UC Davis

Atmospheric rivers (ARs) are large and narrow filaments of poleward horizontal water vapor transport. AR carry over 90% of moisture from the tropics to higher latitudes but cover only between 2% and 10% of the earth’s surface. When ARs are forced upwards frequently lead to heavy precipitation. ARs are associated with up to half of the extreme events in the top 2% of the precipitation and wind distribution across most mid-latitude regions. ARs can lead to hydrological hazards, and a better understanding of AR can help in the study, forecasting, and communication of flooding. Because of its direct relationship with horizontal vapor transport, extreme precipitation, and overall AR impacts over land, the AR size is an important characteristic that needs to be better understood. Furthermore, most of the ARs research work focuses on midlatitudes and polar regions. It is not until recently that ARs in tropical latitudes are starting to generate interest within the scientific AR community. We develop and implement five size estimation methods independent of the AR detection algorithms and use them to characterize the size of ARs. We create North American landfalling AR composites using ERA5 reanalysis data in the 1980-2017 period. To study how AR size changes with future climate scenarios, we use data from the Coupled Model Intercomparison Project 5 and 6 (CMIP5/6) to create historical and future AR composites in the 1950-2100 period. We apply our size estimation methods to study how AR size responds to climate change. Additionally, we use data from the ERA-20C reanalysis to study the relationship between lower latitude ARs and the extreme precipitation in Central-Western Mexico (CWM) during the dry season (November-March) in the 1900-2010 period.

North American landfall ARs (NALFARs) that originate in the Northwest Pacific (WP) (100◦E-180◦E) have larger sizes and are more zonally oriented than those from the Northeast Pacific (EP) (180◦E-240◦E). ARs become smaller through their life cycle, mainly due to reductions in their width. They also become more meridionally oriented towards the end of their life cycle. NALFARs become smaller through their life cycle, mainly due to reductions in their width. They also become more meridionally oriented towards the end of their life cycle. Overall, the size estimation methods developed in this work provide a range of AR areas (between 7x1011m2 and 1013 m2) that is several orders of magnitude narrower than the current estimation by the AR detectors from the Atmospheric River Tracking Method Intercomparison Project (ARTMIP).From a global AR size analysis, we show an increase between 10% and 21% in the background IVT field among CMIP5/6 models.

According to our results, AR width is more sensitive to climate change and has a larger contribution than length to the change in the AR area. We find a mean AR area of 3.15x106 (2.32x106-3.98x106) km2 for historical runs, and 3.42x106 (2.73x106-4.11x106) km2 for future runs. Most size estimation methods and CMIP5/6 models show positive trends in AR area, length, and width, between historical and strong radiative forcing future simulations (CMIP5: RCP-8.5, CMIP6: SSP-858). Regardless of the individual sign in AR size change, the mean AR cross-section water vapor transport increases between 8% and 37% for future simulations. Additionally, our results suggest that NALFARs are more likely to penetrate further inland under climate change.

Regarding landfalling ARs in CWM, our results suggest that more than 25% of the extreme dry-season precipitation is associated with AR-like events, with up to 75% in December and January. This AR-associated precipitation is associated with an enhanced mean vertically integrated water vapor (IWV) and horizontal vapor transport (IVT) fields (30 kg m−2 and IVT 400 kg m−1s−1, respectively). The meteorological state of the atmosphere shows “ideal” conditions for orographic precipitation due to landfalling ARs: a high plume of horizontal vapor transport perpendicular to the mountain range. These events are associated with a weakening of the westward equatorial IVT and a tropospheric wave pattern, observable in the mean sea level pressure and geopotential height anomalies.

We believe that the size estimation methods developed in this work provide statistical constraints for AR size and geometry, and how they change in future climates. These results could help as a reference for tuning existing ARDTs or designing new AR detection algorithms. Furthermore, we demonstrate the relationship between ARs and winter rainfall in CWM. This relationship leaves the question open of how similar are these tropical ARs to the more studied higher latitude ARs and how they will respond in a warming world.