monsoon, and the very prominent Tibetan anticyclone, associated to the Asian
monsoon, that extends westward over North Africa.
The western African monsoon is peculiar, of a much smaller size than the other
two NH monsoon systems, and situated deep in the Tropics. It is associated, with a
lower level Saharan heat trough centered over North Africa, with a Saharan anticyclone in the middle troposphere—under the western side of the Tibetan high- and
with a mid-level jet east of the peripheral southern part of the Saharan high. Unlike
other monsoons, precipitations of the African monsoon locate in the south flank of
the Saharan monsoon high.
The oceanic subtropical anticyclones are outstanding features in seasonal maps of
sea mean level pressure. Located at about 30
latitude in both hemispheres, under the
average position of the subtropical jet stream, they occupy 40% of the surface of the
globe and exist throughout the year. Subtropical anticyclones appear as a maximum
of ψ in lower levels (Fig. 3.18), reaching mean pressure maximum values, except in
the case of the winter Siberian high (Fig. 3.20). Subtropical anticyclones play a
central role in the Earth’s climate system. To a large extent, they connect the tropical
and the midlatitude atmospheric regimes, determine tropical cyclones tracks and
subtropical deserts location, and drive oceanic subtropical gyres and hence also the
warm western boundary currents that carry warm waters to poles.
Extension, position, and intensity of the North Atlantic subtropical anticyclone—
the Azores High—varies throughout the year. In the winter, the Azores High is part
of a zonal belt of high pressure whose primary forcing is attributed to the strong
zonal-mean descent branch of the Hadley global cell in the wintertime Hemisphere.
Planetary-scale waves modify the belt of high pressure; so, the eastern North
America trough shrinks the west flank of the Azores High while the eastern North
Atlantic ridge stretches the east flank. The Mediterranean Sea low-pressure area,
which is a prolongation of the Central Europe trough, interrupts the subtropical belt
of high pressure. This fact is imputed to the high population of Mediterranean
cyclones, partly determined by excursions of the Polar Jet Stream and the
European trough in winter, which favors cyclogenesis over the area (Xoplaki
2002). Poleward of the Azores High, intense pressure gradients can be successively
observed that are associated to the westerly midlatitude winds and the Iceland
subpolar low-pressure zone. The Iceland Low presents a cellular aspect, and is due
to both, thermal effects in lower levels of the relatively warm underlying ocean, and
its position downstream of the major North America trough at the upper troposphere,
where the storm track is located. The Azores High is the only subtropical anticyclone
maintaining in the winter season a certain cellular structure, and forming a meridional pressure dipole together with the Iceland Low. The pattern of variability in the
intensity of this pressure dipole is known as the North Atlantic Oscillation (NAO),
very interrelated to the hemispheric Arctic Oscillation.
The Azores High moves in summer over the Atlantic basin poleward and
expands, primarily due to the contraction of the Iceland Low. This contraction is
caused by the considerable continental warming from 60
to 90
N and the subsequent decrease in temperature difference with the ocean, with the consequent loss of
baroclinicity and reduction of the processes of cyclogenesis. Moreover, in terms of
54
J. M. Sánchez-Laulhé et al.
monsoon, that extends westward over North Africa.
The western African monsoon is peculiar, of a much smaller size than the other
two NH monsoon systems, and situated deep in the Tropics. It is associated, with a
lower level Saharan heat trough centered over North Africa, with a Saharan anticyclone in the middle troposphere—under the western side of the Tibetan high- and
with a mid-level jet east of the peripheral southern part of the Saharan high. Unlike
other monsoons, precipitations of the African monsoon locate in the south flank of
the Saharan monsoon high.
The oceanic subtropical anticyclones are outstanding features in seasonal maps of
sea mean level pressure. Located at about 30
latitude in both hemispheres, under the
average position of the subtropical jet stream, they occupy 40% of the surface of the
globe and exist throughout the year. Subtropical anticyclones appear as a maximum
of ψ in lower levels (Fig. 3.18), reaching mean pressure maximum values, except in
the case of the winter Siberian high (Fig. 3.20). Subtropical anticyclones play a
central role in the Earth’s climate system. To a large extent, they connect the tropical
and the midlatitude atmospheric regimes, determine tropical cyclones tracks and
subtropical deserts location, and drive oceanic subtropical gyres and hence also the
warm western boundary currents that carry warm waters to poles.
Extension, position, and intensity of the North Atlantic subtropical anticyclone—
the Azores High—varies throughout the year. In the winter, the Azores High is part
of a zonal belt of high pressure whose primary forcing is attributed to the strong
zonal-mean descent branch of the Hadley global cell in the wintertime Hemisphere.
Planetary-scale waves modify the belt of high pressure; so, the eastern North
America trough shrinks the west flank of the Azores High while the eastern North
Atlantic ridge stretches the east flank. The Mediterranean Sea low-pressure area,
which is a prolongation of the Central Europe trough, interrupts the subtropical belt
of high pressure. This fact is imputed to the high population of Mediterranean
cyclones, partly determined by excursions of the Polar Jet Stream and the
European trough in winter, which favors cyclogenesis over the area (Xoplaki
2002). Poleward of the Azores High, intense pressure gradients can be successively
observed that are associated to the westerly midlatitude winds and the Iceland
subpolar low-pressure zone. The Iceland Low presents a cellular aspect, and is due
to both, thermal effects in lower levels of the relatively warm underlying ocean, and
its position downstream of the major North America trough at the upper troposphere,
where the storm track is located. The Azores High is the only subtropical anticyclone
maintaining in the winter season a certain cellular structure, and forming a meridional pressure dipole together with the Iceland Low. The pattern of variability in the
intensity of this pressure dipole is known as the North Atlantic Oscillation (NAO),
very interrelated to the hemispheric Arctic Oscillation.
The Azores High moves in summer over the Atlantic basin poleward and
expands, primarily due to the contraction of the Iceland Low. This contraction is
caused by the considerable continental warming from 60
to 90
N and the subsequent decrease in temperature difference with the ocean, with the consequent loss of
baroclinicity and reduction of the processes of cyclogenesis. Moreover, in terms of
54
J. M. Sánchez-Laulhé et al.