The Importance of the Stratospheric Pathway of El Niņo-Southern-Oscillation Teleconnections: A Modeling Study with WACCM

The El Niņo-Southern Oscillation (ENSO) is a major driver of Northern Hemisphere wintertime variability, and the teleconnections pathways via which ENSO impacts the midlatitudes appear much richer that previously believed. In particular, a recent observational study has highlighted the existence of a stratospheric pathway, whereby tropical sea surface temperature anomalies are able to reach upwards from the Pacific into the polar stratosphere and then descend deep into the North Atlantic sector and far into the Eurasian continent.

Here we corroborate the earlier observational study by analyzing an ensemble of model simulations with a state-of-the-art chemistry-climate model, Whole Atmosphere Community Climate Model (WACCM). In addition to having a well-resolved stratosphere, and thus being able to simulate stratospheric variability (viz. sudden warmings), WACCM is coupled to an ocean model and its ENSO simulations are also quite realistic. Using an ensemble of 6 historical simulations, from 1955 to 2005, allows us to greatly improve the statistics, as the observational record is limited to a relatively small number of El Niņo events.

To distinguish the stratospheric pathway from the more familiar Pacific-North-America tropospheric pathway, we simply make composites of El Niņo years with and without sudden warmings. In agreement with the earlier observational study, WACCM shows that ENSO's impacts over the North Atlantic and Eurasia are largely dominated by the stratospheric pathway. This finding confirms the importance of accounting for the stratospheric pathway in producing seasonal forecasts, particularly over the North Atlantic sector and Eurasia.