Sea Ice Advection Research Articles

Atmospheric forcing of the southeast Bering Sea Shelf during 1995–99 in the context of a 40-year historical record

Abstract The atmospheric forcing of the Bering Sea over its eastern shelf is estimated using the 40-year record of daily data from the NCEP/NCAR Reanalysis. This data set includes estimates of the processes responsible for the atmospheric forcing, namely the surface fluxes of momentum, sensible and latent heat, and longwave and shortwave radiation, and therefore permits quantifying effects that previously could be inferred only from the large-scale nature of the flow. The forcing in 1995–1999 is described in detail using daily time series; historical context for these results is provided with seasonal averages for the years 1959–1999. The analysis for winter concentrates on aspects related to the formation and advection of sea ice. Results indicate that the presence of sea ice is strongly related to the net surface-heat fluxes as well as the cross-shelf component of the wind. The 40-year record lacks any discernible long-term trend in the winter forcing and response. There was a notably cold period in the early to middle 1970s, and a warm period from the late 1970s into the early 1980s, but conditions during the 1990s are similar to those in the late 1950s and 1960s. The analysis for the warm season focuses on the mechanisms responsible for the variability in SST warming. Much of the intraseasonal and interannual variability in this warming can be attributed to variations in the downward shortwave radiation (solar heating). The 40-year record does indicate a long-term trend toward increased solar heating, and reduced surface latent-heat fluxes (evaporative cooling). These changes have led to August SSTs in the 1990s that are roughly 1°C warmer than in the 1960s.

An overview of physical processes in the North Water

The presence of a polynya strongly affects biological processes by influencing underwater light levels, water-column stratification, the upwelling of nutrients, and the timing of production cycles. An overview of the existing literature on the oceanography, meteorology and sea-ice conditions of the North Water, a large recurring polynya in northern Baffin Bay, is presented. The North Water is influenced by a cold inflow of Arctic Ocean water from the north and, in some areas, a warmer inflow of Atlantic water from the south. Earlier observations and modeling studies suggested that both wind and ocean advection of sea ice, as well as heat input to the surface waters by either upwelling or mixing, were responsible for the formation and maintenance of the polynya. Recent data indicate the North Water to be primarily generated by the southward drift of ice by winds and currents, as opposed to melting ice by sensible heat input over large areas. Localized sensible heat effects occur in fall, winter and spring along the Greenland side of the polynya.

Paleoceanographic implications of a 90,000 year long diatom record in piston core KH94-3, LM-8 off NE Japan

Q-mode principal component analysis (PCA) of a fossil diatom assemblage reveals the late Quaternary paleoceanography of the NW Pacific margin, based on submarine core KH94-3, LM-8 recovered off the Sanriku coast of NE Japan (38°53.46′N, 143°22.19′E). Overall, diatoms were abundant during interglacials, but low in abundance and poor in preservation during the last glacial, with each oxygen isotope stage having its own characteristic assemblage. The results of PCA are: principal component 1 (PC1) is composed mainly of Thalassionema nitzschioides, which predominated during the coldest oxygen isotopic Stages 4 (75–58 ka) and 2 (25–11 ka), and by allochthonous taxa. The diatom tests are poorly preserved and the diatom mass accumulation rate (DMAR) is the lowest in these stages. PC2 is characterized by ice-related diatoms that predominate in Stage 3 (the interstadial), whereas PC3, which is characterized by Neodenticula seminae and Odontella aurita, shows high values in Stage 1. Diatom assemblages exhibit drastic changes between oxygen isotopic Stages 3 and 1 (11–0 ka). Distinct increases of DMAR occurred during both periods. We infer an intensified advection of sea ice from the Sea of Okhotsk toward the area off Sanriku during Stage 3. This interpretation explains the high DMAR in Stage 3, owing to high productivity at the sea ice edge. Our study can contribute to marine environmental reconstruction since the last glaciation because fewer micropaleontological data exist in the NW Pacific margin for these time intervals, compared with other oceans.

Sea ice dynamics in the Weddell Sea simulated with an optimized model

The advection of sea ice and associated freshwater/salt fluxes in the Weddell Sea in 1986 and 1987 are investigated with a large‐scale dynamic‐thermodynamic sea ice model. The model is validated and optimized by comparison of simulated sea ice trajectories with observed drift paths of six buoys deployed on the Weddell Sea ice. The skill of the model is quantified by an error function that measures the deviations of simulated trajectories from observed 30‐day sea ice drift. A large number of sensitivity studies show how simulated sea ice transports and associated freshwater/salt fluxes respond to variations in physical parameterizations. The model reproduces the observed ice drift well, provided ice dynamics parameters are set to appropriate values. Optimized values for the drag coefficients and for the ice strength parameter are determined by applying the error function to various sensitivity studies with different parameters. The optimized model yields a mean northward sea ice volume export out of the southern Weddell Sea of 1693 km3 in 1986 and 2339 km3 in 1987. This shows the important role of sea ice transport for the freshwater budget of the Weddell Sea and gives an indication of its high interannual variability.

