Feature article

Stratosphere holds potential for predicting NZ climate

The layers of the atmosphere shown schematically. The weather we experience is influenced by the overlying stratospheric layer. Note that the Earth’s protective ozone layer is concentrated in the stratosphere.

How the atmosphere behaves from one day to the next – the weather – can be predicted only a few days in advance. That’s because weather patterns are short-lived; cyclones and anticyclones form and decay over just a few days. Chaotic behaviour in the atmosphere sets a practical limit of about two passing weather systems before predictability is lost. Occasional exceptions are tropical cyclones which may endure for a week or 10 days, but even in those cases, their paths and durability are hard to predict.

So when it comes to climate outlooks covering 2-3 months, we require much more stable patterns of atmospheric circulation, patterns that can be relied on to persist for long enough to enable useful predictions to be made.

Since the 1970s, climate scientists have used the effects of slowly evolving ocean conditions, recognisable in ocean surface temperature patterns and subsurface conditions, to predict shifts in mean climate patterns over land. The best known of these is the El Niño – Southern Oscillation phenomenon.

Since then, scientists worldwide have been sifting through many data sets to find patterns of variability that might be useful for predicting seasonal climate. While the resulting climate dynamics literature abounds with patterns of climate variability, in the end only a few patterns emerged as contenders for climate prediction. Two of these are described as 'rings of climate variability, each circling a pole at high latitudes'. In the southern hemisphere, the ring of climate around the South Pole is referred to as the Southern Annular Mode (SAM), or the High Latitude Mode.

It seems clear that wind patterns in the stratosphere (see figure) are accompanied by similar patterns at ground level. For example, stronger westerly winds at ground level occur when the wind is strong at high altitudes. The pressure patterns associated with SAM tend to alternate every few weeks, in a fairly symmetrical pattern around the South Pole. Higher pressures in the stratosphere over New Zealand (north of about 50° S) and lower pressures over the Pole, lead to more anticyclonic conditions over the country, with weaker westerly wind flow over New Zealand. The alternative phase of the SAM, when the pressure fields are reversed, leads to stronger, more disturbed westerly flow over the country. So we could say that the stratospheric layer 'nudges' tropospheric weather. The sum effect of all those 'nudges' influences seasonal climate near the ground.

The phase changes of the SAM are not predictable more than a few days in advance; however, once changed, the phases tend to persist for a few weeks.

There is some evidence that stratospheric ozone depletion, the popularly termed ozone hole, also leads to a strengthening of circumpolar winds and higher atmospheric pressures in the mid latitude New Zealand region. This may have an effect on the strength of westerlies over New Zealand during spring – they might be expected to be weaker than normal during late winter and early spring when the ozone hole is most pronounced.

As forecasting techniques are developed further, what’s happening in the stratosphere may enable us to forecast climate patterns near the ground with more lead time.

For more information about the Southern Annular Mode, see www.atmos.colostate.edu/ao/introduction.html