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Jane Bauer
and
Seelye Martin

Abstract

During March 1979 field observations in the Bering Sea show that because of the interaction of winds and ocean swell with the ice, the ice edge divides into three distinct zones. First, adjacent to the open ocean is an “edge" zone, 1–15 km in width, which consists of heavily rafted and ridged floes with thicknesses of 1–5 m and measuring 10–20 m on a side. Second is a “transition” zone measuring ∼5 km in width, which consists of rectangular ice floes with thicknesses of ∼0.5 m and measuring 20–40 m on a side. Third is the “interior” zone, which extends over hundreds of kilometers and consists of very large, relatively flat floes with thicknesses of ∼0.3 m. In the edge zone the incident swell causes the floes to fracture, raft and form pressure ridges, resulting in small thick floes. In the transition zone the swell amplitude is reduced to the point that the floes fracture in a rectangular pattern with very little rafting or ridging taking place. In the interior zone the swell amplitude is further reduced such that the waves propagate without fracturing the ice, so that the floes have horizontal dimensions of kilometers. Because of this ice distribution, when strong winds blow off the ice, bands of ice floes form at the ice edge. The reason bands form is that the edge zone ice has a large aerodynamic drag due to the heavy rafting and ridging, so that this ice moves downwind ahead of the rest of the pack. Once this ice moves away from the pack, the combination of aerodynamic drag plus the absorption of wind wave and swell energy leads to the band formation. We observed that these bands, which are on the order of 1 km wide and 10 km in length, move south into warmer water until they melt.

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Seelye Martin
and
Peter Kauffman

Abstract

In an experimental and theoretical study we model a phenomenon which occurs in the summer polar oceans; namely, the melting of flat sheets of either glacial ice or desalinated sea ice which float over sea water held at a temperature above freezing. Our laboratory results show when the solution salinity is such that the temperature of maximum density is below the freezing temperature, or for sea water salinities greater than 25‰, the heat transfer to the ice takes place in three regions. First, just beneath the ice, there is a boundary layer across which the salinity increases almost to its far-field value and the temperature increases linearly. Below this, there is an unstable convective boundary layer, which appears to be part double-diffusive, part pure thermal convection. Finally, there is a region of deep thermal convection. From comparison of a one-dimensional theoretical model of the heat transfer with the laboratory study, we find that the ice melts about twice as fast for this convective case as for a purely diffusive heat transfer model.

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Lon E. Hachmeister
and
Seelye Martin

Abstract

An experimental study of the stability of a progressive internal wave of mode 3 propagating down a long tank filled with a linearly stratified salt solution is conducted over the range 0.45≤ω3/N≤0.80, where ω3 is the paddle frequency and N the Brunt-Väisälä frequency. Our observations show that the generated wave excites resonant triads at frequencies. below and forced triads at frequencies above the paddle frequency. There are two distinct families of resonant triads, corresponding to what Simmons calls set I and set II with modal form (n, n+3, 3) and (n+3, n, 3), n=1, 2, 3,…, respectively. Simmons' theory predicts that the set I triads grow approximately twice as fast as the corresponding triads of set II. In our experiments, we observe the set I triads at low amplitudes, with the set II triads forming as we increase the paddle amplitude. Above the driving frequency, we find only forced waves whose frequency and mode number are the sum of those of the paddle and the more energetic triad members, but whose wavenumber is not that of a free wave. The experiments substantiate Hasselmann's theoretical conjecture: that resonant instabilities alone cannot transfer energy to frequencies above the driving frequency.

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Seelye Martin
and
Esther A. Munoz

Abstract

This paper discusses the behavior of the Arctic Ocean surface air temperature field for 1979–93. Temperatures are derived from a new gridded 6-h, 2-m air temperature dataset called POLES. These gridded air temperatures are estimated from optimal interpolation of temperature inputs from drifting buoys, manned Soviet North Pole (NP) drifting ice stations, coastal land weather stations, and ship reports. In processing the POLES data, the winter and summer properties of the mean NP temperatures are used to discard inaccurate or snow-covered buoys and to remove a summer warm bias. Comparison of the POLES and specific NP temperatures shows that the POLES temperature behaves well and gives better results than other gridded temperature datasets. Maps of the mean seasonal temperatures give realistic results consistent with other published estimates, and plots of the summer advance and retreat of the 0°C isotherm show the expected asymmetry between advance and retreat associated with the open water formation adjacent to the coasts. Also, comparison of the regional POLES observations of the annual onset of melt and freeze with published estimates derived from visible and passive microwave satellite data gives realistic results. However, problems with the dataset arise in the post-1991 period from the termination of the NP stations and a reduction in the number of Siberian and Alaskan weather stations. In spite of these problems, the paper shows that the POLES dataset provides an improved air temperature field for use in numerical sea ice models and for comparison with satellite datasets.

