Stratospheric channels of Rossby wave–triggered inertial instability

Multiyear studies of inertial instability have been, to our knowledge, limited to the upper troposphere, the stratopause, or to selected rocketsonde sites. No long-term stratospheric climatologies using global winds have been performed, possibly owing to doubts concerning the accuracy of tropical winds in these data sets. Without such a global climatology, the significance of individual case studies can be difficult to assess: are these isolated and rare events, or are they snapshots of a more widespread and consequential phenomenon?

Observational, theoretical and numerical studies have linked inertial instability events with Rossby wave breaking in the middle atmosphere. The Rossby wave breaking appears to organize the instability both latitudinally and longitudinally. Rossby wave breaking has been previously studied in a few climatologies. However, these studies typically focus on single levels in the vertical. They do not extend throughout the entire depth of the stratosphere and/or do not examine an extended number of years.

Therefore, we have performed climatologies of inertial instability and Rossby wave breaking using United Kingdom Met Office (MetO) assimilated analyses from November 1991 through March 2004. These climatologies span the entire stratosphere and are used to juxtapose the evolution of the wave breaking and instability in longitude, latitude, altitude and time. Inertial instability is proxied by anomalous geostrophic absolute vorticity, and Rossby wave breaking is identified with reversed meridional gradients of potential vorticity.

The results for the upper and middle stratosphere indicate deep poleward-and-eastward channels of higher inertial instability frequency extending from the central equatorial Pacific in both winter hemispheres. These corridors of enhanced inertial instability frequency are generally found just poleward of regions of frequent Rossby wave breaking, a result confirmed with a case study drawn from the period of the climatology.

Both the poleward-eastward channels and the slight poleward offset of the instability versus the Rossby wave breaking region are evident from the stratopause downward into the middle stratosphere. The frequency of events diminishes with decreasing altitude. Analysis of interannual variability of Rossby wave breaking and inertial instability in the middle stratosphere channels confirms a statistical correlation between them, reinforcing previous results from numerical modeling.

The existence of these chronic channels of heightened Rossby wave-triggered instability raises the possibility of narrowly focused regions of tropical-extratropical interactions in the stratosphere. Because the Rossby wave breaking trigger is anchored spatially and temporally, these results suggest very specific regions and times in which to look for these interactions.

In addition, we have also detected a possible quasi-biennial oscillation signal in selected analysis of the early years of the climatology. This result and its implications for tropical-extratropical interactions will be discussed in more detail at the conference.