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ArticleInsights into water mass origins in the central Arctic Ocean from in-situ dissolved organic matter fluorescence(American Geophysical Union, 2021-06-27) Stedmon, Colin ; Amon, Rainer M. W. ; Bauch, Dorothea ; Bracher, Astrid ; Gonçalves-Araujo, Rafael ; Hoppmann, Mario ; Krishfield, Richard A. ; Laney, Samuel R. ; Rabe, Benjamin ; Reader, Heather ; Granskog, Mats A.The Arctic Ocean receives a large supply of dissolved organic matter (DOM) from its catchment and shelf sediments, which can be traced across much of the basin's upper waters. This signature can potentially be used as a tracer. On the shelf, the combination of river discharge and sea-ice formation, modifies water densities and mixing considerably. These waters are a source of the halocline layer that covers much of the Arctic Ocean, but also contain elevated levels of DOM. Here we demonstrate how this can be used as a supplementary tracer and contribute to evaluating ocean circulation in the Arctic. A fraction of the organic compounds that DOM consists of fluoresce and can be measured using in-situ fluorometers. When deployed on autonomous platforms these provide high temporal and spatial resolution measurements over long periods. The results of an analysis of data derived from several Ice Tethered Profilers (ITPs) offer a unique spatial coverage of the distribution of DOM in the surface 800 m below Arctic sea-ice. Water mass analysis using temperature, salinity and DOM fluorescence, can clearly distinguish between the contribution of Siberian terrestrial DOM and marine DOM from the Chukchi shelf to the waters of the halocline. The findings offer a new approach to trace the distribution of Pacific waters and its export from the Arctic Ocean. Our results indicate the potential to extend the approach to separate freshwater contributions from, sea-ice melt, riverine discharge and the Pacific Ocean.
ArticleOverview of the MOSAiC expedition: physical oceanography(University of California Press, 2022-02-07) Rabe, Benjamin ; Heuzé, Céline ; Regnery, Julia ; Aksenov, Yevgeny ; Allerholt, Jacob ; Athanase, Marylou ; Bai, Youcheng ; Basque, Chris R. ; Bauch, Dorothea ; Baumann, Till M. ; Chen, Dake ; Cole, Sylvia T. ; Craw, Lisa ; Davies, Andrew ; Damm, Ellen ; Dethloff, Klaus ; Divine, Dmitry V. ; Doglioni, Francesca ; Ebert, Falk ; Fang, Ying-Chih ; Fer, Ilker ; Fong, Allison A. ; Gradinger, Rolf ; Granskog, Mats A. ; Graupner, Rainer ; Haas, Christian ; He, Hailun ; Hoppmann, Mario ; Janout, Markus A. ; Kadko, David ; Kanzow, Torsten C. ; Karam, Salar ; Kawaguchi, Yusuke ; Koenig, Zoe ; Kong, Bin ; Krishfield, Richard A. ; Krumpen, Thomas ; Kuhlmey, David ; Kuznetsov, Ivan ; Lan, Musheng ; Laukert, Georgi ; Lei, Ruibo ; Li, Tao ; Torres-Valdes, Sinhue ; Lin, Lina ; Lin, Long ; Liu, Hailong ; Liu, Na ; Loose, Brice ; Ma, Xiaobing ; McKay, Rosalie ; Mallet, Maria ; Mallett, Robbie ; Maslowski, Wieslaw ; Mertens, Christian ; Mohrholz, Volker ; Muilwijk, Morven ; Nicolaus, Marcel ; O’Brien, Jeffrey K. ; Perovich, Donald K. ; Ren, Jian ; Rex, Markus ; Ribeiro, Natalia ; Rinke, Annette ; Schaffer, Janin ; Schuffenhauer, Ingo ; Schulz, Kirstin ; Shupe, Matthew ; Shaw, William J. ; Sokolov, Vladimir T. ; Sommerfeld, Anja ; Spreen, Gunnar ; Stanton, Timothy P. ; Stephens, Mark ; Su, Jie ; Sukhikh, Natalia ; Sundfjord, Arild ; Thomisch, Karolin ; Tippenhauer, Sandra ; Toole, John M. ; Vredenborg, Myriel ; Walter, Maren ; Wang, Hangzhou ; Wang, Lei ; Wang, Yuntao ; Wendisch, Manfred ; Zhao, Jinping ; Zhou, Meng ; Zhu, JialiangArctic Ocean properties and processes are highly relevant to the regional and global coupled climate system, yet still scarcely observed, especially in winter. Team OCEAN conducted a full year of physical oceanography observations as part of the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC), a drift with the Arctic sea ice from October 2019 to September 2020. An international team designed and implemented the program to characterize the Arctic Ocean system in unprecedented detail, from the seafloor to the air-sea ice-ocean interface, from sub-mesoscales to pan-Arctic. The oceanographic measurements were coordinated with the other teams to explore the ocean physics and linkages to the climate and ecosystem. This paper introduces the major components of the physical oceanography program and complements the other team overviews of the MOSAiC observational program. Team OCEAN’s sampling strategy was designed around hydrographic ship-, ice- and autonomous platform-based measurements to improve the understanding of regional circulation and mixing processes. Measurements were carried out both routinely, with a regular schedule, and in response to storms or opening leads. Here we present along-drift time series of hydrographic properties, allowing insights into the seasonal and regional evolution of the water column from winter in the Laptev Sea to early summer in Fram Strait: freshening of the surface, deepening of the mixed layer, increase in temperature and salinity of the Atlantic Water. We also highlight the presence of Canada Basin deep water intrusions and a surface meltwater layer in leads. MOSAiC most likely was the most comprehensive program ever conducted over the ice-covered Arctic Ocean. While data analysis and interpretation are ongoing, the acquired datasets will support a wide range of physical oceanography and multi-disciplinary research. They will provide a significant foundation for assessing and advancing modeling capabilities in the Arctic Ocean.
ArticleArctic 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.
PreprintAn 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, TakashiUnprecedented 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.