Geoscience Reference
In-Depth Information
There is a secondary minimum (or maximum) in tropopause height in October
(December-January). By contrast, the tropopause temperature shows a simple
annual cycle with a minimum of −62°C in January and a maximum of −49°C in
July, apparently following the annual cycle of temperature in the lower stratosphere
as identified for the 100 hPa level (Wilson and Godson,
1963
). E. J. Highwood et al.
(
2000
) also point out that tropopause temperatures in winter are lower over the
Eurasian sector and higher over the Canadian-Atlantic sector.
The existence of a double maximum/minimum in tropopause height (Gaigerov,
1967
; Makhover,
1983
), is attributed to several factors. The primary August max-
imum is attributed to the lag in the heating of the surrounding landmasses, while
the secondary one in January is related to enhanced meridional heat transport by
the planetary waves. The spring minimum is associated with the seasonal weaken-
ing of this transport. The autumn minimum relates to the cooling of the landmasses
and a decrease in cyclone activity compared with late summer. Two patterns of the
annual cycle in tropopause height are identified in different regions. Over North
America and subarctic Siberia there is a simple annual wave (winter/summer -
tropopause pressure maximum/minimum), whereas there is double wave (spring/
autumn - maxima, summer/winter - minima) in the High Arctic, northern Europe,
and western Siberia.
Figure 4.6
shows the spatial pattern of the mean thermally defined tropopause
height (hPa) for 1979-1993 for the annual mean and summer. The first order pat-
tern is that the Arctic tropopause is encountered lower in the atmosphere (i.e., at a
higher pressure) than in lower latitudes. This is consistent with the schematic shown
in
Figure 4.1
. The annual mean height field is nevertheless strongly eccentric near
the Pole with a center of 280 hPa located over Devon-Ellesmere islands. In sum-
mer, the minimum extends from Ellesmere Island to the North Pole, and its average
center is about 290 hPa (Hoinka,
1998
). G. Zangl and K. Hoinka (
2001
) extended
this work to examine the tropopause based on dynamic and thermodynamic criteria.
Their January map is similar to the annual mean. Highwood et al. (
2000
) also obtain
similar results
A key feature of the structure of the atmosphere in middle - high latitudes is the
extent of tropopause folds. These are situated above strong baroclinic zones - associ-
ated with intense cyclones. In these regions, stratospheric air is extruded into the tro-
posphere by intermittent folding of the tropopause (Danielsen,
1968
). The associated
mass removal of stratospheric air is maximized in late spring. The depression of the
tropopause associated with the mean frontal zones is illustrated in
Figure 4.1
.
4.4
The Mid-Tropospheric Circulation
4.4.1
Mean Circulation
The primary feature of the mid-tropospheric (500 hPa) circulation in northern high
latitudes is a well-developed cyclonic vortex during much of the year (
Figure 4.7
).
