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THC abstracts and comments

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  • Mike Doran
    A number of thoughts are brought on by a scanning of abstracts. Of course, as all of you know, I must comment that NONE of these scholars is looking at the
    Message 1 of 1 , May 15, 2005
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      A number of thoughts are brought on by a scanning of abstracts. Of
      course, as all of you know, I must comment that NONE of these
      scholars is looking at the ELECTRICAL implications of salinity
      changes. However, the models or studies do suggest is a number of
      things of significance as far as I am concerned:

      First, the hydrological model of salinity can be overwhelmed by
      airborne transport. IOWs, if the Atlantic is too saline, water
      certainly will sink to the depths and move to the Pacific--over huge
      timescales. But salinity differences can change because increases in
      conductivity bring about storms--that increase the fresh water
      content--WAY faster than the oceans can balance by hydrology.

      Second, wind and clouds cause THC, not just salt content. So if the
      clouds are modulated, so is the THC, as far as I am concerned.

      Third, it isn't so much that the failing of the THC in the Atlantic
      leaves more heat energy in the tropics for cyclonic activity--with
      that I wholly disagree. Rather, when you consider that in 1998 SSTs
      in the Pacific dropped 10 degF IN ONE MONTH, does the realization
      that heat is won or lost very quickly from the surface no matter
      where if there is not proper heat trapping clouds. Therefore, what
      is WAY MORE SIGNIFICANT is the increases in CONDUCTIVITY brought
      about by the decrease in salt sinking of the THC as presently
      situated. That then increases these cyclonic behaviors, as we have
      seen.

      Fourth, in terms of climate STABILITY, there is actually a stability
      in the past glaciation, as well as certainly an even greater
      stability in the present Holeocine. But they are two different
      dynamics, driven by salinity AND temperature of the oceans and the
      conductivity meaning that both sets of conditions contain. While the
      colder ocean is more resistive approximately by a percent per
      degreeF, with ice on the terresphere the ocean has more salt to
      water, and it is, therefore, more conductive. Both glaciated periods
      and less glaciatied periods have, electrically, equillibriums.

      Fifth, this is a MODULATED dynamic by the biosphere, and what is
      modulated ultimately is the earth's magnetic field. This is where
      human activity should be considered--what defects in living earth
      feedback loops are humans causing and how do these changes to
      modulating processes impact these stabilities, seen even in
      glaciations. The danger, of course, is INSTABILITY which leads to a
      Day After Tomorrow event and difficult, less stabile following
      equillibriums.

      Here are the abstracts complied by a power from the Global Warming
      yahoo group:


      Part I . Studies that demonstrate the stability of the Holocene

      The role of the thermohaline circulation in abrupt climate change
      Clark, P.U., Pisias, N.G., Stocker, T.F. and Weaver, A.J. 2002.

      The role of the thermohaline circulation in abrupt climate change.
      Nature 415: 863-869* Department of Geosciences, Oregon State
      University, Corvallis, Oregon 97331, USA † College of Oceanic and
      Atmospheric Sciences, Oregon State University, Corvallis, Oregon
      97331, USA

      ‡ Climate and Environmental Physics, University of Bern, Physics
      Institute, Sidlerstrasse 5, 3012 Bern, Switzerland
      § School of Earth and Ocean Sciences, University of Victoria, PO Box
      3055, Victoria, British Columbia V8W 3P6, Canada

      Correspondence and requests for materials should be addressed to
      P.U.C. (e-mail: clarkp@...).

      The possibility of a reduced Atlantic thermohaline circulation in
      response to increases in greenhouse-gas concentrations has been
      demonstrated in a number of simulations with general circulation
      models of the coupled ocean–atmosphere system. But it remains
      difficult to assess the likelihood of future changes in the
      thermohaline circulation, mainly owing to poorly constrained model
      parameterizations and uncertainties in the response of the climate
      system to greenhouse warming. Analyses of past abrupt climate changes
      help to solve these problems. Data and models both suggest that
      abrupt climate change during the last glaciation originated through
      changes in the Atlantic thermohaline circulation in response to small
      changes in the hydrological cycle. Atmospheric and oceanic responses
      to these changes were then transmitted globally through a number of
      feedbacks. The palaeoclimate data and the model results also indicate
      that the stability of the thermohaline circulation depends on the
      mean climate state.

