The North Atlantic storm track is usually far away from the Alboran Sea,
although the behavior of the storm track is highly variable and Gibraltar is one of
the gates through which Atlantic cyclones enter the Mediterranean basin. In the
Alboran area, the fc field presents a zonal minimum in the NAE sector longitudes,
being f c < 5% in winter and f c ¼ 0 in the summer, corresponding the f c relative
summer maxima over the Iberian Peninsula to the presence of heat lows.
The pattern of frequency density of Fig. 3.21 compares well with the frequency
density (or track density) produced by other studies (e.g., Sinclair 1997; Sickmöller
et al. 2000; Hoskins and Hodges 2002; Dacre and Gray 2009).
Besides midlatitude storms, weather in Europe strongly depends on life cycles of
Rossby waves that propagate along the slowly varying part of the North Atlantic jet
stream (Martius et al. 2010). Early stages of midlatitude storm life cycle are
dominated by the one-way influence of the baroclinic zone on the cyclone, but in
their late stages, cyclones and anticyclones may strongly modify its environment,
causing the jet stream to meander over thousands of kilometers. This wavy pattern
also migrates eastward, forming the transient Rossby waves. The strong meridional
potential vorticity (PV) gradient associated with the jet stream serves as a waveguide
for propagating Rossby waves. Frequently, small disturbances in the jet entrance
region over eastern North America grow and evolve into large-amplitude features in
the European sector where they often break (e.g., Schwierz et al. 2004). Rossby
wave breaking (RWB) leads to the development of upper cold troughs on the
equatorward side, associated to elongated tongues of high PV stratospheric air
(PV > 2 PVU; 1 PVU 1 Â 10
À6 K kg
À1 m
2 s
À1 ). These structures with
anomalously high PV can exert a profound far-field impact on the flow throughout
the troposphere (Hoskins et al. 1985). Frequently, the high PV tongues are further
stretched into narrow filaments, PV filamentation, resulting from tropopause folds,
so-called PV streamers. PV streamers are widely recognized as upper-tropospheric
precursors for Mediterranean cyclogenesis, and as trigger of high impact weather, in
particular heavy precipitation and flooding events, when sufficient moisture is
available within the lower to mid-troposphere. Several studies have related PV
streamer and heavy precipitation over Spain (e.g., Jacobeit 1987; Romero et al.
1999; Nieto et al. 2007; Merino et al. 2016), over the Mediterranean region
(Massacand et al. 1998; Martius et al. 2006, 2007; Raveh-Rubin and Wernli
2015), over Alps (e.g., Martius et al. 2006), and over Africa (Knippertz and Martin
2005). RWB events are also important for the large-scale flow itself as they reinforce
weather regimes such as blocking ridges (Michel and Rivière 2011; Spensberger and
Spengler 2014).
PV streamers eventually break up into distinct upper level cut off low systems
(COLs), completely detached from the main westerly current, associated to high-PV
cutoff vortices air into the troposphere (e.g., Appenzeller and Davies 1992; Nieto
et al. 2008). Along with its slowly southward excursion, a COL usually maintains
the characteristics of a high PV anomaly: Most COLs present a weak or moderate
cyclonic flow in lower tropospheric layers, whose magnitude decreases toward the
earth’s surface, low temperature and reduced static stability in the middle and lower
troposphere. Because of their long-term persistence, cold vortices, interacting with
3 Alboran Sea Area Climate and Weather
59
although the behavior of the storm track is highly variable and Gibraltar is one of
the gates through which Atlantic cyclones enter the Mediterranean basin. In the
Alboran area, the fc field presents a zonal minimum in the NAE sector longitudes,
being f c < 5% in winter and f c ¼ 0 in the summer, corresponding the f c relative
summer maxima over the Iberian Peninsula to the presence of heat lows.
The pattern of frequency density of Fig. 3.21 compares well with the frequency
density (or track density) produced by other studies (e.g., Sinclair 1997; Sickmöller
et al. 2000; Hoskins and Hodges 2002; Dacre and Gray 2009).
Besides midlatitude storms, weather in Europe strongly depends on life cycles of
Rossby waves that propagate along the slowly varying part of the North Atlantic jet
stream (Martius et al. 2010). Early stages of midlatitude storm life cycle are
dominated by the one-way influence of the baroclinic zone on the cyclone, but in
their late stages, cyclones and anticyclones may strongly modify its environment,
causing the jet stream to meander over thousands of kilometers. This wavy pattern
also migrates eastward, forming the transient Rossby waves. The strong meridional
potential vorticity (PV) gradient associated with the jet stream serves as a waveguide
for propagating Rossby waves. Frequently, small disturbances in the jet entrance
region over eastern North America grow and evolve into large-amplitude features in
the European sector where they often break (e.g., Schwierz et al. 2004). Rossby
wave breaking (RWB) leads to the development of upper cold troughs on the
equatorward side, associated to elongated tongues of high PV stratospheric air
(PV > 2 PVU; 1 PVU 1 Â 10
À6 K kg
À1 m
2 s
À1 ). These structures with
anomalously high PV can exert a profound far-field impact on the flow throughout
the troposphere (Hoskins et al. 1985). Frequently, the high PV tongues are further
stretched into narrow filaments, PV filamentation, resulting from tropopause folds,
so-called PV streamers. PV streamers are widely recognized as upper-tropospheric
precursors for Mediterranean cyclogenesis, and as trigger of high impact weather, in
particular heavy precipitation and flooding events, when sufficient moisture is
available within the lower to mid-troposphere. Several studies have related PV
streamer and heavy precipitation over Spain (e.g., Jacobeit 1987; Romero et al.
1999; Nieto et al. 2007; Merino et al. 2016), over the Mediterranean region
(Massacand et al. 1998; Martius et al. 2006, 2007; Raveh-Rubin and Wernli
2015), over Alps (e.g., Martius et al. 2006), and over Africa (Knippertz and Martin
2005). RWB events are also important for the large-scale flow itself as they reinforce
weather regimes such as blocking ridges (Michel and Rivière 2011; Spensberger and
Spengler 2014).
PV streamers eventually break up into distinct upper level cut off low systems
(COLs), completely detached from the main westerly current, associated to high-PV
cutoff vortices air into the troposphere (e.g., Appenzeller and Davies 1992; Nieto
et al. 2008). Along with its slowly southward excursion, a COL usually maintains
the characteristics of a high PV anomaly: Most COLs present a weak or moderate
cyclonic flow in lower tropospheric layers, whose magnitude decreases toward the
earth’s surface, low temperature and reduced static stability in the middle and lower
troposphere. Because of their long-term persistence, cold vortices, interacting with
3 Alboran Sea Area Climate and Weather
59