Sudden stratospheric warmings

An important aspect of variabililty in the stratospheric circulation, and in particular the distortions just mentioned, is sudden stratospheric warmings. Scherhag (1960) first noted these in the 1950s and they were subsequently found to be a characteristic late winter phenomenon (Hare, 1961; Wilson and Godson, 1962, 1963; Labitzke, 1968; 1981). Limpasuvan et al. (2004) provide a valuable composite analysis of the lifecycle of sudden stratospheric warming events in the Northern Hemisphere from the so-called Transformed Eulerian Mean (TEM) framework. The reader is referred to that paper and extensive references therein for details.

The TEM framework is very useful for diagnosing the influence of eddies on the zonal-mean circulation. It is useful to first consider the TEM framework in the context of the troposphere. Briefly, tropospheric eddies transport heat polewards. This acts to reduce the zonal-mean temperature gradient in the extratropics and hence the strength of the zonal-mean winds. Eddies also transport westerly zonal momentum polewards, tending to increase the zonal-mean winds in the extratropics. In the TEM framework, the net effect of these processes at a given latitude can be expressed in terms of the divergence of the Eliassen-Palm (EP) flux. The EP flux has vertical and meridional components. The vertical component of the EP flux is related to the strength of the meridional eddy temperature flux (v'T') while the meridional component is related to strength of the eddy momentum flux (uV), where the primes indicate departures from the zonal mean and the overbars indicate the zonal mean.

For long-term mean winter conditions, through much of the depth of the atmosphere and north of about 30° N, the EP flux divergence is negative (i.e., convergent). It can be shown that EP flux convergence represents a westward force on the zonal wind, i.e., it induces slowing of the winds (Holton, 2004). In a steady state condition, the zonal wind and temperature are constant. Slowing of the zonal-mean wind by the EP flux convergence is balanced by the effects of a residual circulation in the meridional plane that is in a thermally direct sense (clockwise as viewed in a mean cross section with the North Pole to the right). This residual circulation (which is part of the total zonal-mean meridional circulation) is highly efficient in maintaining thermal wind balance in large-scale atmospheric motions, and keeping the ageostrophic flow small in comparison with the geostrophic flow. The Coriolis force on the residual poleward meridional wind speeds up the zonal winds aloft. The Coriolis force on the residual equatorward wind reduces the zonal winds near the surface where the EP flux is divergent. Through continuity, there must be attendant vertical motions. A residual downward motion (adiabatic warming) balances radiative cooling in higher latitudes while upward motion (adiabatic cooling) in lower latitudes balances radiative warming. Departures from the steady state, seen as time changes in the zonal wind and temperature, arise from imbalances between the EP flux divergence/convergence and associated residual circulations. The EP flux divergence pattern is weaker in summer.

The mean circulation and thermal structure of the winter stratosphere examined earlier can be similarly understood from the presence of mean EP flux convergence and meridional-plane circulations associated with the upward propagation of planetary waves. For example, in the extratropics, eddy forcing maintains the observed stratospheric temperature above its radiative equilibrium. Hence there is net radiative cooling, which is balanced by downward motion (adiabatic warming) associated with the residual circulation. In the tropics, the temperature is below radiative equilibrium, consistent with upward motion (adiabatic cooling). In turn, sudden stratospheric warmings (which as the name implies are transient events) can be understood from the anomalous upward propagation of planetary waves.

As outlined by Limpasuvan et al. (2004), when an anomalous vertically propagating planetary wave enters the stratosphere, it imparts an anomalous EP flux convergence. The decelerated westerly flow is brought back toward balance by the Coriolis force acting on the poleward mean residual flow anomaly. This induces adiabatic temperature changes that also try and bring the flow back to balance. Through continuity, the poleward mean meridional flow across the axis of the EP flux forcing requires sinking motion (adiabatic cooling) below and poleward of the forcing region. In turn, there is rising motion (adiabatic cooling) below and equatorward of the forcing region. These meridional-plane circulations quickly weaken the meridional temperature gradient, and give rise to rapid high-latitude stratospheric warming.

In extreme cases, stratospheric temperatures can rise 50 K and the circumpolar vortex can reverse to easterly flow over the span of a few days. The presence of vertically propagating waves, while necessary for sudden stratospheric warmings, is not a sufficient condition. It appears that the stratospheric flow needs to be "preconditioned", such that wave activity is focused toward the polar vortex. The preconditioning occurs when the vortex is poleward of its climatological position (Limpasuvan et al., 2004).

Figure 4.5 illustrates changes in 10 hPa temperature associated with a major warming event that took place between late December 1984 and early January 1985. Temperature fields are given for five-day averages for 17-21 December 1984,

Figure 4.5 The change of 10 hPa temperatures (°C) associated with a sudden stratospheric warming event that occurred during late December 1984 through early January 1985. The plots give mean temperatures for the prewarming (17-21 December), warming (December 27-31) and postwarming (January 6-10) phases as identified by Kodera and Chiba (1995). Results are based on the NCEP/NCAR reanalysis (by the authors).

27-31 December 1984 and 6-10 January 1985. These are identified by Kodera and Chiba (1995) as representing, respectively, the prewarming, warming and postwarming phases of the event. The prewarming stage shows a vortex center located well off the Pole over the Norwegian Sea, with minimum temperatures of about -72 °C. During the warming phase, the vortex breaks down into four centers. The postwarming phase shows further breakdown. The contrast in high-latitude (e.g., north of 70° N) temperatures between the prewarming and the postwarming phase is readily apparent. Between the warming and postwarming phase, the stratospheric zonal-mean zonal winds reversed at 70° N (Kodera and Chiba, 1995).

There is mounting evidence from studies over the past several decades that sudden stratospheric warmings can have pronounced effects on the circulation of the troposphere and surface. Various mechanisms have been offered (Baldwin and Dunkerton, 1999; 2001; Hartmann etal., 2000; Ambaum and Hoskins, 2002; Perlwitz and Harnik, 2003). While they are still being elucidated, the point to be emphasized is the apparent existence of two-way coupling - while upward propagation of planetary waves influences the stratospheric circulation, changes in the stratospheric circulation can influence the troposphere and surface; for example, by changing the conditions for vertical propagation of waves. Baldwin and Dunkerton (2001) find that the surface signature of downward propagating anomalies strongly resembles the surface signature of the Northern Annular Mode (also referred to as the Arctic Oscillation), a mode of atmospheric variability that has pronounced impacts on Arctic climate and has emerged as an important issue in the climate change debate (see Chapter 11).

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