Atmospheric River Dynamics: What Can Water Vapor Age Tell Us About the Moisture Transport Leading to Extreme Precipitation?

Atmospheric rivers (ARs) are thought to be the main driver of extreme precipitation events in the North Atlantic and Pacific, and are responsible for most of the total extratropical poleward moisture transport. They are associated with violent weather and high precipitation that can lead to floods in populous coastal areas. Moreover, the frequency and intensity of ARs is expected to increase with climate change, driven by the rise in atmospheric moisture and precipitation. An important question about ARs is whether they are being supplied primarily from remote subtropical regions, or whether they are recycling water vapor by evaporating and precipitating as they travel northward.

In a previous paper, we implemented water vapor (WV) age tracers in a global circulation model to resolve the WV age spectrum and the age of precipitation in both space and time, which allowed us to study the dynamics of WV age. In this study, we use our novel tracers to test how the mean WV age and the mean age of precipitation can be used to identify and investigate the dynamics behind ARs. We use column integrated water vapor (CWV) wave activity and precipitation (Lu et al., 2017) to track AR features, and show that the mean WV age at the surface and the mean age of precipitation is well correlated to CWV wave activity in wintertime extreme precipitation events over the North Atlantic and Pacific.

From composite images of our WV age tracers and climatological diagnostics, we show how during winter, surface WV age and the age of precipitation are higher than the seasonal average, supporting long range moisture transport by ARs, while in summer, they are lower than average, meaning local sources of water vapor is feeding into convective storms. During winter, tropical WV lifts up and travels poleward via extra tropical cyclones, with convective precipitation removing WV from the lower levels along the way. In the midlatitudes, large scale condensation precipitates most of the subtropical WV. As a result, the age of precipitation and surface WV age are about 2 days over seasonal average at the end of the storm track, matching our estimated advective time scale from the subtropics. Also, the large amount of precipitation reduces the WV age in the upper levels of the atmosphere.  

During summer on the other hand, there are high values of CWV wave activity which could be interpreted to also indicate long range transport. But, lower surface WV age and age of precipitation than average, among other results, indicates that it is due to local evaporation and convective storms recycling the locally available WV along the storm tracks. 

In summary, our results show how our tracers of WV age, which could be implemented relatively simply into more complex climate models, give us a new straightforward tool to analyse the lengthscale WV travels in the atmosphere, helping us understand the dynamics behind WV transport, and its impact on the water cycle with climate change.