Historical perspective

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The major "centers of action" in the Northern Hemisphere circulation - the Azores and Pacific highs, the sub-polar Icelandic and Aleutian lows, and the Siberian High, had been identified in the 1880s by Teisserene de Bort. By 1920, the "Bergen School" had developed ideas on the characteristics of extratropical cyclones. The nature of the general circulation of the atmosphere was interpreted in terms of a three cell model by Bjerknes, Rossby and others. This model includes thermally direct Hadley cells in low latitudes, thermally indirect Ferrell cells in middle latitudes, and thermally direct cells in high latitudes, separated by the subtropical and polar front jet streams.

Inroads were being made on the nature of climate variability. In 1932, Walker and Bliss (1932) published a landmark paper demonstrating that the North Atlantic Oscillation, reflecting covariability in the strengths of the Icelandic Low and Azores High, is a key teleconnection in the Northern Hemisphere, with impacts on the climates of northeastern North America and northern Europe (Chapter 11).

However, a clear understanding of the Arctic circulation lagged behind. The earliest views of the Arctic's atmospheric circulation can be attributed to von Helmholtz (1888). He argued that the Arctic was dominated by a more-or-less permanent anticyclone beneath a tropospheric vortex, a view that prevailed well into the twentieth century. His basic thinking was developed by Hobbs (1910,1926) in his glacial anticyclone theory and further elaborated in a later paper (Hobbs, 1945) that focused on the perceived existence of a "Greenland glacial anticyclone" - a persistent anticyclone over the Greenland Ice Sheet having strong impacts on middle latitude weather. From today's perspective, Hobbs' papers provide for an amusing read.

Misconception regarding the circulation north of the sub-polar centers of action is not surprising given the dearth of observations. It was not until the 1940s and early 1950s that data were sufficient to make more definitive conclusions. Sea level pressure (SLP) analyses in the US Historical Weather Map Series produced during World War II contained strong positive biases prior to the 1930s away from the North Atlantic sector. Smaller errors are apparent up to 1939. As noted by Jones (1987), these maps were prepared by relatively untrained analysts, who tended to extrapolate into the data-poor Arctic with the concept of an Arctic high-pressure cell still in mind.

While the glacial anticyclone paradigm was seemingly put to rest in papers by Dorsey (1945), Matthes (1946) and Matthes and Belmont (1946), Hobbs continued to argue for the Greenland glacial anticyclone for several more years (Hobbs, 1948). Even in the early 1950s, some studies depicted cyclone activity as largely restricted to the periphery of the Arctic Ocean (e.g., Pettersen, 1950). This view may have been influenced by Sverdrup's (1933) observations during the Maud expedition (1918-25) of the frequent passage of cyclones along the fringes of the Arctic Ocean. In a book chapter on Arctic climate published in 1958, F. K. Hare makes use of Pettersen's (1950) map, which for both winter and summer depicts most of the Arctic Ocean as a "Quiet Zone of Minimum Cyclonic Activity". For summer, Hare provides the following description:

The quiet central zone in summer coincides fairly closely with the permanent pack ice of the Arctic Sea. Although a few frontal cyclones appear to cross it, the prevailing state is one of monotonously slack and ill-defined circulation, appropriate enough to what is certainly the world's largest quasi-homogeneous surface. Only along the flanks does cyclonic cloud and rainfall become at all common.

But an epiphany soon followed. The emergence in North America of more modern views of the Arctic circulation appears in the work of research groups at McGill

University led by F. K. Hare (Wilson, 1958; Hare and Orvig, 1958) and the University of Washington led by R. J. Reed (Keegan, 1958; Reed and Kunkel, 1960; Reed, 1962). These studies showed that while anticyclones are common and often persistent features, they are by no means permanent. It was also realized that cyclones can be found anywhere in the Arctic and in all seasons.

In the Soviet Union, by contrast, a relatively modern view had already been formulated in 1945 by Dzerdzeevskii (1945). This study, remarkable both for its insight and in recognition of what must have been difficult wartime working conditions, was based on data from the icebreaker Sedov, the first Soviet drifting ice station, NP-1, and other observations. Dzerdzeevskii correctly concluded that cyclone activity was common over the central Arctic Ocean during summer.

Turning to jet streams, fronts and airmasses, conceptual models of the general circulation through the 1950s and 1960s presented in studies such as those by Palmen (1951), Defant and Taba (1957) and Palmen and Newton (1969) featured a two-front structure associated with the subtropical and polar-front jet streams. This model can be traced back to the pioneering work by Bjerknes, Rossby and others described earlier. However, in the 1950s, the Canadian Meteorological Service adopted a three-front, three-jet stream, four-airmass model (Anderson et al., 1955; Penner, 1955; Galloway, 1958; Mclntyre, 1958). Operational analyses were made via frontal contour charts showing the location of fronts at the 1000, 700 and 500 hPa levels. The wet-bulb potential temperature was selected as the best diagnostic variable for airmass identification. The northernmost fronts represented "Arctic fronts". These analyses served as the basis for numerous climate studies.

