Weather extremes in a changing climate : variability, mechanisms and societal impacts

Extreme weather events and associated impacts have been gaining increasing attention from researchers, stakeholders and the general public. Among extreme weather events, extratropical cyclones (ETCs) do represent one of the major causes of disruption as they are often associated with extreme precipitation and wind. The Iberian Peninsula (IP) is a key area in this regard due to its location, south of the North Atlantic (NA) storm track exit region. This thesis analyses the North-Atlantic cyclones variability, its link to large-scale frontal system variability and to precipitation and wind extremes, in a compound weather extreme perspective. Several precipitation data sources are assessed over the IP, including the last reanalysis product from ECMWF (ERA5). The methodology is based on grouping precipitation days according to anomalies thresholds, considering both the intensity and the spatial extent. The main result is that ERA5 overperforms the other products, although satellite data have very good performances when the precipitation anomalies are large. Therefore, ERA5 is used to compute a novel ranking of concurrent precipitation and wind events for the IP. The top100 concurrent events are detailed in terms of the attending cyclones and the spatial pattern of meteorological extremes. Novel subregional characterization of concurrent precipitation and wind events and associated cyclones is produced for the IP, showing that the northwestern domain is the most affected by concurrent events. Within ETCs, the precipitation and wind spatial patterns, which determine the damage swath, are organized around the cyclone’s center and along the fronts. A methodology to objectively associate precipitation to fronts at a sub-daily basis is applied to gridded data sets, including a novel sensitivity test for the optimal size of the allocating area. This new approach enables a comprehensive analysis of the long-term climatology and trend of frontal precipitation on the NA domain

Weather extremes in a changing climate : variability, mechanisms and societal impacts

Extreme weather events and associated impacts have been gaining increasing attention from researchers, stakeholders and the general public. Among extreme weather events, extratropical cyclones (ETCs) do represent one of the major causes of disruption as they are often associated with extreme precipitation and wind. The Iberian Peninsula (IP) is a key area in this regard due to its location, south of the North Atlantic (NA) storm track exit region. This thesis analyses the North-Atlantic cyclones variability, its link to large-scale frontal system variability and to precipitation and wind extremes, in a compound weather extreme perspective. Several precipitation data sources are assessed over the IP, including the last reanalysis product from ECMWF (ERA5). The methodology is based on grouping precipitation days according to anomalies thresholds, considering both the intensity and the spatial extent. The main result is that ERA5 overperforms the other products, although satellite data have very good performances when the precipitation anomalies are large. Therefore, ERA5 is used to compute a novel ranking of concurrent precipitation and wind events for the IP. The top100 concurrent events are detailed in terms of the attending cyclones and the spatial pattern of meteorological extremes. Novel subregional characterization of concurrent precipitation and wind events and associated cyclones is produced for the IP, showing that the northwestern domain is the most affected by concurrent events. Within ETCs, the precipitation and wind spatial patterns, which determine the damage swath, are organized around the cyclone’s center and along the fronts. A methodology to objectively associate precipitation to fronts at a sub-daily basis is applied to gridded data sets, including a novel sensitivity test for the optimal size of the allocating area. This new approach enables a comprehensive analysis of the long-term climatology and trend of frontal precipitation on the NA domain

Mechanisms of subsurface thermal structure and sea surface thermohaline variabilities in the southwestern tropical Pacific during 1975-85

Major features of the southwestern tropical Pacific (SWTP), defined between 160E-140W and 24S-10S, are brought to light through analysis of surface water samples (23000) and temperature/depth observations (8500), both collected by ship opportunity programs during the 1979-85 period. The mean vertical thermal structure (and its related parameters), mean sea surface temperature (SST) and salinity (SSS) are first portrayed, to further quantify the 1979-85 variability. It is demonstrated that the observed seasonal and interannual variabilities, the latter being associated with the strong 1982-83 El Nino Southern Oscillation (ENSO) event, are mostly governed by specific mechanisms involving varying wind field and rainfall regimes