The Southern Oscillation Index (SOI) has steadily increased over the past two weeks, mainly associated with higher than normal pressures in the tropical Pacific. This is considered to be largely a weather effect, and doesn’t necessarily reflect the boarder climate situation. The latest (10 March) 30-day SOI value is +9.5.
Sustained positive values of the SOI above +8 may indicate a La Niña event, while sustained negative values below −8 may indicate an El Niño event. Values of between about +8 and −8 generally indicate neutral conditions.
今年可算是2003年以来第一个ENSO中性的冬季
“We have just witnessed our first ENSO-neutral winter since 2003-04 (2005-06 was an ENSO-neutral winter, but much closer to La Niña, and dipped into La Niña rankings during March-April). ”
The majority of El Niño-Southern Oscillation (ENSO) indicators have remained neutral since mid-2012. While the surface waters of the eastern Pacific have cooled recently, they are not supported by equally cool waters beneath the surface. Hence climate models suggest a neutral ENSO pattern will persist into the austral spring. However, the development of La Niña in 2013 cannot be totally ruled out yet.
In the tropical Indian Ocean, the current sea surface temperature pattern is typical of a negative Indian Ocean Dipole (IOD) event, and hence the IOD index has been below −0.4 °C since mid-May. If this index remains at, or below, −0.4 °C until mid-to-late July, then 2013 will be classified as a negative IOD year. All climate models surveyed predict a negative IOD during the southern winter-spring period.
A negative IOD during winter-spring increases the chances of above-average rainfall over southern Australia, and over parts of northern Australia it increases the chance of higher humidity. The northwest cloudbands that have tracked across Australia over the past month or two are examples of how a negative IOD can influence southern Australia