St. Lawrence Island polynya: Ice circulation and dense water production

Thesis (Ph.D.) University of Alaska Fairbanks, 1999The 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

Characteristics and transformation of Pacific winter water on the Chukchi Sea shelf in late spring

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Oceans 124 (2019): 7153– 7177, doi: 10.1029/2019JC015261.Data from a late spring survey of the northeast Chukchi Sea are used to investigate various aspects of newly ventilated winter water (NVWW). More than 96% of the water sampled on the shelf was NVWW, the saltiest (densest) of which tended to be in the main flow pathways on the shelf. Nearly all of the hydrographic profiles on the shelf displayed a two‐layer structure, with a surface mixed layer and bottom boundary layer separated by a weak density interface (on the order of 0.02 kg/m3). Using a polynya model to drive a one‐dimensional mixing model, it was demonstrated that, on average, the profiles would become completely homogenized within 14–25 hr when subjected to the March and April heat fluxes. A subset of the profiles would become homogenized when subjected to the May heat fluxes. Since the study domain contained numerous leads within the pack ice—many of them refreezing—and since some of the measured profiles were vertically uniform in density, this suggests that NVWW is formed throughout the Chukchi shelf via convection within small openings in the ice. This is consistent with the result that the salinity signals of the NVWW along the central shelf pathway cannot be explained solely by advection from Bering Strait or via modification within large polynyas. The local convection would be expected to stir nutrients into the water column from the sediments, which explains the high nitrate concentrations observed throughout the shelf. This provides a favorable initial condition for phytoplankton growth on the Chukchi shelf.The authors are indebted to Commanding Officer John Reeves, Executive Officer Gregory Stanclik, Operations Officer Jacob Cass, and the entire crew of the USCGC Healy for their hard work and dedication in making the SUBICE cruise a success. We also acknowledge Scott Hiller for his assistance with Healy's meteorological data. We thank an anonymous reviewer for helpful input that improved the paper. Funding for A. P., R. P., C. N., and F. B. was provided by the National Science Foundation (NSF) under grant PLR‐1303617. K. M. was funded by the Natural Sciences and Engineering Research Council of Canada. K. V. acknowledges the Bergen Research Foundation under Grant BFS2016REK01. K. A. was supported by the NSF grant PLR‐1304563. The CTD and shipboard ADCP data are available from https://www.rvdata.us/search/cruise/HLY1401, and the nutrient data can be accessed from https://arcticdata.io/catalog/view/doi:10.18739/A2RG3Z and http://ocean.stanford.edu/subice/. The shipboard meteorological data reside at http://ocean.stanford.edu/subice/.2020-04-1

The St. Lawrence polynya and the Bering shelf circulation : new observations and a model comparison

Author Posting. © American Geophysical Union, 2006. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 111 (2006): C09023, doi:10.1029/2005JC003268.Using 14 year-long instrumented moorings deployed south of St. Lawrence Island, along with oceanographic drifters, we investigate the circulation over the central Bering shelf and the role of polynyas in forming and disseminating saline waters over the shelf. We focus also on evaluating the Gawarkiewicz and Chapman [1995] model of eddy production within coastal polynyas. Principal results include: 1) The northern central shelf near-surface waters exhibit westward flow carrying low-salinity waters from the Alaskan coast in fall and early winter, with consequences for water mass formation and biological production. 2) Within the St. Lawrence polynya, the freshening effect of winter advection is about half as large as the salting effect of surface brine flux resulting from freezing. 3) Brine production over the Bering shelf occurs primarily offshore, rather than within coastal polynyas, even though ice production per unit area is much larger within the polynyas. 4) We find little evidence for the geostrophic flow adjustment predicted by recent polynya models. 5) In contrast to the theoretical prediction that dense water from the polynya is carried offshore by eddies, we find negligible cross-shelf eddy density fluxes within and surrounding the polynya and very low levels of eddy energy that decreased from fall to winter, even though dense water accumulated within the polynya and large cross-shore density gradients developed. 6) It is possible that dense polynya water was advected downstream of our array before appreciable eddy fluxes materialized.This work was supported by National Science Foundation grant OCE9730697 to the University of Alaska and grant OCE9730823 to the University of Washington. S. M. acknowledges the support of the National Science Foundation under OCE9811097 and of NASA under grant NNG04GM69G. The University of Hamburg contributions were funded by the Bundesminister für Bildung und Wissenschaft. Funding for the drifter deployment was made possible by the North Pacific Research Board, grant NPMRI T2130. Manuscript preparation was additionally supported by Office of Naval Research grants N00014-99-1-0345 and N00014-02-1-0305 to the University of Washington

