The influence
of jet regime on the frequency and distribution of extreme events
Understanding
how the large scale atmospheric circulation affects the distribution of extreme
weather events is clearly important. Intuitively, we expect the
distribution of extreme events to be linked to the large scale flow. In
particular, we expect quantities like wind speed and vorticity to be directly
related to the structure of the jet streams and synoptic storms. In particular,
we expect the distribution of extreme events to be related differently to the
jet under different jet regimes. Since the Southern Hemisphere jet stream
undergoes a sharp seasonal transition, from a dominantly thermally driven
subtropical jet during winter, to an eddy driven mid latitude jet during summer
[e.g Nakamura et al., 2004], we expect the distribution
of extreme events to also vary seasonally. Here we examine extreme values of
upper level cyclonic vorticity. We expect this quantity to be more strongly affected
by the large scale circulation compared to the more traditional fields used for
studying extreme weather events (e.g. surface temperature or precipitation),
but at the same time we expect it to be indicative of extreme weather. We expect extreme vorticity values
to be associated with a deep upper level trough, and with wave breaking and the formation of potential
vorticity (PV) streamers, both of which have been shown to play a role in the
formation of extreme weather conditions (e.g. extreme precipitation [Martius et al., 1006, 2013; Romero et al., 1999; Jacobeit , 1987; Krichak et al.,
2007; Massacand et al., 1998], Brazilian cold surges
[Sprenger et al., 2012]).
We are currently writing a book chapter on the
relation of extreme events to the jet stream and storm tracks, in which we will
describe this relation in the MQG model, in an idealized
GCM and in observations.
Harnik N., O. Lachmy, and Chaim Garfinkel ‘The Influence of Storm Tracks and Jet
Stream Dynamics on the Occurrence of Large-Scale Flow Anomalies and Extreme
Weather and Climate Conditions’. In, Dynamics
and Predictability of Large-Scale, High-Impact Weather and Climate Events. Ed. Li, J., R. Swinbank, H. Volkert and R. Grotjahn. Cambridge
University Press