with John Maunder
The Southern Oscillation Index (SOI) is a standardized index based on the pressure differences observed at sea level between Tahiti and Darwin, Australia.
SOI is a prominent measure of the large-scale fluctuations in atmospheric pressure that occur between the western and eastern tropical Pacific (i.e., the state of the Southern Oscillation) during El Niño and La Niña.
In general, the smoothed SOI time series correspond very well to changes in ocean temperatures in the eastern tropical Pacific. The negative phase of SOI represents lower than normal air pressure in Tahiti and higher than normal air pressure in Darwin. The positive phase of SOI represents higher than normal air pressure in Tahiti and lower than normal air pressure in Darwin.
Prolonged periods of negative SOI values coincide with unusually warm ocean waters across the eastern tropical Pacific, typical of El Niño episodes. In contrast, prolonged periods of positive SOI values coincide with abnormally cold ocean waters in the eastern tropical Pacific, typical of La Niña episodes. Sustained negative SOI values below -8 often indicate El Niño episodes. These negative values are generally accompanied by a sustained warming of the central and eastern tropical Pacific Ocean, a decrease in the strength of the Pacific trade winds.
Sustained positive SOI values above +8 are typical of a La Niña episode. They are associated with stronger Pacific trade winds and warmer sea temperatures north of Australia. The waters of the central and eastern tropical Pacific Ocean become cooler during this time.
The Australian Bureau of Meteorology reported on May 11, 2011 that the El Niño – Southern Oscillation (ENSO) is continuing at neutral levels. The outlook for the climate model currently indicates that this neutral phase will last at least until October.
ENSO’s ocean indicators persist at neutral levels, with Pacific Sea surface temperatures near the long-term average across much of the equatorial region. Below the surface, temperatures are near average, with slightly warmer than average water over much of the subsurface. Atmospheric indicators such as the Southern Oscillation Index (SOI) and cloud configuration are also close to average. The trade winds were stronger than average in the far west, but close to average elsewhere.
The Madden-Julian Oscillation (MJO) is located over the Indian Ocean region. It is expected to move eastward across Australian longitudes over the next fortnight.
Southern Ring Mode (SAM) was positive last week. It should remain positive for the next fortnight.
The Indian Ocean Dipole (IOD) is currently neutral. The outlook for the climate model suggests that the IOD is likely to remain neutral during the first half of winter.
The graph below (from Australian Bureau of Meteorology, BOM) shows monthly SOI values from 1880 to April 2020.
The graph below (from Australian Bureau of Meteorology, BOM) shows monthly SOI values from January 2018 to mid-May 2021.
El Nino and La Nina affect New Zealand (Source Niwa)
During El Niño, New Zealand tends to experience stronger or more frequent westerly winds in the summer, usually resulting in drought in areas of the east coast and more rain in the west. In winter, the winds tend to be more southerly, bringing colder conditions to both land and the surrounding ocean. In the spring and fall, southwest winds are more common.
The events of La Niña have different impacts on New Zealand’s climate. More northeasterly winds are characteristic, which tend to bring wet and rainy conditions to the northeast of the North Island, and reduce rainfall to the south and southwest of the South Island. As a result, some areas, such as central Otago and southern Canterbury, may experience drought in both El Niño and La Niña. Warmer-than-normal temperatures typically occur over much of the country during La Niña, although there are regional and seasonal exceptions.
Although ENSO events have a significant influence on New Zealand’s climate, they account for less than 25 percent of the annual variance in seasonal precipitation and temperature at most New Zealand measurement sites.
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