Geoscience Reference
In-Depth Information
• There is no tropopause at very high latitudes in the winter hemisphere. In
the absence of shortwave (ultraviolet) radiation to absorb, ozone does not
provide a heat source and temperatures continue to decrease with elevation.
• Polar temperature inversions, in which temperatures increase with height
(decreasing pressure), occur near the surface at high latitudes in winter. Again,
in the absence of insolation, the main heat source for the polar atmosphere in
winter is the atmospheric transport of warm air from lower latitudes.
• Zonal mean meridional temperature gradients are larger in the winter
hemisphere than in the summer hemisphere, and larger in middle latitudes
than in the tropics.
• Near the surface, the zonal mean temperature maximum is very close to
the equator during Northern Hemisphere winter and moves farther into
the Northern Hemisphere (to about 25°N) during Northern Hemisphere
summer. In other words, the zonal mean temperature distribution is more
symmetric about the equator in DJF than in JJA due to continentality.
Information about the atmospheric circulation is provided by examining wind
distributions. The east-west component of the wind is referred to as the
zonal
wind
, and it is denoted by
u
. The sign convention is that
u
is positive when the
wind is blowing from west to east, that is, in the same direction as the rotation
of the earth, and negative when the flow is from east to west. The term
easterly
means “from the east,” so it is a synonym for westward. Similarly,
westerly
means “from the west,” or eastward. The north-south component of the wind
is called the
meridional wind
, denoted by
v
. Positive values of
v
denote a
south-
erly
wind, that is, flow from the south to the north, while negative values of
v
denote a
northerly
, or southward, wind.
Figure 2.10
depicts the zonal mean zonal wind, or the east-west wind aver-
aged around all longitudes.
• In the troposphere, the low is primarily westerly (positive) in middle
latitudes and easterly (negative) in the tropics.
• In the midlatitude troposphere, winds become more strongly westerly with
elevation until they reach a maximum at the tropopause. These maxima are
known as the
subtropical westerly jets
.
• The tropical easterly low is more uniform with elevation and so has less
wind shear
than the midlatitude flow.
• The subtropical jets are stronger in the winter hemisphere than in the summer
hemisphere, and are located 15° of latitude closer to the equator in winter.
The zonal mean meridional, or north-south, wind distribution is shown in
Figure 2.11
. Here, the contour interval used is 0.5 m/s, an order of magnitude
smaller than the contour interval that was used to display the zonal mean zonal
wind in
Figure 2.10
. Arrows indicate the wind direction.
According to
Figure 2.11
:
• The greatest values of the zonal mean meridional wind are found in the
upper troposphere (300-100 hPa) in the tropics, and near the surface (below
800 hPa).
• The low-level low in the tropics converges in the meridional direction. The
centerline of this
convergence
is located in the summer hemisphere. During