      Recent modelling ideas postulate an atmosphere–ocean system during
      the last glaciation that was extremely close to a threshold, thus
      requiring very weak freshwater forcing to trigger abrupt changes of
      the THC16, 60. Whether the sequence of abrupt events originates from
      unknown periodic forcing43, 60 or instabilities and feedbacks
      associated with circum-Atlantic ice sheets16, 38 remains an open
      question. Furthermore, qualitative consistence with the
      palaeoclimatic records remains very sensitive to parameter choices in
      these models. The basic question about the origin of abrupt change is
      therefore not solved. However, all model simulations until now point
      towards the key role of the freshwater balance in the Atlantic Ocean.
      More realistic models and improved reconstructions of the various
      components of the hydrological cycle (precipitation, run-off, iceberg
      discharge, sea ice) are urgently needed.

      Paradoxically, although the THC in current models responds to
      freshwater forcings without delay, the largest deglacial meltwater
      event on record, referred to as meltwater pulse 1A (MWP-1A), occurs
      more than 1,000 years before the next significant change in the THC
      associated with the Younger Dryas cold interval39. This paradox may
      be resolved, however, if MWP-1A originated largely from the Antarctic
      Ice Sheet40, where its impact on the Atlantic THC would be
      substantially reduced.

      Draut, A.E., Raymo, M.E., McManus, J.F. and Oppo, D.W. 2003.
      Climate stability during the Pliocene warm period. Paleoceanography
      18: 10.1029/2003PA000889.

      We present a high-resolution climate record from a sediment core
      spanning an 80-ky interval of time during the mid-Pliocene epoch,
      when warmer conditions and lower global ice volume prevailed
      worldwide. Oxygen and carbon isotope analyses were made on benthic
      and planktonic foraminifera from ODP Site 981 in the North Atlantic.
      The amplitude and approximate recurrence interval of suborbital
      variations in these records are comparable to those of Holocene and
      Marine Isotope Stage 11 (MIS 11) records from the North Atlantic. We
      conclude that the mid-Pliocene warm interval was a time of relative
      climatic stability.
      These results suggest that warmer climatic conditions alone may not
      necessarily enhance variability in the climate system, a finding that
      may facilitate predictions of 21st century climatic response to
      anthropogenic warming.

      4.5. Implications for Climatic Behavior During Warm Conditions With
      Low Ice Volume [36] Our high-resolution data points to a relatively
      stable climate during the mid-Pliocene warm period, analogous to
      millennial-scale stability within the Holocene and MIS 11. Millennial-
      scale climate fluctuations appear to occur with reduced amplitude
      during warm episodes [see also Oppo et al., 1998; McManus et al.,
      1999, 2003; McIntyre et al., 2001] and our results are consistent
      with the assertion by
      McManus et al. [1999] that glacial conditions represented by a sea-
      level decrease of _20–50 m must be surpassed in
      order for large-amplitude millennial-scale variability to be evident.

      5. Summary and Conclusions [39] Analyses of d18O and d13C in benthic
      and planktonic foraminifera in an 80-kyr North Atlantic sedimentary
      sequence from the mid-Pliocene warm period reveal that although
      millennial-scale fluctuations exist in these
      records, they occur with low amplitude compared with millennial-scale
      variations that occur during glacial times.
      The mid-Pliocene warm period thus appears to have been an interval of
      relative climatic stability, similar to more
      recent warm intervals such as the Holocene or MIS 11. The Pliocene
      warm period effectively behaves like an
      extended interglacial interval. The results of this work support
      other research that has suggested that warm
      temperatures and low global ice volume may dampen high-frequency
      variations within the climate system,
      imposing relative stability on millennial timescales. These findings
      may facilitate prediction of climatic response and
      sensitivity to anthropogenic perturbations of the hydrologic cycle at
      high northern latitudes.

