Geoscience Reference
In-Depth Information
3.3.2
Longitudinal Variations in Atmospheric Energy Fluxes
The term
−
∇
•
F
A
for the polar cap is identical to the meridional (north/south) com-
ponent on the atmospheric energy flux averaged around the 70
o
N circle divided by
the areas of the polar cap. As is evident in
Figure 3.9
, the convergence of atmo-
spheric energy transport into the polar cap present in all months as shown in ERA-
40 is the result of inflows along some longitudes being incompletely compensated
by outflow along other longitudes, with the strength and relative importance of these
inflows and outflows varying by season. In terms of total atmospheric energy trans-
port (internal heat energy, potential energy, latent heat energy, and kinetic energy;
top panel), there is a prominent region of poleward flow (contributing to flux conver-
gence) centered at about 50
o
W. As shown in
Chapter 4
, this is just east of the axis of
the mean 500 hPa eastern North American trough, pointing to contributions from the
time mean flow and eddy transports associated with the North Atlantic storm track.
These inflows are strongest in winter, at which time the North Atlantic storm track is
most active. However, M. Tsukernik, T. Chase, and M. Serreze (
2004
) note that the
longitude of this winter maximum corresponds roughly to the Norwegian Sea. This
region is characterized by high convective heating rates associated with cold winter
airmasses moving over open ocean waters. It appears that this regional heat source
is an important contributor to the energy budget of the Arctic atmosphere.
To the west, centered at about 110
o
W, lies strong region of equatorward flow
(contributing to flux divergence), also strongest in winter (discussed in
Chapter 4
),
which is associated with the descending leg of the 500 hPa western North American
ridge. Over Eurasia, centered at about 150
o
E, lies a region of inflow during the
cold season, and weak outflow in summer. In winter, this longitude is just down-
stream of the East Asian trough. The trough weakens and shifts east in summer,
helping to account for the outflows in this season. Finally, outflow dominates a
broad region from about 30-90
o
E, with the longitude of the maximum varying by
season. Hence, the peak in the flux convergence seen in winter (
Table 3.1
) is the net
result of stronger inflows over some longitudes winning out over stronger outflows
along other longitudes, with these inflow and outflows linked to large-scale features
of the atmospheric circulation.
Interestingly, the convergence of latent heat energy, viewed by itself (bottom panel of
Plate 4
), peaks not in winter but in summer and early autumn (Serreze et al.,
2007
). The
basic reason for this is that specific humidity is much higher at this time than in winter.
This seasonal peak is primarily the result of strong moisture inflows in four regions,
more than compensating for the strong outflows centered at about 110
o
W. The area of
inflow at around 90
o
E is slightly east of the Urals trough. The area of summer inflow at
about 165
o
W is just east of the East Asian trough. Prominent inflows at about 50
o
W and
near the prime meridian are separated by a region of equatorward flow in summer, and
weak poleward flow in the other seasons. This separation appears to be attributed to the
Greenland ice sheet blocking the inflow of moisture (Serreze et al.,
2007
). Most of the
moisture low occurs at fairly low levels of the atmosphere (fellow 700 hPa, which is
roughly 3000 m). At 70
o
N, the highest ice sheet elevations of about 2,900 m are found
at about 35
o
W longitude.
