Links between blocking circulation and precipitation extremes over Romania in summer

The relationship between atmospheric blocking and extreme precipitation in summer in Romania during the period 1962-2010 is investigated. Percentile-based indices (R90p) for extreme precipitation events are calculated using data recorded at a large number of meteorological stations (98) covering the entire country. The dominant mode of variability of these indices has a monopolar structure and is related to two distinct atmospheric circulation blocking patterns. High frequency of blocking in the 0 degrees-40 degrees E sector is associated with high frequency of extreme precipitation events over Romania. This is due to enhanced synoptic scale activity associated with advection of relatively high potential vorticity from northeast towards Romanian region, as well as with enhanced synoptic scale activity in the Mediterranean region. Enhanced blocking activity in the 50 degrees-70 degrees E sector favours an eastward extension of Atlantic jet, and the associated instabilities are related to extreme precipitation events over large parts of central and Eastern Europe. We argue that these two distinct blocking patterns explain a substantial part of extreme precipitation variability in Romania during summer.