Cathleen A. Geiger

Thickness distribution of Antarctic sea ice

[1] Ship-based observations are used to describe regional and seasonal changes in the thickness distribution and characteristics of sea ice and snow cover thickness around Antarctica. The data set comprises 23,373 observations collected over more than 2 decades of activity and has been compiled as part of the Scientific Committee on Antarctic Research (SCAR) Antarctic Sea Ice Processes and Climate (ASPeCt) program. The results show the seasonal progression of the ice thickness distribution for six regions around the continent together with statistics on the mean thickness, surface ridging, snow cover, and local variability for each region and season. A simple ridge model is used to calculate the total ice thickness from the observations of level ice and surface topography, to provide a best estimate of the total ice mass, including the ridged component. The long-term mean and standard deviation of total sea ice thickness (including ridges) is reported as 0.87 ± 0.91 m, which is 40% greater than the mean level ice thickness of 0.62 m. Analysis of the structure function along north/south and east/west transects revealed lag distances over which sea ice thickness decorrelates to be of the order of 100–300 km, which we use as a basis for presenting near-continuous maps of sea ice and snow cover thickness plotted on a 2.5 � 5.0 grid.

Simulated low‐frequency modes of circulation in the Arctic Ocean

The variability of the Arctic circulation is investigated for a 43 year period (1951–1993) from a coupled ice-ocean model. Empirical orthogonal function (EOF) analysis shows that the variability of the sea surface height (SSH) and vertically integrated transport is organized so that in the leading mode the whole Arctic operates as a single gyre. The mode is associated with the Arctic Oscillation (AO) [Thompson and Wallace, 1998], and it explains over 70% of the variance in the vertically integrated transport and 25% of the SSH variability. The physical interpretation of this mode is derived to arise from its close connection to the Atlantic inflow to the Arctic. The mode shows a major shift toward cyclonic circulation in the end of the 1980s which is associated with a large multiyear pulse of Atlantic water to the Arctic. Thus this event appears as the likely initiation of the Atlantic layer warming observed during the recent years [Carmack et al., 1995]. Overall, the first mode shows strong decadal variability as reported by Proshutinsky and Johnson [1997]. The second mode of the oceanic circulation, which explains 9% of the variance in the transport, contains two gyres with opposing cyclonicity in the Eurasia and Canada basins. It projects onto the North Atlantic Oscillation (NAO) pattern and displays a 14 year cycle which is known to exist in the midlatitude North Atlantic surface temperatures [Deser and Blackmon, 1993]. A further examination reveals that this mode describes the variability of the flow through the Barents Sea, which is modulated by the water mass modification due to the local heat flux variability. The apparent NAO connection is provided by a simultaneous correlation between the time series of this second mode and the leading heat flux mode in the North Atlantic which is associated with NAO.

Spatial and temporal characterization of sea-ice deformation

Abstract In late March 2007 an array of GPS ice drifters was deployed in the Beaufort Sea as part of the Sea Ice Experiment: Dynamic Nature of the Arctic (SEDNA). the drifters were deployed in an array designed to resolve four, nested spatial scales of sea-ice deformation, from 10 to 140 km, with the arrays maintaining appropriate shape for strain-rate calculation until mid-June. In this paper, we test whether sea-ice deformation displays fractal properties in the vicinity of SEDNA. We identify that deformation time series have different spectral properties depending on the spatial scale. At the scales around 100 km, deformation is a red-noise process, indicating the importance of the ice-pack surface forcing in determining the deformation rate of sea ice at this scale. At smaller scales, the deformation becomes an increasingly whiter process (it has pink noise properties), which suggests an increasing role of dissipative processes at smaller scales. At spatial scales of 10–100 km, and sub-daily scales, there is no deformation coherence across scales; coherence only becomes apparent at longer scales greater than 100 km. the lack of coherence at small scales aids in understanding previous observations where correlation between 10km regions adjacent to each other varied widely, with correlation coefficients between –0.3 and 1. This suggests it is not appropriate to think of sea ice as having a decorrelation length scale for deformation. We find that lead scale observations of deformation are required when estimating ice growth in leads and ridging time series. For the two SEDNA arrays, we find coherence between 140 and 20 km scale deformation up to periods of 16 days. This suggests sea-ice deformation displays coherent deformation between 100 km scale and the scale of the Beaufort Sea (of order 1000 km), over synoptic time periods (daily to weekly timescales). Organization of leads at synoptic and larger scales is an emergent feature of the deformation field that is caused by the smooth variation of surface forcing (wind) on the ice pack.

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

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 highfrequency 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.

Motion Tracking of Discontinuous Sea Ice

The purpose of this paper is twofold: 1) to develop a high-resolution sea ice motion tracking system at the geospatial mesoscale (1-100 km2) and 2) to propose an algorithm that measures motion at close proximity to discontinuous regions. Here, we present a motion tracking system that computes differential motion at 400 m resolution and validate the accuracy/precision of this system via four studies. The first study measures the accuracy against displacements measured from in situ Global Positioning System (GPS) buoys deployed at the Sea-ice Experiment: Dynamic Nature of the Arctic (SEDNA) and the Surface HEat Budget of the Arctic Ocean (SHEBA) experiments. The estimates are found to be statistically comparable with GPS, with an average error of 361.9 and 600.6 m for the experiments, respectively. The second study compares the estimated displacements to those measured by the RADARSAT Geophysical Processing System. A precision error of 75.7 m is found between the two motion tracking systems. The third study uses intensity warping of randomly sampled measurements to evaluate discontinuous motion tracking. A one-tailed Wilcoxon signed rank test is used to validate these measurements at α = 0.01. Results from this paper prove that anisotropic smoothing produces significantly smaller errors at discontinuous locations (W = 4240 and p <; 0.001) over conventional isotropic smoothing. The fourth study compares displacements measured by anisotropic smoothing against manual measurements. This paper demonstrates an average reduction of the estimation error by 50 m with the use of anisotropic smoothing over the conventional isotropic smoothing.

