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ArticleIrminger Current Anticyclones in the Labrador Sea observed in the hydrographic record, 1990-2004(Sears Foundation for Marine Research, 2009-05) Rykova, Tatiana A. ; Straneo, Fiamma ; Lilly, Jonathan M. ; Yashayaev, IgorA significant fraction of the lateral heat transport into the Labrador Sea's interior, needed to balance the net heat loss to the atmosphere, is attributed to the Irminger Current Anticyclones. These mesoscale eddies advect warm, salty boundary current water, of subtropical origin, from the boundary current to the interior— but when or how they release their anomalous heat content has not been previously investigated. In this study, we discuss the seasonal and interannual evolution of these anticyclones as inferred from the analysis of hydrographic data from the Labrador Sea from 1990 to 2004. The 29 identified anticyclones fall into two categories, which we refer to as unconvected and convected. Unconvected anticyclones have properties that are close to those of the boundary current, including a fresh surface layer, and they are found near the boundaries and never observed in winter. Convected anticyclones, on the other hand, contain a mixed layer, lack a freshwater cap and are observed throughout the year. Using a one-dimensional mixing model, it is shown that the convected eddies are those Irminger Current Anticyclones that have been modified by the large winter buoyancy loss of the region. This provides evidence that such eddies can survive the strong winter buoyancy loss in the Labrador Sea and that their anomalous heat and salt content is not trivially mixed into the Sea's interior. Finally, we observe a clear trend in the eddies' properties toward warmer and saltier conditions after 1997 reflecting changes in the source waters and the reduced atmospheric forcing over the Labrador Sea.
ArticleRole of Greenland freshwater anomaly in the recent freshening of the subpolar North Atlantic(American Geophysical Union, 2019-04-26) Dukhovskoy, Dmitry S. ; Yashayaev, Igor ; Proshutinsky, Andrey ; Bamber, Jonathan L. ; Bashmachnikov, Igor ; Chassignet, Eric P. ; Lee, Craig M. ; Tedstone, AndrewThe cumulative Greenland freshwater flux anomaly has exceeded 5,000 km3 since the 1990s. The volume of this surplus freshwater is expected to cause substantial freshening in the North Atlantic. Analysis of hydrographic observations in the subpolar seas reveals freshening signals in the 2010s. The sources of this freshening are yet to be determined. In this study, the relationship between the surplus Greenland freshwater flux and this freshening is tested by analyzing the propagation of the Greenland freshwater anomaly and its impact on salinity in the subpolar North Atlantic based on observational data and numerical experiments with and without the Greenland runoff. A passive tracer is continuously released during the simulations at freshwater sources along the coast of Greenland to track the Greenland freshwater anomaly. Tracer budget analysis shows that 44% of the volume of the Greenland freshwater anomaly is retained in the subpolar North Atlantic by the end of the simulation. This volume is sufficient to cause strong freshening in the subpolar seas if it stays in the upper 50–100 m. However, in the model the anomaly is mixed down to several hundred meters of the water column resulting in smaller magnitudes of freshening compared to the observations. Therefore, the simulations suggest that the accelerated Greenland melting would not be sufficient to cause the observed freshening in the subpolar seas and other sources of freshwater have contributed to the freshening. Impacts on salinity in the subpolar seas of the freshwater transport through Fram Strait and precipitation are discussed.
