Nobre Carolina

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  • Article
    The North Icelandic Jet and its relationship to the North Icelandic Irminger Current
    (Sears Foundation for Marine Research, 2017-09-01) Pickart, Robert S. ; Spall, Michael A. ; Torres, Daniel J. ; Våge, Kjetil ; Valdimarsson, Héðinn ; Nobre, Carolina ; Moore, G. W. K. ; Jonsson, Steingrimur ; Mastropole, Dana M.
    Shipboard hydrographic and velocity sections are used to quantify aspects of the North Icelandic Jet (NIJ), which transports dense overflow water to Denmark Strait, and the North Icelandic Irminger Current (NIIC), which imports Atlantic water to the Iceland Sea. The mean transports of the two currents are comparable, in line with previous notions that there is a local overturning cell in the Iceland Sea that transforms the Atlantic water to dense overflow water. As the NIJ and NIIC flow along the north side of Iceland, they appear to share a common front when the bottom topography steers them close together, but even when they are separate there is a poleward flow inshore of the NIJ. The interannual variability in salinity of the inflowing NIIC is in phase with that of the outflowing NIJ. It is suggested, however, that the NIIC signal does not dictate that of the NIJ. Instead, the combination of liquid and solid freshwater flux from the east Greenland boundary can account for the observed net freshening of the NIIC to the NIJ for the densest half of the overturning circulation in the northwest Iceland Sea. This implies that the remaining overturning must occur in a different geographic area, consistent with earlier model results. The year-to-year variability in salinity of the NIJ can be explained by applying annual anomalies of evaporation minus precipitation over the Iceland Sea to a one-dimensional mixing model. These anomalies vary in phase with the wind stress curl over the North Atlantic subpolar gyre, which previous studies have shown drives the interannual variation in salinity of the inflowing NIIC.
  • Article
    Characteristics and transformation of Pacific winter water on the Chukchi Sea shelf in late spring
    (American Geophysical Union, 2019-10-14) Pacini, Astrid ; Moore, G. W. K. ; Pickart, Robert S. ; Nobre, Carolina ; Bahr, Frank B. ; Vage, Kjetil ; Arrigo, Kevin R.
    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.
  • Article
    Under-ice phytoplankton blooms inhibited by spring convective mixing in refreezing leads
    (John Wiley & Sons, 2018-01-07) Lowry, Kate E. ; Pickart, Robert S. ; Selz, Virginia ; Mills, Matthew M. ; Pacini, Astrid ; Lewis, Kate M. ; Joy-Warren, Hannah L. ; Nobre, Carolina ; van Dijken, Gert L. ; Grondin, Pierre-Luc ; Ferland, Joannie ; Arrigo, Kevin R.
    Spring phytoplankton growth in polar marine ecosystems is limited by light availability beneath ice-covered waters, particularly early in the season prior to snowmelt and melt pond formation. Leads of open water increase light transmission to the ice-covered ocean and are sites of air-sea exchange. We explore the role of leads in controlling phytoplankton bloom dynamics within the sea ice zone of the Arctic Ocean. Data are presented from spring measurements in the Chukchi Sea during the Study of Under-ice Blooms In the Chukchi Ecosystem (SUBICE) program in May and June 2014. We observed that fully consolidated sea ice supported modest under-ice blooms, while waters beneath sea ice with leads had significantly lower phytoplankton biomass, despite high nutrient availability. Through an analysis of hydrographic and biological properties, we attribute this counterintuitive finding to springtime convective mixing in refreezing leads of open water. Our results demonstrate that waters beneath loosely consolidated sea ice (84–95% ice concentration) had weak stratification and were frequently mixed below the critical depth (the depth at which depth-integrated production balances depth-integrated respiration). These findings are supported by theoretical model calculations of under-ice light, primary production, and critical depth at varied lead fractions. The model demonstrates that under-ice blooms can form even beneath snow-covered sea ice in the absence of mixing but not in more deeply mixed waters beneath sea ice with refreezing leads. Future estimates of primary production should account for these phytoplankton dynamics in ice-covered waters.
