Thursday 04 Feb 2016: The background state of the atmosphere and slow modes of variability (GAFD Seminar)
Prof. John Methven - University of Reading
In the last decade, western Europe has seen a number of extreme seasons. For example, anomalously high precipitation totals for the summers of 2007 and 2012, and winter 2013/14, 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 a distinct structure and intrinsic phase speed derived from global conservation properties. The framework is suitable to examine climate and its interactions with large-amplitude disturbances, and makes a clean partition between adiabatic and non-conservative processes.
The theory of wave-mean flow interaction requires a partition of the atmospheric flow into a notional background state and perturbations to it. The evolution of both components and their diagnosed ``interaction'' depends upon the partition. Here, the background state is defined in terms of two fundamental integral properties of the full flow: mass and circulation enclosed by potential vorticity (PV) contours within isentropic layers. The background state, also known as the Modified Lagrangian Mean (MLM), is defined as a zonally symmetric state where the PV contours enclose the same mass and circulation as the full state. For adiabatic and frictionless flow, the integrals are all invariant and therefore the MLM state is a steady solution of the primitive equations. Here a new calculation called equivalent latitude iteration with PV inversion (ELIPVI) is used to obtain the MLM state where the position of the lower boundary is obtained as part of the solution. This is central to tropospheric variability, baroclinicity and Rossby wave behaviour. In this partition, all the time-dependence in the adiabatic flow is put into the perturbations which can be described by two wave activity conservation laws valid at large amplitude for pseudomomentum and pseudoenergy.
Modes of variability in the perturbations are extracted using the empirical normal mode (ENM) technique. This combines amounts to EOF analysis using pseudomomentum as the norm. For a linear system the ENMs would equal the dynamical normal modes since they are also orthogonal under this norm. However, the technique is applied to the atmosphere and for the first time the boundary contribution to wave activity is taken into account. 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. Periods of westward, stationary (blocked) or eastward propagation ("zonal regime") of Rossby waves are shown to be related to the speed obtained.