Simulation of sea ice transport through Fram Strait: Natural variability and sensitivity to forcing

The interannual variability of the sea ice transport through Fram Strait is simulated with a dynamic‐thermodynamic sea ice model. Forcing with daily varying wind fields for the 7‐year period 1986–1992 causes a high variability of sea ice drift on timescales from days to years. Annual means of simulated ice transport through Pram Strait differ up to a factor of 2. Additional sensitivity studies investigate the response of sea ice transports to variations of the prescribed atmospheric and oceanic forcing. Wind speed, ocean current speed, air temperature, and precipitation rate are systematically varied over a wide range. The model predicts an almost linear relation of ice transport with wind speed and ocean current, a strong, nonlinear relation with air temperature, and a rather small sensitivity to changes in precipitation. The results show that the interannual variability of wind forcing causes considerable variations of sea ice export through Fram Strait. The fluxes of freshwater and negative latent heat associated with the sea ice transport can significantly affect the ocean circulation in the Greenland Sea and in the North Atlantic. This shows how variations of the ocean circulation are coupled to the variability of the atmosphere by the mechanism of sea ice advection. To adequately represent these important interactions in the coupled system atmosphere‐cryosphere‐ocean, both the dynamics and the thermodynamics of sea ice must be included in climate models.

The second Hadley Centre coupled ocean-atmosphere GCM: model description, spinup and validation

This study describes a new coupled ocean-atmosphere general circulation model (OAGCM) developed for studies of climate change and results from a hindcast experiment. The model includes various physical and technical improvements relative to an earlier version of the Hadley Centre OAGCM. A coupled spinup process is used to bring the model to equilibrium. Compared to uncoupled spinup methods this is computationally more expensive, but helps to counter climate drift arising from inadequate sampling of short time scale coupled variability when the components are equilibrated separately. Including sea ice advection and enhancing reference surface salinities in high southern latitudes in austral winter to promote bottom water formation during spinup appears to have stabilized the high-latitude drift exhibited in the earlier model’s control run. In the present study, the atmospheric control climate is stable on multi-century time scales with a drift in global average surface air temperature of only +0.016 K/century, despite a small residual drift in the deep ocean. The control climate is an improvement over the earlier model in several respects, notably in its variability on short time scales. Two anomaly runs are presented incorporating estimated forcing changes over the period 1860 to 1990 arising from greenhouse gases alone and from greenhouse gases plus the radiative scattering effect of sulphate aerosols. These allow validation of the model against the instrumental climate record. Inclusion of aerosol forcing gives a significantly better simulation of historical temperature patterns, although comparisons against recent sea ice trends are equivocal. These studies emphasize the potential importance of including additional forcing terms apart from greenhouse gases in climate simulations, and refining estimates of their spatial distribution and magnitude.

Multiparameter AVHRR-Derived Products for Arctic Climate Studies

Abstract Generation and sample applications of an integrated set of remotely sensed products for investigations of Arctic climate are described. Cloud fraction, ice surface temperature, surface albedo, downwelling radiative fluxes, ice motion vectors, and cloud properties such as optical depth, phase, and droplet effective radius are estimated from calibrated and navigated AVHRR 1.1-km imagery of the Arctic Beaufort Sea region for June 1992 through July 1993. The processing strategy and characteristics of the products are reviewed. The utility of this type of multiparameter dataset for modeling applications and process studies is illustrated using simple examples of an albedo parameterization, sensible heat flux calculation, and sea ice advection.

Winter plankton assemblage in the ice edge zone of the Weddell and scotia seas: composition, biomass and spatial distributions

As part of the Antarctic Marine Ecosystem Research in the Ice Edge Zone (AMERIEZ) program, we examined the biomass and distribution of phytoplankton and protozooplankton at an advancing ice edge in the Weddell and Scotia Seas during the early austral winter. The advance of ice cover, local melting of sea ice and advection of water masses, possibly from lower latitude regions, were the main sources of variability in the physical regime of the ice-edge zone. Analysis of the plankton assemblage showed phytoplankton biomass (PPC) in the upper 100 m ranging from 100 to 272 mg C m −2 and protozooplankton biomass (PZC) ranging from 177 to 410 mg C m −2. Autotrophic dinoflagellates dominated phytoplankton stocks, followed by other autotrophic nanoflagellates and diatoms in decreasing biomass. Heterotrophic flagellates dominated protozooplankton biomass followed by ciliates and sarcodines. The biomass of major groups comprising the planktonic assemblage was similar at most stations with the exception of one station where diatoms predominated. Integrated PPC and PZC showed no relationship to water column stability. Integrated autotrophic flagellate biomass was higher at open water stations than at ice-covered stations, but none of the other integrated group biomasses showed variations that could be related to ice cover or water mass characteristics. Analysis of discrete-depth samples indicated that all groups (except the sarcodines) showed near surface maxima, that total PPC, diatoms and autotrophic flagellates showed higher biomasses at ice-edge and open water stations than at ice-covered stations and that none of the heterotrophs showed variations related to ice cover. Some groups, however, showed differences that could be related to water mass characteristics, but this was less evident than was the effect of depth or ice cover. Comparison of species assemblages of diatoms and choanoflagellates among water column stations indicated variations that could be related to the differing cruise legs and water mass characteristics. Similar diatom assemblages were found in both ice and water, but higher concentrations occured in the ice assemblages. A few diatom species were found to be indicator species for ice versus water assemblages and to distinguish among the varying hydrographic regimes. Although phytoplankton stocks were higher at non ice-covered than at ice-covered stations, we were not able to see distinct differences between ice-edge stations and those north of the furthest ice extent. We hypothesize that advection of sea ice into water above the freezing point and subsequent melting of ice probably affected much of our study area, so that any effects of “enhanced production” in the ice-edge zone would have been difficult to resolve. Moreover, absolute primary production was very low, and based on the trophic composition of the planktonic assemblage and production estimates from our AMERIEZ colleagues, we concluded that neither algal nor bacterial production was sufficient to produce an enrichment of protozooplankton stocks in the ice-edge zone. Calculations of a carbon budget suggested bacterial production was a significant proportion of total production and that the nano- and microheterotrophs must predominate in the utilization of both phyto- and bacterioplankton production at the winter ice edge. An analysis of species assemblages suggested little advection of populations from lower latitude regions and supports the contention that material was apparently released from sea ice during localized melting events. This input of carbon biomass and detritus from ice may supply the carbon needed to support the high concentrations of heterotrophs observed in our study, but this interpretation is confounded because ice-edge heterotrophic plankton populations also may be enriched by seeding from sea ice.

