The seasonal mean atmospheric general circulation can be considered as the sum
of two components, a zonally symmetric one represented by the zonal mean winds
and another zonally asymmetric one formed by standing planetary waves originated
by orography and land–sea temperature contrasts (Hoskins and Karoly 1981; Held
et al. 2002). The asymmetric component takes the form of persistent highs and lows
in low levels, which, for example, shape midlatitudes and subtropical dry zones
(Broccoli and Manabe 1992; Rodwell and Hoskins 1996, 2001) and guide
extratropical storms.
The main elements of the general circulation can be described by both, the stream
function, ψ, and the velocity potential, χ. The atmospheric flow is largely determined
by the ψ: winds follow streamlines, leaving ψ higher values to the right, and being
their intensities proportional to the separation between streamlines. As for vertical
movements, there are ascents in tropospheric regions with relative minimums of χ in
upper levels of the troposphere and relative maximums in low levels, and the
opposite for descents. The maps in Fig. 3.18 (obtained from the ERA-40 dataset;
Kållberg et al. 2007) show ψ and χ at 200 hPa, and 850 hPa for troposphere in
winter, December to February, and in summer, June to August. It is perceivable from
the streamlines at 200 hPa the presence of planetary (wavelength of the order of the
Earth’s radius) stationary waves, and a predominance of westerly winds, especially
intense for winter of each hemisphere. Zones of intense ψ gradient correspond to the
mean position of jet streams. Only in the equatorial zone of the maritime continent
and Africa in DJF, and in the most equatorial zone of the boreal hemisphere in JJA,
due to the intense anticyclone of the South Asian summer monsoon, the wind is from
the East.
In low levels, westerlies are also the predominant winds in middle latitudes
(Fig. 3.18b, d), although less intense than at high levels because of the strong west
vertical wind shear due to the sharp southward temperature gradient. In the Tropics,
however, easterly winds, called trade winds, dominate throughout the year. Between
middle and low latitudes, there exists a zonal belt of ψ maximum that corresponds to
the subtropical anticyclonic zone.
Subtropical jet streams, in Northern Hemispheric (NH) winter (SJ in Fig. 3.19a) at
about 30
N, separate two different flow regimes, the mid-high latitudes regime and
the tropical regime. The NH stationary waves present, in both extratropical and
tropical latitudes, three major troughs and three major ridges/anticyclones at high
levels, with an abrupt change in the longitudinal phase across 30
N. Tropical
troughs (ridges) and extratropical ridges (troughs) align in north–south direction,
with strong subtropical jet streams between major extratropical troughs and major
tropical ridges and the storm tracks immediately on the northern flank of the strong
subtropical jet over the oceans. In the NAE sector, a strong subtropical jet stream (ST
in Fig. 3.19a) occurs between the extratropical North America trough and the
tropical western North Atlantic ridge (T and R, respectively in Fig. 3.19a). The
subtropical jet weakens eastward because of the relatively low sea surface temperatures (SSTs) in the equatorial Atlantic Ocean, and it peaks up between the Central
Europe-Mediterranean Sea trough and the East Africa ridge.
50
J. M. Sánchez-Laulhé et al.
of two components, a zonally symmetric one represented by the zonal mean winds
and another zonally asymmetric one formed by standing planetary waves originated
by orography and land–sea temperature contrasts (Hoskins and Karoly 1981; Held
et al. 2002). The asymmetric component takes the form of persistent highs and lows
in low levels, which, for example, shape midlatitudes and subtropical dry zones
(Broccoli and Manabe 1992; Rodwell and Hoskins 1996, 2001) and guide
extratropical storms.
The main elements of the general circulation can be described by both, the stream
function, ψ, and the velocity potential, χ. The atmospheric flow is largely determined
by the ψ: winds follow streamlines, leaving ψ higher values to the right, and being
their intensities proportional to the separation between streamlines. As for vertical
movements, there are ascents in tropospheric regions with relative minimums of χ in
upper levels of the troposphere and relative maximums in low levels, and the
opposite for descents. The maps in Fig. 3.18 (obtained from the ERA-40 dataset;
Kållberg et al. 2007) show ψ and χ at 200 hPa, and 850 hPa for troposphere in
winter, December to February, and in summer, June to August. It is perceivable from
the streamlines at 200 hPa the presence of planetary (wavelength of the order of the
Earth’s radius) stationary waves, and a predominance of westerly winds, especially
intense for winter of each hemisphere. Zones of intense ψ gradient correspond to the
mean position of jet streams. Only in the equatorial zone of the maritime continent
and Africa in DJF, and in the most equatorial zone of the boreal hemisphere in JJA,
due to the intense anticyclone of the South Asian summer monsoon, the wind is from
the East.
In low levels, westerlies are also the predominant winds in middle latitudes
(Fig. 3.18b, d), although less intense than at high levels because of the strong west
vertical wind shear due to the sharp southward temperature gradient. In the Tropics,
however, easterly winds, called trade winds, dominate throughout the year. Between
middle and low latitudes, there exists a zonal belt of ψ maximum that corresponds to
the subtropical anticyclonic zone.
Subtropical jet streams, in Northern Hemispheric (NH) winter (SJ in Fig. 3.19a) at
about 30
N, separate two different flow regimes, the mid-high latitudes regime and
the tropical regime. The NH stationary waves present, in both extratropical and
tropical latitudes, three major troughs and three major ridges/anticyclones at high
levels, with an abrupt change in the longitudinal phase across 30
N. Tropical
troughs (ridges) and extratropical ridges (troughs) align in north–south direction,
with strong subtropical jet streams between major extratropical troughs and major
tropical ridges and the storm tracks immediately on the northern flank of the strong
subtropical jet over the oceans. In the NAE sector, a strong subtropical jet stream (ST
in Fig. 3.19a) occurs between the extratropical North America trough and the
tropical western North Atlantic ridge (T and R, respectively in Fig. 3.19a). The
subtropical jet weakens eastward because of the relatively low sea surface temperatures (SSTs) in the equatorial Atlantic Ocean, and it peaks up between the Central
Europe-Mediterranean Sea trough and the East Africa ridge.
50
J. M. Sánchez-Laulhé et al.