Geoscience Reference
In-Depth Information
(a)
FERREL
HADLEY
HADLEY
FERREL
2
0
4
0
−
2
0
6
12
16
8
−
2
−
1
4
0
[V]
(DJF)
8
0
10
60S
30S
30N
60N
(b)
0
Figure 9.4
Mean latitude
average circulation of the
atmosphere (a) December to
February, (b) June to August.
Values on the streamlines are
total mass circulation
between that streamline and
the zero streamline. (From
Rassmusson
et al
.,1993,
published with permission.)
FERREL
HADLEY
HADLEY
FERREL
2
0
−
16
4
−
1
0
6
−
4
0
−
8
−
12
2
[V]
(JJA)
2
8
10
60S
30S
0
30N
60N
Equator
neighboring latitudes must flow in to replace it and that the easterly component of
the trade winds was associated with the rotation of the Earth. The idea of
symmetrical cells on either side of the equator became accepted but it was later
realized that in fact there are several cells on either side of the equator, see Fig. 9.3.
However, because the latitude of maximum solar heating is strongly seasonal,
there is a strong seasonality in the Hadley cell and circulation is only briefly
symmetrical at the spring and fall equinoxes. Otherwise, the dominant latitudinal
circulation comprises three main cells, with the strong equatorial Hadley cell being
of central importance, see Fig. 9.4. There is a strong, single summer Hadley cell in
each hemisphere with the location of rising air shifting with the thermal equator.
This is balanced by falling air in the winter hemisphere. The regular seasonal shift
in the location of rising and falling air between the two hemispheres means the
circulation pattern appears symmetrical as an annual average.
Mean low-level circulation
Outside the tropics, the rotation of the Earth is a fundamental influence on atmos-
pheric general circulation, see Fig. 9.5a. Near-surface wind flow, which would oth-
erwise be purely pressure driven, includes circular flows induced by the Coriolis
force. In the northern hemisphere, the circulation induced is clockwise around
regions of high pressure and counterclockwise around regions of low pressure. In
the southern hemisphere the circulation sense is reversed, i.e., circulation is coun-
terclockwise around high pressure and clockwise around low pressure. In the
