Concurrent Hydroclimatic Hazards from Catchment to Global Scales
Authors: De Luca Paolo 
Published: March 2020

Interactions between multiple hazards can cause socio-economic damages that exceed those expected by the individual hazard components. Over the past decade, the multi-hazards paradigm has emerged to the extent that the Sendai Framework for Disaster Risk Reduction 2015-2030 advocated a multi-hazard approach. This thesis examines three types of concurrent hydroclimatic hazards that can occur at catchment to global scales. The first multi-hazard is the link between multi-basin flooding (MBF) and extra-tropical cyclones (ETCs) over Great Britain during the period 1975-2014. Results show that during the most geographically widespread MBF episode, up to 108 river catchments (or ~46% of the study area) recorded a peak flow annual maximum within a 16-day window. Most extreme MBF episodes were linked to cyclonic Lamb Weather Types (LWTs), atmospheric rivers and very severe gales. These episodes were associated with significant socio-economic impacts due to widespread flooding. The second hazard was observed (1971-2000) and projected (2011-2100) LWTs, whose seasonal frequency and persistence are associated with multi-hazards over the British Isles. Daily sea-level-pressure data from two reanalyses products, one subjective weather pattern catalogue and an ensemble of 10 Atmosphere-Ocean General Circulation Models (AOGCMs) were used to compute LWTs. Results showed that the AOGCMs are overall able to reproduce historical weather pattern persistence, which, along with annual frequency, is projected to significantly increase anticyclonic and decrease cyclonic LWTs, in summer and autumn respectively. This implies a higher risk of drought, heatwaves and air pollution events in summer but reduced likelihood of flooding and severe gales in autumn by the end of the 21st century. By 2100, the AOGCMs suggest an increased risk of concurrent flood-wind hazards during winter. In summer, the strength of the nocturnal Urban Heat Island (UHI) of London is expected to intensify by the end of the century, contributing to higher chances of combined heatwave-air pollution events. The third type of multi-hazard investigated was the spatio-temporal concurrence of global wet and dry hydrological extremes, during the 1950-2014 period. The analysis was conducted using the monthly self-calibrated Palmer Drought Severity Index based on the Penman-Monteith model (sc_PDSI_pm). Results showed that the land area impacted by extreme dry and wet-dry events significantly increased over the observational period. The most geographically widespread wet-dry event covered a total area of 21 million km2 (or 14% of the global land area) with documented flood and drought impacts over diverse regions. Two new metrics were developed to provide more insight into the combined wet and dry hazards: the wet-dry (WD) ratio and the extreme transitions (ET) time interval. The former quantifies the predominance of wet or dry extremes over a given area, whereas the ET measures the average separation time between the opposite extremes (i.e. between wet to dry or dry to wet transitions). The WD-ratio reveals a predominance of wet over dry extremes in the USA, northern and southern south America, northern Europe, north Africa, western China and most of Australia. The ET median for wet to dry is ~27 months, and 21 months for dry to wet. Global correlations between wet-dry hydrological extremes and El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO) and American Multidecadal Oscillation (AMO) were also investigated. ENSO and PDO showed similar correlation patterns, with the former significantly impacting a larger area. On the other hand, the AMO showed an almost inverse spatial correlation pattern, with an overall larger area impacted. The findings presented in this thesis could be informative for emergency responders and relief agencies, disaster risk reduction practitioners, and (re)insurance companies. For instance, multi-basin flooding co-occurring with ETCs could overwhelm emergency response that depends on support from neighbouring regions that are similarly affected. Economic damages could exceed those insured by households and businesses. Projected rises in nocturnal UHI intensity in London could exacerbate heat-stress and, when combined with episodes of poor air quality, increase the likelihood of health problems amongst vulnerable groups. Furthermore, concurrent wet and dry hydrological extremes could be significant for organizations with global assets or sensitive supply chains, and the hydropower, agricultural and transport sectors. Global maps generated of major wet-dry events and the WD-ratio could also be integrated into a seasonal forecasting product, to help stakeholders in hedging the risk. Key opportunities for further research on multi-hazards are future hydroclimatic projections in the light of anthropogenic climate change and the application of new statistical techniques that could help in discerning the driving physical processes.