The relation between Arctic sea ice surface elevation and draft: A case study using coincident AUV sonar and airborne scanning laser

[1] Data are presented from a survey by airborne scanning laser profilometer and an AUV‐mounted, upward looking swath sonar in the spring Beaufort Sea. The air‐snow (surface elevation) and water‐ice (draft) surfaces were mapped at 1 × 1 m resolution over a 300 × 300 m area. Data were separated into level and deformed ice fractions using the surface roughness of the sonar data. The relation (R = d/f ) between draft, d, and surface elevation, f, was then examined. Correlation between top and bottom surfaces was essentially zero at full resolution, requiring averaging over patches of at least 11 m diameter to constrain the relation largely because of the significant error (∼15 cm) of the laser instrument. Level ice points were concentrated in two core regions, corresponding to level FY ice and refrozen leads, with variations in R attributed primarily to positive snow thickness variability. Deformed ice displayed a more diffuse “cloud,” with draft having a more important role in determining R because of wider deformed features underwater. Averaging over footprints similar to satellite altimeters showed the mean surface elevation (typical of ICESat) to be stable with averaging scale, with R = 3.4 (level) and R = 4.2 (deformed). The “minimum elevation within a footprint” characteristic reported for CryoSat was less stable, significantly overestimating R for level ice (R >5 ) and deformed ice (R > 6). The mean draft difference between measurements and isostasy suggests 70 m as an isostatic length scale for level ice. The isostatic scale for deformed ice appears to be longer than accessible with these data (>300 m).

Large-scale comparison between buoy and SSM/I drift and deformation in the Eurasian Basin during winter 1992–1993

Detecting a wireless terminal being taken from a facility. In a first embodiment, a base station is assigned to each exit from the facility. The base station uses a directional antenna which only communicates with wireless terminals within the exit area. When a wireless terminal enters the exit area and registers on the base station, the base station transmits the registration information to a wireless switching system that immediately places a telephone call to the user of the wireless terminal to inform them that they have not turned in their wireless terminal. Alernatively, in a third embodiment, after the wireless terminal is registered, the base station rather than the wireless switching system places a telephone call to the user of the wireless terminal to inform them that they have not turned in their wireless terminal. In a second embodiment of the invention, a transmission signal separate from the frequencies utilized for voice and data communication by the wireless switching system is utilized to alert the wireless terminal to the fact that the wireless terminal is in an exit area. The wireless terminal is responsive to this alerting to place a call to the wireless switching system to inform the wireless switching system of this fact. The wireless switching system then alerts the user via the wireless terminal.

Discontinuous Non-Rigid Motion Analysis of Sea Ice using C-Band Synthetic Aperture Radar Satellite Imagery

Sea-ice motion consists of complex non-rigid motions involving continuous, piece-wise continuous and discrete particle motion. Techniques for estimating non-rigid motion of sea ice from pairs of satellite images (generally spaced three days apart) are still in the developmental stages. For interior Arctic and Antarctic pack ice, the continuum assumption begins to fail below the 5 km scale with evidence of discontinuities already revealed in models and remote sensing products in the form of abrupt changes in magnitude and direction of the differential velocity. Using a hierarchical multi-scale phase-correlation method and profiting from known limitations of cross correlation methods, we incorporate the identification of discontinuities into our motion estimation algorithm, thereby descending below the continuum threshold to examine the phenomenon of discontinuous non-rigid sea-ice motion.

Role of Ice Dynamics in the Sea Ice Mass Balance

Over the past decade, the Arctic Ocean and Beaufort Sea ice pack has been less extensive and thinner than has been observed during the previous 35 years [e.g., Wadhams and Davis, 2000; Tucker et al., 2001; Rothrock et al., 1999; Parkinson and Cavalieri, 2002; Comiso, 2002]. During the summers of 2007 and 2008, the ice extents for both the Beaufort Sea and the Northern Hemisphere were the lowest on record. Mechanisms causing recent sea ice change in the Pacific Arctic and the Beaufort Sea are under investigation on many fronts [e.g., Drobot and Maslanik, 2003; Shimada et al., 2006]; the mechanisms include increased ocean surface warming due to Pacific Ocean water inflow to the region and variability in meteorological and surface conditions. However, in most studies addressing these events, the impact of sea ice dynamics, specifically deformation, has not been measured in detail.

Towards estimation of dense disparities from stereo images containing large textureless regions

Stereo algorithms for structure reconstruction demand accurate disparities with low mismatch errors and false positives. Mismatch errors in large textureless regions force most accurate algorithms to be sparse, with disparities known only in textured regions. We propose a novel method which uses characteristics of the multi-valued disparity to segregate image regions into unambiguous, occluded but textured and regions with low color variation. The disparity in the unambiguous region is calculated using stable matching with local disparity filtering. The disparity is interpolated into other regions by diffusion using unstructured triangulation and method of finite elements for rapid convergence. The boundary conditions for each of the region are appropriately modified so that accurate discontinuities in the disparity are preserved. A comparison of our method with some existing methods through experiments reveal that this algorithm indeed performs significantly better in producing dense accurate disparities.