The recurrence of high-pressure centres (type B) increases, while the frequency of type E weather is lower than in the developing phase. Also, a slight increase in zonal circulation can be observed, although its recurrence is lower than during general conditions. The attenuation phase (10 days) can

be characterised by the restoration of the overall circulation frequency with even more intense western (32% compared to 23% for the overall circulation) flows (type A), Vorinostat supplier which bring cooler and moister air. The northern flow (type D) is also very favourable for dry period attenuation, while the recurrence of the high-pressure centre (type B) decreased (Figure 2). The regional differences are not so specific, although some peculiarities can be identified. The influence of the meridional flows (types E and F) on dry period development and a persisting phase is higher in the west than in the other regions, whereas in south-eastern and north-eastern Lithuania high-pressure centres (type B) are a more frequent cause of dry periods. The largest regional difference is determined during the attenuation phase. A

very strong shift from meridional to zonal circulation (Table 2) in south-eastern Lithuania is observed. Meanwhile, a strong increase in the northern flow (type D) during the attenuation phase is common to all parts of the country. The NAO and AO indices for the periods 30 days prior to dry period events have been grouped into three different clusters. The first group indicates a prevailing negative NAO/AO phase during these periods, the second a stepwise weakening of the positive NAO/AO phase,

and the third a different mean course for the NAO and AO selleck screening library indices. A strong positive NAO index after the first 15 days drops Non-specific serine/threonine protein kinase close to zero, while the AO index shows a permanent strengthening of the positive AO phase during the first 20 days, followed by a slight weakening (Figure 3). The first cluster aggregates the highest number of dry period preconditions – 55%, the second – 25%, and the third – 20%. The AO index in most of the cases shows a greater magnitude than the NAO, and also the number of members in every cluster is distributed more evenly in the case of AO than in the NAO. For this reason all composites for every cluster were made only for AO cluster members. Composite analysis of 500 hPa height anomalies calculated as a mean field for every AO index time series cluster reveals three different preconditioning circulation patterns before dry period events. The first one resembles a typical summer western European blocking pattern, which tends to propagate further eastwards. However, this mean field aggregates different synoptic development patterns (Figure 4a). Many of the events included in this pattern represent the slow movement of an upper high from the mid-Atlantic to western Europe, while others depict the slow development of a cut-off-low over the western Mediterranean, or the retreat of an upper low from Scandinavia to northern Asia.