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Global setting – October 2015

6 November 2015

Atmospheric and oceanic anomalies in the equatorial Pacific reflect strong El Niño conditions. Sea surface temperature anomalies in the central and eastern Pacific have increased since September and are close to or exceed +2.5oC in places, whereas sub-surface temperature anomalies exceed +6oC in the far eastern tropical Pacific.

The atmosphere is well coupled to these ocean anomalies: the Southern Oscillation Index (SOI) is strongly negative (-2.0 for October 2015), and westerly wind anomalies (implying weaker easterly trade-winds) have intensified further since September. Convection and rainfall is suppressed over Indonesia, while enhanced convective activity and rainfall are observed in the central and eastern equatorial Pacific.

International guidance indicates that El Niño is certain (100% chance) to continue over the next three months. The current event is slightly weaker than the 1997/98 El Niño (the strongest since 1950) at this stage, but is expected to intensify further and peak in the summer months.

For November 2015 - January 2016, above normal pressure is forecast to the north and west of New Zealand, while below normal pressure is expected to the south of the country.  This circulation pattern is likely to be accompanied by anomalous west-southwesterly wind flows - a signature of El Niño conditions.

 

Sea Surface Temperatures

Sea surface temperatures are forecast to be normal or below normal to the west of the country, and below normal to the east of New Zealand.

Differences from average global sea surface temperatures for 4-31 October 2015. Map courtesy of NOAA Climate Diagnostics Centre (http://www.esrl.noaa.gov/psd/map/images/sst/sst.anom.month.gif)
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: October SOI -2.0; August to October average -1.9.
Differences from average October surface temperatures in the seas around New Zealand.

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