The Southern Annular Mode and New Zealand Climate
The 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. The SAM involves alternating windiness and storm activity between the mid-latitudes, where New Zealand is located, and higher latitudes, such as over the Southern Ocean and Antarctica.
Positive SAM
In its positive phase, the SAM is associated with higher than normal air pressure in the New Zealand region. This tends to bring relatively light winds and tranquil weather conditions. Meanwhile, windiness and storm activity is typically increased over the Southern Ocean and Antarctica.
Negative SAM
In its negative phase, the SAM usually results in lower than normal air pressure around New Zealand and higher pressure well to the south of the country. Westerlies tend to increase around New Zealand, increasing the risk for unsettled weather conditions. The weather tends to be a bit calmer in the Southern Ocean, where westerlies are weaker than normal.
Latest observations
Recent trends in the SAM
Between January 2010 and October 2019, the SAM spent 59% of days in its positive phase. The negative phase occurred the remaining 41% of the time.
During 2017 and 2018, the positive phase was observed 66% and 65% of the time, respectively. This was associated with New Zealand’s hottest summer on record (December 2017-Feburary 2018) and the country’s hottest month on record (January 2018).
During the summer of 2017-2018, the SAM was positive 96% of the time, or 86 days had positive values and just 4 had negative values.
SAM distribution by decade
|
1980-89 |
1990-99 |
2000-09 |
2010-19 | |
|
% days with positive SAM |
48% |
56% |
51% |
59% |
|
% days with negative SAM |
52% |
44% |
49% |
41% |
Data: National Centers for Environmental Prediction