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Antony K. Liu
,
Seelye Martin
, and
Ronald Kwok

Abstract

This paper demonstrates the use of wavelet transforms in the tracking of sequential ice features in the ERS-1 synthetic aperture radar (SAR) imagery, especially in situations where feature correlation techniques fail to yield reasonable results. Examples include the evolution of the St. Lawrence polynya and summer sea ice change in the Beaufort Sea. For the polynya, the evolution of the region of young ice growth surrounding a polynya can be easily tracked by wavelet analysis due to the large backscatter difference between the young and old ice. Also within the polynya, a 2D fast Fourier transform (FFT) is used to identify the extent of the Langmuir circulation region, which is coincident with the wave-agitated frazil ice growth region, where the sea ice experiences its fastest growth. Therefore, the combination of wavelet and FFT analysis of SAR images provides for the large-scale monitoring of different polynya features. For summer ice, previous work shows that this is the most difficult period for ice trackers due to the lack of features on the sea ice cover. The multiscale wavelet analysis shows that this method delineates the detailed floe shapes during this period, so that between consecutive images, the floe translation and rotation can be estimated.

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Ignatius G. Rigor
,
Roger L. Colony
, and
Seelye Martin

Abstract

The statistics of surface air temperature observations obtained from buoys, manned drifting stations, and meteorological land stations in the Arctic during 1979–97 are analyzed. Although the basic statistics agree with what has been published in various climatologies, the seasonal correlation length scales between the observations are shorter than the annual correlation length scales, especially during summer when the inhomogeneity between the ice-covered ocean and the land is most apparent. During autumn, winter, and spring, the monthly mean correlation length scales are approximately constant at about 1000 km; during summer, the length scales are much shorter, that is, as low as 300 km. These revised scales are particularly important in the optimal interpolation of data on surface air temperature (SAT) and are used in the analysis of an improved SAT dataset called International Arctic Buoy Programme/Polar Exchange at the Sea Surface (IABP/POLES). Compared to observations from land stations and the Russian North Pole drift stations, the IABP/POLES dataset has higher correlations and lower rms errors than previous SAT fields and provides better temperature estimates, especially during summer in the marginal ice zones. In addition, the revised correlation length scales allow data taken at interior land stations to be included in the optimal interpretation analysis without introducing land biases to grid points over the ocean. The new analysis provides 12-h fields of air temperatures on a 100-km rectangular grid for all land and ocean areas of the Arctic region for the years 1979–97.

The IABP/POLES dataset is then used to study spatial and temporal variations in SAT. This dataset shows that on average melt begins in the marginal seas by the first week of June and advances rapidly over the Arctic Ocean, reaching the pole by 19 June, 2 weeks later. Freeze begins at the pole on 16 August, and the freeze isotherm advances more slowly than the melt isotherm. Freeze returns to the marginal seas a month later than at the pole, on 21 September. Near the North Pole, the melt season length is about 58 days, while near the margin, the melt season is about 100 days. A trend of +1°C (decade)−1 is found during winter in the eastern Arctic Ocean, but a trend of −1°C (decade)−1 is found in the western Arctic Ocean. During spring, almost the entire Arctic shows significant warming trends. In the eastern Arctic Ocean this warming is as much as 2°C (decade)−1. The spring warming is associated with a trend toward a lengthening of the melt season in the eastern Arctic. The western Arctic, however, shows a slight shortening of the melt season. These changes in surface air temperature over the Arctic Ocean are related to the Arctic Oscillation, which accounts for more than half of the surface air temperature trends over Alaska, Eurasia, and the eastern Arctic Ocean but less than half in the western Arctic Ocean.

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