An Investigation of the Presence of Atmospheric Rivers During the Planetary-Scale Wave Life Cycle and Their Role in Arctic Warming

Atmospheric rivers are a ubiquitous feature of Earth's climate system and have been identified as being one of the most important mechanisms by which the atmosphere transports moisture poleward. In extreme cases, this poleward moisture transport can penetrate into the Arctic and contribute to Arctic warming through enhanced cloud cover and downward infrared radiation. Oftentimes, these extreme events are associated with Rossby wave breaking and an amplification of the planetary-scale waves. Recently it has been shown that the planetary-scale wave life cycle is triggered by tropical convection over the Pacific warm pool region and followed by distant Arctic warming, referred to as the Tropically Excited Arctic warMing (TEAM) mechanism. Furthermore, climatological moisture fluxes during December-March show planetary-scale moisture flux divergence over the tropics and planetary-scale moisture flux convergence in the high latitudes. Therefore, to explain this observed climatology, it is hypothesized that during the planetary-scale wave life cycle atmospheric rivers serve as moisture conduits between the tropics and high latitudes and thereby play a critical role in Arctic warming induced by tropical convection. To investigate atmospheric rivers during the planetary-scale wave life cycle, daily ERA-Interim reanalysis data is used to separate the extreme moisture fluxes associated with atmospheric rivers from background moisture fluxes. Compared to climatology, there are several features of interest over the Pacific. First, there is evidence of a tropical source region of moisture in the southwest Pacific that flows northeastward and converges in the subtropics along trailing cold fronts. Second, in conjunction with the presence of a weakened subtropical high over the central Pacific, transient extratropical systems and their cold fronts act as convergence zones that focus and narrow these atmospheric rivers into their warm conveyor belts. Third, these rivers are diverted northwestward in association with cyclonic Rossby wave breaking, delivering anomalously large moisture fluxes through the Bering Strait into the Arctic. To corroborate that the extreme moisture fluxes found in daily ERA-Interim data are not merely streamlines but rather represent the trajectories of parcels that travel from the tropics to the high latitudes, the Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT) is employed. Finally, to test if tropical convection can trigger the planetary-scale wave life cycle, the Geophysical Fluid Dynamics Laboratory (GFDL) spectral dynamical core is run with prescribed idealized heating over the warm pool and a passive tracer that corresponds to climatological values of December-March specific humidity. It is found that not only does this prescribed heating lead to a growth of the planetary-scale waves, it also results in the filamentation of the tracer over the Pacific Ocean with anomalously low values of the tracer found over the tropics and high values over the Arctic. These findings collectively suggest that atmospheric rivers during the planetary-scale wave life cycle play an important role in explaining the observed climatology because they serve as moisture conduits between regions of planetary-scale moisture flux divergence in the tropics and planetary-scale moisture flux convergence in the high latitudes. Furthermore, as the Pacific Ocean exhibits more La-Niņa-like tropical convection over the warm pool region, it can be expected that the planetary-scale wave life cycle and its attendant atmospheric rivers will become more frequent and thereby contribute to Arctic amplification.