Now showing items 1-13 of 13

• #### Brine Percolation, Flooding And Snow Ice Formation On Antarctic Sea Ice

Modelling studies of brine percolation, flooding, and snow ice formation on Antarctic sea ice were undertaken to (1) determine the influence of brine transport processes on the salinity, porosity, and stable isotopic composition of snow ice and the underlying ice, (2) explain the range of salinities and isotopic composition observed in ice cores, and to provide a better estimate of the contribution of snow ice to the thickness of the winter pack ice, (3) better understand the microstructural controls on brine percolation and its effects on the properties of sea ice, and (4) understand the effects of meteorological forcing on snow ice formation and development of the ice cover. Snow ice thickness is most dependent on snow accumulation rates. Once snow ice begins to form on a floe, most of the subsequent thickening is due to snow ice formation. Results show that percolation in winter sea ice is most likely an inhomogeneous process. Flooding most likely occurs rapidly through localized regions of high permeability, such as in large, open brine drainage channels or cracks. Simulations of the freezing of a flooded slush layer show that focussing of thermohaline convection may form porous drainage channels in the ice and snow. These channels allow rapid desalination of the slush and exchange of H218O depleted brine with sea water. Significant positive shifts in delta18O are possible in the slush layer. This process can explain the range of delta18O observed in ice cores. Based on these results, a cutoff of delta18O < -2� is recommended for snow ice identification in the Ross, Amundsen, and Bellingshausen seas. Such a cutoff puts the amount of snow ice observed at 6--18% of the ice thickness. Although flooding appears to occur through spatially restricted regions of the ice, the precise nature of the flow and factors controlling onset of percolation are unclear.
• #### Circulation variability in the Bering Sea

Sea surface height anomalies measured by the GEOSAT radar to develop an improved circulation scheme for the deep basin of the Bering Sea and to make inferences about the dynamics controlling the circulation. A conceptual model of the circulation in the Bering Sea is presented in three manuscripts. The first provides a description of a large eddy in the Alaskan Stream. This eddy is shown to influence circulation in the southern Aleutian Basin. The second manuscript reports on an empirical orthogonal function analysis of averaged sea surface height anomalies. The analysis is interpreted to describe the superposition of two principal circulation schemes. The former describes annual period, basin scale cyclonic circulation. The latter scheme reflects the southwestward propagation of $\sim$1.9 year period, gyre scale baroclinic long waves across the Aleutian Basin. The final paper reports on observations of topographic planetary waves associated with the Bering Slope Current. The model provides a possible resolution of some of the discrepancies between previously published circulation schemes for the Bering Sea.
• #### Distribution of large calanoid copepods in relation to physical oceanographic conditions and foraging auklets in the western Aleutian Islands

Acoustic measurements and net sampling were used to estimate zooplankton abundance and biomass relative to water mass types and flow fields in the western Aleutian Islands during June and July, 1992 and 1993. Observations are interpreted relative to the distribution and abundance of least auklets (Aethia pusilla), which forage on zooplankton. Highest zooplankton biomass (up to 7 g m$\sp{-3}$) occurred during June 1992, in the pycnocline separating the upper mixed layer from the cold intermediate layer north of a front separating Bering Sea and Alaska Stream water. The large calanoid Neocalanus flemingeri had highest abundance but the larger Neocalanus cristatus accounted for most of the biomass. N. cristatus and N. flemingeri were absent south of the Bering Sea front, where the community was dominated by Neocalanus plumchrus and Eucalanus bungii. Auklets were foraging almost exclusively north of the Bering Sea front. Neocalanus spp. abundance in the upper mixed layer was much lower in July 1993, than in June 1992. Neocalanus occurred primarily in scattered aggregates near the pycnocline over Bering Sea Intermediate water and at the surface in Pacific water. Auklets shifted their foraging activities to passes and shelf areas among the islands, where tidally generated divergences and convergences upwelled and concentrated prey into patches in the mixed layer. Elevated densities of Neocalanus were observed in convergence zones in Delarov Pass and over a ridge south of Kiska Island. Convergence zones were identified by intense sound scattering from entrained bubbles and by deceleration of the horizontal velocity components in acoustic doppler current data, a record of current speed and direction beneath the vessel. Densities of auklet prey in the study area during June were apparently influenced by the position of the front between Bering Sea and Pacific water masses. The position of the front was influenced by Alaska Stream flow anomalies lasting for several years. Prey densities on the shelves and in the passes during July were influenced by tidal currents at spatial scales of tens of meters to ten kilometers and lasting one tidal cycle.
• #### Numerical modeling study of the circulation in the Gulf of Alaska

