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23 Factsheet 12: ENSO and IPO

Ohiwa Split
ENSO is measured in terms of the east–west
pressure difference, the Southern Oscillation
Index (SOI), which is a scaled form of the
difference in mean sea-level pressure between
Tahiti and Darwin.  The plot above shows monthly
SOI values for the last 20 years.  El Niños occur
when the SOI is persistently lower than –1
(and La Niña when the SOI is persistently greater
than + 1), ie, above or below the shaded grey area.

Natural fluctuations in New Zealand’s climate are influenced by two key natural cycles, operating over timescales of years, the El Niño-Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO).  Both these natural phenomena operate over the entire Pacific Ocean and beyond, and cause fluctuations in the prevailing Trade Winds and in the strength of the subtropical high-pressure belt.  El Niño events occur irregularly, about 3–7 years apart, and there can be large variability in the intensity of individual events.  They typically become established in April or May and persist for about a year thereafter.

During El Niño conditions New Zealand experiences:

  • more westerly winds
  • slightly high wave conditions off the southwest coast of the South Island
  • depressed sea levels
  • lower likelihood of ex-tropical cyclones affecting New Zealand.

During La Niña conditions New Zealand experiences:

  • more northeasterly winds
  • slightly higher wave conditions off the northeast coast of the North Island

  • higher sea levels

    higher likelihood of ex-tropical cyclones affecting New Zealand.

The IPO is a long-lived Pacific-wide natural fluctuation that causes relatively abrupt ‘shifts’ in circulation patterns within the Pacific Ocean that can last for two to three decades.  It is strongest in the northern Pacific but affects New Zealand’s climate.  There are two phases of IPO, a negative phase and a positive phase.  Three phases have been identified since the 1920s: a positive phase (1922–44), a negative phase (1946–77), and another positive phase (1978 to possibly 1998).

Ohiwa Split
IPO is defined by an index based on
sea-surface temperatures (SST).  The IPO
changes phase every 20–30 years.

Positive IPO phases are characterised by:

  • an increased tendency for El Niño events
  • a decreased rate of sea-level rise
  • increased westerly winds and anticyclones in the north Tasman
  • a tendency for beaches on the northeast coastline of the North Island to accrete
  • possibly less frequent and smaller storm surge events
  • drier conditions in the north and east.

Negative phases of IPO are characterised by:

  • an increased tendency for La Niña events
  • an increased rate of sea-level rise
  • weaker westerlies, more easterlies and northeasterlies over northern New Zealand
  • a tendency for beaches on the northeast coastline of the North Island to erode
  • possibly more frequent and larger storm surge events.

 

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