Northerly Wind Events Research Articles

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.

Temperature evolution of the upper ocean in the Greenland Sea January to March 1989

Tomographic data obtained during early 1989 in the Greenland Sea have been analyzed at 4–8 hour resolution to give the range‐averaged vertical temperature evolution in the upper 500m for a 106km path. The tomographic inversions used both ray travel time data and normal mode group velocity data in order to maximize near‐surface resolution. Two major events are apparent in the results. The first is the warming of a cold (−1.9°C) 100m thick surface layer, and the second, 10 days later, is the cooling of a relatively warm (−0.9°C) subsurface layer between 300m and 500m depth. This warm subsurface layer is a critical source of salinity and buoyancy for deep convection. The surface layer warming is consistent with a mixed layer deepening over a portion of the path, bringing up water from below. Special Sensor Microwave Imager (SSM/I) ice data indicate that the local ice field disappears 3–4 days after the surface warming. The cooling of the warm 300m to 500m layer is also consistent with a vertical process. There is no ice cover at this time, and so surface heat fluxes are large. A northerly wind event occurs at the onset of the cooling of the 300–500 m layer, suggesting that wind‐induced mixing may have played a role in initiating the process. There is evidence of southward flow advecting warm water into the area both before and after the two events studied in detail here.

Sediment-transport events on the northern California continental shelf during the 1990–1991 STRESS experiment

Measurements of currents and light transmission were made at bottom tripods and moorings arrayed across the northern California continental shelf along the Coastal Ocean Dynamics Experiment (CODE) “C” transect as part of the 1990–1991 Sediment Transport Events on Shelves and Slopes (STRESS) experiment. In combination with meteorological and wave data from the National Data Buoy Center Buoy 46013, these measurements provide information about the physical forcing and resultant resuspension and transport of bottom material between 21 November and 8 March. Sixteen events were identified in the wave, wind and current-meter records for this period. Only two were local storms with southerly winds, but they caused about half of the seasonal net transport. Seven were swell events that combined long-period waves generated by distant storms with local currents. At the 90-m site, swells interacted with the mean northward flow to produce northward transport. During six northerly wind events, upwelling-favorable winds often were sufficient to slow or reverse the mean northward flow and thus caused southward transport. A single current event, which produced moderate southward transport, was observed at the 130-m site. Net transport during the winter experiment was offshore at all sites, northward at the inner- and mid-shelf sites, but southward at the outer-shelf site. The results suggest that local storms with southerly winds may dominate seasonal transport, as on the Washington shelf, but significant transport also can occur during fair weather and during periods of northerly winds.

Circulation and mixing in a coastal upwelling embayment; Gulf of Arauco, Chile

An analysis is presented of a set of fine scale current, temperature and salinity profiles from a coastal upwelling embayment, the Gulf of Arauco, Chile (37°S) during the summer upwelling season in January 1985. These profiles were obtained with a new instrument, GISMO, developed at the Oceanographic Institute, Gothenburg, Sweden. Typical circulation and mixing patterns were identified for “normal” southerly wind conditions and for northerly wind events, which occur regularly during the upwelling season. Strong events of shear intensification and related internal mixing were observed after a northerly wind event and interpreted in terms of passing internal Kelvin wave fronts. Nutrient flux into the surface layers due to this particular mixing mechanism may represent an important contribution for maintaining the tremendous biological production in the study area.