Future of landfalling atmospheric rivers with extreme precipitation in British Columbia

Abstract:

Synoptic weather patterns that enhance water vapour transport over North Pacific Ocean are common triggers for autumn flooding events in coastal British Columbia (BC). The bulk of this water vapor transport takes place through narrow corridors known as atmospheric rivers (ARs) that occur within the warm conveyor belt of extratropical cyclones. Global climate models (GCMs) simulate ARs to varying degrees, however, all GCMs can simulate the synoptic patterns that favor ARs development. Thus by looking into the synoptic patterns in GCMs one can indirectly investigate future changes in frequency and intensity of AR events, a goal that we have set for this study. First we evaluate the performance of five GCMs in simulating the synoptic patterns responsible for AR-extreme events (landfalling ARs in BC with extreme precipitation over the province). As reference data we used four reanalysis data-sets for the period 1979-2010. With the use of a clustering algorithm we identified characteristic daily patterns of integrated vapour transport (IVT) over the North Pacific Ocean, and further identified the IVT patterns linked to AR-extreme events. We find that for the period 1974-2005 all five GCMs are relatively unsuccessful in simulating the frequencies of these patterns, as well as total precipitation during the AR-extreme events. Despite the large range of projections and uncertainties, better performing GCMs agree in their projections for the end of the 21st century, simulating more frequent AR-extreme events, as well higher precipitation totals during these events. In the best performing model ensemble of three GCMs, the projections according to RCP4.5 emission scenario reveal an increase of AR-extreme events from 18% per extended fall season (Sep-Dec) in 1974-2005 to 22% in 2070-2100, while the seasonal (Sep-Dec) AR-extreme total precipitation averaged over BC increases by 44% between the two periods.