The Stratospheric Sudden Warming (SSW) is a phenomenon characterized by a rapid increase in polar
temperatures in the stratosphere. The temperature increases more than a few dozen degrees in a few
days in the boreal winter. In some cases the westerly polar night jet disappears and easterly winds
appear during the warming. The warming is called a "Minor Warming", when the polar temperature increases more than 25 degrees in a period
of a week or less at any stratospheric level. If the zonal mean temperature increases poleward from
60 degrees latitude and the net zonal mean zonal winds become easterly at 60 degrees latitude at
10 hPa (32 km) or below, it is classified as a "Major Warming".
The SSW was discovered in 1952 by Scherhag (1952), but it took some time before the mechanism was
theoretically identified by Matsuno (1971). The SSW is caused by a rapid amplification of planetary
waves propagating upward from the troposphere. Planetary waves deposit westward momentum and create
a strong meridional circulation which produces a large warming in the polar stratosphere due to
adiabatic heating (e.g., McIntyre 1982).
Interaction between Stratosphere and Troposphere
The relationship between the stratosphere and the troposphere has widely been recognized. During
winter, tropospheric planetary waves propagate into the stratosphere along the westerly jet (e.g.,
Charney and Drazin 1961). More recently, the converse relationship that the zonal mean zonal wind
anomalies slowly propagate from the subtropical upper stratosphere to the polar region of the lower
stratosphere and the troposphere during the boreal winter, is also noted (Kodera et al. 1990). It
has been shown that SSWs occur in association with slowly propagating zonal mean zonal wind anomalies,
and the related changes in the troposphere exhibits the Annular Mode (AO) (Thompson and Wallace 1998)
like structure (Kodera et al. 2000). Baldwin and Dunkerton (1999) also showed that the downward
propagation of the AO from the stratosphere to the troposphere occurs in association with SSWs.
E-P Flux Analysis
As mentioned above, the stratospheric and the tropospheric circulation are connected with each other
through wave mean flow interactions. The Eliassen-Palm (E-P) flux (Eliassen and Palm 1961) is widely
used to characterize the wave activity. The direction of the E-P flux is proportional to the group
velocity and indicates the direction of the propagation of the waves. Approximately, the vertical and
horizontal components of the E-P flux are proportional to the eddy heat and momentum flux, respectively.
The divergence of E-P flux is proportional to the northward flux of quasi-geostrophic potential vorticity,
so that it is a direct measure of the total forcing of the zonal mean flow by eddies. Therefore, the E-P
flux and its divergence are important and useful to diagnose planetary waves propagation as well as the
effective mean zonal force induced by the waves. Some figures on this web page display E-P fluxes under
different forms, such as meridional cross sections and time series, for monitoring the stratospheric
circulation.