The Use of AVHRR Thermal Infrared Imagery to Determine Sea Ice Thickness within the Chukchi Polynya

Sea ice thickness changes over a nine-day period are determined for the Chukchi Polynya using Maykut's (1986) and Kuhn et al.'s (1975) theoretical predictive models. The models relate ice thickness to sea ice surface temperature, air temperature, wind speed, and sea water temperature. Sea ice surface temperatures are derived from AVHRR imagery and meteorological observations are taken from the synoptic weather station at Barrow, Alaska. The Maykut equation yields results that appear to be realistic for the ice thickness distribution within the polynya at the beginning stages of polynya formation. Ice thickness calculations at the later stages of polynya formation are partially invalidated by the movement of large floes to the oldest part of the polynya in response to a wind from the northeast. Such a major disturbance on the surface of the polynya complicates the identification of the type and thickness of ice that is forming. These results offer encouragement for the prospects of future field studies to validate and refine the technique and for the extension of the technique to calculation of heat transfer and salt rejection within the Chukchi Polynya and other polynyas.Key words: polynya, Chukchi Sea, ice thickness,A VHRR imagery, surface heat transfer, ice growthMots clés: polynia, mer des Tchouktches, épaisseur de la glace, images prises au radar perfectionné à trés haute résolution, transfert thermique de surface, croissance de la glac

Investigation of ice formation and water mass modification in eastern Laptev Sea polynyas by means of satellites and models

Salt expelled during the formation of ice in polynyas leads to a downward precipitation of brine that causes thermohaline convection and erodes the density stratification of the water column. In this thesis we investigate by means of flux models and satellite data the ability of the Western New Siberian (WNS) flaw polynya to modify the stratification of the water column and to form saline bottom water. The accuracy of existent microwave satellite-based polynya monitoring methods is assessed by a comparison of derived estimates with airborne electromagnetic ice thickness measurements and aerial photographs taken across the polynya. The cross-validation indicates that in the narrow flaw polynyas of the Laptev Sea the coarse resolution of commonly used microwave channel combinations provokes errors through mixed signals at the fast and pack ice edges. Likewise, the accuracy of flux models is tested by comparing model results to ice thickness and ice production estimates derived from high-resolution thermal infrared satellite observations. We find that if a realistic fast ice boundary and parameterization of the collection depth H is used and if the movement of the pack ice edge is prescribed correctly, the model is an appropriate tool for studying polynya dynamics and estimating associated fluxes. Hence, a flux model is used to examine the effect of ice production on the stratification of the water column. The ability of the polynya to form dense shelf bottom water is investigated by adding the brine released during an except ionally strong WNS polynya event in 2004 to the average winter density stratification of the water body. Owing to the strong density stratification and the apparent lack of extreme polynya events in the eastern Laptev Sea, we find the likelihood of convective mixing down to the bottom to be extremely low. We conclude that the recently observed breakdown of the stratification during polynya events is therefore predominantly related to wind- and tidally-driven turbulent mixing

Advances in understanding and parameterization of small-scalephysical processes in the marine Arctic climate system: a review

The Arctic climate system includes numerous highly interactive small-scale physical processes in the atmosphere, sea ice, and ocean. During and since the International Polar Year 2007–2009, significant advances have been made in understanding these processes. Here, these recent advances are reviewed, synthesized, and discussed. In atmospheric physics, the primary advances have been in cloud physics, radiative transfer, mesoscale cyclones, coastal, and fjordic processes as well as in boundary layer processes and surface fluxes. In sea ice and its snow cover, advances have been made in understanding of the surface albedo and its relationships with snow properties, the internal structure of sea ice, the heat and salt transfer in ice, the formation of superimposed ice and snow ice, and the small-scale dynamics of sea ice. For the ocean, significant advances have been related to exchange processes at the ice–ocean interface, diapycnal mixing, double-diffusive convection, tidal currents and diurnal resonance. Despite this recent progress, some of these small-scale physical processes are still not sufficiently understood: these include wave–turbulence interactions in the atmosphere and ocean, the exchange of heat and salt at the ice–ocean interface, and the mechanical weakening of sea ice. Many other processes are reasonably well understood as stand-alone processes but the challenge is to understand their interactions with and impacts and feedbacks on other processes. Uncertainty in the parameterization of small-scale processes continues to be among the greatest challenges facing climate modelling, particularly in high latitudes. Further improvements in parameterization require new year-round field campaigns on the Arctic sea ice, closely combined with satellite remote sensing studies and numerical model experiments.publishedVersio

