Interactions between anomalous wave activity and a strong El Nino

Sam Lillo & David Parsons
University of Oklahoma
10 Nov, 1:10 pm in 2155

The winter of 2015-16 featured a strong El Nino with enhanced tropical convection in the central Pacific, forcing classic anomalous Hadley cell and Walker circulations. In addition,  poleward Rossby wave dispersion. Meanwhile, the North Pacific was characterized by significant synoptic-scale Rossby wave activity emerging from Asia, including multiple wave packets tracking around the globe during February and March. The interaction of one of these packets with the ENSO-driven subtropical anticyclone resulted in a large wavebreak over the East Pacific, leading to the deepening of a trough over Mexico of unprecedented amplitude on 10 March 2016. This trough had significant ramifications at the surface, including an intense cold wave across central Mexico and snow as far south as Guadalajara.

Overall, short-wave Rossby wave activity was more prevalent this past winter across the extratropical Pacific than during past strong El Nino winters. The background flow in the mid-latitudes diverged from canonical strong Nino conditions, with a well-defined waveguide north of the typical zone of refraction in the subtropical jet. The waveguide acted to trap poleward-propagating wave activity from the tropics and maintain the coherence of Rossby wave packets in the mid-latitude jet. Motivated by these observations, we explore implications in high impact weather, as well as its predictability during this past winter.