Earth as a Planet: Atmosphere and Oceans 175
3.3 Observed Global-Scale Circulation
As described earlier, the atmospheric circulation organizes
primarily into a pattern of east–west winds, and perhaps the
most notable feature is the eastward-blowing jet streams in
the midlatitudes of each hemisphere (Fig. 2). In a longi-
tudinal and seasonal average, the winter hemisphere wind
maximum reaches 40 m s−^1 at 30◦latitude, and the summer
hemisphere wind maximum reaches 20–30 m s−^1 at 40–50◦
latitude. In between these eastward wind maxima, from
latitude 20◦Nto20◦S the tropospheric winds blow weakly
westward. The jet streams are broadly distributed in height,
with peak speeds at about 12-km altitude. Although the
longitudinally and seasonally averaged winds exhibit only a
single tropospheric eastward-wind maximum in each hemi-
sphere, instantaneous 3-dimensional snapshots of the at-
mosphere illustrate that there often exist two distinct jet
streams, the subtropical jet at∼ 30 ◦latitude and the polar
jet at∼ 50 ◦latitude. These jets are relatively narrow—a few
hundred km in latitudinal extent—and can reach speeds up
to 100 m s−^1. However, the intense jet cores are usually
less than a few thousand kilometers in longitudinal extent
(often residing over continental areas such as eastern Asia
and eastern North America), and the jets typically exhibit
wide, time-variable wavelike fluctuations in position. When
FIGURE 2 Longitudinally averaged zonal (i.e., east–west) winds
in Earth’s troposphere, showing the midlatitude maxima
associated with the jet streams. (From Hurrell et al., 1998.)
averaged over longitude and time, these variations in the
individual jet streams smear into the single eastward maxi-
mum evident in each hemisphere in Fig. 2.
Although the east–west winds dominate the time-
averaged circulation, weaker vertical and latitudinal mo-
tion are required to transport energy from the equator to
the poles. Broadly speaking, this transport occurs in two dis-
tinct modes. In the tropics exists a direct thermal overturn-
ing circulation called theHadley cell, where, on average,
air rises near the equator, moves poleward, and descends.
This is an extremely efficient means of transporting heat
and contributes to the horizontally homogenized temper-
atures that exist in the tropics. However, planetary rota-
tion prevents the Hadley cell from extending all the way
to the poles (to conserve angular momentum about the
rotation axis, equatorial air would accelerate eastward to
extreme speeds as it approached the pole, a phenomenon
that is dynamically inhibited). On Earth, the Hadley cell
extends to latitudes of∼ 30 ◦. Poleward of∼ 30 ◦, the surface
temperatures decrease rapidly toward the pole; this is the
location of the subtropical jet. Although planetary rotation
inhibits the Hadley cell in this region, north–south mo-
tions still occur via a complex 3-dimensional process called
baroclinic instability. Meanders on the jet stream grow,
pushing cold high-latitude air under warm low-latitude
air in confined regions∼1000–5000 km across. These in-
stabilities grow, mature, and decay over∼5 day periods;
new ones form as old ones disappear. These structures
evolve to form regions of sharp thermal gradient called
fronts,as well as 1000–5000 km long arc-shaped clouds and
precipitation that dominate much of the winter weather
in the United States, Europe, and other midlatitude
regions.
Water vapor in Earth’s troposphere greatly accentuates
convective activity because latent heat is liberated when
moist air is raised above its lifting–condensation level, and
this further increases the buoyancy of the rising air, leading
to moist convection. Towering thunderstorms get their en-
ergy from this process, and hurricanes are the most dramatic
and best-organized examples of moist convection. Hurri-
canes occur only on Earth because only Earth provides the
necessary combination of high humidity and surface fric-
tion. Surface friction is required to cause air to spiral into
the center of the hurricane, where it is then forced upward
past its lifting–condensation level.
The Hadley cell exerts a strong control over weather in
the tropics. The upward transport in the ascending branch
of the Hadley circulation occurs almost entirely in local-
ized thunderstorms whose convective towers cover only a
small fraction (perhaps∼1%) of the total horizontal area
of the tropics. Because this ascending branch resides near
the equator, equatorial regions receive abundant rainfall, al-
lowing the development of tropical rainforests in Southeast
Asia/Indonesia, Brazil, and central Africa.