UC Irvine

Atmospheric rivers (ARs) are filamentary features that play a leading role in the poleward transport of atmospheric moisture and in the global redistribution of heat from the tropics. When they cross over land (so-called landfall), they are a major source of wintertime precipitation, particularly over the western coastline of North America. The extreme precipitation and flooding that sometimes accompany landfalling ARs can have severe socio-economic consequences. Despite advances in observational networks on land, the large-scale mechanisms influencing AR behavior and landfalling intensity are poorly understood. This dissertation aims to better characterize their present-day behavior and projected response to climate change over the North Pacific basin so as to improve forecasts of their impact at landfall.

Composites of dynamical fields using thirty years of the Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis are made following the eastward progression of ARs. A close relationship exists between the extratropical upper tropospheric dynamics, particularly anticyclonic Rossby wave breaking, and lower tropospheric moisture transport. Comparison between the strongest and the weakest ARs show consistent differences in both the intensity of moisture transport and the scale and rate of development of anticyclonic Rossby wave breaking. The strong relationship of landfalling ARs to anticyclonic Rossby wave breaking persists in a case study analysis of long-duration landfalling events.

Landfalling ARs are evaluated in historical (1980 - 2004) simulations from 28 models participating in fifth phase of the Coupled Model Intercomparison Project (CMIP5) and compared to the MERRA and ERA-Interim reanalyses. Few models correctly resolve the frequency distribution, interannual variability in number and amplitude of moisture flux, and median landfalling latitude. The response of a subset of high performing models to projected warming is investigated using Representative Concentration Pathway (RCP) 8.5 (2070 - 2099) projections. Selected models show a broadening of the frequency distribution, with the largest increase in frequency occurring equatorward of peak historical frequency. The equatorward increase in peak historical frequency is co-located with increases in the 850- and 250-hPa zonal winds. The moisture flux response to warming is mostly thermodynamic, but equatorward of its peak distribution, it is dominated by a dynamic response.