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Global setting – April 2015

Warming of the sea surface across the equatorial Pacific Ocean continued in April 2015, building upon the warmer than normal waters observed in previous months. These patterns, in combination with weaker trade winds and increasing cloudiness near, and to the east of, the International Date Line are consistent with weak El Niño conditions. However, further coupling between the ocean and atmosphere is needed for El Niño to become established.

The international guidance monitored by NIWA indicates the probability of El Niño formally commencing over the next three months (May – July 2015) is close to 80% (up from 60% last month). Nonetheless, it is important to recognize that the skill of ENSO forecasts issued at this time of year during the northern hemisphere spring tends to be lower than at other times of the year (the so-called “spring predictability barrier”). NIWA’s National Climate Centre will maintain a close watch on the developing conditions.

During May - July 2015, higher than normal mean sea level pressures are expected over the New Zealand region, extending across the Tasman Sea. Lower than normal mean sea level pressures are expected well south of the country.  This circulation pattern is likely to produce more south-westerly quarter wind flows than normal.

Sea Surface Temperatures

Coastal waters are forecast to be in the near average to above average temperature range for the May-July 2015 period.

Differences from average global sea surface temperatures for 5 April to 2 May 2015. Map courtesy of NOAA Climate Diagnostics Centre (http://www.cdc.noaa.gov/map/images/sst/sst.anom.month.gif).
Differences from normal April surface temperatures in the seas around New Zealand.
Monthly values of the Southern Oscillation Index (SOI), a measure of changes in atmospheric pressures across the Pacific, and the 3-month mean (black line). SOI mean values: April SOI -0.2; February to April average -0.4.

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