south of the Atlas mountain ridge enters the central part of the basin from south-west.
However, in case of strong European blocking, retrogressive upper-level cut-off
lows/PV maxima of high impact can affect Alboran.
In cases of prevailing blocking regime over the European–Atlantic sector, and/or
in case of negative NAO, cyclonic activity in the Mediterranean is generally
enhanced. The period between 1957 and 1980 was marked by a significant increase
in Northern Atlantic moderate-to-deep cyclones frequency. During this period,
linked to positive-to-negative shifts of the Atlantic Multidecadal Oscillation
(AMO; Knight et al. 2005; Trenberth et al. 2017) between 1957 and 1980 (Varino
et al. 2018), polar regions underwent a significant cooling over the whole troposphere that increased and shifted poleward the midlatitude meridional temperature
gradient and the baroclinicity.
The high variability of the Atlantic storm track has a profound impact on the
Mediterranean hydroclimate (e.g., Corte-Real et al. 1995; Trigo et al. 2000;
Dunkeloh and Jacobeit 2003; Xoplaki et al. 2004; Krichak and Alpert 2005).
There are indications of a poleward shift of the Atlantic storm track and a
strengthening north of the British Isles, and a weakening of the Mediterranean
storm track with the climate change (Bengtsson et al. 2006; Pinto et al. 2007;
Zappa et al. 2013). For the period 1961–1999, Ziv et al. (2013) found a latetwentieth-century decrease in the number of Mediterranean cyclones. This was in
agreement with Trigo et al. (2000), which attributed the concomitant decline in the
Mediterranean rainfall to the weakening of Mediterranean cyclones. However,
according to Kelley et al. (2011, 2012) and Hoerling et al. (2012), the late-twentiethcentury drying was dominated by natural variability of the NAO, so that it is not
necessary to appeal as a cause of this drying to rising greenhouse gases.
3.2.4 Euro-Atlantic Weather Regimes
Although the extratropical planetary-scale wave patterns are geographically
anchored, they do change in time, either because the heating patterns in the atmosphere vary or because of internal (chaotic) processes. The results are robust
circulations, called weather regimes, which have time scales longer than that associated with developing baroclinic systems but shorter than a season. Weather
regimes have a typical 6–10 day nominal persistence and are spatially well defined
(typically the width of an oceanic basin) and limited in number. They could be
understood as envelopes for daily atmospheric variability. While the day-to-day
meteorological fluctuations can be described in terms of temporal transition between
regimes, the year-to-year (or longer timescale) climate fluctuations can be interpreted
as changes in their frequency of occurrence provided the hypothesis of long-term
quasi-stationary climate (Cassou 2009). A consequence of the transient behavior of
the atmospheric planetary waves is that anomalies in climate on seasonal time scales
typically occur over large geographic regions. Some regions may be cooler or
perhaps drier than average, while at the same time thousands of kilometers away,
62
J. M. Sánchez-Laulhé et al.
However, in case of strong European blocking, retrogressive upper-level cut-off
lows/PV maxima of high impact can affect Alboran.
In cases of prevailing blocking regime over the European–Atlantic sector, and/or
in case of negative NAO, cyclonic activity in the Mediterranean is generally
enhanced. The period between 1957 and 1980 was marked by a significant increase
in Northern Atlantic moderate-to-deep cyclones frequency. During this period,
linked to positive-to-negative shifts of the Atlantic Multidecadal Oscillation
(AMO; Knight et al. 2005; Trenberth et al. 2017) between 1957 and 1980 (Varino
et al. 2018), polar regions underwent a significant cooling over the whole troposphere that increased and shifted poleward the midlatitude meridional temperature
gradient and the baroclinicity.
The high variability of the Atlantic storm track has a profound impact on the
Mediterranean hydroclimate (e.g., Corte-Real et al. 1995; Trigo et al. 2000;
Dunkeloh and Jacobeit 2003; Xoplaki et al. 2004; Krichak and Alpert 2005).
There are indications of a poleward shift of the Atlantic storm track and a
strengthening north of the British Isles, and a weakening of the Mediterranean
storm track with the climate change (Bengtsson et al. 2006; Pinto et al. 2007;
Zappa et al. 2013). For the period 1961–1999, Ziv et al. (2013) found a latetwentieth-century decrease in the number of Mediterranean cyclones. This was in
agreement with Trigo et al. (2000), which attributed the concomitant decline in the
Mediterranean rainfall to the weakening of Mediterranean cyclones. However,
according to Kelley et al. (2011, 2012) and Hoerling et al. (2012), the late-twentiethcentury drying was dominated by natural variability of the NAO, so that it is not
necessary to appeal as a cause of this drying to rising greenhouse gases.
3.2.4 Euro-Atlantic Weather Regimes
Although the extratropical planetary-scale wave patterns are geographically
anchored, they do change in time, either because the heating patterns in the atmosphere vary or because of internal (chaotic) processes. The results are robust
circulations, called weather regimes, which have time scales longer than that associated with developing baroclinic systems but shorter than a season. Weather
regimes have a typical 6–10 day nominal persistence and are spatially well defined
(typically the width of an oceanic basin) and limited in number. They could be
understood as envelopes for daily atmospheric variability. While the day-to-day
meteorological fluctuations can be described in terms of temporal transition between
regimes, the year-to-year (or longer timescale) climate fluctuations can be interpreted
as changes in their frequency of occurrence provided the hypothesis of long-term
quasi-stationary climate (Cassou 2009). A consequence of the transient behavior of
the atmospheric planetary waves is that anomalies in climate on seasonal time scales
typically occur over large geographic regions. Some regions may be cooler or
perhaps drier than average, while at the same time thousands of kilometers away,
62
J. M. Sánchez-Laulhé et al.