Quantifying Storm-Track Variability and Impacts Using Accumulated Cyclone Track Activity

Extratropical cyclones that propagate across the mid-latitude storm tracks give rise to much of the significant weather in the mid-latitudes, including extreme precipitation and high winds. Multiple storm track activity metrics have been used to quantify storm track activity, including Eulerian variance/covariance statistics and Lagrangian cyclone track statistics. Nevertheless, to date there has not been a systematic evaluation regarding how these different storm track metrics relate to each other, as well as how these metrics relate to mean flow variability and weather impacts. Such an assessment is important because previous studies have shown that different storm track metrics display rather different changes under global warming, thus it is essential to understand their similarities and differences.

In this presentation, a new storm track activity metric, the “Accumulated Cyclone Track Activity”, will be introduced. This metric is derived from Lagrangian cyclone track statistics, and includes information on the variations of both cyclone frequency and intensity at each location within a month. We will show that this metric correlates strongly with local weather impacts including variations in precipitation and high winds over most mid-latitude regions – these relationships being much stronger than those between weather impacts and traditional Lagrangian storm track activity metrics such as cyclone frequency, cyclone intensity, or Eulerian storm track metrics such as sea level pressure and meridional wind variance.

The leading modes of variability of the Northern Hemisphere storm tracks based on this new metric are examined, together with the accompanying large scale flow anomalies and the associated weather impacts. Currently CMIP5 model simulations are being examined to assess model fidelity in simulating this metric and its impacts, as well as its projected change under global warming.