A series of numerical experiments are performed to simulate the Gulf of Alaska circulation and to examine the dynamical ocean response to the annual mean and seasonal forcing using a primitive equation model (Semtner 1974). The model domain encompasses the North Pacific north of 45$\sp\circ$ N and east of 180$\sp\circ$ and is surrounded by artificial walls in the south and west. Biharmonic diffusion is used in the interior to excite mesoscale eddies. A sponge layer with high Laplacian diffusion is incorporated near the western boundary. Horizontal resolution of 30$\sp\prime$ x 20$\sp\prime$ and 20 vertical levels are used to resolve the mesoscale topography and eddies. Wind stress computed from sea level atmospheric pressure and temperature and salinity data of Levitus (1982) are used. A diagnostic model produces a circulation in the Gulf of Alaska which agrees with observed patterns. In a three-layer flat-bottom baroclinic model, baroclinic Rossby waves propagate at 0.8 cm/sec and it takes a decade for spin-up to be completed. Baroclinic models forced by the annual mean wind and thermohaline forcings show the generation of eddies by baroclinic instability. The eddies in the flat-bottom model have a period of 75 days and are interpreted as barotropic Rossby waves. In the model with topography, the period of dominant eddies is 3-4 years and they are interpreted as baroclinic Rossby waves. Anticyclonic eddies near Sitka show similar characteristics as the Sitka eddy. They propagate westward and cause meanders in the Alaska Stream near Kodiak Island. The abnormal shift of the Alaska gyre in 1981 is probably due to the presence of one of these anticyclonic eddies. A flat-bottom model with seasonal forcing shows a large seasonal variability. When bottom topography is present, however, seasonal response is greatly reduced due to the dissipation of barotropic response by bottom topography. The seasonal baroclinic model shows a similar seasonal variability to the seasonal barotropic model indicating that the seasonal response is mainly barotropic. Eddies are also excited in the seasonal case and are almost identical to those of the annual mean case.
• #### Numerical modeling study of the circulation of the Greenland Sea

This study is a simulation of the circulation of the Greenland Sea aimed at modeling some of the issues related to the Great Salinity Anomaly (GSA) and deep water formation using a primitive equation ocean general circulation model (Semtner, 1974). The features of the model include: (1) a high resolution, (2) real topography, (3) open boundaries at the south and north, and (4) temporally variable wind and thermohaline forcing. The model is used to study: (1) the spreading of a fresh water anomaly, (2) the mechanisms of cross frontal mixing that lead to deep water formation, (3) the general circulation of the deep and upper layers of the ocean and their dependence on wind and thermohaline forcing, and (4) the possible implications of meso-scale and large-scale variability on climate change. One of the major results of this work is the simulation of continental shelf waves propagating along the shelf slope of Greenland between 77$\sp\circ$N and 72$\sp\circ$N. Waves with a subinertial period of 17.2 hrs, a wavelength of 363 km, a phase speed of 586 cm/s and a group velocity of 409 cm/s, are found. Possible mechanism for generation of shelf waves is presented. It is suggested that some energy related with wave activity may support cross-frontal mixing in the East Greenland Current (EGC), where formation of the two main sources of North Atlantic Deep Water (e.g. Norwegian Sea Deep Water and Denmark Strait Overflow Water) have been reported. The results from the GSA simulation suggest that during the early stage of the GSA (e.g. during its propagation with the EGC to the south, in the late 1960s) when no observations are available, the fresh water signal is not being mixed into the interior circulation of the Greenland Sea gyre. The second experiment, representing recirculation of the GSA from the North Atlantic back into the Greenland Sea, in the late 1970s, shows freshening in the Greenland Sea gyre of comparable magnitudes ($-$0.05 to $-$0.1 psu) to the observed ones. These results agree with the earlier indirect measurements (Rhein, 1991; Schlosser et al., 1991) indicating dramatic reduction of deep water renewal in the Greenland Sea in the late 1970s and early 1980s. From the general circulation experiments it has been found that the ocean response to seasonal forcing is mainly barotropic. This implies a strong topographic control in the distribution of currents and hydrographic variables. Most of the areas of topographic steering which are simulated in the region have been reported in the literature. The so-called Molloy Deep eddy shows its direct dependence on the large scale dynamics affecting the northward flow of the West Spitsbergen Current (WSC), controlling this way a net mass transport into the Arctic Ocean. Simulations with different wind forcing suggest dependence of the Greenland Sea gyre circulation on the variations with time of the local wind forcing. Results indicate that monthly mean wind stress forcing probably underestimate wind forcing in the model. Analysis of surface, intermediate and deep ocean velocity fields compare reasonably well with observations.
• #### On the dynamics of the Alaska coastal current

