Polar Vortex Oscillation: A View from an Isentropic PV-Latitude Lagrangian Coordinate

Following contours of constant potential vorticity (PV) on isentropic surfaces derived from the NCEP/NCAR reanalysis II dataset for the period of 1979-2003, we have mapped the 3-D daily isentropic PV field to PV-latitude grids on isentropic surfaces forming a semi-Lagrangian theta-PVLAT coordinate. We then transform the original 3-D daily isentropic analyses to 2-D fields in the theta-PVLAT coordinate by averaging along PV-latitudes. Because of conservation properties of both q and PV, the advective tendencies of PV are naturally filtered out in the semi-Lagrangian theta-PVLAT coordinate. As a result, the temporal variability in the theta-PVLAT coordinate mainly reflects irreversible nonlinear dynamic mixings and large-scale thermal forcings and is expected to have a time scale much longer than individual weather events.

The leading EOF mode of the daily Northern Hemisphere PV anomalies in the theta-PVLAT coordinate explains about 69% of the total variance of daily PV anomalies, and is highly correlated with the daily NAM index (about 0.71). It describes a polar vortex oscillation (PVO) between a strong polar vortex (a positive phase event of the annular mode) and a weak polar vorex (a stratosphere warming event ). In spite of using daily data, the time series of PVO (the PVO index) has a remarkable long time scale and exhibits only 1-2 PVO events within a winter season. The interannual variability of the PVO index is associated with both the duration variations of PVO events and their intensity variations. Based on the PVO index, we have constructed “relative-intensity-based” composite maps for various meteorological variables to describe the fundamental temporal and spatial phase relationships among different variables as the PVO evolves. In comparison with the conventional temporal composite analysis, the “relative-intensity-based” composite method takes into consideration of both duration and intensity variations from one PVO event to another. Due to the smoothness of the PVO index, the temporal evolution of PVO events can be faithfully captured by the relative-intensity-based composite analysis.

The composite analysis in the theta-PVLAT coordinate captures the well-known features of PVO events, such as the downward propagation of PV anomalies from the stratosphere to the troposphere in high latitudes, and the much faster transition from a stronger polar vortex to a weaker one in comparison to the building-up/recovery of the polar vortex. Moreover, it identifies a systematic poleward propagation of anomalies in the upper stratosphere rooted from deep tropics prior to the beginning of the downward propagation of anomalies. We will discuss the role of the “stratosphere bridge” that links tropical thermal forcing and high-latitude circulation in winter season as well as its implications for climate predictions.