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Recent news headlines suggest the Gulf Stream current could shut down in just a few years--or perhaps a few decades--bringing about a catastrophic change in global climate. Will the Gulf […]

ABSTRACTS The Iceland Greenland Seas Project (IGP) is a coordinated atmosphere-ocean research program investigating climate processes in the source region of the densest waters of the Atlantic meridional overturning circulation. [...]

The Atlantic meridional overturning circulation (AMOC) is a system of ocean currents that has an essential role in Earth's climate, redistributing heat and influencing the carbon cycle.sup.1, 2. The AMOC has been shown to be weakening in recent years.sup.1; this decline may reflect decadal-scale variability in convection in the Labrador Sea, but short observational datasets preclude a longer-term perspective on the modern state and variability of Labrador Sea convection and the AMOC.sup.1, 3-5. Here we provide several lines of palaeo-oceanographic evidence that Labrador Sea deep convection and the AMOC have been anomalously weak over the past 150 years or so (since the end of the Little Ice Age, LIA, approximately ad 1850) compared with the preceding 1,500 years. Our palaeoclimate reconstructions indicate that the transition occurred either as a predominantly abrupt shift towards the end of the LIA, or as a more gradual, continued decline over the past 150 years; this ambiguity probably arises from non-AMOC influences on the various proxies or from the different sensitivities of these proxies to individual components of the AMOC. We suggest that enhanced freshwater fluxes from the Arctic and Nordic seas towards the end of the LIA--sourced from melting glaciers and thickened sea ice that developed earlier in the LIA--weakened Labrador Sea convection and the AMOC. The lack of a subsequent recovery may have resulted from hysteresis or from twentieth-century melting of the Greenland Ice Sheet.sup.6. Our results suggest that recent decadal variability in Labrador Sea convection and the AMOC has occurred during an atypical, weak background state. Future work should aim to constrain the roles of internal climate variability and early anthropogenic forcing in the AMOC weakening described here.Palaeoclimate records show that the Atlantic meridional overturning circulation weakened substantially at the end of the Little Ice Age, probably in response to enhanced freshwater fluxes from the Arctic and Nordic seas., Author(s): David J. R. Thornalley [sup.1] [sup.2] , Delia W. Oppo [sup.2] , Pablo Ortega [sup.3] , Jon I. Robson [sup.3] , Chris M. Brierley [sup.1] , Renee Davis [sup.1] [...]

The Arctic Ocean has an important role in Earth's climate, both through surface processes (1) such as sea-ice formation and transport, and through the production and export of waters at [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2010.07.009 Byline: Pascale Lherminier (a), Herle Mercier (a), Thierry Huck (a), Claire Gourcuff (a), Fiz F. Perez (b), Pascal Morin (c), Artem Sarafanov (d), Anastasia Falina (d) Abstract: The horizontal circulation of the subpolar gyre and the Meridional Overturning Circulation (MOC) are investigated here by comparing two snapshots of the North Atlantic as delivered by two hydrographic sections between Greenland and Portugal. The corresponding cruises were carried out in June-July 2002 and June-July 2004 on R/V Thalassa within the framework of the Ovide project. The absolute transports in June 2004 are described in detail, and then compared with transports in June 2002. The MOC (in density coordinates), driven by the volume balance between the northward North Atlantic Current (NAC) and the net southward export of dense water from the subpolar gyre, did not change: MOC.sub.I=16.3[+ or -]2.4Sv (1Sv=10.sup.6 m.sup.3 s.sup.-1). Its upper limb, above I.sub.1=32.1, is decomposed into two main branches, the Eastern NAC (ENAC) and the Western NAC (WNAC), that transport about 8Sv each. In the lower limb of the MOC, we find a 4-5Sv increase in the cyclonic circulation of the subpolar gyre between June 2002 and 2004, affecting mainly the intermediate water without changing the MOC.sub.I amplitude. Accordingly, the 14[+ or -]2Sv transport over Reykjanes Ridge in June 2004 (between 58.sup.a50'N and Iceland) is estimated to have been 4-5Sv stronger than in June 2002. Sustaining this observation, a relatively warm and salty anomaly coming from the Iceland Basin was found in the East Greenland-Irminger Current (EGIC) in June 2004, along with a modified vertical structure of the transport that shows a 4-5Sv intensification of the net southward flow in the corresponding layer. Overall, in June 2004, the EGIC (from the surface to I.sub.0=27.8) is found at 23.7[+ or -]1.4Sv in June 2004, and the Deep Western Boundary Current (DWBC) below sums up to 11.2[+ or -]1.7Sv, so that the western boundary current is [approximately equal to]5Sv stronger than in June 2002. Author Affiliation: (a) Laboratoire de Physique des Oceans, UMR6523, Ifremer, CNRS, IRD, UBO, Plouzane, France (b) Instituto de Investigaciones Marinas (CSIC), Vigo, Spain (c) Adaptation et Diversite en Milieu Marin, UMR7144, Station Biologique de Roscoff, CNRS, UPMC, Roscoff, France (d) P.P. Shirshov Institute of Oceanology, Moscow, Russia Article History: Received 9 November 2009; Revised 9 July 2010; Accepted 13 July 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2009.05.009 Byline: Susan L. Hautala (a), H. Paul Johnson (a), Douglas E. Hammond (b) Keywords: North Pacific; Thermohaline circulation; Abyssal circulation; Geothermal heating; Water masses; Inverse methods Abstract: A combination of beta spiral and minimum length inverse methods, along with a compilation of historical and recent high-resolution CTD data, are used to produce a quantitative estimate of the subthermocline circulation in Cascadia Basin. Flow in the North Pacific Deep Water, from 900-1900m, is characterized by a basin-scale anticyclonic gyre. Below 2000m, two water masses are present within the basin interior, distinguished by different potential temperature-salinity lines. These water masses, referred to as Cascadia Basin Bottom Water (CBBW) and Cascadia Basin Deep Water (CBDW), are separated by a transition zone at about 2400m depth. Below the depth where it freely communicates with the broader North Pacific, Cascadia Basin is renewed by northward flow through deep gaps in the Blanco Fracture Zone that feeds the lower limb of a vertical circulation cell within the CBBW. Lower CBBW gradually warms and returns to the south at lighter density. Isopycnal layer renewal times, based on combined lateral and diapycnal advective fluxes, increase upwards from the bottom. The densest layer, existing in the southeast quadrant of the basin below [approximately equal to]2850m, has an advective flushing time of 0.6 years. The total volume flushing time for the entire CBBW is 2.4 years, corresponding to an average water parcel residence time of 4.7 years. Geothermal heating at the Cascadia Basin seafloor produces a characteristic bottom-intensified temperature anomaly and plays an important role in the conversion of cold bottom water to lighter density within the CBBW. Although covering only about 0.05% of the global seafloor, the combined effects of bottom heat flux and diapycnal mixing within Cascadia Basin provide about 2-3% of the total required global input to the upward branch of the global thermohaline circulation. Author Affiliation: (a) University of Washington, School of Oceanography, Box 357940, Seattle, WA 98195, USA (b) University of Southern California, Department of Earth Sciences, 3651 Trousdale Pkwy, Los Angeles, CA 90089, USA Article History: Received 18 December 2008; Revised 11 May 2009; Accepted 18 May 2009