Abstract ID: 230
Relating the properties of quasi-stationary Rossby waves to the jet that they live on
Lead Author: John Methven
University of Reading, United Kingdom
Keywords: Persistent weather, Modes of variability, Wave activity, Jetstream, Phase speed
Abstract: In recent decades, western Europe has seen a number of extreme
seasons. For example, anomalously high precipitation totals for the
summers of 2007 and 2012 as well as the exceptionally high temperature
of summer 2003 and the coldness of winter 2009/10. One common feature
in all these examples is the existence of persistent, near stationary
Rossby wave patterns on the tropopause. Here a rigorous framework is
used to extract these “”slow modes of variability”” from
data in such a way that each mode has an intrinsic phase speed related
only to its spatial structure, in a similar way that a bell rings with a
characteristic note. The phase speed is derived from global
conservation properties. The question addressed here is how these slow
modes of variability relate to the structure of the background state jet
that they live on.
Idealised experiments using a global primitive equation model are
constructed where the only forcing is a weak relaxation to a zonally
symmetric balanced background state which includes a sloping tropopause
and westerly jet with a latitude that can be controlled by a single parameter.
The jet is baroclinically unstable and so wave activity is sustained through
repeated growth of baroclinic waves and their nonlinear life cycles.
Modes of variability in the perturbations are extracted from global data
using the empirical normal mode (ENM) technique. This amounts to
EOF analysis using pseudomomentum (wave activity) as the norm.
For a linear system the ENMs would equal the dynamical
normal modes since they are also orthogonal under this norm. Although
the system is nonlinear, only a few ENMs dominate the variability and
the ratio of pseudoenergy to pseudomomentum yields a unique phase
speed for each. It is shown how the properties of the dominant modes
depend on the quasi-equilibrium background state jet strength and latitude.
In this way seasons with anomalously persistent mid-latitude Rossby wave
activity may be related to the interannual variability in the background
state zonal flow.
Dominic Jones (University of Albany)
Thomas Frame (University of Reading)
Paul Berrisford (ECMWF)