Dynamical Driving of Transient Interference with the Climatological Stationary Wave

The dynamics of interference between transient eddies and climatological stationary eddies in the Northern Hemisphere is investigated. The amplitude and sign of the interference is represented by the Stationary Wave Index (SWI), which is calculated by projecting the daily 300-hPa streamfunction anomaly field onto the 300-hPa climatological stationary wave between 15°N and 75°N. ERA-Interim reanalysis data for the years 1979 to 2013 is used. Different atmospheric variables are regressed against the SWI to investigate the dynamical processes that drive constructive and destructive interference between the stationary and transient eddies, as well as the impact of this interference on the extratropical circulation and surface air temperature.

It is shown that constructive interference can arise from processes confined to midlatitudes and can be further amplified by processes that take place in the tropics and the Arctic. With regard to the driving by processes outside of midlatitudes, it is found that constructive interference follows an enhancement and localization of Warm Pool tropical convection and a loss of sea ice over the Barents and Kara Seas. When the tropical and Arctic influences are small, one impact of constructive interference is warming of the Arctic in the vicinity of Alaska. When the Warm Pool convection is active, most of the Arctic undergoes a warming, with the largest amplitude warming occurring over the Barents and Kara Seas. The constructive interference that arises from the loss of Arctic sea ice has a rather different impact, as it is followed by a disruption of the stratospheric polar vortex. The above features are reversed when strong destructive interference takes place.