SAM’s in charge
The Southern Annular Mode (SAM) is one of New Zealand’s main climate drivers, and its influence over winter has been clear to see if you know what to look for.
Heading into spring, it looks set to remain the key player driving our weather patterns in the short-term. The SAM not only plays a huge role in the weather in NZ, but also in climate variability throughout the Southern Hemisphere. The word annular means ring-shaped, and in the context of the Southern Annular Mode refers to a belt of westerly winds which circle the South Pole. The strength and displacement of this belt northwards or southwards from its normal position, the mode, indicate the phase of the SAM and can help us deduce trends in the weather and climate.
In NZ, when SAM is in its positive phase, the belt of westerlies contracts to the South Pole. Frequent highs sit over the South Island and near the Chatham Islands, producing fairly settled weather over the country, overall, with prevailing northerly airflows and warmer temperatures.
In its negative phase, the belt of westerlies is displaced further northward and washes over NZ; wetter, windier and cooler southwesterly conditions prevail, with frequent fronts and lows.
THE INFLUENCE OF SAM OVER WINTER
A sustained period of positive SAM occurred in January, February and March 2018 as seen in Figure 1; it was no accident that this coincided with our heatwave summer.
During late autumn and through winter, semi-regular switching of the SAM from its positive to negative phase has been evident. This led to changeable weather maps in the NZ region; in turn we have seen fluctuations in both rainfall and temperature across the country.
Whilst generally quite a mild winter, we have had several wintery blasts from the south. The temperature in Waikato has regularly fluctuated throughout the winter months, and you can notice a loose correspondence between sustained fluctuations in the SAM and the temperature anomaly at Hamilton as seen in Figure 2. Whilst not necessarily warmer when in a positive phase, and cooler when in a negative phase, the shape of the temperature graph does ‘respond’ to the trajectory of the SAM.
A notable cold snap with a sustained southerly air flow over the North Island occurred at the start of September. From September 3 until 12 Wellington had only a couple hours of northerly winds on the morning of September 10, with the maximum temperature recorded there over the nine-day stretch a miserly 11.6°C. This compared with its average maximum temperature of 13.4 degrees for September and ran pretty close to Wellington’s record spell of southerlies of 10 days and 10 hours which occurred in 1960. A decent cold snap in mid-September also brought low and deep snowfall to Fiordland, Southland and Central Otago, with snow lying in both Queenstown and Arrowtown. At Cardrona ski field, 54cm of fresh snow was reported in 24 hours. Noticeably, the SAM took a sharp tumble from strongly positive in the preceding days before the storm.
WHAT NEXT FOR SPRING?
The SAM will likely continue to dominate proceedings as we head deeper into spring, although the semi-regular switching of the SAM looks unlikely to last much beyond the end of September. The long-range models signal potential for a prolonged phase of positive SAM and blocking highs.
El Nino may also play a modest part late in the year, although all indications show any response here in NZ will be rather muted.