  • Article
    Water properties, heat and volume fluxes of Pacific water in Barrow Canyon during summer 2010
    (Elsevier, 2015-04-25) Itoh, Motoyo ; Pickart, Robert S. ; Kikuchi, Takashi ; Fukamachi, Yasushi ; Ohshima, Kay I. ; Simizu, Daisuke ; Arrigo, Kevin R. ; Vagle, Svein ; He, Jianfeng ; Ashjian, Carin J. ; Mathis, Jeremy T. ; Nishino, Shigeto ; Nobre, Carolina
    Over the past few decades, sea ice retreat during summer has been enhanced in the Pacific sector of the Arctic basin, likely due in part to increasing summertime heat flux of Pacific-origin water from the Bering Strait. Barrow Canyon, in the northeast Chukchi Sea, is a major conduit through which the Pacific-origin water enters the Arctic basin. This paper presents results from 6 repeat high-resolution shipboard hydrographic/velocity sections occupied across Barrow Canyon in summer 2010. The different Pacific water masses feeding the canyon – Alaskan coastal water (ACW), summer Bering Sea water (BSW), and Pacific winter water (PWW) – all displayed significant intra-seasonal variability. Net volume transports through the canyon were between 0.96 and 1.70 Sv poleward, consisting of 0.41–0.98 Sv of warm Pacific water (ACW and BSW) and 0.28–0.65 Sv of PWW. The poleward heat flux also varied strongly, ranging from 8.56 TW to 24.56 TW, mainly due to the change in temperature of the warm Pacific water. Using supplemental mooring data from the core of the warm water, along with wind data from the Pt. Barrow weather station, we derive and assess a proxy for estimating heat flux in the canyon for the summer time period, which is when most of the heat passes northward towards the basin. The average heat flux for 2010 was estimated to be 3.34 TW, which is as large as the previous record maximum in 2007. This amount of heat could melt 315,000 km2 of 1-meter thick ice, which likely contributed to significant summer sea ice retreat in the Pacific sector of the Arctic Ocean.
  • Article
    The relationship between patterns of benthic fauna and zooplankton in the Chukchi Sea and physical forcing
    (The Oceanography Society, 2015-09) Pisareva, Maria N. ; Pickart, Robert S. ; Iken, Katrin ; Ershova, Elizaveta A. ; Grebmeier, Jacqueline M. ; Cooper, Lee W. ; Bluhm, Bodil A. ; Nobre, Carolina ; Hopcroft, Russell R. ; Hu, Haoguo ; Wang, Jia ; Ashjian, Carin J. ; Kosobokova, Ksenia N. ; Whitledge, Terry E.
    Using data from a number of summer surveys of the Chukchi Sea over the past decade, we investigate aspects in which the benthic fauna, sediment structure, and zooplankton there are related to circulation patterns and shelf hydrographic conditions. A flow speed map is constructed that reveals the major pathways on the shelf. Regions of enhanced flow speed are dictated by lateral constrictions—in particular, Bering Strait and Barrow and Herald Canyons—and by sloping topography near coastlines. For the most part, benthic epifaunal and macrofaunal suspension feeders are found in high flow regimes, while deposit feeders are located in regions of weaker flow. The major exceptions are in Bering Strait, where benthic sampling was underrepresented, and in Herald Canyon where the pattern is inexplicably reversed. Sediment grain size is also largely consistent with variations in flow speed on the shelf. Data from three biophysical surveys of the Chukchi Sea, carried out as part of the Russian-American Long-term Census of the Arctic program, reveal close relationships between the water masses and the zooplankton communities on the shelf. Variations in atmospheric forcing, particularly wind, during the three sampling periods caused significant changes in the lateral and vertical distributions of the summer and winter water masses. These water mass changes, in turn, were reflected in the amounts and species of zooplankton observed throughout the shelf in each survey. Our study highlights the close relationship between physical drivers (wind forcing, water masses, circulation, and sediment type) in the Chukchi Sea and the biological signals in the benthos and the plankton on a variety of time scales.