Atmospheric circulation patterns during glacial inception: A possible candidate

QUATERNARY

RESEARCH

21,

105-110

(1984)

SHORT PAPERS Atmospheric Circulation Patterns during Glacial Inception: A Possible Candidate THOMAS Department

J. CROWLEY~

of Physics, University of Missouri, St. Louis, Missouri 63121 Received April 22, 1983

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 “Greenjand A~QY~.’ (GA) atmospheric circulation type ~may be a candidate for the at@ospheric circulation type required during glacial inception. The pattern is an amplification, with only minor phase shiftsof the present average winter circulation pattern in the extratropical Northern Hemisphere. Southerly flow in the northwest Atlantic is associated with warm ocean temperatures, low sea ice in the Davis Strait, and increased precipitation over northeast Canada. Evidence from modeling of the present climate indicates that the GA pattern could be maintained by increased snow cover over eastern North America. Enhanced snow cover, due to decreased Northern Hemisphere summer insolation, could cause a similar response on an ice-age time scale.

The subject of atmospheric circulation patterns during glacial inception has attracted much interest over the years (e.g., Lamb and Woodroffe, 1971; Flohn, 1974; Williams, 1975; Andrews and Barry, 1978). Knowledge of such patterns provides valuable information about how glacial stages began. It is the purpose of this paper to update some aspects of this subject by incorporating recent results from paleoclimatology, climate modeling, and diagnostic studies of present atmospheric circulation patterns. The analysis may serve as a guide for future quantitative investigations of circulation changes on ice-age time scales. It will be assumed that studies of atmospheric circulation patterns on short time scales (seasonal, interannual) provide some insight into possible patterns on ice-age time scales. This assumption has obvious limitations (cf. Barry, 1981), but the approach has been applied with some success to the interpretation of climatic trends on decadal, centennial, and millennial time scales (San’ Present address: Climate Dynamics Program, National Science Foundation, 1800 G St. NW, Washington. D.C. 20550.

chez and Kutzbach, 1974; Diaz and Andrews, 1982; Douglas et al., 1982). A convenient starting point for the analysis involves the indications of warm seasurface temperatures (SST) in the northwest Atlantic during major phases of ice growth (Ruddiman and McIntyre, 1979; Ruddiman et al., 1980). The thermal contrast of cold land/warm sea should result in increased coastal baroclinicity, thereby sustaining or increasing moisture transport into northeast Canada during the winter months (cf. Barry, 1966; Brinkmann and Barry, 1972). Such transport has been thought necessary to maintain snow accumulation rates compatible with those indicated by the assumed ice-volume record of 180 (cf. Andrews and Mahaffy, 1976, and Weertman, 1976, with Shackleton, 1977). However, recent work (Budd, 1981; Andrews, 1982; Dodge et al., 1983) has called into question the reliability of the St80 record as an ice-volume indicator. Thus, it may be necessary to consider only (1) whether a cold land/warm sea pattern occurs at any time during the present; and (2) how the pattern off northeast Canada fits into the larger-scale atmospheric circula-

105 0033-5894184 $3.00 Copyright B 1984 by the University of Washington. AU rights of reproduction in any form reserved.

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tion pattern of the extratropical Northern Hemisphere. Most of the present winter circulation patterns in the extratropical Northern Hemisphere can be described by a relatively small number of atmospheric circulation types. For example, van Loon and Rogers (1978) have shown that 72% of the Januaries between 1899 and 1975 are characterized by one of four circulation types. One of the types is of particular interest (Fig. 1). It is called the “Greenland Above” (GA) circulation pattern-so named because anomalies include above-average temperatures in Greenland and (usually) western North America, coupled with below-average temperatures in western Europe and (usually) eastern North America (see also Dickson and Namias, 1976; Wallace and Gutzler, 1981). Surface pressures

J. CROWLEY

are relatively high in the normal area of the Icelandic low. There are two preferred Atlantic storm tracks, one along the east coast of North America into the Labrador Sea, the other eastward across 35”-40”N into the Mediterranean (Dickson and Namias, 1976; Rogers and van Loon, 1979). The severe winter of 1976-1977 in eastern North America was a “Greenland Above” winter (Rogers and van Loon, 1979). The pattern has prevailed during 19% of the Januaries between 1899 and 1975 (van Loon and Rogers, 1978). An important feature of the GA circulation pattern involves the accentuated upper-level ridge over the western North Atlantic. The pattern is associated with southerly flow at the surface, warm SST in the northwest Atlantic, low sea-ice conditions in the Davis Strait, and increased precipitation over northeast Canada (Dickson and Namias, 1976; Rogers and van Loon, 1979). These features suggest that the GA pattern might be an ideal candidate for the type of circulation required during times of glacial inception. Tsuchiya (1964) made substantially the same suggestion based on a more limited analysis of wintertime circulation patterns. There are other features of the GA pattern that merit comment: (1) The pattern is principally due to an amplification, without significant phase shifts, of the “normal” winter ridgetrough pattern over North America (Namias, 1978). That is, the pattern is marked by relative simplicity-a desirable feature in any glacial model. FIG. 1. An example of the mean geostrophic flow (2) The temperature response in western during “Greenland Above” winters. This example Europe is usually the same as that in shows flow along the 2800-m contour of the 700-mb eastern North America; both regions have surface for the winter of 1976-1977 (after Namias, below-average values. Thus, the GA pat1978). This flow pattern is typical of other GA winters (Rogers and van Loon, 1979). Dotted region south of tern could possibly account for glaciation Iceland indicates area of positive SST anomalies for in both North America and Europe. GA winters (compared to average SST between 1854 (3) The zonally asymmetric circulation and 1967; from Rogers and van Loon (1979)). Regions pattern may enhance climate sensitivity with horizontal lines in eastern Canada indicate hy(Hartmann and Short, 1979); i.e., a relapothesized areas of ice growth during glacial inception (see text). tively large climatic response may result

