A coupled climate model simulation of the Last Glacial Maximum, Part 1: transient multi-decadal response

Abstract.A coupled atmosphere–ocean–sea ice–land surface climate system model developed at the Canadian Centre for Climate Modelling and Analysis (CCCma) is used to investigate the response to glacial ice-sheet topography and decreased CO2 representative of the last glacial maximum (LGM, roughly 21,000 years before present). Imposing these glacial boundary conditions leads to a strong and rapid climate adjustment, first in the atmosphere, and then in the ocean. We describe the model and boundary conditions and analyze the initial transient response. In a subsequent paper we analyze the near-equilibrium response. During the transient phase, surface air temperature rapidly decreases with time. The rapid cooling over the Laurentide and Fennoscandian ice sheets introduces a strong inter-hemispheric asymmetry. Sea surface temperature also decreases and after 80 years of integration, ocean surface temperature has decreased by 3 °C and global surface air temperature by 5 °C. The expected overall cooling is contrasted by marked regions of unexpected localized ocean surface warming and the retreat of sea ice in the northern North Atlantic and in the high latitude Southern Ocean. This strong transient behavior, associated with more vigorous oceanic convection at these high latitudes, represents the localization of the initial response in areas remote from the direct forcing change. The strength of the North Atlantic (Southern Ocean) overturning stream function, associated with the formation of North Atlantic Deep Water (Antarctic Bottom Water), increases from 12 to 36 Sv (2 to 26 Sv). There is a marked increase in the Antarctic Circumpolar Current transport as well (from 80 to 140 Sv), even though the wind stress decreases over the Southern Ocean, presumably as a consequence of the enhanced bottom pressure torque due to an increase in the density and the formation of Antarctic Bottom Water. Both precipitation and evaporation decrease on average with time and the change in the hydrological cycle modifies sea surface salinity (SSS). SSS increases in the Arctic Ocean due to a decrease in river runoff while it decreases in the North Atlantic due to an increase in river runoff and an increase in precipitation minus evaporation. The initial (and near equilibrium) climate response to LGM forcing in the fully coupled atmosphere–ocean climate model is quite different from that in the associated atmosphere-mixed layer ocean model due to very strong oceanic feedbacks. This is especially evident where oceanic convection is active. The anomalously warm surface temperature, especially in the Southern Ocean, and the marked increase in the overturning circulation are transient features of the response to imposed LGM boundary conditions which moderate or reverse as the system approaches equilibrium (as described in the companion paper). Other differences with the mixed layer simulation include a considerably colder tropical Pacific and the development of a La Niña-like response both of which are absent in the mixed layer simulations.

[1]  W. Peltier,et al.  Ice Age Paleotopography , 1994, Science.

[2]  A. Mcintyre,et al.  Seasonal reconstructions of the earth's surface at the last glacial maximum , 1981 .

[3]  R. Peterson,et al.  Volume Transport of the Antarctic Circumpolar Current from Bottom Pressure Measurements , 1985 .

[4]  Carl Wunsch,et al.  An estimate of global ocean circulation and heat fluxes , 1996, Nature.

[5]  Anthony J. Broccoli,et al.  A coupled model study of the Last Glacial Maximum: Was part of the North Atlantic relatively warm? , 2001 .

[6]  G. Boer,et al.  A transient climate change simulation with greenhouse gas and aerosol forcing: experimental design and comparison with the instrumental record for the twentieth century , 2000 .

[7]  C. Lorius,et al.  Vostok ice core provides 160,000-year record of atmospheric CO2 , 1987, Nature.

[8]  Norman A. McFarlane,et al.  The Effect of Orographically Excited Gravity Wave Drag on the General Circulation of the Lower Stratosphere and Troposphere , 1987 .

[9]  Richard G. Fairbanks,et al.  Tropical Temperature Variations Since 20,000 Years Ago: Modulating Interhemispheric Climate Change , 1994, Science.

[10]  Edward A. Boyle,et al.  North Atlantic thermohaline circulation during the past 20,000 years linked to high-latitude surface temperature , 1987, Nature.

[11]  Arnold L. Gordon,et al.  Interocean Exchange of Thermocline Water , 1986 .

[12]  A. Kitoh,et al.  A simulation of the Last Glacial Maximum with a coupled atmosphere‐ocean GCM , 2001 .

[13]  Carl Wunsch,et al.  Improved estimates of global ocean circulation, heat transport and mixing from hydrographic data , 2000, Nature.

