Role of the Polar-night Jet Oscillation on the formation of the Arctic Oscillation in the Northern Hemisphere winter

The coupling between the Polar-night Jet Oscillation (PJO) and the Arctic Oscillation (AO) was examined using observational data. Positive and negative AO indices tended to appear within a specific phase of the time evolution of the PJO, and the lifetime of the AO tended to be longer when the AO was coupled with the PJO. We performed lagged regression analyses for Eulerian mean wave forcing to examine how the AO signal appears with the time evolution of the PJO. We found that the AO signal at the surface is created through meridional circulation driven by the combined effect of mechanical and thermal eddy forcing at some stage of the PJO. The effect of these wave forcings on sea level pressure (SLP) changes was investigated using a zonal-mean quasi-geostrophic model on the sphere. Approximately 40% of the polar SLP change was found to come from thermal forcing when the AO-like signal appeared and two thirds from the stratosphere, whereas 60% of the polar SLP change was a result of mechanical forcing that originated almost in the troposphere. The contribution from the wave of the zonal wave number 1 component was very important in both forcings, particularly for SLP in the polar cap region. Waves of zonal wave number 2 and 3 components also contributed significantly to tropospheric forcing.