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from a relatively small perturbation (e.g., orbital forcing). It should be noted, however, that the significance of zonal asymmetry for climate sensitivity has been contested by some authors (Oerlemans, 1980; North et al., 1981). (4) The GA pattern may represent a preferred mode of atmospheric circulation. Climate simulations with a general circulation model indicate that a circulation pattern similar to GA occurs in the absence of any anomalous forcing (Lau, 1981). This response may indicate the presence of multiple equilibrium states in the atmosphere (cf. Charney and Straus, 1980). The coincidence of a preferred mode of atmospheric oscillation with the postulated mode for glacial inception may imply that a pathway already exists in the atmosphere for moving from an interglacial to a glacial state. The general sparseness of land records for times of glacial inception precludes a rigorous test of the GA hypothesis. However, inferred higher accumulation rates in the Greenland ice cores are consistent with a GA circulation pattern (Andrews et al., 1974; Dansgaard et al., 1982). Evidence in Atlantic deep-sea cores of drier conditions in western equatorial Africa (Pokras and Mix, 1983) is also consistent with that predicted by GA teleconnections to lower latitudes (Meehl and van Loon, 1979). The generally northward location of the intertropical *convergence zone (ITCZ) during GA winters produces precipitation deficits in the equatorial regions. A similar northward shift of the ITCZ may also have occurred during the ice buildup at the end of al80 Stage 3; Street and Grove (1979) illustrate relatively high lake levels in the subSahara during that time. Records from the Little Ice Age and an interstade also provide some evidence for the occurrence of a circulation pattern similar to GA. Although these times were not intervals of ice growth, their analysis may still supply useful information as to whether certain circulation types are preferentially

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occurring during cooler intervals. Tree-ring records indicate a greater frequency of the GA pattern during parts of the Little Ice Age (Fritts et al., 1979). Higher temperatures over Greenland (Dansgaard et al., 1975) and higher pressures over the North Atlantic (Gribbin and Lamb, 1978, Fig. 4.3) are also consistent with a GA pattern during this time. Teleconnections of the GA pattern with lower latitudes (Namias, 1972; Hastenrath, 1976, 1978; Meehl and van Loon, 1979; Rogers and van Loon, 1979) indicate that regional climatic anomalies are particularly coherent in the interval from about 1790 to 1820. The GA pattern correlates presently with increased precipitation in the Mediterranean and the Sahel. Compilations of historical data by Nicholson (1980; see also Lamb, 1977, Fig. 17.21, p. 469) indicate increased moisture in these regions during the early 1800s. There is also some evidence for a zonally asymmetric circulation pattern during an intermediate stage of glaciation: al80 Stage 3. Although this interval was an interstadial, the Laurentide Ice Sheet was still large enough to occupy the lowlands of the St. Lawrence River valley (McDonald and Shilts, 1971; cf. Andrews et al., 1983). Dreimanis and Raukas (1975) have noted that warm interglacial-like climates prevailed at the same time in western North America, eastern Europe, and western Siberia. In fact, the regions of interstadial warmth coincide closely with regions of anomalous southerly flow in the present GA pattern (Fig. 1; cf. Wallace and Gutzler, 1981, p. 804). There is some theoretical justification for postulating the presence of the GA flow pattern during glacial growth. Namias (1978) has suggested that, through ice-albedo feedback, increased snow cover sometimes tends to anchor the jet stream, with a resultant amplification of the preexisting pattern. Figure 1 illustrates possible locations of ice cover during glacial inception (Barry et al., 1975; Andrews and

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Barry, 1978; cf. Williams, 1978). The increased coverage is triggered by low Northern Hemisphere insolation values in the summer (e.g., Berger, 1978). The coincidence of the snowfields with the climatological-mean winter trough may cause an amplification of the circulation into a GA pattern during subsequent winters. Although this model has not been tested on ice-age time scales, some calculations have been made on shorter time scales (Roads, 1981). Preliminary indications are that the ice-albedo feedback in a nonlinear, primitive-equation model produces a response similar to that conjectured by Namias. The calculated resonance between the perturbed and background states (cf. Roads, 1982) may also indicate a source of enhanced sensitivity for ice-age models. Other analyses indicate that the GA circulation type may also be forced from lower latitudes (Horel and Wallace, 1981; van Loon and Madden, 1981), but no mechanism for doing so on ice-age time scales is apparent. The above discussion may be summarized as follows: It is postulated that t,he cold land/warm sea pattern in the northwest Atlantic during glacial growth may reflect the present “Greenland Above” atmospheric circulation type. This type represents a simple amplification of the “normal” ridge-trough system over North America and Europe. Evidence from modeling of the present climate suggests that the GA pattern could be maintained by increased snow cover over eastern North America. Enhanced snow cover, due to decreased-insolation, might cause a similar response on ice-age time scales. ACKNOWLEDGMENTS I thank R. G. Barry, P. B. James, W. F. Ruddiman, and T. Webb III for their helpful comments. Figures were drafted by G. J. McClure, and Susan Swyers typed the manuscript.

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