speaking, weather regimes are classes of seasonal large-scale atmospheric circulation patterns gathered together from a similarity criterion, that are defined by their
mean conditions, by their variances, and by their frequency of occurrence.
The weather regimes are characterized by a strong longitudinal dependence with
maximum temporal variance over the northern oceans, especially during boreal
winter. Different methods can be applied to obtain the weather regimes over a
given geographical domain all of them leading to similar patterns (Michelangeli
et al. 1995).
Figure 3.23a–d shows the four wintertime (December–March) weather regimes
of the NAE sector, obtained by Cassou et al. (2010) through cluster analysis from
ERA40 reanalysis 500 hPa geopotential (Z500) anomaly maps over 1958–2002. The
first two clusters (Fig. 3.23a, b) capture the negative and the positive phases of the
NAO (NAOÀ and NAO+, respectively). The third cluster (Fig. 3.23c), Anticyclonic
Ridge regime, displays a strong anticyclonic ridge off Western Europe almost
covering the entire basin, which features the northward extension of the Azores
high. The fourth cluster (Fig. 3.23d), the Scandinavian Blocking regime, exhibits a
zonal pressure dipole between Greenland and Scandinavia with a clear southeastward extension of low-pressure anomalies toward the Iberian Peninsula. The temporal evolution of the number of days attributed to a given regime over the complete
winter season is given in Fig. 3.23e–h. NAE sector weather regimes are a significant
driver of the intraseasonal, interannual, and multidecadal variability of the Atlantic
Ocean fields.
The Atlantic Ridge (AR) regime is reminiscent of the so-called East Atlantic
teleconnection pattern (negative phase, Barnston and Livezey 1987) viewed as a
Euro-Atlantic wave train. It displays an anticyclonic ridge (positive Z500 anomalies)
over the Central-North Atlantic Ocean, and cyclonic conditions over Central Europe
and the Mediterranean (negative Z500 anomalies, Fig. 3.23c). In the winter, temperature variability in the Mediterranean region is to a large extent explained by the
AR pattern (Sáenz et al. 2001). During AR regime, the localization of the Azores
High shifts northwards, increasing significantly the probability of cold extremes
occurrence over the Iberian Peninsula. Concerning Mediterranean precipitation
variability, the AR influence is less distinct, but there are some coupled circulation
rainfall patterns during winter that are being moderately correlated with it (Dunkeloh
and Jacobeit 2003).
The Scandinavian Blocking regime is reminiscent of the Scandinavian
teleconnection in its positive phase pattern, characterized by a strong anomalous
height anomaly over Northern Europe (Tyrlis and Hoskins 2008) and a mild deeper
trough extending south-eastward from the Labrador Sea to the Iberian Peninsula.
This regime is especially important for Mediterranean winter rainfall variability. It
enhances Mediterranean cyclogenesis and leads to widespread above-average precipitation in the Mediterranean region (Xoplaki 2002), and to events of extreme
rainfall in the Spanish Mediterranean coast included Alboran.
The NAO is primarily a north–south dipole characterized by simultaneous out-ofphase zonally elongated sea-level pressure (SLP) anomalies between temperate and
high latitudes over the North Atlantic. The NAO, with centers of action near Iceland
64
J. M. Sánchez-Laulhé et al.
mean conditions, by their variances, and by their frequency of occurrence.
The weather regimes are characterized by a strong longitudinal dependence with
maximum temporal variance over the northern oceans, especially during boreal
winter. Different methods can be applied to obtain the weather regimes over a
given geographical domain all of them leading to similar patterns (Michelangeli
et al. 1995).
Figure 3.23a–d shows the four wintertime (December–March) weather regimes
of the NAE sector, obtained by Cassou et al. (2010) through cluster analysis from
ERA40 reanalysis 500 hPa geopotential (Z500) anomaly maps over 1958–2002. The
first two clusters (Fig. 3.23a, b) capture the negative and the positive phases of the
NAO (NAOÀ and NAO+, respectively). The third cluster (Fig. 3.23c), Anticyclonic
Ridge regime, displays a strong anticyclonic ridge off Western Europe almost
covering the entire basin, which features the northward extension of the Azores
high. The fourth cluster (Fig. 3.23d), the Scandinavian Blocking regime, exhibits a
zonal pressure dipole between Greenland and Scandinavia with a clear southeastward extension of low-pressure anomalies toward the Iberian Peninsula. The temporal evolution of the number of days attributed to a given regime over the complete
winter season is given in Fig. 3.23e–h. NAE sector weather regimes are a significant
driver of the intraseasonal, interannual, and multidecadal variability of the Atlantic
Ocean fields.
The Atlantic Ridge (AR) regime is reminiscent of the so-called East Atlantic
teleconnection pattern (negative phase, Barnston and Livezey 1987) viewed as a
Euro-Atlantic wave train. It displays an anticyclonic ridge (positive Z500 anomalies)
over the Central-North Atlantic Ocean, and cyclonic conditions over Central Europe
and the Mediterranean (negative Z500 anomalies, Fig. 3.23c). In the winter, temperature variability in the Mediterranean region is to a large extent explained by the
AR pattern (Sáenz et al. 2001). During AR regime, the localization of the Azores
High shifts northwards, increasing significantly the probability of cold extremes
occurrence over the Iberian Peninsula. Concerning Mediterranean precipitation
variability, the AR influence is less distinct, but there are some coupled circulation
rainfall patterns during winter that are being moderately correlated with it (Dunkeloh
and Jacobeit 2003).
The Scandinavian Blocking regime is reminiscent of the Scandinavian
teleconnection in its positive phase pattern, characterized by a strong anomalous
height anomaly over Northern Europe (Tyrlis and Hoskins 2008) and a mild deeper
trough extending south-eastward from the Labrador Sea to the Iberian Peninsula.
This regime is especially important for Mediterranean winter rainfall variability. It
enhances Mediterranean cyclogenesis and leads to widespread above-average precipitation in the Mediterranean region (Xoplaki 2002), and to events of extreme
rainfall in the Spanish Mediterranean coast included Alboran.
The NAO is primarily a north–south dipole characterized by simultaneous out-ofphase zonally elongated sea-level pressure (SLP) anomalies between temperate and
high latitudes over the North Atlantic. The NAO, with centers of action near Iceland
64
J. M. Sánchez-Laulhé et al.