of polynya heat losses, yielding the potential of improved estimates of Arctic polynya productivity.

The effect of severe storms on the ice cover of the northern Tatarskiy Strait

The Tatarskiy Strait is the northernmost region of the Japan Sea, with an ice-covered area in winter of about 4 × 104 km2. This study uses the daily passive microwave images of the region from the special sensor microwave imager (SSM/I), which was launched in 1987, to estimate the production of ice and Japan Sea bottom water. In winter, the prevailing winds in the strait are northerly and cold; these conditions create a region of reduced ice concentration in the northern strait, which leads to an enhanced ice production throughout the season. In addition to these prevailing winds, each winter one or two severe storms generate very strong northerly winds and cold temperatures in the strait. These storms create a large transient polynya in the strait at a time simultaneous with the greatest heat flux out of the open water. Calculation of the ice growth from open water in the northern strait yields about 25 km3 of ice per season, of which severe storms provide about 25%. The total ice production is sufficient to form about 5–12 × 102 km3 of the Japan Sea bottom water, which is about 50–100% of the renewal rate required from 14C data. Because the oxygen bottom layer thickness decreased between 1969 and 1984, the study also investigates the frequency of the severe storms in the northern strait during 1966–1990 and finds that these storms are about 3 times as frequent at the beginning of this period than at the end. This suggests the storms may play a role in the generation of the Japan Sea bottom water.

Recent observations of a spring-summer surface warming over the Arctic Ocean

For the Arctic Ocean, a 1961–1990 trend analysis of the 2-m, 6-hourly air temperatures from the Russian North Pole (NP) drifting ice stations shows a significant warming in May and June, or seasonally in summer. In this analysis, if we choose temperatures from only those stations which report at least 95% of the time, and define an anomaly field by removal of a mean temperature field, then for both the temperature and anomaly fields, we obtain statistically significant May and June warmings of respectively 0.8 and 0.4°C/decade, and a summer seasonal warming of 0.2°C/decade. For the other seasons, although our trends are not statistically significant, they match the trends derived for the same period from the Arctic land stations.

The effect of possible Taylor columns on the summer ice retreat in the Chukchi Sea

The Chukchi Sea topography consists of a broad, flat, 50-m-deep plain, with two prominent shoals, Herald and Hanna, which have horizontal dimensions of about 100 km and rise to depths of about 30 m. Herald Shoal in particular is a prominent, isolated seamount which, because of the warm water flux in summer through Bering Strait, has a warm 0.1 m s−1 flow incident upon it. Examination of active and passive microwave imagery of the region for 1992–1994 shows that as the general ice cover recedes, the ice remains preferentially over Herald Shoal for 3–4 weeks after the surrounding ice has melted. A scale analysis suggests that the cause of the ice persistence is the formation of a Taylor column over the shoal, which traps cold water and ice above it. A similar trapping of ice occurs over Hanna Shoal and on its eastern slope, but the response is complicated by a very different topography and the northward flow down Barrow Canyon.

A laboratory study of the effect of frost flowers on C band radar backscatter from sea ice

C band images of Arctic sea ice taken by the ERS 1 synthetic aperture radar show transitory regions of enhanced radar backscatter from young sea ice. Published field observations associate this increase with frost flower growth and the capture of blowing snow by the flowers. To investigate the first part of this phenomenon, we carried out a laboratory experiment on the response of C band radar backscatter to frost flowers growing on the surface of newly formed saline ice. The experiment took place in a 5 m by 7 m by 1.2 m deep saline water pool located in a two-story indoor refrigerated facility at the Cold Regions Research and Engineering Laboratory. Sodium chloride ice was grown in this pool at an air temperature of −28°C. The frost flowers first appeared on the ice surface as dendrites and then changed to needles as the ice sheet grew thicker and the surface temperatures became colder. The frost flowers reached to a height of 10–15 mm, and beneath each cluster of frost flowers a slush layer formed to a thickness of approximately 4 mm. Far-field radar measurements of the backscatter from the ice were made at incident angles from 20° to 40° and at approximately 6-hour intervals throughout the 3-day period of the experiment. A backscatter minimum occurred early in the flower growth at the time coincident with an abrupt doubling in the ice surface salinity. Once the full flower coverage was achieved, we removed first the crystal flowers and then the slush layer from the ice surface. The results for these cases show that the crystals have little impact on the backscatter, while the underlying slush patches yield a backscatter increase of 3–5 dB over that of bare ice. The laboratory results suggest that this relative backscatter increase of approximately 5 dB can be used as an index to mark the full areal coverage of frost flowers.

Observations of short‐period ice floe accelerations during leg II of the Polarbjørn drift

During leg II of the Polarbjorn drift, we measured the orthogonal components of acceleration in the vertical and horizontal directions for the frequency interval 0.04–5 Hz, and the compass heading, at three sites adjacent to the ship. During days 310–329, the ice surrounding the ship underwent a transition from large floes with scales of 1–10 km, to small, broken floes with scales of 10–100 m. The deformation and fracture associated with this transition generated significant accelerations which were associated with ice shear, ridging, and ice floe collisions. For the shearing between two large floes, we observed stick-slip accelerations with 0.4-Hz frequencies and 3 mm s−2 amplitudes. For ridging, we observed two kinds of behavior, depending on floe size. For ridging of large kilometer-scale floes, we observed 1-Hz sinusoidal oscillations with 10 mm s−2 amplitudes, while for 100-m-scale floes, the spectra of the accelerations were white. When the field of 50- to 100-m-scale floes was divergent, ice collisions with amplitudes as large as 150 mm s−2 dominated; for two of the three sites these collisions excited the resonant bobbing and rocking frequency of the floe. Our observations ended on day 329, when the ice converged, the floes were apparently crushed, and the accelerometers ceased operation.