[14]  A. Gordon,et al.  Southern ocean winter mixed layer , 1990 .

[15]  Timothy P. Boyer,et al.  World Ocean Atlas 1994. Volume 4. Temperature , 1994 .

[16]  P. Delecluse,et al.  Measurements within the Pacific-Indian oceans throughflow region , 1994 .

[17]  K. Bryan,et al.  A water mass model of the world ocean circulation , 1979 .

[18]  J. Houghton,et al.  Climate change 2001 : the scientific basis , 2001 .

[19]  Gokhan Danabasoglu,et al.  Sensitivity of the global ocean circulation to parameterizations of mesoscale tracer transports , 1995 .

[20]  M. Mccartney,et al.  Transequatorial Flow of Antarctic Bottom Water in the Western Atlantic Ocean: Abyssal Geostrophy at the Equator , 1993 .

[21]  S. Rintoul.  South Atlantic interbasin exchange , 1991 .

[22]  S. Philander,et al.  The climate of the Last Glacial Maximum: Results from a coupled atmosphere-ocean general circulation model , 1999 .

[23]  G. Siedler,et al.  Path of the North Atlantic Deep Water in the Brazil Basin , 1998 .

[24]  P. Gent,et al.  Isopycnal mixing in ocean circulation models , 1990 .

[25]  T. Yamagata,et al.  Pacific low‐latitude western boundary currents and the Indonesian throughflow , 1996 .

[26]  R. Pollard,et al.  Structure and transport of the Antarctic Circumpolar Current and Agulhas Return Current at 40°E , 1993 .

[27]  Donald J. Cavalieri,et al.  Arctic and Antarctic Sea Ice, 1978-1987: Satellite Passive-Microwave Observations and Analysis , 1992 .

[28]  Gregory C. Johnson,et al.  Circulation, mixing, and production of Antarctic Bottom Water , 1999 .

[29]  G. Boer,et al.  CMIP1 evaluation and intercomparison of coupled climate models , 2001 .

[30]  R. Fairbanks.  A 17,000-year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation , 1989, Nature.

[31]  A. Broccoli.  Tropical Cooling at the Last Glacial Maximum: An Atmosphere-Mixed Layer Ocean Model Simulation , 2000 .

[32]  A. Henderson‐sellers,et al.  A global archive of land cover and soils data for use in general circulation climate models , 1985 .

[33]  J. Blanchet,et al.  The Canadian Climate Centre Second-Generation General Circulation Model and Its Equilibrium Climate , 1992 .

[34]  W. Zenk,et al.  Circulation and Variability at the Southern Boundary of the Brazil Basin , 1999 .

[35]  Stefan Rahmstorf,et al.  Simulation of modern and glacial climates with a coupled global model of intermediate complexity , 1998, Nature.

[36]  E. Mosley‐Thompson,et al.  Late Glacial Stage and Holocene Tropical Ice Core Records from Huascar�n, Peru , 1995, Science.

[37]  W. Schmitz,et al.  On the North Atlantic Circulation , 1993 .

[38]  W. Cai.  Circulation driven by observed surface thermohaline fields in a coarse resolution ocean general circ , 1994 .

[39]  G. Boer,et al.  A transient climate change simulation with greenhouse gas and aerosol forcing: projected climate to the twenty-first century , 2000 .

[40]  A. Semtner.  A MODEL FOR THE THERMODYNAMIC GROWTH OF SEA ICE IN NUMERICAL INVESTIGATIONS OF CLIMATE , 1975 .

[41]  G. Boer,et al.  Warming asymmetry in climate change simulations , 2001 .

[42]  R. Dickson,et al.  The production of North Atlantic Deep Water: Sources, rates, and pathways , 1994 .

[43]  Syukuro Manabe,et al.  Transient responses of a coupled ocean-atmosphere model to gradual changes of atmospheric CO2 , 1991 .

[44]  A. Weaver,et al.  The Canadian Centre for Climate Modelling and Analysis global coupled model and its climate , 2000 .

[45]  D. Rind,et al.  Terrestrial Conditions at the Last Glacial Maximum and CLIMAP Sea-Surface Temperature Estimates: Are They Consistent? , 1985, Quaternary Research.

[46]  W. Peltier,et al.  Global water balance and atmospheric water vapour transport at last glacial maximum: climate simulations with the Canadian Climate Centre for Modelling and Analysis atmospheric general circulation model , 2000 .

[47]  Timothy P. Boyer,et al.  World Ocean Atlas 1994. Volume 3. Salinity , 1994 .