intensity relative to the zonal mean pressure, Azores High reaches its peak. In this
season, the Azores High shows a cell-type configuration with a ridgeline following a
northeast–southwest orientation, approximately similar to that of the African Atlantic coastline. The western Mediterranean basin is dominated by the expanded
subtropical ridge that toward the east is displaced southward by a trough extending
northwest from the Arabian Gulf, which is a prolongation of the Indian summer
monsoon depression. A tight pressure gradient along the coast appears between the
Azores High and the Saharan thermal trough and, to a lesser extent, between the
Azores High and the Iberian thermal trough.
On eastern flanks of subtropical anticyclones there exist regions of strong atmospheric descent, notably intense in summer, showing a maximum (minimum) of χ in
upper (lower) levels (Fig. 3.18), determining mostly dry weather. Similar summer
descent regions of subtropical anticyclones appear on coastal regions of California,
Chile, southwest Africa, and Australia, and the Mediterranean basin. The climates of
all these regions, including the Alboran sea area, can be classified as “Mediterranean
type.” Where skies are not clear, maritime clouds are of low type, mostly stratocumulus developed in the maritime boundary layer (MBL). Above the MBL, there is a
relatively shallow layer—either isothermal or containing a subsidence inversion—
that acts as a convectively stable interface separating the MBL air from the potentially, or actually, warmer, middle troposphere air. This layer has very low relative
humidity, due to the predominant, faint but persistent, large-scale subsidence prevent
cloudiness formation or maintenance.
There is no unanimity on the primary forcing mechanism involved in the formation and intensification of subtropical anticyclones in summer. Rodwell and Hoskins
(2001) suggested that the Mediterranean-type climates may be induced remotely by
the monsoon to the east, attributing as primary forcing for the air adiabatic descent
over the eastern North Atlantic, the interaction between the large scale Rossby wave
forced by the diabatic warming of the Asian monsoon and the midlatitude westerlies.
Below the descent, feedbacks, involving SST and MBL clouds, further enhance
descent over these regions. However, other studies (Miyasaka and Nakamura 2005;
Nakamura 2012; Wu and Liu 2003) suggested that the formation and variation of
Azores High are mainly determined by the spatial distribution and variations of
diabatic heating, producing a tight thermal contrast between the heated dry landmasses over the western portion of the North-Western Africa and the relatively cool
ocean off the coast.
The summer atmospheric circulation over the Mediterranean basin is characterized by localized intense subsidence and low-level northerlies over the central to
eastern portion of the basin, presenting χ another maximum (minimum) of in upper
(lower) levels (Fig. 3.18). This subsidence was attributed by Simpson et al. (2015) to
the influence of the elevated terrain of the Middle East on the Mediterranean
summertime circulation. Between the strong descents and low-level northerlies
over both the eastern Atlantic and the central to eastern Mediterranean, there is an
area of ascent, that include Alboran, attributed to the interaction between the Atlas
Mountains Range and the predominant low-level easterly winds. Descents areas in
summer on the Mediterranean and subtropical East Atlantic are marked by “C”
3 Alboran Sea Area Climate and Weather
55
season, the Azores High shows a cell-type configuration with a ridgeline following a
northeast–southwest orientation, approximately similar to that of the African Atlantic coastline. The western Mediterranean basin is dominated by the expanded
subtropical ridge that toward the east is displaced southward by a trough extending
northwest from the Arabian Gulf, which is a prolongation of the Indian summer
monsoon depression. A tight pressure gradient along the coast appears between the
Azores High and the Saharan thermal trough and, to a lesser extent, between the
Azores High and the Iberian thermal trough.
On eastern flanks of subtropical anticyclones there exist regions of strong atmospheric descent, notably intense in summer, showing a maximum (minimum) of χ in
upper (lower) levels (Fig. 3.18), determining mostly dry weather. Similar summer
descent regions of subtropical anticyclones appear on coastal regions of California,
Chile, southwest Africa, and Australia, and the Mediterranean basin. The climates of
all these regions, including the Alboran sea area, can be classified as “Mediterranean
type.” Where skies are not clear, maritime clouds are of low type, mostly stratocumulus developed in the maritime boundary layer (MBL). Above the MBL, there is a
relatively shallow layer—either isothermal or containing a subsidence inversion—
that acts as a convectively stable interface separating the MBL air from the potentially, or actually, warmer, middle troposphere air. This layer has very low relative
humidity, due to the predominant, faint but persistent, large-scale subsidence prevent
cloudiness formation or maintenance.
There is no unanimity on the primary forcing mechanism involved in the formation and intensification of subtropical anticyclones in summer. Rodwell and Hoskins
(2001) suggested that the Mediterranean-type climates may be induced remotely by
the monsoon to the east, attributing as primary forcing for the air adiabatic descent
over the eastern North Atlantic, the interaction between the large scale Rossby wave
forced by the diabatic warming of the Asian monsoon and the midlatitude westerlies.
Below the descent, feedbacks, involving SST and MBL clouds, further enhance
descent over these regions. However, other studies (Miyasaka and Nakamura 2005;
Nakamura 2012; Wu and Liu 2003) suggested that the formation and variation of
Azores High are mainly determined by the spatial distribution and variations of
diabatic heating, producing a tight thermal contrast between the heated dry landmasses over the western portion of the North-Western Africa and the relatively cool
ocean off the coast.
The summer atmospheric circulation over the Mediterranean basin is characterized by localized intense subsidence and low-level northerlies over the central to
eastern portion of the basin, presenting χ another maximum (minimum) of in upper
(lower) levels (Fig. 3.18). This subsidence was attributed by Simpson et al. (2015) to
the influence of the elevated terrain of the Middle East on the Mediterranean
summertime circulation. Between the strong descents and low-level northerlies
over both the eastern Atlantic and the central to eastern Mediterranean, there is an
area of ascent, that include Alboran, attributed to the interaction between the Atlas
Mountains Range and the predominant low-level easterly winds. Descents areas in
summer on the Mediterranean and subtropical East Atlantic are marked by “C”
3 Alboran Sea Area Climate and Weather
55