Impacts of anomalous mid-latitude wave activity during a strong El Niño

Sam Lillo

School of Meteorology, University of Oklahoma, Norman, OK

Thursday, Oct 13, 2016, 11:00 am
DSRC Room 1D403

Abstract

The winter of 2015-16 featured a strong El Niño with enhanced tropical convection in the central Pacific, forcing the classic anomalous Hadley cell and poleward Rossby wave dispersion. This classic response in the tropics interacted with a background flow in the mid-latitudes that diverged from the canonical strong El Niño climatology. 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.

The anomalous mid-latitude wave activity potentially also had impacts in the stratosphere. The state of the tropical and subtropical stratosphere was controlled largely by the strongest westerly QBO on record. The waveguide for mid-latitude tropospheric wave activity typically weakens above the tropopause, however, the anomalous westerly QBO provided an extended continuum into the lower to mid stratosphere for the mid-latitude wave activity to propagate upward and equatorward. Beginning during the 2015-16 winter, the QBO evolution took an anomalous turn from the reliable pattern that has been observed for decades. The convergent wave activity resulted in westerly momentum fluxing out of the mid stratosphere, and easterlies developing prematurely. As a result, the westerly waveguide in the tropics was cut-off, and the descending easterly phase of the QBO was, for all intents and purposes, skipped over during this cycle.