Southern Annular Mode

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

This plot shows the daily values of the SAM over the last 3 years. It also provides the latest monthly value of the SAM and the percentage of time the SAM has spent in its positive and negative phase in the current year. Data: National Centers for Environmental Prediction; updated once monthly

Recent trends in the SAM

Between January 2010 and October 2019, the SAM spent 60% of days in its positive phase. The negative phase occurred the remaining 40% 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%

60%

% days with negative SAM

52%

44%

49%

40%

Data: National Centers for Environmental Prediction

Further reading and useful links

Contact

Meteorologist/Forecaster
A schematic of the positive phase of the SAM. Areas of above normal pressure are shaded in red while areas of below normal pressure are shaded in blue. [NIWA]
A schematic of the negative phase of the SAM. Areas of above normal pressure are shaded in red while areas of below normal pressure are shaded in blue. [NIWA]