      Nature 409, 153 – 158 (11 January 2001); doi:10.1038/35051500

      Rapid changes of glacial climate simulated in a coupled climate
      model ANDREY GANOPOLSKI AND STEFAN RAHMSTORF
      Potsdam Institute for Climate Impact Research, PO Box 60 12 03, 14412
      Potsdam, Germany Correspondence and requests for materials should be
      addressed to A.G. (e-mail: ganopolski@...).

      Abrupt changes in climate, termed Dansgaard–Oeschger and Heinrich
      events, have punctuated the last glacial period ( 100–10 kyr ago) but
      not the Holocene (the past 10 kyr). Here we use an intermediate-
      complexity climate model to investigate the stability of glacial
      climate, and we find that only one mode of Atlantic Ocean circulation
      is stable: a cold mode with deep water formation in the Atlantic
      Ocean south of Iceland. However, a 'warm' circulation mode similar to
      the present-day Atlantic Ocean is only marginally unstable, and
      temporary transitions to this warm mode can easily be triggered. This
      leads to abrupt warm events in the model which share many
      characteristics of the observed Dansgaard–Oeschger events. For a
      large freshwater input (such as a large release of icebergs), the
      model's deep water formation is temporarily switched off, causing no
      strong cooling in Greenland but warming in Antarctica, as is observed
      for Heinrich events. Our stability analysis provides an explanation
      why glacial climate is much more variable than Holocene climate.

      Sediment-Color Record from the Northeast Atlantic Reveals Patterns of
      Millennial-Scale Climate Variability during the Past 500,000 Years

      Quaternary Research January 2002, vol. 57, no. 1, pp. 49-57(9)

      Helmke J.P. [1]; Schulz M. [2]; Bauch H.A. [3]

      [1] GEOMAR Research Center for Marine Geosciences, Wischhofstr. 1-3,
      Kiel, D-24148, Germany [2] Institute for Geosciences, University of
      Kiel, Olshausenstrasse 40, Kiel, D-24118, Germany [3] Alfred-Wegener
      Institute for Polar and Marine Research, Bremerhaven,
      Columbusstrasse, D-27568, Germany

      Abstract:

      A 500,000-yr-long deep-sea sediment-color record from the Northeast
      Atlantic was investigated to reconstruct the evolution of late
      Pleistocene climate variability on millennial time scales. Variations
      of the red–green color intensity are probably caused by climatically
      induced changes in the ice-rafted input of red-colored iron-bearing
      terrigenous material to the core site. The resolution of the age
      model impedes the detection of distinct spectral features at sub-
      Milankovitch periodicities. Hence, millennial-scale climate
      variability is quantified as time-dependent variance of the high-pass
      filtered color time series. The course of the estimated variance
      shows distinct patterns, which can be linked to continental ice mass.
      During the past 500,000 yr, large-amplitude millennial-scale climate
      variability occurs only if continental ice mass exceeds a threshold
      level, equivalent to sea level at approximately 40% of the lowering
      during the last glacial maximum. © 2002 University of Washington.

      Part II GCM studies

      doi: 10.1175/1520-0442(2000)013<1809:L>2.0.CO;2
      Journal of Climate: Vol. 13, No. 11, pp. 1809–1813.

