The Role of Weather Processes in Arctic Warming

The Tropically Excited Arctic warMing mechanism (TEAM) proposes that localized tropical convection excites planetary-scale waves which transport sensible and latent heat into the Arctic. The resulting heat flux convergence leads to an increase in downward infrared radiation and Arctic surface warming. Reanalysis data is used to conduct a suite of observational analyses and idealized model simulations. Spectral decomposition shows that synoptic-scale (zonal wavenumbers 4-7) waves are the primary agent for sensible heat transport within the mid-latitude, whereas heat transport from the mid-latitude to the Arctic is carried out by planetary-scale (zonal wavenumbers 1-3) waves. Insight into the latter waves is attained by examining planetary-scale wave life cycles. It is shown that localized tropical convection over the Maritime Continent precedes the peak of the planetary-scale wave life cycle by ~10 to 14 days. The tropical convection excites a Rossby wave train over the North Pacific that amplifies the climatological December-March stationary waves. These planetary-scale waves are baroclinic and transport sensible and latent heat poleward. During the planetary-scale wave life cycle, synoptic-scale waves are diverted northward over the central North Pacific. The warm conveyor belts associated with the synoptic-scale waves channel moisture from the subtropics into atmospheric rivers which ascend as they move poleward and penetrate into the Arctic near the Bering Strait. The planetary-scale wave life cycle ceases as ridging over Alaska retrogrades westward. The ridging blocks any additional moisture transport into the Arctic. However, sensible and latent heat remain elevated over the Arctic which enhances downward infrared radiation and maintains warm surface temperatures. The physical picture presented here illustrates the importance of weather-producing processes – tropical convection, synoptic-scale storms, wave breaking, and atmospheric rivers – toward understanding Arctic warming, an impressive long-term change that is currently taking place.