ArticleTime series measurements of transient tracers and tracer-derived transport in the Deep Western Boundary Current between the Labrador Sea and the subtropical Atlantic Ocean at Line W(John Wiley & Sons, 2016-11-10) Smith, John N. ; Smethie, William M. ; Yashayaev, Igor ; Curry, Ruth G. ; Azetsu-Scott, KumikoTime series measurements of the nuclear fuel reprocessing tracer 129I and the gas ventilation tracer CFC-11 were undertaken on the AR7W section in the Labrador Sea (1997–2014) and on Line W (2004–2014), located over the US continental slope off Cape Cod, to determine advection and mixing time scales for the transport of Denmark Strait Overflow Water (DSOW) within the Deep Western Boundary Current (DWBC). Tracer measurements were also conducted in 2010 over the continental rise southeast of Bermuda to intercept the equatorward flow of DSOW by interior pathways. The Labrador Sea tracer and hydrographic time series data were used as input functions in a boundary current model that employs transit time distributions to simulate the effects of mixing and advection on downstream tracer distributions. Model simulations of tracer levels in the boundary current core and adjacent interior (shoulder) region with which mixing occurs were compared with the Line W time series measurements to determine boundary current model parameters. These results indicate that DSOW is transported from the Labrador Sea to Line W via the DWBC on a time scale of 5–6 years corresponding to a mean flow velocity of 2.7 cm/s while mixing between the core and interior regions occurs with a time constant of 2.6 years. A tracer section over the southern flank of the Bermuda rise indicates that the flow of DSOW that separated from the DWBC had undergone transport through interior pathways on a time scale of 9 years with a mixing time constant of 4 years.
ArticleSubpolar North Atlantic western boundary density anomalies and the Meridional Overturning Circulation(Nature Research, 2021-05-24) Li, Feili ; Lozier, M. Susan ; Bacon, Sheldon ; Bower, Amy S. ; Cunningham, Stuart A. ; de Jong, Marieke F. ; deYoung, Brad ; Fraser, Neil ; Fried, Nora ; Han, Guoqi ; Holliday, Naomi Penny ; Holte, James W. ; Houpert, Loïc ; Inall, Mark E. ; Johns, William E. ; Jones, Sam ; Johnson, Clare ; Karstensen, Johannes ; Le Bras, Isabela A. ; Lherminier, Pascale ; Lin, Xiaopei ; Mercier, Herlé ; Oltmanns, Marilena ; Pacini, Astrid ; Petit, Tillys ; Pickart, Robert S. ; Rayner, Darren ; Straneo, Fiamma ; Thierry, Virginie ; Visbeck, Martin ; Yashayaev, Igor ; Zhou, ChunChanges in the Atlantic Meridional Overturning Circulation, which have the potential to drive societally-important climate impacts, have traditionally been linked to the strength of deep water formation in the subpolar North Atlantic. Yet there is neither clear observational evidence nor agreement among models about how changes in deep water formation influence overturning. Here, we use data from a trans-basin mooring array (OSNAP—Overturning in the Subpolar North Atlantic Program) to show that winter convection during 2014–2018 in the interior basin had minimal impact on density changes in the deep western boundary currents in the subpolar basins. Contrary to previous modeling studies, we find no discernable relationship between western boundary changes and subpolar overturning variability over the observational time scales. Our results require a reconsideration of the notion of deep western boundary changes representing overturning characteristics, with implications for constraining the source of overturning variability within and downstream of the subpolar region.
ArticleTracking Labrador Sea Water property signals along the Deep Western Boundary Current(John Wiley & Sons, 2017-07-03) Le Bras, Isabela A. ; Yashayaev, Igor ; Toole, John M.Observations of the Deep Western Boundary Current (DWBC) at Line W on the western North Atlantic continental slope southeast of Cape Cod from 1995 to 2014 reveal water mass changes that are consistent with changes in source water properties upstream in the Labrador Sea. This is most evident in the cold, dense, and deep class of Labrador Sea Water (dLSW) that was created and progressively replenished and deepened by recurring winter convection during the severe winters of 1987–1994. The arrival of this record cold, fresh, and low potential vorticity anomaly at Line W lags its formation in the Labrador Sea by 3–7 years. Complementary observations along the path of the DWBC provide further evidence that this anomaly is advected along the boundary and indicate that stirring between the boundary and the interior intensifies south of the Flemish Cap. Finally, the consistency of the data with realistic advective and mixing time scales is assessed using the Waugh and Hall (2005) model framework. The data are found to be best represented by a mean transit time of 5 years from the Labrador Sea to Line W, with a leading order role for both advection by the DWBC and mixing between the boundary flow and interior waters.