NASA Oceanic Processes Program annual review

Current flight projects and definition studies, descriptions of individual research activities, and a bibliography of referred journal articles appearing within the past two years are contained

Moisture Flux Estimates Derived from EOS Aqua Data in the Arctic

The Arctic sea ice acts as a barrier between the ocean and atmosphere inhibiting the exchange of heat, momentum, and moisture. Recently, the ice pack has been decreasing in area and concentration. This diminished sea ice coverage could potentially allow for larger moisture fluxes that affect surface energy budgets, the occurrence of clouds, and the near-surface humidity and temperature. Currently, reanalyses are known to produce large errors and biases in the Arctic, warranting improved moisture flux algorithms and input data. Using the Monin-Obukhov similarity theory, with adjustments made to better suit the conditions of the Arctic, and observations from NASA's EOS Aqua satellite, specifically the AIRS and AMSR-E instruments, the daily moisture flux is calculated from 2003-2011. The moisture flux is studied for a series of North Water polynya events between 2003-2009 to test the accuracy of the Aqua products and our algorithm. Using in situ data we validated moisture flux results, finding an error of 20.3%, improving the moisture flux accuracy compared to other climate models. The moisture flux for the entire Arctic was studied to look for inter-annual variations and was compared to changes in the sea ice. Instead of an expected increase in the moisture flux due to a declining sea ice pack, there has been a 15% decrease. On a regional scale and based on their average moisture flux, the Chukchi/Beaufort Seas, Laptev/E. Siberian Seas, Canadian Archipelago and Central Arctic are increasing, between 2.1 and 4.8 %/yr. Increases are due to the changes in the ice concentration, which allows for the surface temperatures to increase substantially in the fall and winter months when the amount of moisture exchanged is highest. The Kara/Barents Seas, E. Greenland Sea and Baffin Bay are decreasing, between 0.53 and 9.2 %/yr. These regions have areas of open water year round, and their exchanges of moisture are due mostly to smaller differences in surface and 2 m specific humidities. The contribution of the sea ice zone to the total moisture flux (from the open ocean and sea ice zone) has increased by 3.6% because the amount of open water within the sea ice zone has increased by 4.3%

Ice Formation in the Arctic Ocean: Observed Processes and Climate Feedbacks

The Arctic seasonal ice zone undergoes dramatic changes throughout the year. In the summer, ice cover melts off, leaving the ocean surface exposed to the atmosphere. Incident solar radiation warms the surface mixed layer. At the end of the summer season, the mixed layer must then cool to the freezing point in order for ice to form at the surface. This thesis explores the impact of that heat throughout the annual cycle, but especially on first-year ice growth. Through analysis of CTD profiles and buoy data, it is found that summer heat gets into the summer halocline, where it is trapped through the fall season cooling process. The mixed layer heat (estimated from an empirical relationship derived from direct measurements and atmospheric and geographic parameters along with sea surface temperatures) is well correlated with freeze-up date. The results presented herein suggest that delayed freeze-up does result in thinner ice cover at the end of the winter season, in part because the maximum ice growth rate is consistent (approximately 12 cm/day) between observations from Antarctic polynyas with exceptionally high ice production and the few observations that exist in the Arctic. Finally, from analysis of end-of-season first year ice thickness distributions, it is clear that the summer heat trapped in the summer halocline is released over the course of the winter, limiting ice growth in addition to that caused by the delay in freeze-up. In all, it is likely that as the Arctic becomes increasingly dominated by first-year ice, the thinning trend will continue though at a slower rate than in the current transition from multi- to first-year ice cover

Science program for an imaging radar receiving station in Alaska. Report of the science working group

It is argued that there would be broad scientific benefit in establishing in Alaska an imaging radar receiving station that would collect data from the European Space Agency's Remote Sensing Satellite, ERS-1. This station would acquire imagery of the ice cover from the American territorial waters of the Beaufort, Chukchi, and Bering Seas. This station, in conjunction with similar stations proposed for Kiruna, Sweden, and Prince Albert, Canada would provide synoptic coverage of nearly the entire Arctic. The value of such coverage to aspects of oceanography, geology, glaciology, and botany is considered