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Northern hemisphere blockings and their impacts over the european continent : historical overview and associated mechanisms
Publication . Sousa, Pedro M; Trigo, Ricardo M., 1967-; Barriopedro, David
Blocking high pressure systems are large-scale atmospheric circulation patterns with meteorological impacts that vary across regions and seasons, depending on the blocking location, spatial characteristics and temporal length. While blocking episodes are an important component of intra-seasonal and inter-annual variability at mid-latitudes, particularly in Europe, previous studies have mostly focused on characterizing European blocking impacts on either winter or summer seasons. In this thesis, a thorough characterization of Euro-Atlantic blocking occurrence within different longitudinal sectors (Atlantic, European and Russian) was performed, followed by a comprehensive analysis of their seasonal impacts on temperature and on the precipitation regimes of the European continent and specific sub-regions, with particular emphasis on Iberia, and on extreme events. In order to distinguish high-latitude blocking from other common high pressure systems affecting Europe, namely sub-tropical ridges, a novel ridge detection scheme was developed. Ridges do not require a wave-breaking occurrence as blockings do, although they are frequent precursors of wave-breaking, which may lead to blocking. Thus, as an additional novelty, this thesis also characterizes the distinctive seasonal impacts associated with sub-tropical ridges occurring at different longitudinal sectors of the Euro-Atlantic region. Finally, an assessment of the involved mechanisms behind the temperature and precipitation responses to blocking is also carried out. This included for precipitation the role of cyclonic activity (storm-tracks and cut-off lows), moisture transport, and large-scale atmospheric instability, while for temperature the focus was on the role played by horizontal advection, subsidence and imbalances in various radiation budgets. This distinction clarifies that most extreme heat episodes in southern Europe and Mediterranean areas should not be attributed to blockings, but rather to ridges. In central and northern areas of the continent, both regimes are responsible for warm conditions in summer, due to enhanced radiative heating and increased subsidence. During winter, blocking and ridges lead to opposite temperature responses. Blocking reinforces cold northerly advection in its eastern flank, thus promoting European cold winter spells, especially those located in the eastern Atlantic and western Europe, while mild Atlantic flows associated to ridge patterns result in warmer conditions. Regarding the impacts on precipitation, blocking and ridges are associated with a marked northsouth dipole for the three considered longitudinal sectors of occurrence. While blocking patterns force a split of the storm-track, ridges are associated with a stronger zonal flow at higher latitudes. Thus, negative (positive) precipitation anomalies during blocks occur at higher (lower) latitudes. Enhanced atmospheric instability and cyclonic activity south of blocking centers relate very well with increased rainfall in southern Europe, including Iberia, where torrential regimes are more relevant in the precipitation totals. This dipole is reversed during ridges, which lead to dry conditions in southern Europe. The seasonal analysis further reveals that winters characterized by high frequencies of blocking (ridge) occurrence present above (below) average snow covered soils. Blocking/ridge impacts on temperature and precipitation have also additional effects on meteorological and climate extreme events with large socio economic impacts. Thus, the results show a blocking/ridge role at different time-scales in controlling a significant part of wildfire inter annual variability in Mediterranean areas. Meteorological pre-conditioning raises vegetation stress during dry winters, while increased ridge and blocking occurrence in summer fosters meteorological conditions which are prone to fire ignition and spread. Finally, outputs from state-of-the-art General Circulation Models raise the challenge to develop further these automated schemes for blocking/ridges detection, as their representation in climate models is crucial to understand impacts towards the later decades of the 21st century. Summing up, the main novelties and achievements of this thesis are: 1) the clear disentanglement between blocks and ridges; 2) the seasonal analyses of specific regional impacts resulting from different locations of blocking/ridge structures; 3) the detailed analysis of dynamical/physical forcing mechanisms associated to each considered weather pattern.
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Fundação para a Ciência e a Tecnologia
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SFRH
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SFRH/BD/84395/2012