      Tropical Stabilization of the Thermohaline Circulation in a
      Greenhouse Warming Simulation

      M. Latif, E. Roeckner, U. Mikolajewicz, and R. Voss Max-Planck-
      Institut für Meteorologie, Hamburg, Germany (Manuscript received 22
      November 1999, 23 December 1999)

      ABSTRACT

      Most global climate models simulate a weakening of the North Atlantic
      thermohaline circulation (THC) in response to enhanced greenhouse
      warming. Both surface warming and freshening in high latitudes, the
      so-called sinking region, contribute to the weakening of the THC.
      Some models even simulate a complete breakdown of the THC at
      sufficiently strong forcing. Here results are presented from a state-
      of-the-art global climate model that does not simulate a weakening of
      the THC in response to greenhouse warming. Large-scale air–sea
      interactions in the Tropics, similar to those operating during
      present-day El Niños, lead to anomalously high salinities in the
      tropical Atlantic. These are advected into the sinking region,
      thereby increasing the surface density and compensating the effects
      of the local warming and freshening.


      JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. D21, PAGES 27,335–
      27,354, 2001

      Effects of glacial meltwater in the GISS coupled atmosphere-ocean
      model, 1, North Atlantic Deep Water response
      D. Rind Goddard Institute for Space Studies at Columbia University,
      New York, New York P. deMenocal Goddard Institute for Space Studies
      at Columbia University, New York, New York G. Russell Goddard
      Institute for Space Studies at Columbia University, New York, New
      York S. Sheth Goddard Institute for Space Studies at Columbia
      University, New York, New York D. Collins Goddard Institute for Space
      Studies at Columbia University, New York, New York G. Schmidt Goddard
      Institute for Space Studies at Columbia University, New York, New
      York J. Teller Goddard Institute for Space Studies at Columbia
      University, New York, New York

      Abstract

      Varying magnitudes of freshwater input through the St. Lawrence are
      added to different versions of the GISS coupled atmosphere-ocean
      model. The results show a generally linear response in percentage
      North Atlantic Deep Water (NADW) reduction to the volume of water
      added without any obvious threshold effects. When the estimated best
      guess freshwater input for the Allerod-Younger Dryas interval is
      added, only small reductions in NADW production occur in less than a
      century, with complete cessation requiring more than 300 years.

      Comment: Study shows no threshold. Utilizes realistic quantities.

      Bleck, R. and Sun, S. 2004. Diagnostics of the oceanic thermohaline
      circulation in a coupled climate model. Global and Planetary Change
      40: 233-248.

      Excerpt from Instroduction: 1. Introduction
      The need to distinguish natural variability from manmade trends in
      the earth's climate system provides
      a driving force for the development not only of coupled atmosphere–
      ocean–land models, but also of
      tools useful for diagnosing a plethora of interaction modes in the
      coupled system. One particularly striking
      example of an interaction is the possible slowdown of the Atlantic
      branch of the oceanic meridional
      overturning circulation (MOC) during global warming.

      Such a slowdown is projected by most of today's climate models
      according to results compiled in Fig.
      9.21 of IPCC (2001). Sun and Bleck (2001b) recently reported on
      coupled simulations carried out with the GISS atmospheric
      model and a hybrid-isopycnic coordinate ocean model called HYCOM
      (Bleck, 2002). In their simulation,
      the Atlantic MOC maintains its strength as atmospheric CO2
      concentration gradually increases at
      the rate of 1% per year to twice its initial value, remaining
      constant thereafter.

      Wu, P., Wood, R. and Stott, P. 2004. Does the recent freshening
      trend in the North Atlantic indicate a weakening thermohaline
      circulation? Geophysical Research Letters 31: 10.1029/2003GL018584.

      4. Conclusions
      [9] We have analysed an ensemble of four HadCM3 simulations forced
      with all historical external (anthropogenic
      and natural) forcings. The model has reproduced a systematic
      freshening in the deep North Atlantic Ocean
      during the past four decades very similar to the reports of an
      observed freshening trend by Dickson et al. [2002] and
      Curry et al. [2003]. From the model simulations, we can trace such a
      freshening trend to the Arctic Ocean where sea
      surface salinity undergoes a large decreasing trend due to melt ice
      and river runoff during the same period. However,
      we do not find a decreasing trend of the North Atlantic THC. On the
      contrary, the THC has an upward trend
      diagnostically associated with an increased north-south upper ocean
      density gradient between the sub-polar North
      Atlantic and the mid-low latitudes.

