A quantitative understanding of stratosphere-troposphere coupling is important for three main reasons:

1. On intraseasonal time scales, weather, storm tracks, and phase of the Northern Annular Mode (NAM) are affected by variability of the stratosphere (e.g., Baldwin and Dunkerton, 2001; Thompson et al., 2005). The slowly varying stratosphere provides an element of improved predictability on a time scale of at least two months, with a magnitude approaching that of ENSO (Thompson et al., 2005).
2. As the climate of the stratosphere has changed, the positions of tropospheric jets have shifted and the width of the tropics has expanded (e.g., Thompson and Solomon, 2002; Gillett and Thompson, 2003; Son et al., 2008; Lu et al., 2009). Depletion of the ozone layer over Antarctica has caused a poleward shift of wind and precipitation patterns (Perlwitz, 2011). The depletion of Antarctic ozone occurs primarily during late winter/early spring, causing a cooling of the polar stratosphere owing to reduced absorption of ultraviolet radiation. This cooling leads to a delayed summertime response in the lower atmosphere, characterized by a poleward shift of the jet stream. Kang et al. (2011) concluded that roughly one third of the recent Australian drought can be attributed to stratospheric ozone loss.
3. In the future, we can expect both radiative and dynamical stratospheric processes to impact surface climate, the ocean, and the biosphere. Altered surface winds and jets, in turn, affect both land climate and the ocean circulation, and therefore marine biology and chemical processes. Such changes, when they occur in regions sensitive to precipitation, could have major impacts on the viability of both ecosystems and human society in those regions. Without a good understanding of how and why the troposphere is affected, it is challenging to interpret climate model results, making it difficult to predict how future stratospheric changes will couple downward to the surface.

Dynamical and observational basis

The troposphere influences the stratosphere mainly through a variety of atmospheric waves that propagate upward. Recent evidence shows that the stratosphere organizes this chaotic wave forcing from below to create long-lived changes in the stratospheric circulation. These stratospheric changes can feed back to affect weather and climate in the troposphere. In the Northern Hemisphere, the connection can be described as a link between the strength of the stratospheric polar vortex and the dominant pattern of surface weather variability, the NAM (Northern Annual Mode).

When the NAM (which is very similar to the North Atlantic Oscillation) is positive, pressures are lower than normal over the polar cap but higher at low latitudes, with stronger westerlies at mid-latitudes, especially across the Atlantic. Northern Europe and much of the United States are warmer and wetter than average, and Southern Europe is drier than average. Variations in the strength of the polar vortex appear to induce changes to the surface NAM (Figure 1), but the dynamical processes are not well understood.

However, it may be possible to use stratospheric information (or forecast models with well-represented stratospheres) to improve weather forecasts beyond the seven to ten-day limit of weather prediction models (Figure 2) .

A better understanding of stratosphere-troposphere coupling may help to better predict not only weather on monthly and seasonal time scales, but also the climatic effects of greenhouse gas increases, stratospheric ozone depletion, solar changes and volcanoes. The SPARC DynVar activity is a key component of the stratosphere–troposphere coupling theme. It promotes the development and use of coupled atmosphere-ocean-sea-ice general circulation models to understand how does the stratospheric circulation affects the tropospheric mean climate, climate variability, and climate change.