ArticleArgo data 1999-2019: two million temperature-salinity profiles and subsurface velocity observations from a global array of profiling floats.(Frontiers Media, 2020-09-15) Wong, Annie P. S. ; Wijffels, Susan E. ; Riser, Stephen C. ; Pouliquen, Sylvie ; Hosoda, Shigeki ; Roemmich, Dean ; Gilson, John ; Johnson, Gregory C. ; Martini, Kim I. ; Murphy, David J. ; Scanderbeg, Megan ; Udaya Bhaskar, T. V. S. ; Buck, Justin J. H. ; Merceur, Frederic ; Carval, Thierry ; Maze, Guillaume ; Cabanes, Cécile ; André, Xavier ; Poffa, Noé ; Yashayaev, Igor ; Barker, Paul M. ; Guinehut, Stéphanie ; Belbeoch, Mathieu ; Ignaszewski, Mark ; Baringer, Molly O. ; Schmid, Claudia ; Lyman, John ; McTaggart, Kristene E. ; Purkey, Sarah G. ; Zilberman, Nathalie ; Alkire, Matthew ; Swift, Dana ; Owens, W. Brechner ; Jayne, Steven R. ; Hersh, Cora ; Robbins, Pelle E. ; West-Mack, Deb ; Bahr, Frank B. ; Yoshida, Sachiko ; Sutton, Philip J. H. ; Cancouët, Romain ; Coatanoan, Christine ; Dobbler, Delphine ; Garcia Juan, Andrea ; Gourrion, Jérôme ; Kolodziejczyk, Nicolas ; Bernard, Vincent ; Bourlès, Bernard ; Claustre, Hervé ; d’Ortenzio, Fabrizio ; Le Reste, Serge ; Le Traon, Pierre-Yves ; Rannou, Jean-Philippe ; Saout-Grit, Carole ; Speich, Sabrina ; Thierry, Virginie ; Verbrugge, Nathalie ; Angel-Benavides, Ingrid M. ; Klein, Birgit ; Notarstefano, Giulio ; Poulain, Pierre Marie ; Vélez-Belchí, Pedro ; Suga, Toshio ; Ando, Kentaro ; Iwasaska, Naoto ; Kobayashi, Taiyo ; Masuda, Shuhei ; Oka, Eitarou ; Sato, Kanako ; Nakamura, Tomoaki ; Sato, Katsunari ; Takatsuki, Yasushi ; Yoshida, Takashi ; Cowley, Rebecca ; Lovell, Jenny L. ; Oke, Peter ; van Wijk, Esmee ; Carse, Fiona ; Donnelly, Matthew ; Gould, W. John ; Gowers, Katie ; King, Brian A. ; Loch, Stephen G. ; Mowat, Mary ; Turton, Jon ; Pattabhi Rama Rao, Eluri ; Ravichandran, M. ; Freeland, Howard ; Gaboury, Isabelle ; Gilbert, Denis ; Greenan, Blair J. W. ; Ouellet, Mathieu ; Ross, Tetjana ; Tran, Anh ; Dong, Mingmei ; Liu, Zenghong ; Xu, Jianping ; Kang, KiRyong ; Jo, HyeongJun ; Kim, Sung-Dae ; Park, Hyuk-MinIn the past two decades, the Argo Program has collected, processed, and distributed over two million vertical profiles of temperature and salinity from the upper two kilometers of the global ocean. A similar number of subsurface velocity observations near 1,000 dbar have also been collected. This paper recounts the history of the global Argo Program, from its aspiration arising out of the World Ocean Circulation Experiment, to the development and implementation of its instrumentation and telecommunication systems, and the various technical problems encountered. We describe the Argo data system and its quality control procedures, and the gradual changes in the vertical resolution and spatial coverage of Argo data from 1999 to 2019. The accuracies of the float data have been assessed by comparison with high-quality shipboard measurements, and are concluded to be 0.002°C for temperature, 2.4 dbar for pressure, and 0.01 PSS-78 for salinity, after delayed-mode adjustments. Finally, the challenges faced by the vision of an expanding Argo Program beyond 2020 are discussed.
