SangHoon Lee

Variability of Warm Deep Water Inflow in a Submarine Trough on the Amundsen Sea Shelf

The ice shelves in the Amundsen Sea are thinning rapidly, and the main reason for their decline appears nto be warm ocean currents circulating below the ice shelves and melting these from below. Ocean currents ntransportwarm densewater ontothe shelf,channeledby bathymetric troughs leadingto the deep inner basins. nA hydrographic mooring equipped with an upward-looking ADCP has been placed in one of these troughs on nthe central Amundsen shelf. The two years (2010/11) of mooring data are here used to characterize the inflow nof warm deep water to the deep shelf basins. During both years, the warm layer thickness and temperature npeaked in austral fall. The along-trough velocity is dominated by strong fluctuations that do not vary in the nvertical. These fluctuations are correlated with the local wind, with eastward wind over the shelf and shelf nbreak giving flow toward the ice shelves. In addition, there is a persistent flow of dense lower Circumpolar nDeep Water (CDW) toward the ice shelves in the bottom layer. This bottom-intensified flow appears to be ndriven by buoyancy forces rather than the shelfbreak wind. The years of 2010 and 2011 were characterized by na comparatively stationary Amundsen Sea low, and hence there were no strong eastward winds during winter nthat could drive an upwelling of warm water along the shelf break. Regardless of this, there was a persistent nflow of lower CDW in the bottom layer during the two years. The average heat transport toward the ice nshelves in the trough was estimated from the mooring data to be 0.95 TW.

Circulation and Modification of Warm Deep Water on the Central Amundsen Shelf

AbstractThe circulation pathways and subsurface cooling and freshening of warm deep water on the central Amundsen Sea shelf are deduced from hydrographic transects and four subsurface moorings. The Amundsen Sea continental shelf is intersected by the Dotson trough (DT), leading from the outer shelf to the deep basins on the inner shelf. During the measurement period, warm deep water was observed to flow southward on the eastern side of DT in approximate geostrophic balance. A northward outflow from the shelf was also observed along the bottom in the western side of DT. Estimates of the flow rate suggest that up to one-third of the inflowing warm deep water leaves the shelf area below the thermocline in this deep outflow. The deep current was 1.2°C colder and 0.3 psu fresher than the inflow, but still warm, salty, and dense compared to the overlying water mass. The temperature and salinity properties suggest that the cooling and freshening process is induced by subsurface melting of glacial ice, possibly f...

Some Implications of Ekman Layer Dynamics for Cross-Shelf Exchange in the Amundsen Sea

Theexchangeofwarm,saltyseawateracrossthecontinentalshelvesoffWestAntarcticaleadstosubsurface glacial melting at the interface between the ocean and the West Antarctic Ice Sheet. One mechanism that contributesto the cross-shelf transportis Ekmantransportinduced by along-slope currentsoverthe slope and shelf break. An investigation of this process is applied to the Amundsen Sea shelfbreak region, using recently acquiredandhistoricalfielddatatoguidetheanalyses.Along-slopecurrentswereobservedattransectsacross the eastern and western reaches of the Amundsen slope. Currents in the east flowed eastward, and currents farther west flowed westward. Under the eastward-flowing currents, hydrographic isolines sloped upward paralleling the seabed. In this layer, declining buoyancy forces rather than friction were bringing the velocity to zero at the seabed.Thebasin water in the easternpart ofthe shelfwas dominatedby wateroriginating from 800‐1000-m depth off shelf, suggesting that transport of such water across the shelf frequently occurs. The authors show that arrested Ekman layers mechanism can supply deep water to the shelf break in the eastern section, where it has access to the shelf. Because no unmodified off-shelf water was found on the shelf in the western part, bottom layer Ekman transport does not appear a likely mechanism for delivery of warm deep water to the western shelf area. Warming of the warm bottom water was most pronounced on the western shelf, where the deep-water temperature increased by 0.68C during the past decade.

Tintinnid ciliates of Amundsen Sea (Antarctica) plankton communities

The Amundsen Sea has been described as one of the most productive and dynamic pelagic systems in Antarctica and is one of the least studied. Based on samples from 15 stations in the Amundsen Sea, we describe for the first time the composition of the tintinnid ciliate assemblage of the microzooplankton. We compared the species compositions of coastal polynya sites, where the phytoplankton communities are dominated by Phaeocystis, to those of the offshore deep water sites, which are dominated by diatoms. We found a total of 15 species. Polynya sites were dominated by a few species of tintinnids, mostly those endemic to the Southern Ocean. In contrast, the deep-water sites contained many widespread tintinnid species, which are known from a wide variety of systems as well as other areas of the Southern Ocean. We examined polymorphism known to characterize the Antarctic tintinnid species Cymatocylis affinis/convallaria and Codonellopsis gaussi. We found that the types or forms found appeared unrelated to the type of microplankton community, defined by the identity of the dominant phytoplankton taxa. However, the number of different morphotypes found at a site appeared related to the overall concentration of the species, suggesting that different morphologies, previously considered distinct species, may simply be developmental stages.

Morphological and Ribosomal DNA‐based Characterization of Six Antarctic Ciliate Morphospecies from the Amundsen Sea with Phylogenetic Analyses

We characterized six tintinnid ciliates from Antarctic waters using molecular markers and morphological traits: Amphorellopsis quinquealata, Codonellopsis gaussi, Cymatocylis convallaria, Cy. calyciformis, Cy. drygalskii, and Laackmanniella prolongata. The 100% similarity in SSU‐ITS1‐5.8S rDNA‐ITS2‐partial LSU rDNA sequences among Cy. convallaria, Cy. calyciformis, and Cy. drygalskii is supportive of synonymy. Codonellopsis gaussi and L. prolongata also showed high levels of similarity in SSU rDNA (99.83%) and the D2 domain of LSU rDNA (95.77%), suggesting that they are closely related. Phylogenetic analysis placed Cymatocylis in the Rhabdonellidae, Amphorellopsis in the Tintinnidae and L. prolongata/Co. gaussi within the Dictyocystidae.

Subinertial Oscillations on the Amundsen Sea Shelf, Antarctica

AbstractMooring data from the western flank of Dotson trough, Amundsen Sea shelf region, show the presence of barotropic oscillations with a period of 40–80 h. The oscillations are visible in velocity, temperature, salinity, and pressure and are comparable to tides in magnitude. The period of the oscillations corresponds to topographic Rossby waves of low group velocity and a wavelength of about 40 km, that is, the half-width of the channel. It is suggested that these resonant topographic Rossby waves cause the observed peak in the wave spectra. The observations show that sparse CTD data from this region should be treated with caution and need to be complemented with moorings or yo-yo stations in order to give a representative picture for the hydrography.