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  • American Meteorological Society  (4)
  • 2005-2009  (4)
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  • 1
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    Last Glacial Maximum and Holocene Climate in CCSM3 (2006)
    American Meteorological Society
    Details
    Publication Date: 2006-06-01
    Description: The climate sensitivity of the Community Climate System Model version 3 (CCSM3) is studied for two past climate forcings, the Last Glacial Maximum (LGM) and the mid-Holocene. The LGM, approximately 21 000 yr ago, is a glacial period with large changes in the greenhouse gases, sea level, and ice sheets. The mid-Holocene, approximately 6000 yr ago, occurred during the current interglacial with primary changes in the seasonal solar irradiance. The LGM CCSM3 simulation has a global cooling of 4.5°C compared to preindustrial (PI) conditions with amplification of this cooling at high latitudes and over the continental ice sheets present at LGM. Tropical sea surface temperature (SST) cools by 1.7°C and tropical land temperature cools by 2.6°C on average. Simulations with the CCSM3 slab ocean model suggest that about half of the global cooling is explained by the reduced LGM concentration of atmospheric CO2 (∼50% of present-day concentrations). There is an increase in the Antarctic Circumpolar Current and Antarctic Bottom Water formation, and with increased ocean stratification, somewhat weaker and much shallower North Atlantic Deep Water. The mid-Holocene CCSM3 simulation has a global, annual cooling of less than 0.1°C compared to the PI simulation. Much larger and significant changes occur regionally and seasonally, including a more intense northern African summer monsoon, reduced Arctic sea ice in all months, and weaker ENSO variability.
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 2
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    Response of Thermohaline Circulation to Freshwater Forcing under Present-Day and LGM Conditions (2008)
    American Meteorological Society
    Details
    Publication Date: 2008-05-15
    Description: Responses of the thermohaline circulation (THC) to freshwater forcing (hosing) in the subpolar North Atlantic Ocean under present-day and the last glacial maximum (LGM) conditions are investigated using the National Center for Atmospheric Research Community Climate System Model versions 2 and 3. Three sets of simulations are analyzed, with each set including a control run and a freshwater hosing run. The first two sets are under present-day conditions with an open and closed Bering Strait. The third one is under LGM conditions, which has a closed Bering Strait. Results show that the THC nearly collapses in all three hosing runs when the freshwater forcing is turned on. The full recovery of the THC, however, is at least a century earlier in the open Bering Strait run than the closed Bering Strait and LGM runs. This is because the excessive freshwater is diverged almost equally toward north and south from the subpolar North Atlantic when the Bering Strait is open. A significant portion of the freshwater flowing northward into the Arctic exits into the North Pacific via a reversed Bering Strait Throughflow, which accelerates the THC recovery. When the Bering Strait is closed, this Arctic to Pacific transport is absent and freshwater can only be removed through the southern end of the North Atlantic. Together with the surface freshwater excess due to precipitation, evaporation, river runoff, and melting ice in the closed Bering Strait experiments after the hosing, the removal of the excessive freshwater takes longer, and this slows the recovery of the THC. Although the background conditions are quite different between the present-day closed Bering Strait run and the LGM run, the THC responds to the freshwater forcing added in the North Atlantic in a very similar manner.
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 3
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    Climate Sensitivity of Moderate- and Low-Resolution Versions of CCSM3 to Preindustrial Forcings (2006)
    American Meteorological Society
    Details
    Publication Date: 2006-06-01
    Description: Preindustrial (PI) simulations of the Community Climate System Model version 3 (CCSM3) at two resolutions, a moderate and a low resolution, are described and compared to the standard controls for present-day (PD) simulations. Because of computational efficiency, the moderate- and low-resolution versions of CCSM3 may be appropriate for climate change studies requiring simulations of the order of hundreds to thousands of years. The PI simulations provide the basis for comparison for proxy records that represent average late Holocene conditions. When forced with PI trace gases, aerosols, and solar irradiance estimates, both resolutions have a global cooling of 1.2°–1.3°C, increased sea ice in both hemispheres, and less precipitation near the equator and at midlatitudes as compared to simulations using PD forcing. The response to PI forcings differs in the two resolutions for North Atlantic meridional overturning circulation (MOC), the Antarctic Circumpolar Current (ACC), and ENSO. The moderate-resolution CCSM3 has enhanced ACC, North Atlantic MOC, and tropical Pacific ENSO variability for PI forcings as compared to PD. The low-resolution CCSM3 with more extensive sea ice and colder climate at high northern latitudes in the PD simulation shows less sensitivity of the North Atlantic MOC to PI forcing. ENSO variability and the strength of the ACC do not increase with PI forcing in the low-resolution CCSM3.
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 4
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    Global-Scale Energy and Freshwater Balance in Glacial Climate: A Comparison of Three PMIP2 LGM Simulations (2008)
    American Meteorological Society
    Details
    Publication Date: 2008-10-01
    Description: Three coupled atmosphere–ocean general circulation model (AOGCM) simulations of the Last Glacial Maximum (LGM: about 21 000 yr before present), conducted under the protocol of the second phase of the Paleoclimate Modelling Intercomparison Project (PMIP2), have been analyzed from a viewpoint of large-scale energy and freshwater balance. Atmospheric latent heat (LH) transport decreases at most latitudes due to reduced water vapor content in the lower troposphere, and dry static energy (DSE) transport in northern midlatitudes increases and changes the intensity contrast between the Pacific and Atlantic regions due to enhanced stationary waves over the North American ice sheets. In low latitudes, even with an intensified Hadley circulation in the Northern Hemisphere (NH), reduced DSE transport by the mean zonal circulation as well as a reduced equatorward LH transport is observed. The oceanic heat transport at NH midlatitudes increases owing to intensified subpolar gyres, and the Atlantic heat transport at low latitudes increases in all models whether or not meridional overturning circulation (MOC) intensifies. As a result, total poleward energy transport at the LGM increases in NH mid- and low latitudes in all models. Oceanic freshwater transport decreases, compensating for the response of the atmospheric water vapor transport. These responses in the atmosphere and ocean make the northern North Atlantic Ocean cold and relatively fresh, and the Southern Ocean relatively warm and saline. This is a common and robust feature in all models. The resultant ocean densities and ocean MOC response, however, show model dependency.
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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