      [10] This freshening trend does not seem to be consistent with an
      anthropogenically forced climate change scenario.
      Although climate models project a freshening trend due to intensified
      global hydrological cycle for the early part of the
      21st century under global warming, the freshening trend comes
      together with a weakening THC and a collapse of
      Labrador Sea convection. Both the observed and the simulated
      freshening for the past four decades are associated
      with the NEADW as well as the LSW. Instead of collapsing, we see an
      intensified Labrador Sea convection in both
      model and observations. In the model runs this is not associated with
      any trend in the NAO. Our analysis based
      on model simulations does not seem to support an interpretation of
      the observed freshening trend as an early signal of
      climate change due to human activities.



      Part III Alternative theories to the Hydrological/Temperature theory

      Nature 405, 775 – 778 (15 June 2000); doi:10.1038/35015531

      Significant dissipation of tidal energy in the deep ocean inferred
      from satellite altimeter data

      G. D. EGBERT* AND R. D. RAY†

      * College of Oceanic and Atmospheric Sciences, Oregon State
      University, Corvallis, Oregon 97331, USA
      † NASA Goddard Space Flight Center, Code 926, Greenbelt, Maryland
      20771, USA Correspondence and requests for materials should be
      addressed to G.D.E. (e-mail: egbert@...).

      How and where the ocean tides dissipate their energy are long-
      standing questions that have consequences ranging from the history of
      the Moon to the mixing of the oceans. Historically, the principal
      sink of tidal energy has been thought to be bottom friction in
      shallow seas. There has long been suggestive evidence, however, that
      tidal dissipation also occurs in the open ocean through the
      scattering by ocean-bottom topography of surface tides into internal
      waves, but estimates of the magnitude of this possible sink have
      varied widely. Here we use satellite altimeter data from
      Topex/Poseidon to map empirically the tidal energy dissipation. We
      show that approximately 1012 watts—that is, 1 TW, representing 25–30%
      of the total dissipation—occurs in the deep ocean, generally near
      areas of rough topography. Of the estimated 2 TW of mixing energy
      required to maintain the large-scale thermohaline circulation of the
      ocean, one-half could therefore be provided by the tides, with the
      other half coming from action on the surface of the ocean.


      OCEANOGRAPHY:
      What Is the Thermohaline Circulation?
      Carl Wunsch*
      The author is in the Department of Earth, Atmospheric and Planetary
      Sciences, Massachusetts Institute of Technology, Cambridge, MA 01239,
      USA. E-mail: cwunsch@...

      Excerpt:

      If the flow is integrated zonally in the ocean (see the figure), one
      notices what is best called a meridional overturning circulation
      (MOC) (3). Features such as the Gulf Stream mass flux at high
      latitudes that is associated, at least loosely, with regions of
      severe heat loss to the atmosphere. In these regions, the fluid
      becomes dense and convectively unstable; the downward flux and
      subsequent lateral flow thus appear to be driven by thermal and
      evaporative forcing from the atmosphere. The ocean seems to act like
      a heat engine, in analogy to the atmosphere. Some authors apparently
      think of this convective mode of motion as the thermohaline
      circulation. But results of the past few years suggest that such a
      convectively driven mass flux is impossible. There are several lines
      of argument. The first goes back to Sandström (4), who pointed out
      that when a fluid is heated and cooled at the same pressure (or
      heated at a lower pressure), no significant work can be extracted
      from the flow, with the region below the cold source becoming
      homogeneous.

      . . .

      The conclusion from this and other lines of evidence is that the
      ocean's mass flux is sustained primarily by the wind, and secondarily
      by tidal forcing. Both in models and the real ocean, surface buoyancy
      boundary conditions strongly influence the transport of heat and
      salt, because the fluid must become dense enough to sink, but these
      boundary conditions do not actually drive the circulation.

      . . .

      Timmermann, A. and Goosse, H. 2004. Is the wind stress forcing
      essential for the meriodional overturning circulation? Geophysical
      Research Letters 31: 10.1029/2003GL018777.