Observing the advection of sea ice in the Weddell Sea using buoy and satellite passive microwave data

This study examines the progress and behavior of an area of sea ice as it drifts from the southwestern Weddell Sea by using data from four buoys tracked by Nimbus 6 and concurrent ice concentrations retrieved from Nimbus 7 scanning multichannel microwave radiometer data. It covers the entire growth season of 1980. The overall drift characteristics, and their relationship to ice edge displacement, are examined within the framework of four zones. Three phases are identified in the large‐scale behavior of the Weddell sea ice cover, namely, a rapid equatorward and eastward advance, a quasi‐equilibrium phase, and a period of rapid recession. A broad division is made between the sectors to the east and west of 35°–40°W in terms of the nature of ice edge advance. Transient departures from the mean northward advection are linked to the passage of cyclones; certain such events may be significant in terms of the formation and maintenance of the region of highly consolidated perennial ice observed in the western Weddell Sea. Mean daily drift rates (up to 0.9 m s−1) from within the central‐western Weddell pack are high in 1980, particularly early in the growth season and when compared with rates to the west of 50°W. Outbreaks of cold continental air alternate with incursions of relatively warm air from the north; warm conditions are recorded as far as 1200 km in from the ice edge in winter. Closed loops in the buoy trajectories, which are clockwise to the south of 63°S, reverse to become anticlockwise to the north. Mid‐ocean islands impede the eastward progress of the ice. A coherence is noted in the response of the buoys to the passage of storms, even though the buoys separated by a distance of over 100 km.

On deep convection in the Weddell Gyre

In March 1983 a region (horizontal scale > 200 km) of convectively cooled and freshened Weddell Deep Water ( γ max < 0°C) was observed central Weddell Sea. The extremely low salt and nutrient contents of the upper 1000 m of the cold spot suggest that its initial water column originated from the Antarctic Coastal Current. The deep convection was not related to a large-scale polynya. The remaining sea ice cover impeded the ventilation of deep waters. Compared to the 1983 cold spot, the cold spot associated with the Weddell Polynya had a considerably higher salt content, even though lower than that of the Weddell Warm Regime. Hydrographic characteristics suggest that the Weddell Polynya cold spot originated from the Weddell Cold Regime. Alternatively, deep convection in the Weddell Polynya was accompanied by a strong advection of sea ice and surface water. An initial preconditioning for deep convection seems to have occurred in a water column with a shallow main pycnocline by an enrichment of the upper layers in salinity.

The size of wind‐driven coastal polynyas

The sizes of wind‐generated coastal polynyas have been observed to be nearly constant for steady atmospheric conditions owing to the balance between the advection of sea ice away from the coast and the area‐averaged production rate of new ice. A simple model is used to explore the relationship of several environmental parameters to the maximum size attained by the polynya and the speed at which the maximum is reached for a given atmospheric event. The model results suggest that size is strongly a function of air temperature, such that colder air produces a smaller polynya for a given offshore wind velocity. However, size is only moderately a function of wind speed, especially for winds greater than 10 m s−1, since increasing the speed increases both the advection rate and the ice production rate. The model results are compared to observations made around a coastal polynya during February 1982 and 1983 along the southern coast of St. Lawrence Island in the northern Bering Sea and during February 1985 along the southern coast of the Seward Peninsula. The model correctly predicts the general maximum dimensions of these winter polynyas, although the atmospheric stationarity assumptions limit the usefulness of the predictions of the speed at which the maximum is reached. The results of this study suggest that the contribution of heat from the coastal ocean to the high‐latitude winter atmosphere is a self‐limiting process proportional to the amount of time the wind‐driven ice drift has a component normal to the coast. This has important implications for the interpretation of satellite imagery for ice‐covered oceans and for understanding high‐latitude climate dynamics.