Sigrid Salo

Arctic sea ice as a granular plastic

An important consideration in understanding sea ice mechanics is the integration of observed sea ice behavior on a floe neighborhood scale (1–10 km) into ice dynamics on a regional scale O(50 km). We investigate sea ice kinematics from October 1993 through April 1994 using relative motions from 13 drifting buoys with Global Positioning System navigation in a 20-km array centered on the Sea Ice Mechanics Initiative ice camp, and we compare these motions to synthetic aperture radar (SAR)- derived ice velocities over a 100- by 500-km region in the Beaufort Sea. There is excellent correspondence between the deformation of the buoy array and that from the SAR. Inferred ice dynamics from analysis of the two major northerly wind convergence events of the winter are consistent with a granular hardening plastic conceptual model for Beaufort sea ice. Under continued northerly winds the ice from the Alaskan shore to the camp failed in shear and convergence, in a progressive manner away from the coast. The continuum scale O(10 km) is an order of magnitude larger than the grain, i.e., floe, size O(1 km). The ice motion often forms aggregates of 20–200 km separated by narrow (<10 km) shear zones, similar to granular materials. At moderate forcing, i.e., wind stress multiplied by fetch, the ice appears to fail along slip lines that occur at an acute angle to each other and to the direction of the wind forcing, characteristic of a plastic material at critical state. With longer fetch the ice appears to fail in compression, perpendicular to the wind direction. Sea ice appeared to harden on a regional scale after the first event. During the second northerly wind event there was a sea ice breakout toward the west, apparently due to a lack of lateral confining stress. Our observations suggest that the ice floes advect through relatively stationary stress fields, created by the wind forcing and coastal boundaries. For example, while the SAR and advanced very high resolution radiometer images indicated the presence of the shear feature at the same geographic location for nearly a week, buoys would show shearing only for several days as they transited across the region of shear. There is a high correspondence between the major internal ice deformation events and persistent weather patterns on a 3- to 5-day temporal scale. This implies that SAR data collection and analysis for regional sea ice dynamics should be consistent with the wind forcing and have a sampling of less than 3 days.

Salinity signature of the Pacific Decadal Oscillation

Three sites in the North Pacific have temperature and salinity observations in most months for several years before and after 1977. The Gulf of Alaska station (57°N, 148°W) showed a 2°C warming and a 0.6 freshening in salinity at 10 m depth in the 1980s compared to the 1970s. OWS PAPA (50°N, 145°W) and PAPA line station 7 (49.1°N, 132.4°W) show warming of 0.6°C and 0.9°C, with no major salinity change. The decrease in density and increase in stratification in the Gulf of Alaska after 1977 corresponds primarily to a decrease in salinity in the upper 150 m. We propose that while the Pacific Decadal Oscillation has an east/west character in temperature, the salinity signature will have a NNW/SSE character, similar to the pattern of interannual variability in precipitation.

Is there a ‘‘new normal’’ climate in the Beaufort Sea?

Since 2007, environmental conditions in the Beaufort Sea have appeared to be consistently different from those in the past. Is a “new normal” climate emerging in the region? Sea-surface temperatures (SSTs) have been notably warm during the summer, leading to delayed freeze-up in the fall along with large surface air temperature (SAT) anomalies due to the release of stored ocean heat to the atmosphere. In the autumn of 2011 and 2012, SST and SAT anomalies in Arctic marginal seas were the largest observed in the Northern Hemisphere. Since 2007, there has been an increase in easterly winds, which has helped set the stage for Arctic amplification by advecting sea ice out of the region and enhancing surface stratification due to the offshore transport of fresh water from the large Mackenzie River discharge plume. These winds are linked to an intensification of the Beaufort High and are evident throughout the troposphere. Their occurrence has undoubtedly contributed to the acceleration of sea-ice loss and surface warming in the Beaufort Sea, with additional impacts likely throughout the ecosystem.

