Continental Hydrology, Rapid Climate Continental Hydrology, Rapid Climate Change, and the Intensity of the Change, and the Intensity of the

Atlantic MOC: Insights from Atlantic MOC: Insights from PaleoclimatologyPaleoclimatology

W.R. PeltierW.R. Peltier

Department of PhysicsDepartment of Physics

University of TorontoUniversity of Toronto

WOCE derived N-S salinity section through the Atlantic Ocean Basin

From McManus et al. (Nature, 2004)

North Americanice cover andpro-glacial lakesjust prior to theonset of theYounger-Dryascold reversal

OutlineOutline Motivation: Although the origin of the Younger-Dryas cooling Motivation: Although the origin of the Younger-Dryas cooling

event is widely understood to have been due to a sharp event is widely understood to have been due to a sharp reduction in the strength of the Atlantic MOC, there is no reduction in the strength of the Atlantic MOC, there is no consensus as to the geographical routing of the runoff from the consensus as to the geographical routing of the runoff from the continents that was the ultimate cause.continents that was the ultimate cause.

Constraining models of continental deglaciation: geological, Constraining models of continental deglaciation: geological, geophysical and geodetic observations.geophysical and geodetic observations.

Runoff routing during North American deglaciation: detailed Runoff routing during North American deglaciation: detailed ice mechanical reconstructions of ice sheet evolution—AN ice mechanical reconstructions of ice sheet evolution—AN ARCTIC TRIGGER FOR THE Y-D.ARCTIC TRIGGER FOR THE Y-D.

Arctic vs. Atlantic “hosing” and the shutdown of the MOC.Arctic vs. Atlantic “hosing” and the shutdown of the MOC. Summary Summary

Quantitative Constraints onContinental Scale Ice Sheet Reconstructions

In the Torontodata base ofpost glacial RSLhistories there are several hundred 14C dated Holocene time series that may be employedto calibrate a model of continental scale ice sheet evolution, as well as, from A. Dyke, a refined data set of margin positions

From PhysicsToday, 2002

The present dayrate of global sea level risepredicted toexist as a consequence ofglacial isostaticadjustmentalone, bothincluding (a) and excluding (b) the influenceof rotationalfeedback. Thedifference is shown in (c) and geoid height timedependence in(d).

The dual satellite GravityRecovery and ClimateExperiment ( GRACE )is now in space and isexpected to providefundamental insights intothe magnitude and spatialvariability of the modern rate of global sea level rise.

Space Geodetic constraints on ice Space Geodetic constraints on ice thicknessthickness

Absolute Gravity ConstraintsAbsolute Gravity Constraints

The sedimentcover mapemployed in theice sheet modellingis one developed for seismologicalapplications by Laske and Mastersat Scripps

The Data Constrained ModelOf the Evolution of the North American Ice Sheet Complex

This is fromthe recentlypublishedanalyses ofTarasov andPeltier (2004,QSR 23, 359-388) in whichan ensemble ofPMIP LGM runs were employed, together with the GRIP Del18O record and NCEP data, to force the ISM

The three domedstructure impliedby the analysesof Dyke and Prest is obtainedas a consequenceof the geographicaldisribution of thesurface sediment which enables fastflow to occur inregions such asHudson Strait which is known tohave been inhabited by anintense ice stream.

It is the geodetic data derived from the green points that provide the necessaryconstraints over thecontinental interior.Coupled with actionof the basalsediment cover themodel then deliversa well definedKeewatin Dome

The presence ofthis KeewatinDome of theLGM LIS wasfirst suggestedin the analyses Art Dyke andVictor Prest ofthe GeologicalSurvey ofCanada in 1987.

Modelling the Run-off ofMeltwater from the Continents

The main requirementsfor an accuratecomputation of therunoff of meltwaterfom the continentduring deglaciation include (1) an accuratedigital elevation model for modern topography(that shown is the USGS hydrologically selfconsistent model), and(2) an accurate modelof the glacial isostaticadjustment process, asdiscussed previously,

The main result of the detailed runoffcomputation is theprediction of the occurrence of anintense pulse ofMeltwater into theArctic Oceanprecisely at the timeof onset of theYounger-Dryas coldreversal! This is fromTarasov and Peltier,Nature, June 2, 2005.

The impact of an Arctic meltwater pulse upon the strength of the Atlantic thermohaline circulation

Spin-up tostatistical equilibriumof the NCAR CSM1.4under both modernand LGM boundaryconditions. The LGMrun employs Vostocktrace gases, ICE-4Gtopography and LGMorbital forcing

An initial test of the NCAR CSM model.Modern and LGMEquilibria of the Atlantic MOC. TheLGM equilibrium isPredicted to have been~40% weaker thanModern, in accord withThe recently publishedPa/Th isotopic Measurements ofMcManus et al, 2004.See Peltier and Solheim, 2004, QSR 23.

MODMMOD

LGM

DIFF

Results of the CMIP/PMIP Water hosing experiments in CSM1.4

This is from Peltier, Stastna and Vettoretti,submission to the“water hosing” intercomprison tests beingconducted under theauspices of the WCRP.Part of a paper nowsubmitted to the Journalof Climate. The unperturbed state isModern.

It is clearly importantthat the model properly accounts for the known regions ofdeep water formationin the GIN Seas and the Labrador Sea in theabsence of the appliedfreshwater forcing.

Note that thewestern boundaryundercurrent, which is driven bythe NADW formation process,is eliminated by thesurface fresheningof the N Atlanticwhereas the winddriven Gulf Streampersists.

Suppose, ratherthan fresheningthe North Atlanticat 1 Sv for a periodof 100 years, wewere to applyexactly the sameforcing to theArctic Ocean in theBeaufort Sea, assuggested by theexplicit analysisof continentalrunoff ?

Scaled results forthe strength of the MOC and forsea ice concentration forAtlantic hosing at1 Sv for 100 yearsand for Arctic hosing for the same period of time atthe same strength.Note: results areessentially identical!

The point here is thatjust as the NorthAtlantic Drift willtransport a surfacefreshwater anomalyinto the region of deep water formationif this anomaly is imposed to the south ofGreenland, so the Trans-Polar Drift will transporta fresh water anomaly added to the surface of the Arctic Ocean through Fram Strait andonto the region in whichdeep water would otherwise form.

The End