Karcher Michael

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Karcher
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Michael
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  • Article
    Arctic Ocean basin liquid freshwater storage trend 1992–2012
    (John Wiley & Sons, 2014-02-12) Rabe, Benjamin ; Karcher, Michael ; Kauker, Frank ; Schauer, Ursula ; Toole, John M. ; Krishfield, Richard A. ; Pisarev, Sergey ; Kikuchi, Takashi ; Su, J.
    Freshwater in the Arctic Ocean plays an important role in the regional ocean circulation, sea ice, and global climate. From salinity observed by a variety of platforms, we are able, for the first time, to estimate a statistically reliable liquid freshwater trend from monthly gridded fields over all upper Arctic Ocean basins. From 1992 to 2012 this trend was 600±300 km3 yr−1. A numerical model agrees very well with the observed freshwater changes. A decrease in salinity made up about two thirds of the freshwater trend and a thickening of the upper layer up to one third. The Arctic Ocean Oscillation index, a measure for the regional wind stress curl, correlated well with our freshwater time series. No clear relation to Arctic Oscillation or Arctic Dipole indices could be found. Following other observational studies, an increased Bering Strait freshwater import to the Arctic Ocean, a decreased Davis Strait export, and enhanced net sea ice melt could have played an important role in the freshwater trend we observed.
  • Article
    Arctic pathways of Pacific Water : Arctic Ocean Model Intercomparison experiments
    (John Wiley & Sons, 2016-01-08) Aksenov, Yevgeny ; Karcher, Michael ; Proshutinsky, Andrey ; Gerdes, Rudiger ; de Cuevas, Beverly ; Golubeva, Elena ; Kauker, Frank ; Nguyen, An T. ; Platov, Gennady A. ; Wadley, Martin ; Watanabe, Eiji ; Coward, Andrew C. ; Nurser, A. J. George
    Pacific Water (PW) enters the Arctic Ocean through Bering Strait and brings in heat, fresh water, and nutrients from the northern Bering Sea. The circulation of PW in the central Arctic Ocean is only partially understood due to the lack of observations. In this paper, pathways of PW are investigated using simulations with six state-of-the art regional and global Ocean General Circulation Models (OGCMs). In the simulations, PW is tracked by a passive tracer, released in Bering Strait. Simulated PW spreads from the Bering Strait region in three major branches. One of them starts in the Barrow Canyon, bringing PW along the continental slope of Alaska into the Canadian Straits and then into Baffin Bay. The second begins in the vicinity of the Herald Canyon and transports PW along the continental slope of the East Siberian Sea into the Transpolar Drift, and then through Fram Strait and the Greenland Sea. The third branch begins near the Herald Shoal and the central Chukchi shelf and brings PW into the Beaufort Gyre. In the models, the wind, acting via Ekman pumping, drives the seasonal and interannual variability of PW in the Canadian Basin of the Arctic Ocean. The wind affects the simulated PW pathways by changing the vertical shear of the relative vorticity of the ocean flow in the Canada Basin.
  • Article
    Radium isotopes across the Arctic Ocean show time scales of water mass ventilation and increasing shelf inputs
    (John Wiley & Sons, 2018-07-13) Rutgers van der Loeff, Michiel M. ; Kipp, Lauren ; Charette, Matthew A. ; Moore, Willard S. ; Black, Erin E. ; Stimac, Ingrid ; Charkin, Alexander ; Bauch, Dorothea ; Valk, Ole ; Karcher, Michael ; Krumpen, Thomas ; Casacuberta, Nuria ; Smethie, William M. ; Rember, Robert
    The first full transarctic section of 228Ra in surface waters measured during GEOTRACES cruises PS94 and HLY1502 (2015) shows a consistent distribution with maximum activities in the transpolar drift. Activities in the central Arctic have increased from 2007 through 2011 to 2015. The increased 228Ra input is attributed to stronger wave action on shelves resulting from a longer ice‐free season. A concomitant decrease in the 228Th/228Ra ratio likely results from more rapid transit of surface waters depleted in 228Th by scavenging over the shelf. The 228Ra activities observed in intermediate waters (<1,500 m) in the Amundsen Basin are explained by ventilation with shelf water on a time scale of about 15–18 years, in good agreement with estimates based on SF6 and 129I/236U. The 228Th excess below the mixed layer up to 1,500 m depth can complement 234Th and 210Po as tracers of export production, after correction for the inherent excess resulting from the similarity of 228Ra and 228Th decay times. We show with a Th/Ra profile model that the 228Th/228Ra ratio below 1,500 m is inappropriate for this purpose because it is a delicate balance between horizontal supply of 228Ra and vertical flux of particulate 228Th. The accumulation of 226Ra in the deep Makarov Basin is not associated with an accumulation of Ba and can therefore be attributed to supply from decay of 230Th in the bottom sediment. We estimate a ventilation time of 480 years for the deep Makarov‐Canada Basin, in good agreement with previous estimates using other tracers.
  • Preprint
    An assessment of Arctic Ocean freshwater content changes from the 1990s to the 2006-2008 period
    ( 2010-12-10) Rabe, Benjamin ; Karcher, Michael ; Schauer, Ursula ; Toole, John M. ; Krishfield, Richard A. ; Pisarev, Sergey ; Kauker, Frank ; Gerdes, Rudiger ; Kikuchi, Takashi
    Unprecedented summer-season sampling of the Arctic Ocean during the period 2006−2008 makes possible a quasi-synoptic estimate of liquid freshwater (LFW) inventories in the Arctic Ocean basins. In comparison to observations from 1992−1999, LFW content relative to a salinity of 35 in the layer from the surface to the 34 isohaline increased by 8400 ± 2000 km3 in the Arctic Ocean (water depth greater than 500m). This is close to the annual export of freshwater (liquid and solid) from the Arctic Ocean reported in the literature. Observations and a model simulation show regional variations in LFW were both due to changes in the depth of the lower halocline, often forced by regional wind-induced Ekman pumping, and a mean freshening of the water column above this depth, associated with an increased net sea ice melt and advection of increased amounts of river water from the Siberian shelves. Over the whole Arctic Ocean, changes in the observed mean salinity above the 34 isohaline dominated estimated changes in LFW content; the contribution to LFW change by bounding isohaline depth changes was less than a quarter of the salinity contribution, and non-linear effects due to both factors were negligible.