The Alaska Coastal Current (ACC) in the northern Gulf of Alaska is a wind- and buoyancy-driven near-surface jet primarily maintained by the horizontal salinity gradient due to fresh water entering at the coast. It serves as the major source of fresh water to the North Pacific Ocean. The buoyancy driving force is the major focus of this investigation. The study area is situated just "downstream" of Prince William Sound (PWS), a large estuary whose surface outflow is seen to occupy a narrow inshore band after joining the ACC. The effect of this band appears to be the formation of an occasional double maximum in the ACC. The period focused on in this study was selected on the basis of weak windstress but large fresh water input in order to emphasize the buoyancy forcing. The TS characteristics and a water mass tracing technique are used to separate the thermal and haline signals in the buoyancy forcing and to track the origin and fate of the source waters of the study area. The buoyancy driving force is shown to be primarily haline, with temperature playing a secondary, moderating role. Because of the large topographic variability and sloping density interfaces, and in order to exploit the available data, a diagnostic model retaining the baroclinicity and bottom topography terms was chosen to study the dynamics. Model premises are verified by results from hydrographic surveys, moored current meters, and a profiling current meter. The model predicts a midshelf region of negligible sealevel gradient, with a nearshore ($\approx$70 km wide) band over which the sealevel changes by about 25 cm. The sloping surface drives a strong ($\approx$100 cm/s) surface flow, which decreases to zero and reverses below about 100 m due to the opposing baroclinic pressure gradient. The flow splits around a shoal region. The onshore portion joins the outflow from PWS and accelerates downstream forming a double maximum. The offshore segment forms a large meander before rejoining the rest of the ACC, advecting midshelf water shoreward. The momentum balance is dominated by the JEBAT terms, which primarily determine the flow along and across contours of f/H.
• #### Snow and ice thickness distributions in the south polar Pacific Ocean

Two sets of snow and ice thickness data of antarctic sea ice are presented in this study: drilling profiles on individual sea ice floes and spatially more extensive shipboard observations are investigated in order to reveal spatial and temporal characteristics of sea ice. The data were acquired between 1993 and 1995 in the Ross and the Amundsen/Bellinghausen Seas, two different regions of the south polar Pacific Ocean. Sea ice characteristics as well as their spatial distribution and temporal evolution are derived from the data sets. Strong regional trends are observed in ice thickness distributions of the drilling data. A spatial pattern is detected in the snow and ice thickness data as they were sorted according to distance from the ice edge. Flooding on sea ice floes is a widespread phenomenon in Antarctica and relevant for snow ice growth. Flooding is shown to correlate highly with snow loading, while it was much less distinctly correlated to ridging. Isostatic balance was tested on individual sea ice floes. Deviation from isostatic balance was shown to occur locally but was negligible on averaging scales of a few meters. With the help of ice thickness and roughness criteria an unbiased and reproducible ice classification scheme is developed. A combination of different methods from spectral and spatial statistics was used to describe the surface roughness characteristics of the three ice groups in detail. As a practical outcome from the roughness characteristics, air, water and ice surface drag coefficients were derived. With the goal to enhance sampling efficiency, the methodology of data sampling is investigated and optimized strategies are presented. Finally, a statistical ice thickness model managed to explain regional differences in the shape of the ice thickness distribution and therefore the relative significance of ice growth and development processes for a certain region.
• #### St. Lawrence Island polynya: Ice circulation and dense water production

The St. Lawrence Island polynya (SLIP) opens every winter off the coast of St. Lawrence Island as winds move ice away from the shore. The SLIP is an important site for production of the dense water that flows northward through the Bering Strait to help maintain the Arctic Ocean halocline. Winter 1991/1992 ERS-1 SAR, thermal infrared, and passive microwave imagery are analyzed in combination with regional climate system and analytical simulations to investigate SLIP ice circulation, heat fluxes, and dense water production. Emphasis is on the February 1992 southern SLIP event. Satellite-based measurements show this polynya extended ~165km offshore and ~100km along shore at maximum extent. ERS-1 SAR GPS-derived ice motion indicated maximum ice speeds of ~30km day -1 during polynya expansion. Ice along the polynya boundary drifted parallel to the wind at 3--4% of the wind speed during north/northeasterly winds >7m s-1 Heat fluxes associated with the SLIP varied depending on method of calculation, but indicated increasing trends during polynya development. Associated ice production rates of 4.218.9cm day-1 were computed via different models. Dense water production, derived from ice production rates and polynya size, ranged from 0.011--0.017Sv, suggesting that the SLIP could account for 19--27% of the Bering Sea's contribution and 1--2% of the total Arctic contribution to Arctic Ocean halocline maintenance. Although the regional climate system model generated the SLIP on the same time scales as observed, a larger polynya resulted. The simulated polynya's heat and moisture impact was observed to at least 800mb, reaching 50km downstream. During periods of sustained winds, ice circulation was similar to that observed. Incorporation of a "barotropic" ocean component suggested that ocean circulation may be an important ice circulation forcing mechanism at the SLIP, especially during periods of weak winds, as inclusion greatly improved the simulated ice circulation. The "barotropic" ocean also improved polynya shape and extent. If regional climate changes alter the existence of polynyas like the SLIP, changes in the Arctic Ocean halocline might occur. Additional in situ observations and better fully-coupled atmosphere-ice-ocean models are needed to further ascertain the impact of polynyas on the overall Arctic climate system.