Ice Edge Location Research Articles

Oceanographic and meteorological effects on autumn sea-ice distribution in the western Arctic

Oceanographic, meteorological and sea-ice data were obtained from the northern Bering Sea and Chukchi Sea during the autumns of 1987 and 1988. Ice-edge location was observed from ships and via AVHRR satellite data, and ice-drift information was obtained from ARGOS-tracked drift buoys. Meteorological data were obtained from ships, from an ARGOS-tracked meteorological station and from synoptic charts. The ice edge was significantly farther south in 1988 than during other years and impacted the Alaskan coastline. In 1987, the ice edge was, conversely, anomalously far north. Ice melt-back in certain regions, such as along the Alaskan coast and in Herald Canyon, was due to input from warm ocean currents. The larger-scale interannual differences in ice extent were, however, due to interannual differences in the regional winds. In particular, the anomalous and extreme southward extent of the ice edge during 1988 was due to northerly to northwesterly winds, which held the summer pack ice against the the beach. Meltwater from this ice salt-stratified the upper water column, so that the ice eventually became effectively insulated against vertical flux of heat from the underlying warm water in the coastal current.

Oceanographic and meteorological effects on autumn sea-ice distribution in the western Arctic

Oceanographic, meteorological and sea-ice data were obtained from the northern Bering Sea and Chukchi Sea during the autumns of 1987 and 1988. Ice-edge location was observed from ships and via AVHRR satellite data, and ice-drift information was obtained from ARGOS-tracked drift buoys. Meteorological data were obtained from ships, from an ARGOS-tracked meteorological station and from synoptic charts. The ice edge was significantly farther south in 1988 than during other years and impacted the Alaskan coastline. In 1987, the ice edge was, conversely, anomalously far north. Ice melt-back in certain regions, such as along the Alaskan coast and in Herald Canyon, was due to input from warm ocean currents. The larger-scale interannual differences in ice extent were, however, due to interannual differences in the regional winds. In particular, the anomalous and extreme southward extent of the ice edge during 1988 was due to northerly to northwesterly winds, which held the summer pack ice against the the beach. Meltwater from this ice salt-stratified the upper water column, so that the ice eventually became effectively insulated against vertical flux of heat from the underlying warm water in the coastal current.

Seasonal Arctic Sea-Ice Simulations With A Prognostic Ice-Ocean Model

The seasonal cycle of sea ice, especially the ice margin location in the East Greenland region, is significantly affected by ocean circulation. The ocean circulation in turn can be altered by ice dynamics which cause large amounts of ice to be transported to the ice margin to be melted, thus stratifying the ocean there. By responding to wind changes, the ice dynamics can also create rapid melting or freezing events which can destabilize the ocean.In an earlier study, Hibler and Bryan (1987) carried out a diagnostic simulation of the Arctic ice-ocean system in which a coupled ice-ocean circulation model was integrated for about five years beginning with mean annual estimates by Levitus (1982) of the observed temperature and salinity fields. As a consequence of this short integration, the mean baroclinic circulation of the ocean deviated little from the initial fields, although seasonal and local effects due to the interactive models were simulated. One particularly interesting result of this study was the presence of fluctuations of oceanic heat flux at the ice margin, which appeared to coincide with strong wind events occurring over a few days which induced periods of freezing.With this diagnostic model, good results for the location of the ice margin were obtained. However, a global budget analysis indicated that the net northward heat transport through the Faero–Shetland passage was not adequate to balance the heat loss to the atmosphere sustained by the ocean in the fifth year. Moreover, a 20-year simulation without diagnostic terms showed a degraduation of the baroclinic fields in the Arctic Basin possibly due to the very weak wind stress used for this particular years's wind forcing, or perhaps due to excessive damping in the ocean due to computational requirements imposed by the coarse grid.As a first step in the development of a higher-resolution fully interactive prognostic model, we have modified this model and carried out two prognostic simulations of the Arctic ice ocean system by employing 50-year integrations. The ocean model used for this study is essentially that of Hibler and Bryan (1987). However, the boundary conditions, atmospheric forcing, and ice model have been changed. In particular, a much more robust wind forcing was obtained by replacing the monthly mean wind fields with a 30-year means in order to obtain a seasonal forcing closer to climatology. With regard to the ice rheology, a cavitating fluid model in spherical coordinates which fully conserves ice mass and air sea heat exchanges was employed. The idea here is to attenuate less of the stress into the ocean so that even though the circulation is somewhat sluggish due to large viscous damping, a reasonable current field for the Arctic Basin might be obtained.Two types of prognostic circulation experiments were carried out with this model using different southern boundary conditions. In one case, a diagnostic relaxation near the boundary as used by Hibler and Bryan (1987) was employed. In this case, heat mass and salt transports through the southern boundary are essentially simulated. In the second case, the net burotropic flow through the Faero-Shetland passage and Denmark Strait were specified with the baroclinic transports partially specified by diagnostic relaxation terms. The results from both these models are analyzed with special attention to the ice edge location and the character of the baroclinic fields in the Arctic Basin.

