A dynamical analysis of atmospheric river behavior in two different climates

Atmospheric rivers (ARs) are filamentary features that play a leading role in the transport of atmospheric moisture poleward. They develop over synoptic timescales and form spatially narrow plumes of moisture that can stretch over thousands of kilometers in the lower troposphere. Meteorological case studies have tied landfalling ARs most notably to severe flooding and precipitation events along the western coast of North America.

Previous composite analysis has shown the close association between upper tropospheric dynamical fields and the intensity and landfalling latitude of moisture transport within ARs. Projected changes in the north-south variability of the mid-latitude jet over the North Pacific suggest the potential for a meridionally extended AR landfalling region. These changes have implications for the response of precipitation patterns along the coastline to warming. To investigate the link between jet variability and changes in landfalling AR behavior, an analysis of the response of tropospheric Rossby wave breaking to warming and its association with changes in landfalling AR behavior is performed for the eastern North Pacific using daily output from 10 different model simulations (multiple ensembles when available) from the Coupled Model Intercomparison Project, Phase 5 (CMIP5).

For each model, a climatology of landfalling AR dates is created, in which landfalling dates were identified as lower tropospheric high wind and moisture features with extended geometry. These climatologies were then evaluated in historical (1980 - 2005) simulations and compared to MERRA and ERA-Interim reanalysis fields. An evaluation of model performance suggests that the bias in AR frequency is related to upper-level zonal wind bias. Decomposition of moisture flux over the region into thermodynamic and dynamic responses to warming in RCP 8.5 (2070 - 2100) simulations shows that while a large portion of the change in AR behavior is linked to atmospheric moisture increases, this response is localized over regions where AR frequency is climatologically the highest. To investigate the effect of a meridionally variable jet with warming, the location and scale of Rossby wave breaking in each model is detected using a previously reported automated detection algorithm. The association between the breaking response to warming and variability in AR behavior are investigated.