and over the subtropical Atlantic from the Azores across the Iberian Peninsula, is the
dominant mode of winter climate variability in the North Atlantic region (Hurrell
1995; Hurrell and Van Loon 1997; Cassou et al. 2004). The NAO is strongest in the
winter, but it is also evident during the whole year in the Northern Hemisphere
(Barnston and Livezey 1987), although it is less dominant and has both smaller
amplitude and spatial extent. Accordingly, the associated fluctuations of surface
pressure, temperature, and precipitation occur throughout the year [e.g., Folland
et al. 2009 discuss the summer NAO]. NAO presents a strong interannual to
interdecadal variability that is visible in the temporal evolution of the number of
days attributed to NAO+ and NAOÀ over a complete winter season showed in
Fig. 3.23e, f.
The NAO plays an important role in the interannual variability of European and
North African climate (e.g., Marshall et al. 2001) and it is the most important
mechanism responsible for the strong interannual precipitation variability observed
in the western Mediterranean region (e.g., Rodríguez-Puebla et al. 1998; Trigo et al.
2002), particularly during the winter months, in which this pattern is more active
(Hurrell et al. 2003).
In the winter, across much of the NH, and specifically across wide regions of the
North Atlantic Ocean, Europe, and the Mediterranean, surface air temperatures and
SSTs are significantly correlated with NAO variability. In the NAO+ phase, the
pressure minimum associated with the Iceland Low is deepened, while the pressure
maximum associated with the Azores High is enhanced. This phase leads to
reinforced and northward displaced westerly winds favoring warmer than normal
temperatures in central (Wibig and Głowicki 2002) and northern Europe, and over
the northern part of the Mediterranean region, and cooler conditions over the
southern part (Xoplaki 2002; Trigo et al. 2002, 2006), with an inverse pattern for
the NAOÀ. However, several studies have shown that the NAO influence on
European temperatures is not constant over time (Jacobeit et al. 2001; Slonosky
et al. 2001; Slonosky and Yiou 2002). Although most studies do not return significant correlations NAO-air temperature in the Alboran area, Báez et al. (2013) have
found a significant relation between SST in the Alboran Sea and NAO and AO of
previous seasons.
More important is the impact on precipitation variability due to corresponding
changes in storm track activity. Changes in the mean circulation patterns over the
North Atlantic are accompanied by changes in the intensity and number of storms,
and their paths, that are reflected by important changes in transport and convergence
of atmospheric moisture and, thus, distribution of evaporation and precipitation.
During positive NAO phases, there is a northward shift of the storm track with
enhanced activity. This leads, in general, to wet conditions from Iceland to northern
Europe, although the greatest variations in the precipitation may occur over
the Atlantic, as pointed out by Scaife et al. (2005), and tend to dry conditions from
the Azores across the Iberian Peninsula, northwestern Africa, and over large parts of
the western and northern Mediterranean (Marshall et al. 2001). The anomalies are
reversed for the negative phase (e.g., Ulbrich et al. 1999; Trigo et al. 2004, 2006).
NAOÀ induces above-normal precipitation in southern Europe, particularly over the
3 Alboran Sea Area Climate and Weather
65
dominant mode of winter climate variability in the North Atlantic region (Hurrell
1995; Hurrell and Van Loon 1997; Cassou et al. 2004). The NAO is strongest in the
winter, but it is also evident during the whole year in the Northern Hemisphere
(Barnston and Livezey 1987), although it is less dominant and has both smaller
amplitude and spatial extent. Accordingly, the associated fluctuations of surface
pressure, temperature, and precipitation occur throughout the year [e.g., Folland
et al. 2009 discuss the summer NAO]. NAO presents a strong interannual to
interdecadal variability that is visible in the temporal evolution of the number of
days attributed to NAO+ and NAOÀ over a complete winter season showed in
Fig. 3.23e, f.
The NAO plays an important role in the interannual variability of European and
North African climate (e.g., Marshall et al. 2001) and it is the most important
mechanism responsible for the strong interannual precipitation variability observed
in the western Mediterranean region (e.g., Rodríguez-Puebla et al. 1998; Trigo et al.
2002), particularly during the winter months, in which this pattern is more active
(Hurrell et al. 2003).
In the winter, across much of the NH, and specifically across wide regions of the
North Atlantic Ocean, Europe, and the Mediterranean, surface air temperatures and
SSTs are significantly correlated with NAO variability. In the NAO+ phase, the
pressure minimum associated with the Iceland Low is deepened, while the pressure
maximum associated with the Azores High is enhanced. This phase leads to
reinforced and northward displaced westerly winds favoring warmer than normal
temperatures in central (Wibig and Głowicki 2002) and northern Europe, and over
the northern part of the Mediterranean region, and cooler conditions over the
southern part (Xoplaki 2002; Trigo et al. 2002, 2006), with an inverse pattern for
the NAOÀ. However, several studies have shown that the NAO influence on
European temperatures is not constant over time (Jacobeit et al. 2001; Slonosky
et al. 2001; Slonosky and Yiou 2002). Although most studies do not return significant correlations NAO-air temperature in the Alboran area, Báez et al. (2013) have
found a significant relation between SST in the Alboran Sea and NAO and AO of
previous seasons.
More important is the impact on precipitation variability due to corresponding
changes in storm track activity. Changes in the mean circulation patterns over the
North Atlantic are accompanied by changes in the intensity and number of storms,
and their paths, that are reflected by important changes in transport and convergence
of atmospheric moisture and, thus, distribution of evaporation and precipitation.
During positive NAO phases, there is a northward shift of the storm track with
enhanced activity. This leads, in general, to wet conditions from Iceland to northern
Europe, although the greatest variations in the precipitation may occur over
the Atlantic, as pointed out by Scaife et al. (2005), and tend to dry conditions from
the Azores across the Iberian Peninsula, northwestern Africa, and over large parts of
the western and northern Mediterranean (Marshall et al. 2001). The anomalies are
reversed for the negative phase (e.g., Ulbrich et al. 1999; Trigo et al. 2004, 2006).
NAOÀ induces above-normal precipitation in southern Europe, particularly over the
3 Alboran Sea Area Climate and Weather
65