      Abstract

      We use a global coupled atmosphere-ocean sea-ice model of
      intermediate complexity to demonstrate that wind-forcing is a crucial
      element to sustain meridional overturning flow in the Atlantic.
      Neglecting wind-stress in our multi-century-long simulations leads to
      a complete shutdown of the conveyor belt circulation. This result may
      have tremendous impacts for an assessment of the sensitivity of 2-d
      climate models which typically do not capture wind-driven gyres. It
      is argued that wind effects may be a key element in determining the
      fate and length of a collapsed THC state. Possible paleo implications
      will be discussed.


      GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 6, 1329,
      doi:10.1029/2002GL016564, 2003

      Freshwater teleconnections and ocean thermohaline circulation

      Dan Seidov and Bernd J. Haupt Environment Institute, Pennsylvania
      State University, University Park, Pennsylvania, USA Received 5
      November 2002; accepted 16 January 2003; published 27 March 2003
      .
      [1] Asymmetry of the Atlantic and Pacific sea surface salinity (SSS)
      is recognized as an important element of the
      global ocean thermohaline circulation. However, a threshold of such
      asymmetry that may trigger a true global deepocean
      conveyor has not yet been examined. A combined effect of the Atlantic-
      Pacific and the Southern Ocean
      surface salinity asymmetries also has not yet been clearly shown. We
      address these issues and conclude that Atlantic-
      Pacific SSS asymmetry is one of the most critical elements for
      maintaining the global ocean conveyor. Our experiments
      suggest, albeit preliminary, that high-latitudinal freshwater
      impacts, as a mechanism of altering global ocean
      thermohaline circulation, may be less effective than interbasin
      freshwater communications.


      Atlantic deep circulation controlled by freshening in the Southern
      Ocean

      Oleg A. Saenko and Andrew J. Weaver
      School of Earth and Ocean Sciences, University of Victoria, Victoria,
      Canada

      Andreas Schmittner
      Institut fu¨ r Geowissenschaften, Universita¨t Kiel, Kiel, Germany

      GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 14, 1754,
      doi:10.1029/2003GL017681, 2003

      From the Conclusions:

      [19] Our results are in line with results of Stocker et al. [1992]
      and more recent results of Seidov and Haupt [2003]
      on the effect of salinity contrast between the North Atlantic and the
      Southern Ocean on the Atlantic MOC. As we show,
      however, the magnitude of this salinity contrast can, in turn, be
      controlled by the ocean circulation itself through a
      positive feedback between the circulation and salinity. [20] Our
      results may have implications for understanding
      the response of the Atlantic MOC in global warming scenarios. It has
      often been pointed out that in a warmer
      climate, an intensified hydrological cycle would weaken the MOC by
      transporting more moisture northward. Our results
      suggest that the intensified hydrological cycle could also tend to
      stabilize the MOC by transporting more moisture
      southward.

      Part IV Odds and Ends

      Reconstructing, Monitoring, and Predicting Multidecadal-Scale Changes
      in the North Atlantic Thermohaline Circulation with Sea Surface
      Temperature

      M. Latif
      Institut für Meereskunde, Kiel, Germany E. Roeckner, M. Botzet, M.
      Esch, H. Haak, S. Hagemann, J. Jungclaus, S. Legutke, S. Marsland,
      and U. Mikolajewicz Max-Planck-Institut für Meteorologie, Hamburg,
      Germany J. Mitchell Met Office, Hadley Centre for Climate Prediction
      and Research, Bracknell, Berkshire, United Kingdom (Manuscript
      received 26 November 2002, in final form 17 October 2003)