ArticleAbsolute velocity along the AR7W section in the Labrador Sea(Elsevier B.V., 2012-11-19) Hall, Melinda M. ; Torres, Daniel J. ; Yashayaev, IgorNearly every spring since 1990, hydrographic data have been collected along a section in the Labrador Sea known as AR7W. Since 1995, lowered acoustic doppler current profiler (LADCP) data have also been collected. In this work we use data from six of these sections, spanning the time period 1995 through 2008, to determine absolute velocity across AR7W and analyze the main features of the general circulation in the area. We find that absolute velocity fields are characterized by strong, nearly barotropic flows all along the section, meaning there is no “level of no motion” for geostrophic velocity calculations. There is strong variability from year to year, especially in the strength of the boundary currents at each end; nevertheless, combining data from.all 6 sections yields a well-organized velocity field resembling that presented by Pickart and Spall (2007), except that our velocities tend to be stronger: there is a cyclonic boundary current system with offshore recirculations at both ends of the line; the interior is filled with virtually uniform, top-to-bottom bands of velocity with alternating signs. At the southwestern end of the section, the LADCP data reveal a dual core of the Labrador Current at times when horizontal resolution is adequate. At the northeastern end, the location of the recirculation offshore of the boundary current is bimodal, and hence the apparent width of the boundary current is bimodal as well. In the middle of the section, we have found a bottom current carrying overflow waters along the Northwest Atlantic Mid-Ocean Channel, suggesting one of various possible fast routes for those waters to reach the central Labrador Sea. We have used the hydrographic data to compute geostrophic velocities, referenced to the LADCP profiles, as well as to compute ocean heat transport across AR7W for four of our sections. For all but one year, these fluxes are comparable to the mean air–sea heat flux that occurs between AR7W and Davis Strait from December to May (O(50–80 TW)), and much larger than the annual average values (O(10–20 TW)).
ArticleTime scales of the Greenland freshwater anomaly in the subpolar North Atlantic(American Meteorological Society, 2021-10-15) Dukhovskoy, Dmitry S. ; Yashayaev, Igor ; Chassignet, Eric P. ; Myers, Paul G. ; Platov, Gennady A. ; Proshutinsky, AndreyThe impact of increasing Greenland freshwater discharge on the subpolar North Atlantic (SPNA) remains unknown as there are uncertainties associated with the time scales of the Greenland freshwater anomaly (GFWA) in the SPNA. Results from numerical simulations tracking GFWA and an analytical approach are employed to estimate the response time, suggesting that a decadal time scale (13 years) is required for the SPNA to adjust for increasing GFWA. Analytical solutions obtained for a long-lasting increase of freshwater discharge show a non-steady-state response of the SPNA with increasing content of the GFWA. In contrast, solutions for a short-lived pulse of freshwater demonstrate different responses of the SPNA with a rapid increase of freshwater in the domain followed by an exponential decay after the pulse has passed. The derived theoretical relation between time scales shows that residence time scales are time dependent for a non-steady-state case and asymptote the response time scale with time. The residence time of the GFWA deduced from Lagrangian experiments is close to and smaller than the response time, in agreement with the theory. The Lagrangian analysis shows dependence of the residence time on the entrance route of the GFWA and on the depth. The fraction of the GFWA exported through Davis Strait has limited impact on the interior basins, whereas the fraction entering the SPNA from the southwest Greenland shelf spreads into the interior regions. In both cases, the residence time of the GFWA increases with depth demonstrating long persistence of the freshwater anomaly in the subsurface layers.