Climate Variability

Abstract

T he Southern Annular Mode (or SAM) is a ring of climate variability that encircles the South Pole and extends out to the latitudes of New Zealand. (Its counterpart, the NAM, centres on the North Pole and affects climate in the northern hemisphere.) The SAM involves alternating changes in windiness and storm activity between the middle latitudes, where New Zealand lies (40-50˚S50˚S), and higher latitudes, over the southern oceans and Antarctic sea ice zone (50-70˚S70˚S). In its positive phase, the SAM is associated with relatively light winds and more settled weather over New Zealand latitudes, together with enhanced westerly winds over the southern oceans. In the opposite (negative) phase, the westerlies increase over New Zealand, with more unsettled weather, while windiness and storm activity ease over the southern oceans. How the SAM works The form of the SAM in its positive phase is shown below. The colours show where pressures are either lower (blue) or higher (red) than normal. In between high and low, wind changes follow the contours, blowing more strongly than normal over the southern oceans in the positive phase. In the negative phase, pressures are higher than normal over Antarctica, and the westerly winds are weaker than normal over the southern oceans. The Southern Annular Mode and New Zealand climate We certainly know it when a southerly from Antarctica blows across New Zealand; Jim Renwick and David Thompson are investigating how the South Pole affects our weather in more subtle ways. We see a red blob of high pressure over the South Island in the positive SAM, which is associated with relatively light winds over the country. The negative SAM would show a blue region of low pressure over the South Island, with unsettled weather and stronger than normal westerly winds over most of the country, north of the low centre. The SAM was first identified in the 1970s. On a week-to-week basis, it flips between states-causing either windier or calmer weather over New Zealand latitudes-in an unpredictable way, apparently at random. Though these phase changes of the SAM cannot be predicted more than a few days in advance, once changed, the phases tend to persist for several weeks. In recent years, scientists have noticed a trend in the SAM towards more periods of the positive phase, with a tendency towards strong westerlies over the southern oceans and lighter winds over the middle latitudes. The trend appears to be related to the Antarctic ozone hole, and the influence of the stratosphere on the weather lower down. The question is: what effect is this having on weather and climate in New Zealand? Tracking down trends in New Zealand During the summer of December 2005-February 2006, we examined rainfall and temperature data from stations around New Zealand, looking for days when the SAM was strongly positive or strongly negative. Then we compared daily climate statistics over New Zealand for the two extremes of the SAM.