Winter

Figure 4.12 Frontal frequencies (top, fronts per day) and mean 850-hPa temperature gradients (bottom, K (100 km)-1) for winter and summer. White areas in the frontal frequency maps represent regions with known problems related to extreme topography and surface heating. Areas where the mean temperature gradient exceeds 0.5 K (100 km)-1 are shaded (adapted from Serreze et al., 2001, by permission of AMS).

Figure 4.12 Frontal frequencies (top, fronts per day) and mean 850-hPa temperature gradients (bottom, K (100 km)-1) for winter and summer. White areas in the frontal frequency maps represent regions with known problems related to extreme topography and surface heating. Areas where the mean temperature gradient exceeds 0.5 K (100 km)-1 are shaded (adapted from Serreze et al., 2001, by permission of AMS).

David And Gladys Wright House Plans
Figure 4.12 (Cont.)

associated with the East Asian jet and the Pacific storm track. From comparison with the mean temperature gradient field, one can see how the maxima in frontal frequency are placed on the warm side of these mean baroclinic zones. For example, the frontal zone over the Atlantic Basin lies south of the maximum temperature gradient for this area. The winter gradient fields also exhibit features associated with topography and land-ocean contrasts. Consistent with the analysis of cyclone activity, there is a high frequency of fronts east and northeast of Greenland. There is separation between this area of frequent frontal activity and the relative maximum in fronts further south associated with the eastern North American jet. One could hence consider the area east and northeast of Greenland in the context of a separate wintertime Arctic frontal zone.

Spring (not shown) exibits less frontal activity over the Atlantic Basin, and northward migration of the Pacific Basin feature. Frequencies are high over northwestern North America, with a general shift in activity toward the interior. A local maximum is found centered at about 65° N, which at the resolution of the NCEP/NCAR data is essentially co-located with the Brooks and Mackenzie ranges. There is some separation between the frontal activity over Alaska and that to the south in the Pacific Basin.

In summer, the relative maximum in frontal frequencies over Alaska is well expressed. Of particular interest is the development of a pronounced, zonally oriented band of high frontal frequencies over northern Eurasia, which essentially extends across the continent. Frontal frequencies are highest over eastern Eurasia, corresponding to the Yana, Indigirka and Kolyma valleys, which are bounded to the east, west and south by fairly high topography. The Eurasian and Alaskan features are broadly in agreement with the regions of high summer frontal frequency shown by Reed and Kunkel (1960), with the orientation of the Eurasian feature similar to the summer Arctic frontal zone plotted by Krebs and Barry (1970). The Eurasian and Alaskan frontal zones are associated with mean baroclinic zones aligned roughly along the Arctic Ocean coastline. Reed and Kunkel (1960) view the summer Arctic frontal zone in Eurasia as distinct from the region of high frontal frequencies in middle latitudes of the Pacific Basin. This separation is clearly captured in the automated frontal analysis. The well-defined Eurasian frontal zone seen in summer breaks down in autumn while the high-latitude feature over Alaska is still present in a weakened form. The Alaskan feature, while best expressed in summer, hence persists throughout the year.

Development of the summer Arctic frontal zone is interpreted as a manifestation of: (1) differential heating between the Arctic Ocean and snow-free land; (2) sharpening of the baroclinicity by coastal orography. These conclusions are drawn from the observation that in summer one sees development of a strong temperature gradient along the coast, and that the frontal zone is best pronounced where topography can "trap" the cold Arctic Ocean air. The differential heating link was first suggested by Dzerdzeevskii (1945) and later by Kurashima (1968), while the concept of a topographic link can be traced back to Reed and Kunkel (1960). Further support for topographic controls comes from the modeling study of Lynch et al. (2001b) who examined frontal activity over Alaska. From comparisons between Figures 4.11 and 4.12 there is a close correspondence between the preferred areas of summer cyclogenesis over northeastern Eurasia and Alaska/Yukon and the relative maxima in summer frontal frequencies.