Seasonal Arctic Sea-Ice Simulations With A Prognostic Ice-Ocean Model

The seasonal cycle of sea ice, especially the ice margin location in the East Greenland region, is significantly affected by ocean circulation. The ocean circulation in turn can be altered by ice dynamics which cause large amounts of ice to be transported to the ice margin to be melted, thus stratifying the ocean there. By responding to wind changes, the ice dynamics can also create rapid melting or freezing events which can destabilize the ocean. In an earlier study, Hibler and Bryan (1987) carried out a diagnostic simulation of the Arctic ice-ocean system in which a coupled ice-ocean circulation model was integrated for about five years beginning with mean annual estimates by Levitus (1982) of the observed temperature and salinity fields. As a consequence of this short integration, the mean baroclinic circulation of the ocean deviated little from the initial fields, although seasonal and local effects due to the interactive models were simulated. One particularly interesting result of this study was the presence of fluctuations of oceanic heat flux at the ice margin, which appeared to coincide with strong wind events occurring over a few days which induced periods of freezing. With this diagnostic model, good results for the location of the ice margin were obtained. However, a global budget analysis indicated that the net northward heat transport through the Faero–Shetland passage was not adequate to balance the heat loss to the atmosphere sustained by the ocean in the fifth year. Moreover, a 20-year simulation without diagnostic terms showed a degraduation of the baroclinic fields in the Arctic Basin possibly due to the very weak wind stress used for this particular years's wind forcing, or perhaps due to excessive damping in the ocean due to computational requirements imposed by the coarse grid. As a first step in the development of a higher-resolution fully interactive prognostic model, we have modified this model and carried out two prognostic simulations of the Arctic ice ocean system by employing 50-year integrations. The ocean model used for this study is essentially that of Hibler and Bryan (1987). However, the boundary conditions, atmospheric forcing, and ice model have been changed. In particular, a much more robust wind forcing was obtained by replacing the monthly mean wind fields with a 30-year means in order to obtain a seasonal forcing closer to climatology. With regard to the ice rheology, a cavitating fluid model in spherical coordinates which fully conserves ice mass and air sea heat exchanges was employed. The idea here is to attenuate less of the stress into the ocean so that even though the circulation is somewhat sluggish due to large viscous damping, a reasonable current field for the Arctic Basin might be obtained. Two types of prognostic circulation experiments were carried out with this model using different southern boundary conditions. In one case, a diagnostic relaxation near the boundary as used by Hibler and Bryan (1987) was employed. In this case, heat mass and salt transports through the southern boundary are essentially simulated. In the second case, the net burotropic flow through the Faero-Shetland passage and Denmark Strait were specified with the baroclinic transports partially specified by diagnostic relaxation terms. The results from both these models are analyzed with special attention to the ice edge location and the character of the baroclinic fields in the Arctic Basin.

