Abstract:Models of atmospheric circulation in the North Atlantic sector during glacial inception can be expanded to a hemispheric scale with the aid of diagnostic studies of the present climate. The present “Greenland Above” (GA) atmospheric circulation type may be a candidate for the atmospheric circulation type required during glacial inception. The pattern is an amplification, with only minor phase shifts, of the present average winter circulation pattern in the extratropical Northern Hemisphere. Southerly flow in t… Show more
“…Data also suggest that reversals were more frequent during the first half of the 1800s and probably throughout the Little Ice Age than they have been in this century. This conclusion supports Crowley's (1984) contention that such an anomalous pressure pattern (he specifically cites the GA circulation mode) is typical of periods of global cooling and periods of onset of Quaternary glaciations.…”
Reversals in the usual monthly mean sea level pressure (SLP) gradient over the North Atlantic during winter-an extreme mode of the North Atlantic Oscillation-are documented back to 1867. Such events are typically associated with wellbelow-normal temperatures in north-west Europe and above-normal precipitation amounts in the western Mediterranean area. Also noted during these events is the tendency for above-normal temperatures in western Greenland and belownormal temperatures in the eastern United States.Temporal changes in the frequency of SLP reversals relate well to trends in the strength of the zonal wind and the frequency of blocking in the north-east Atlantic area. Part of the fluctuations in temperature in western Europe during the last century can be explained by these changes in circulation. Nevertheless, major trends in winter temperature over this period, particularly the early 20th century rise in temperature, remain when adjustments are made to account for the effects of circulation changes during reversal winters. This suggests that mechanisms other than changes in the frequency of circulation regimes also play an important role in determining hemispheric temperature trends on these time scales.
“…Data also suggest that reversals were more frequent during the first half of the 1800s and probably throughout the Little Ice Age than they have been in this century. This conclusion supports Crowley's (1984) contention that such an anomalous pressure pattern (he specifically cites the GA circulation mode) is typical of periods of global cooling and periods of onset of Quaternary glaciations.…”
Reversals in the usual monthly mean sea level pressure (SLP) gradient over the North Atlantic during winter-an extreme mode of the North Atlantic Oscillation-are documented back to 1867. Such events are typically associated with wellbelow-normal temperatures in north-west Europe and above-normal precipitation amounts in the western Mediterranean area. Also noted during these events is the tendency for above-normal temperatures in western Greenland and belownormal temperatures in the eastern United States.Temporal changes in the frequency of SLP reversals relate well to trends in the strength of the zonal wind and the frequency of blocking in the north-east Atlantic area. Part of the fluctuations in temperature in western Europe during the last century can be explained by these changes in circulation. Nevertheless, major trends in winter temperature over this period, particularly the early 20th century rise in temperature, remain when adjustments are made to account for the effects of circulation changes during reversal winters. This suggests that mechanisms other than changes in the frequency of circulation regimes also play an important role in determining hemispheric temperature trends on these time scales.
“…Tw o broad and contrasting scenarios have been proposed. In the first, glaciation is associated w ith relatively w arm conditions so as to produce increased accum ulation over the surface of the expanding snow-ice surface (Johnson & M cC lure 1976;R u d d im an & M cIn ty re 1979;Crowley 1984). In an alternative reconstruction (D enton & H ughes 1983; D enton et al 1986), severe conditions during the onset on glaciation result in the freeze-up of the inner seas and channels of A rctic C a n a d a : in o ther words, the glacial equilibrium line falls from present values of betw een 400 and 1200 m (Andrews & M iller 1972) to sea level.…”
Section: (D) Glaciation and Oceanographymentioning
The outer east coast of Baffin Island is characterized by a series of sedimentary forelands. These contain a variety of litho- and biofacies associated with glacial marine and marine deposition into sea levels higher than those of the present. These high relative sea levels were associated with glacial isostatic loading and unloading of the crust by the NE sector of the Laurentide Ice Sheet. On the basis of amino acid epimerization ratios, eleven chronologically distinct units are delimited. The youngest unit is less than or equal to 10 ka, but all others are at or beyond the limits of radiocarbon dating. Based on biostratigraphy and amino acid data, the oldest units exposed in the forelands may be Pliocene in age. Molluscs, Foraminifera, and the palynology of buried soils and organics, indicate that the vast bulk of the exposed sequences contain floras and faunas that represent environments
warmer
than those at present. An analysis of modern and fossil pollen spectra suggests a steady decrease in low arctic conditions throughout the Quaternary.
“…The major features of this departure field are the positive (+100 m) anomaly over southern Greenland and the larger (−400 m) negative anomaly around the Great Lakes. The selection of this long-wave circulation pattern for the ice-age reconstruction is supported by varied observational and theoretical evidence reviewed by Crowley (1984). Fig.…”
Section: An Application Of the Accumulation Mddel To Ice-age Conditionsmentioning
A relatively simple, three-parameter model is used to simulate the annual precipitation (accumulation) distribution for the Greenland ice sheet and surrounding regions. The three parameters are (1) the flux of relative vorticity at the 500 mbar level (a measure of cyclonic activity), (2) atmospheric moisture content, and (3) surface terrain. The climatological (1946–79) precipitation distribution predicted by the model displays major features of the observed distribution derived from pit studies. However, the model suggests that, due to changes in storm tracks during this period of 33 a, accumulation distribution maps based on pit studies for varying periods of record may not be representative of a true mean for a uniform period of record. The model is then applied to reconstructed ice-age conditions. Compared to present conditions, accumulation reductions of 60¾ or more are indicated for much of the southern half of Greenland; only slight reductions are noted for northern Greenland.
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