Tropospheric Precursors to Large AO Events on the Subseasonal Timescale

The Arctic Oscillation (AO) is the dominant mode of Northern Hemisphere winter climate variability. An accurate prediction of the AO could lead to improved seasonal temperature forecasts over the major population centers in eastern North America and Europe as it explains the largest fraction of temperature variance of any other climate mode over these regions. But forecasts of an increase in amplitude or even a phase reversal of the AO can lead to improved temperature forecasts on the weekly and monthly time scales. Many tropospheric AO events are preceded by stratospheric AO events and even earlier in time by anomalous upward energy flux associated with Rossby waves in the troposphere.

Here we identify lower tropospheric circulation anomalies and anomalous upward energy fluxes that precede large negative AO events in both the troposphere and the stratosphere by four to as much as six weeks. Composite analysis of stratospheric warming events identifies regional tropospheric precursors, which precede stratospheric warmings. The tropospheric precursor is found to vary when compositing over polar vortex displacements and splits separately. Prior to vortex displacements the main sea level pressure anomaly center of the tropospheric precursor is associated with the Siberian high. Prior to vortex splits a similar anomaly center is identified in the tropospheric precursor, but it is weaker and appears to be more strongly related to a shift in the storm tracks. Differences in the sea level pressure anomalies in the North Atlantic and the North Pacific are also observed when comparing the precursors prior to vortex displacements and splits. Identification of unique tropospheric precursors to stratospheric warming/cooling and subsequently tropospheric AO events can help to improve our understanding of troposphere-stratosphere coupling and lead to improvements to forecasts on the subseasonal timescale.