Regional sensible and radiative heat flux estimates for the winter Arctic during the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment

We estimate the influence of the mosaic distribution of surface temperatures of sea ice on regional surface sensible and radiative heat fluxes on the basis of advanced very high resolution radiometer (AVHRR) temperatures. The AVHRR data were used to derive ∼1 km2 surface temperature values for 100×100 km2 regions. Regional flux estimates are compared to direct flux measurements taken during the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment in the Beaufort Sea. We describe 48 cases of clear-sky conditions between December 1997 and February 1998. The distribution of surface temperatures within each region is skewed: most temperatures fall near the value observed at the ice camp, but the distribution has a warm tail corresponding to subregions with thinner ice. Sensible heat fluxes at the ice camp were downward, from the air to the ice. Although the camp was representative of the majority of the region, the upward flux from thin ice generally balanced the downward flux to thicker ice, suggesting a regional equilibrium of near-zero sensible heat flux. This was true whether the net downward flux at the camp was large or small or whether the ice was less compact, as in January, or more compact, as in December and February. The only exception to this generalization occurred when the ice was under strong compression. The net regional radiative loss from the ice surface was ∼22% greater than that measured on the floe at the SHEBA camp. This increase is due to the fourth-power temperature dependence of the radiative flux acting on the skewed distribution of surface temperatures. The implications for modeling are that sea ice tends toward small net surface sensible heat flux on an aggregate scale, an emergent property of the system, and that parameterizing the regional mosaic of surface temperatures to resolve correctly the radiative balance in winter is important.

Physical and biological factors influencing the spatial distribution of age-0 walleye pollock (Theragra chalcogramma) around the Pribilof Islands, Bering Sea

Abstract The waters around the Pribilof Islands, in the southeast Bering Sea, are a main nursery area for age-0 pollock. Each summer, the islands are surrounded by a well-mixed inshore region, separated from a stratified offshore region by a frontal zone. To study the spatial distribution of age-0 pollock around this frontal structure in relation to physical and biological factors that are likely to influence it, such as advection, age-0 pollock feeding, and predation, samples were collected during September of four consecutive years, 1994–97, along two transects. Samples collected included water column hydrography and currents, acoustic backscatter, and groundfish predator density. Our analysis suggested that different mechanisms may be involved in controlling age-0 pollock distribution north and south of the islands. On the shelf area north of the islands, high age-0 pollock density was significantly associated with areas of high potential for growth only in years or portions of the frontal transect in which predator numbers were relatively low, indicating the importance of predation in controlling fish distribution in this area. In contrast, south of the islands, age-0 pollock distribution was associated more with prey availability, which appeared to be determined by vertical spatial overlap between predators and prey. Moreover, south of the islands, the stronger geostrophic currents, typical of the slope region, were more likely to affect the overall standing biomass of juvenile pollock, by constantly advecting fish away from the area.

Antarctic icebergs: A significant natural ocean sound source in the Southern Hemisphere

In late 2007, two massive icebergs, C19a and B15a, drifted into open water and slowly disintegrated in the southernmost Pacific Ocean. Archived acoustic records show that the high-intensity underwater sounds accompanying this breakup increased ocean noise levels at mid-to-equatorial latitudes over a period of ∼1.5 years. More typically, seasonal variations in ocean noise, which are characterized by austral summer-highs and winter-lows, appear to be modulated by the annual cycle of Antarctic iceberg drift and subsequent disintegration. This seasonal pattern is observed in all three Oceans of the Southern Hemisphere. The life cycle of Antarctic icebergs affects not only marine ecosystem but also the sound environment in far-reaching areas and must be accounted for in any effort to isolate anthropogenic or climate-induced noise contributions to the ocean soundscape.

Timing of ice retreat alters seabird abundances and distributions in the southeast Bering Sea.

Timing of spring sea-ice retreat shapes the southeast Bering Sea food web. We compared summer seabird densities and average bathymetry depth distributions between years with early (typically warm) and late (typically cold) ice retreat. Averaged over all seabird species, densities in early-ice-retreat-years were 10.1% (95% CI: 1.1–47.9%) of that in late-ice-retreat-years. In early-ice-retreat-years, surface-foraging species had increased numbers over the middle shelf (50–150 m) and reduced numbers over the shelf slope (200–500 m). Pursuit-diving seabirds showed a less clear trend. Euphausiids and the copepod Calanus marshallae/glacialis were 2.4 and 18.1 times less abundant in early-ice-retreat-years, respectively, whereas age-0 walleye pollock Gadus chalcogrammus near-surface densities were 51× higher in early-ice-retreat-years. Our results suggest a mechanistic understanding of how present and future changes in sea-ice-retreat timing may affect top predators like seabirds in the southeastern Bering Sea.

Direct evidence for northward flow on the northwestern Bering Sea shelf

During the summer of 1994, a satellite-tracked drifter transited from the southeastern Bering Sea slope through Bering Strait by a route westward along the slope and then northward through Anadyr Canyon and Strait. The trajectory emphasizes the importance of a western location of northward flow on the Bering Sea shelf. The transit time was 2 months from Cape Navarin to Bering Strait with northward drift velocities of 5–40 cm/s.