New Zealand’s climate varies with fluctuations in the prevailing westerlies, and in the strength of the subtropical high-pressure belt. Many of these are short-lived or random. Others are linked to general variations over the southern hemisphere or Pacific Ocean. These are persistent and predictable to some degree.
The Antarctic Oscillation (also known as the High Latitude Mode) is the alternate weakening (negative phase) and strengthening (positive phase) of the westerlies, roughly every month. Over the last 30 years there has been a trend towards a stronger positive phase – stronger westerly winds at latitude 50° south. This has been attributed to increased greenhouse gases and ozone depletion in the stratosphere.
El Niño–Southern Oscillation
The most important climate patterns in New Zealand come from the central Pacific Ocean.
The Southern Oscillation, or the El Niño–Southern Oscillation, is a tropical, two-phase pattern that affects air pressure, winds, sea temperature and rainfall. It follows an irregular 3–7-year cycle.
In the El Niño phase, the easterly trade winds weaken. Sea temperatures in the eastern tropical Pacific become several degrees warmer than normal. There is a systematic eastward shift of heat and moisture out into the Pacific. Australia experiences higher air pressure and drought, whereas New Zealand experiences stronger than normal south-westerly winds. This generally results in lower temperatures for New Zealand, and drier conditions in the north-east of the country.
The La Niña phase is essentially the opposite of El Niño. New Zealand experiences more north-easterly flows, higher temperatures, and wetter conditions in the north and east of the North Island. Air pressure tends to be higher than normal over the South Island, which can lead to drought conditions in the south, and also in the Wanganui–Manawatū district. Therefore there can be droughts in both El Niño and La Niña phases.
Unusually warm or cold sea temperatures influence weather systems passing over a region. This can affect the climate far away. For example, warmer conditions in the subtropical Indian Ocean in autumn often result in high-pressure systems north of New Zealand in winter. There is a stronger north-westerly airflow and increased rainfall on the West Coast of the South Island.
Interdecadal Pacific Oscillation
Between 1978 and 1998, El Niño events increased. There has been much debate about whether this was a result of global warming.
Another possible explanation is the natural variations in the climate over decades (10-year periods). The Interdecadal Pacific Oscillation is a 15–30-year cycle that affects parts of the Pacific Basin.
Effects on temperature and rainfall
Three phases have been identified during the 20th century – a positive phase (1922–44), a negative phase (1946–77), and a positive phase (1978–98):
- The positive phase brings higher sea temperatures in the tropical Pacific (more like El Niño conditions) and colder conditions in the north Pacific. Around New Zealand, the sea temperatures tend to be lower, and westerly winds stronger, bringing rain to the south-west.
- The negative phase reverses these effects, with warmer sea temperatures around New Zealand.
Major fluctuations in New Zealand’s temperature and rainfall appear to coincide with these phases. Increased temperatures around 1950 occurred soon after the second change, from a positive to a negative phase. Distinct rainfall changes (wetter in the south-west, drier in the north-east) coincided with the switch from negative to positive in the late 1970s.