Measurements of ambient noise “hot spots” along the ice edge in the Greenland Sea Marginal Ice Zone

During July 1987, an experiment was conducted to measure ambient noise along the ice edge in the Greenland Sea. Ambient noise “hot spots,” concentrated areas of relatively high noise levels, were located along the ice edge using a towed array. Ambient noise levels were then recorded using sonobuoys deployed from P3 aircraft. The sonobuoy data were averaged in time to obtain the noise spectra below 2 kHz. The temperature structure of the area was determined by XBT and AXBT buoys and the ice edge location was determined from coincident satellite photos, 90-Ghz microwave imagery, and P3 radar ice edge maps. The relationship between the environmental and acoustic data suggests the existence of hot spots which are (1) primarily related to wind and sea state and (2) primarily related to ice edge compaction by eddy flow.

The application of present and future satellite remote sensors to oceanographic acoustics

An ensemble of active and passive satellite remote sensors are presently operating or will be launched in the near future that provide high resolution detailed oceanographic information on a synoptic scale. These sensors operate in the visible, infrared, and microwave region of the electromagnetic spectrum and include the NOAA 7/8 (TIROS), NIMBUS-7, LANDSAT, SPOT, and GEOS satellites presently in orbit as well as the planned ESA ERS-1, NROSS, TOPEX, RADARSAT, and JERS-1 satellite launches. The information provided by these satellites includes not only ocean surface and air/sea environmental information such as wind speed and direction, gravity wave spectral estimates, surface water height, salinity, temperature, and water vapor content, but also information about the interior of the water column (i.e., fronts, upwelling, and internal waves). Additional information about the ice-covered ocean can also be provided by these satellite data. The sea ice information includes; ice edge location, ice concentration, floe size distributions, ice type, and ice kinematics. The merging of remote sensing data with acoustical data can potentially provide new insight into understanding the interior structure of the ocean. The use of acoustical data to aid in the interpretation of remote sensing oceanographic data can in turn increase the use of the satellite information.

All‐weather millimeter wave imagery in the marginal ice zone

In a recent series of flights over the marginal ice zone in the Greenland Sea the Naval Research Laboratory demonstrated that millimeter wavelength radiometry can be used for the remote determination of sea ice edge properties under all weather conditions. The imaging measurements, obtained at 90 GHz (3.3 mm) and 140 GHz (2.1 mm) from high altitude through heavy cloud conditions, can be used to delineate the sea ice edge, to locate polynyas and leads, to measure ice concentration, to determine ice type, age, and thickness, and to quantify the sea surface wind and atmospheric properties. Sea ice images were also obtained at 220 GHz (1.4 mm) but only under clear atmospheric conditions. The location, movement, and concentration of the sea ice edge greatly affects the dynamics of ice‐ocean circulation and air‐sea interaction. This information is fundamental for accurate long‐term weather prediction and for ship operations. These are the first measurements of sea ice ever obtained at 140 and 220 GHz.

Antarctic sea ice and related atmospheric circulation during FGGE

Anomalies in zonally averaged and regional Antarctic sea ice extent during the FGGE year (1978–79) are outlined. Interest is centred on the early months of 1979 when substantially less than normal ice occurred in the Ross Sea sector, and more than normal in the Weddell Sea sector. The latter anomaly was maintained and extended eastward to the African sector during the winter and spring of 1979. The ice anomalies are found to be associated with contrasting general circulation conditions described primarily by indices of the strength of the mid-latitude westerlies and regional poleward anomalies in the displacement of the subtropical high pressure belt. In general however, the track, intensity and frequency of cyclones in relation to both the ice edge location and to the prevailing thermal conditions at the ocean surface appear to be the most significant factors affecting the development and maintenance of the ice anomalies.

Norwegian Remote Sensing Experiment in a Marginal Ice Zone

The Norwegian Remote Sensing Experiment in the marginal ice zone north of Svalbard took place in fall 1979. Coordinated passive and active microwave measurements were obtained from shipborne, airborne, and satellite instruments together with in situ observations. The obtained spectra of emissivity (frequency range, 5 to 100 gigahertz) should improve identification of ice types and estimates of ice concentration. Mesoscale features along the ice edge were revealed by a 1.215-gigahertz synthetic aperture radar. Ice edge location by the Nimbus 7 scanning multichannel microwave radiometer was shown to be accurate to within 10 kilometers.