Why Are Stratospheric Sudden Warmings Sudden (and Intermittent)?

Abstract

This paper examines the role of wave–mean flow interaction in the onset and suddenness of stratospheric sudden warmings (SSWs). Evidence is presented that SSWs are, on average, a threshold behavior of finite-amplitude Rossby waves arising from the competition between an increasing wave activity A and a decreasing zonal-mean zonal wind u¯. The competition puts a limit to the wave activity flux that a stationary Rossby wave can transmit upward. A rapid, spontaneous vortex breakdown occurs once the upwelling wave activity flux reaches the limit, or equivalently, once u¯ drops below a certain fraction of uREF, a wave-free, reference-state wind inverted from the zonalized quasigeostrophic potential vorticity. This fraction is 0.5 in theory and about 0.3 in reanalyses. We propose r≡u¯/uREF as a local, instantaneous measure of the proximity to vortex breakdown (i.e., preconditioning). The ratio r generally stays above the threshold during strong-vortex winters until a pronounced final warming, whereas during weak-vortex winters it approaches the threshold early in the season, culminating in a precipitous drop in midwinter as SSWs form. The essence of the threshold behavior is captured by a semiempirical 1D model of SSWs, similar to the “traffic jam” model of Nakamura and Huang for atmospheric blocking. This model predicts salient features of SSWs including rapid vortex breakdown and downward migration of the wave activity/zonal wind anomalies, with analytical expressions for the respective time scales. The model’s response to a variety of transient wave forcing and damping is discussed.

The Problem of Diagnosing Jet Waveguidability in the Presence of Large-Amplitude Eddies

“…where L is a linear elliptic differential operator in f and z and the right-hand side (rhs) involves meridional derivatives of ðu 1 AÞ as well as vertical derivatives of u [see Eq. ( 11) in Nakamura and Solomon 2010]. The solution of (20) effectively reduces the strengh of U in comparison to what one would obtain from (19).…”

“…The essential steps from the thought experiment are quantified in section 4 through numerical calculations, allowing us to show the occurrence of artifacts when applying linear theory to the large-amplitude late stage of the evolution. In the same section we suggest an alternative method for defining a background state based on the idea of the zonalization of potential vorticity contours, which essentially recovers the small-amplitude early stage of the evolution (Nakamura and Zhu 2010;Methven and Berrisford 2015). As we will show, the two background states may yield vastly different results in terms of waveguidability, and it will be argued that the results from the zonal-mean background state have to be considered as artifacts.…”

“…We refer to this method as ''zonalization'' and the soobtained background state as the ''zonalized background state'' (cf. Nakamura and Zhu 2010;Methven and Berrisford 2015). No matter how strong or how nonlinear the eddies are, the zonalized background state is always identical to the underlying eddy-free reference state as long as the dynamics are conservative.…”

“…First, this strategy guarantees consistency with these earlier studies, which are all based on horizontal ray tracing in the barotropic model. Second, it avoids ambiguities in the definition of the zonalized state that arise for three-dimensional flow (Nakamura and Solomon 2010). The problem here is that the zonalization procedure in three dimensions requires a choice to be made at the lower boundary.…”

“…This issue indicates one of the limitations of the barotropic model, namely, the fact that it does not allow a zonally symmetric residual circulation in the meridional plane owing to its reduced dimensionality. In a more realistic model with three-dimensional geometry, the eddy forcing would induce such a residual circulation, which in turn would partly compensate the impact of the eddies on the mean flow (Andrews et al 1987;Nakamura and Solomon 2010). It is impossible to realistically represent the effect of the residual circulation in the barotropic framework, but we try to estimate the modifications to be expected using an ad hoc procedure.…”