85th AMS Annual Meeting

Monday, 10 January 2005: 9:45 AM
Very High Resolution Modeling Studies of the Last Glacial Maximum
Tom J Crowley, Duke University, Durham, NC; and S. J. Kim, W. T. H. Hyde, D. J. Erickson, and P. Duffy
2004 marks the 30th anniversary of the first GCM study of the last glacial maximum. Much has been done since that time, and Suki Manabe in particular played a key role in the early stages of modeling by outlining some fundamental responses of the system that are as valid today as the day they were published. To date almost all global modeling studies have been limited to T42 resolution – a resolution that prohibits satisfactory resolution of some atmospheric dynamical features, especially where orography is involved. We have simulated the ice age atmospheric circulation at a T170 resolution (~75 km) using the NCAR CCM3 model. A number of very interesting features are being discovered – which at the time of submission of this abstract have only been partially analyzed. For example, there is not only a split of the jet around the huge Laurentide Ice Sheet, but a “companion” upper level low develops south of the Laurentide “block”. This configuration leads to very deep incursions of midlatitude air into the tropics – in the DJF mean the trough dips to the Nicaragua/Mexican border. Such a configuration should have led to outbreaks of cold air into the deepest tropics; such outbreaks should have significantly modified tropical vegetation, even though mean conditions may have been well above freezing. Interpretation of paleo pollen data may well have been strongly biased such events. The northward penetration of the jet over Alaska resulted in model temperatures comparable to the present – a result consistent with the absence of evidence for any ice in that region during the glacial. On the North Atlantic side in summer, a very strong zone of cyclogenesis develops in the lee of the ice sheet; strong winds feeding into the low result in high values of heat flux that should have at the minimum retarded advance of the ice sheet and probably resulted in substantial melting along the margins. As meltwater would be in close proximity to the Gulf Stream system, some of the low salinity events implicated in thermohaline circulation collapse could actually have come from this source rather than from the Mississippi or St. Lawrence River systems. In the tropics the Indian monsoon is very substantially weakened, in accordance with other models and geological data. The decreased precipitation over the Indian subcontinent was compensated by increased precipitation in the southern Indian Ocean tropics. In fact the maximum precipitation in the tropics was located in this zone – in contrast to the western Pacific maximum at present. Tropical cooling on land is on the average of 4-5 C greater than present, - generally more so than lower resolution GCMs but in accordance with some geological data. Tropical snowline lowering is also comparable to what is observed. Further work on these results should enable not only a better understanding of ice age data but an opportunity to study weather dynamics on “an alternate Earth”.

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