Weather warning archives reveal spatio-temporal hot spots of compound natural hazards

Individual natural hazards may be combined in different ways, leading to cascading or co-occurring effects, turning them into compound hazards. However, assessment of individual as well as compound hazards is often hampered by short or incomplete observational records of actual hazards, and records of various hazards that do not easily combine. In this study we propose an alternative way to detect potential risk of compound natural hazards via archived severe weather warnings. We investigate weather warnings in Sweden from 2011 to 2020 regarding their distributions and frequencies in time (at daily level) and space (at warning district level) from both an individual and compound perspective. We illustrate the methodology and results by focusing on compound flood-related risk, generated by combinations of heavy rainfall, high streamflow and high sea level, and contextualize with two actual compound flood events in Sweden. We find compound fluvial and coastal flood risk primarily along the southwest coast during the winter half year as well as compound fluvio-pluvial flood risk during the summer half year. The results show that severe weather warnings can be used to assess the frequency and compounding nature of natural hazards, as well as to identify actual cases for further investigation, and we encourage similar investigations elsewhere

The Stratospheric Polar Vortex and Surface Effects : The Case of the North American 2018/19 Cold Winter

A severe cold air outbreak hit the US and parts of Canada in January 2019, leaving behind many casualties where at least 21 people died as a consequence. According to Insurance Business America, the event cost the US about 1 billion dollars. In the Midwest, surface temperatures dipped to the lowest on record in decades, reaching -32 degrees C in Chicago, Illinois, and down to -48 degrees C wind chill temperature in Cotton and Dakota, Minnesota, giving rise to broad media attention. A zonal wavenumber 1-3 planetary wave forcing caused a sudden stratospheric warming, with a displacement followed by a split of the polar vortex at the beginning of 2019. The common downward progression of the stratospheric anomalies stalled at the tropopause and, thus, they did not reach tropospheric levels. Instead, the stratospheric trough, developing in a barotropic fashion around 70 degrees W, turned the usually baroclinic structure of the Aleutian high quasi-barotropic. In response, upward propagating waves over the North Pacific were reflected at its lower stratospheric, eastward tilting edge toward North America. Channeled by a dipole structure of positive and negative eddy geopotential height anomalies, the waves converged at the center of the latter and thereby strengthened the circulation anomalies responsible for the severely cold surface temperatures in most of the Midwest and Northeast US