Vertical cross sections of the mean temperature gradient and winds throughout the troposphere provide further insight. Figure 4.13 shows cross sections at longitude 140° E. This longitude cuts through the region where the summer Eurasian frontal zone is best expressed. The zonal wind cross section for January shows a single jet with a core velocity of about 75 m s-1. The tropospheric meridional temperature gradient is maximized at about 300 hPa (negative values meaning temperature decreases to the north). While bearing characteristics of the subtropical jet, the effects of statistical averaging with polar front jets are evident in the baroclinicity at lower levels and its extension to the north. Note the high-latitude baroclinicity at lower stratospheric levels, capturing the lower end of the polar night stratospheric jet discussed earlier. At this longitude, the Arctic frontal zone is best expressed during June. The June zonal wind cross section is dominated by a jet at about 35° N, about half as strong as that for winter. Note, however, the development of a weaker, separate 300 hPa wind maximum (10 m s-1) at 70° N. This can be associated with a distinct high-latitude baroclinic zone extending to 400 hPa, maximized at low levels close to the latitude of the Arctic Ocean coast.

Figure 4.13 Mean zonal winds (ms-1) (top panels) and meridional temperature gradient (K (100 km)-1) (bottom panels) from the equator to the Pole at 140° E for January and June. Positive values are shaded (from Serreze et al., 2001, by permission of AMS).
Figure 4.14 Mean zonal winds (m s-1) (top panels) and meridional temperature gradient (K (100 km)-1) (bottom panels) from the equator to the Pole at 140° W for January and June. Positive values are shaded (from Serreze et al., 2001, by permission of AMS).

The cross section in Figure 4.14 cuts through Alaska at longitude 140° W, where the high-latitude frontal zone is again well expressed. For January, a single jet (30 ms-1) is found at about 30° N. The lower end of the stratospheric polar night jet is again seen at about 70° N. By sharp contrast, June reveals a subtropical jet at about 25° N, a polar jet at about 45° N and a third Arctic jet at about 70° N. Similar to January, there is a fairly sharp, low-level baroclinic zone at high latitudes, but it is shifted north to about 70° N, again close to the latitude of the Arctic Ocean coast.

In eastern North America, the high-latitude summer frontal zone is not well expressed. At 80° W (not shown), both the winter and summer cross sections show a single jet. There is no evidence of a pronounced summer baroclinic zone along the Arctic coast. This follows in that the Canadian Arctic Archipelago is a heterogeneous landscape with a combination of land and ice-covered channels, acting to inhibit amplification of the baroclinicity such as seen along the Eurasian coast where the land-ocean boundary is well defined.

As first noted by Bryson (1966), the extent of Arctic air in summer, as represented by the median location of the Arctic front, also corresponds well with the northern limit of boreal forest. Barry (1967) confirmed this relationship for Canada west of

Hudson Bay, but noted the existence of a broad forest-tundra ecotone over LabradorUngava. Krebs and Barry (1970) also demonstrated the close spatial association of the median location of the Arctic front over northern Eurasia in summer and the forest-tundra boundary. Larsen (1974) examined these ideas further for Canada. However, Hare (1968) pointed out that the relationships need not represent cause and effect -both the climatic and biotic patterns may be a consequence of atmospheric circulation conditions determined by radiative controls. This view was explored further by Hare and Ritchie (1972) in an analysis of latitudinal radiational gradients.

Subsequently, Pielke and Vidale (1996) proposed that the forest-tundra boundary may itself help to determine the airmass contrasts and therefore the frontal location. Drawing on findings from BOREAS, they suggested that stronger heating of the boreal forest from its lower albedo is not compensated by an increase in transpiration, even with the larger leaf area index of the forest. The heterogeneity ("patchiness") of the boreal forest landscape gives rise to mesoscale circulations, which help to mix the heat upwards, giving rise to a deep thermal contrast. While a significant role of vegetation on regional and circumpolar climates on a range of time scales finds support in other studies (e.g., Bonan et al., 1992, 1995; Foley et al, 1994; Lynch et al, 1999a), the available evidence as reviewed above points to the treeline as primarily a response to the position of the Arctic frontal zone. North of the frontal zone, it is simply too cold for trees to grow in summer, although extreme winter temperature minima may be a further physiological control.

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