Abstract:
Variability in the observed Northern and Southern Hemisphere storm tracks is dominated by "pulsing" and "latitudinally shifting" modes in sectors of each hemisphere. These modes are useful simplifications of the full complexity of storm track variability and are ubiquitous in different variables, at different levels in the troposphere and on in the storm track variability on different time scales. Variability associated with the pulsing mode almost always accounts for a large fraction of the variance than the latitudinally shifting mode, but the latitudinally shifting mode is more strongly associated with persistent anomalies in the background flow and with dominant climate variability patterns. At monthly time scales, both the pulsing and latitudinally shifting modes of storm track variability are associated with zonal wind anomalies consistent with eddy-forced acceleration of the zonal wind. At daily time scales, the pulsing mode is associated with an evolution culminating in zonal wind anomalies and eddy activity that shift poleward over time. In model simulations associated with climate change time scales of variability, measures of storm track intensity near the level of the jet indicate suppressed eddy amplitude in cold climates relative to warm climates, whereas measures in the lower troposphere indicate an enhancement of eddy activity. Investigating the seasonality and dynamics of these offsetting tendencies between upper and lower tropospheric levels and the relationship between the dominant modes of storm track variability and the background flow remain as motivations for future work.