Serreze et al. (2001) provide a summary of some of the evolving thought regarding high-latitude fronts. Arctic fronts were very much evident in vertical cross sections presented in early synoptic atlases (Boville et al., 1959). More modern studies using aircraft data collected during the winter season under the Arctic Gas and Aerosol Program (Shapiro etal., 1984; Raatz etal., 1985; Shapiro, 1985; Shapiro etal., 1987b) left little doubt regarding the reality of separate Arctic jet streams. These and other observations were used by Shapiro et al. (1987b) in a "revised" three-front model (Figure 4.1), which harks back to the Canadian scheme.

Some early studies of Arctic fronts include those of Reed (1960) using Northern Hemisphere weather maps, Bryson (1966) using streamlines and airmass analysis for North America, and Barry (1967), who as a young and enthusiastic researcher examined the preferred location of Arctic fronts over North America for January, April, July and October using the Canadian frontal analyses. In a study for Eurasia, Krebs and Barry (1970) took a somewhat different approach of plotting the northernmost frontal boundaries depicted on Northern Hemisphere daily synoptic weather maps for July 1952-6 to determine the median location, as well as the quartiles and outer deciles of the latitudinal distribution of fronts in relation to the forest-tundra ecotone. They also analyzed the latitudinal gradient of 1000-500 hPa thickness.

In the Canadian three-front model, Arctic fronts were objectively differentiated from polar fronts. But what if one instead plotted on a map the location of all fronts over some

Figure 4.1 The meridional structure of the tropopause. The potential vorticity discontinuity tropopause is shown by heavy solid lines, with the stratosphere shaded. The primary frontal zones are bounded by heavy dotted lines. The 40 m s-1 isotach (thin dashed line) encircles the cores of the primary jet streams (Ja, Arctic; Jp, Polar; Js, Subtropical). The secondary (thermal) tropopause is indicated by the heavy dashed line. Major tropospheric and stratospheric airmasses, tropopause surfaces and selected wind systems are labeled. Cross sections at any given time along any given longitude may differ greatly from this idealized model (from Shapiro et al., 1987b, by permission of AMS).

Figure 4.1 The meridional structure of the tropopause. The potential vorticity discontinuity tropopause is shown by heavy solid lines, with the stratosphere shaded. The primary frontal zones are bounded by heavy dotted lines. The 40 m s-1 isotach (thin dashed line) encircles the cores of the primary jet streams (Ja, Arctic; Jp, Polar; Js, Subtropical). The secondary (thermal) tropopause is indicated by the heavy dashed line. Major tropospheric and stratospheric airmasses, tropopause surfaces and selected wind systems are labeled. Cross sections at any given time along any given longitude may differ greatly from this idealized model (from Shapiro et al., 1987b, by permission of AMS).

time period? Would one find geographically preferred regions of high-latitude frontal activity separate from the major locus of polar fronts? That this is indeed the case finds its origins in the remarkable study of Dzerdzeevskii (1945) and a subsequent paper by Reed and Kunkel (1960). The latter study was based on fronts plotted on summer (June-August) sea level pressure analyses for the period 1952-6. It revealed a belt of high frontal frequencies extending along the northern shores of Siberia and Alaska and southeastward across Canada. Reed and Kunkel argued that low frontal frequencies over Kamchatka and the high frequencies off Japan "make it abundantly clear that the polar front remains separate from, and well to the south of, the Arctic frontal zone" (p. 496). Their analysis of winter frontal frequencies failed to show a separate high-latitude feature. Consequently, the Arctic frontal zone as a geographical feature was considered to exist in summer only, in agreement with Dzerdzeevskii (1945). We will return to the problem of the Arctic frontal zone later.

Investigation of stratospheric dynamics only became possible around 1960 when a sufficient number of sounding balloon ascents began to reach the 25 hPa level (~25 km)

and higher on a routine basis. The first detailed characterizations of the circulation in the Arctic stratosphere were prepared by the Arctic Meteorology Research Group at McGill University (Hare, 1960a, b; Hare, 1961) and the Institute of Meteorology at the Free University of Berlin under R. Scherhag (1960). Murgatroyd (1969) presents an early overview of stratospheric structure and dynamics based on balloon and rocket data. Efforts were intensified through the Climatic Impacts Assessment Program in the United States in the 1970s, due to the need to assess the effects of supersonic aircraft on the stratosphere (Reiter, 1975). From the late 1970s onward the availability of data from satellite sounders has greatly augmented our knowledge of the structure and composition of the stratosphere and the problem of stratospheric ozone depletion. In the past decade, it has become increasingly recognized that the circulations of the stratosphere and troposphere are intimately connected, and that these connections may be a key component of Arctic climate variability.

With a historical framework established, we review the basic elements of the Arctic circulation. While some issues of temporal variability are introduced, such as sudden stratospheric warmings, the North Atlantic Oscillation, and the newer paradigm of the Northern Annular Mode (also referred to as the Arctic Oscillation), this discussion is largely reserved for Chapter 11.

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