      ABSTRACT
      Sea surface temperature (SST) observations in the North Atlantic
      indicate the existence of strong multidecadal variability with a
      unique spatial structure. It is shown by means of a new global
      climate model, which does not employ flux adjustments, that the
      multidecadal SST variability is closely related to variations in the
      North Atlantic thermohaline circulation (THC). The close
      correspondence between the North Atlantic SST and THC variabilities
      allows, in conjunction with the dynamical inertia of the THC, for the
      prediction of the slowly varying component of the North Atlantic
      climate system. It is shown additionally that past variations of the
      North Atlantic THC can be reconstructed from a simple North Atlantic
      SST index and that future, anthropogenically forced changes in the
      THC can be easily monitored by observing SSTs. The latter is
      confirmed by another state-of-the-art global climate model. Finally,
      the strong multidecadal variability may mask an anthropogenic signal
      in the North Atlantic for some decades.


      PALEOCEANOGRAPHY, VOL. 18, NO. 4, 1079, doi:10.1029/2002PA000840, 2003

      Meltwater flooding events in the Gulf of Mexico revisited:
      Implications for rapid climate changes during the last deglaciation
      Paul Aharon Department of Geological Sciences, University of Alabama,
      Tuscaloosa, Alabama, USA

      Abstract

      North American freshwater runoff records have been used to support
      the case that climate flickers were caused by shutdowns of the ocean
      thermohaline circulation (THC) resulting from reversals of meltwater
      discharges. Inconsistencies in the documentation of these meltwater
      switches, however, continue to fuel the debate on the cause/s of the
      oscillatory nature of the deglacial climate. New oxygen and carbon
      isotope records from the northern Gulf of Mexico depict in
      exceptional detail the succession of meltwater floods and pauses
      through the southern routing during the interval 16 to 8.9 ka (14C
      years BP; ka, kiloannum). The records underscore the bimodal role
      played by the Gulf of Mexico as a destination of meltwater discharges
      from the receding Laurentide Ice Sheet. The evidence indicates that
      the Gulf of Mexico acted as the principal source of superfloods at
      13.4, 12.6, and 11.9 ka that reached the North Atlantic and
      contributed significantly to density stratification, disruption of
      ocean ventilation, and cold reversals. Gulf of Mexico lapsed into
      a "relief valve" position in post-Younger Dryas time, when meltwater
      discharges were rerouted south at 9.9, 9.7, 9.4, and 9.1 ka, thus
      temporarily interrupting North Atlantic-bound freshwater discharges
      from Lake Agassiz. The history of meltwater events in the Gulf of
      Mexico contradicts the model that meltwater flow via the eastern
      outlets into the North Atlantic disrupted the ocean THC, causing
      cooling, while diversions to the Gulf of Mexico via the Mississippi
      River enhanced THC and warming.
      Received 20 August 2002; accepted 9 July 2003; published 8 October
      2003.


      Decadal variability in the outflow from the Nordic seas to the deep
      Atlantic Ocean

      SHELDON BACON1

      Room 256/43, Southampton Oceanography Centre, Empress Dock,
      Southampton SO14 3ZH, UK Correspondence and requests for materials
      should be addressed to the author (e-mail: S.Bacon@...).
      Nature 394, 871 – 874 (27 August 1998); doi:10.1038/29736

      The global thermohaline circulation is the oceanic overturning mode,
      which is manifested in the North Atlantic Ocean as northward-flowing
      surface waters which sink in the Nordic (Greenland, Iceland and
      Norwegian) seas and return southwards—after overflowing the Greenland–
      Scotland ridge—as deep water. This process has been termed
      the 'conveyor belt', and is believed to keep Europe 5–8 °C warmer
      than it would be if the conveyor were to shut down. The variability
      of today's conveyor belt is therefore an important component of
      climate regulation. The Nordic seas are the only Northern Hemisphere
      source of deep water and a previous study has revealed no long-term
      variability in the outflow of deep water from the Nordic seas to the
      Atlantic Ocean. Here I use flows derived from hydrographic data to
      show that this outflow has approximately doubled, and then returned
      to previous values, over the past four decades. I present evidence
      which suggests that this variability is forced by variability in
      polar air temperature, which in turn may be connected to the recently
      reported Arctic warming.
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