Geoscience Reference
In-Depth Information
Figure 4.13.
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
o
W for
January and June. Positive values are shaded (from Serreze et al.,
2001
, by permission
of AMS).
was explored further by Hare and J. Ritchie (
1972
) in an analysis of latitudinal
radiational gradients.
Subsequently, R. Pielke and P. Vidale (
1996
) proposed that the forest/tundra
boundary may itself help to determine the airmass contrasts and therefore the fron-
tal location. Drawing on findings from BOREAS, they suggested that stronger heat-
ing 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 upward, giving rise to a deep thermal contrast. Although
a significant role of vegetation on regional and circumpolar climates on a range
of time scales finds support in other studies (e.g., Bonan, Pollard, and Thompson,
1992
; Bonan, Chapin, and Thompson,
1995
; Foley et al.,
1994
; Lynch et al.,
1999a
),
the available evidence as previously reviewed 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.
