Geoscience Reference
In-Depth Information
Bor
e
al
w
inter
Aus
t
ra
l
w
int
er
10
100
200
350
500
700
900
1000
10
90°N
45°N
0°
45°S
90°S
90°N
45°N
0°
45°S
90°S
Fig. 5
Longitude-averaged cross sections of annual zonal winds as represented in ECMWF cli-
matological data. The latitude-pressure fields of December/January/February (boreal winter,
left
panel
) and June/July/August (austral winter,
right panel
) have been averaged to one respective
cross section
10
Motion N
Motion S
5
0
−5
Motion global
Matter global
−10
Feb
Apr
Jun
Aug
Oct
Dec
Fig. 6
Constituents of long-term axial atmospheric angular momentum at the annual frequency:
motion term of each hemisphere (
dashed line
and
dotdashed line
), sum of hemispheric motion
terms (
red line
) and global matter term (
blue line
). Underlying data: ECMWF climatological data
at monthly intervals
signals: weak easterly winds (or negative
u
-values) prevail in the tropical regions,
while westerly jet streams are found over most of the extratropics (Salstein
2002
),
centered at levels near 200hPa. The jet stream phenomenon of each hemisphere is
subject to an annual cycle and peaks in its respective winter months. As a result
of the asymmetric global distribution of the continents, the signal in the Northern
Hemisphere is of larger annual variability than that of the Southern Hemisphere. This
difference in magnitude is also evident from Fig.
6
, which depicts long-term mean
axial AAM that has been obtained from the aforementioned climatological dataset.
Obviously, the motion term of the Southern Hemisphere is only about 40% that of
the Northern, and of opposite sign, so that the annual amplitude of the global rela-
tive angular momentum does not exceed 25
10
24
kgm
2
s
−
1
. Pressure fluctuations
provide a relatively minor contribution of about 5-10% to the annual cycle of axial
AAM, see e.g. Gross et al. (
2004
).
·
