Global setting: August 2017
ENSO (El Niño – Southern Oscillation) neutral conditions were still present in the tropical Pacific during August 2017. However, like July, several oceanic and atmospheric patterns, such as the Southern Oscillation Index (SOI) and decreasing sea surface temperatures (SSTs) in the tropical Pacific, leaned toward La Niña. The latest weekly sea-surface and ocean subsurface temperatures in the equatorial Pacific Ocean are slightly below normal (i.e. on the La Niña side of neutral). The Southern Oscillation Index (SOI) is currently positive (+0.4, i.e. on the La Niña side of neutral) and large-scale circulation anomalies along the equator are consistent with patterns usually associated with a La Niña state. On the other hand, La Niña-like rainfall and convection anomalies in the west Pacific were not as strong in August as they were in July.
International guidance favours a persistence of ENSO neutral conditions over the next three-month period (65% chance for September – November 2017). While ENSO-neutral conditions remain the most likely outcome through the end of 2017, recent observations indicate that the ocean and atmosphere may continue to periodically exhibit La Niña-like signatures.
Sea Surface Temperatures
Coastal waters remain generally warmer than average all around the country and positive anomalies increased during the final week of August, especially near the eastern and northern North Island. The anomaly in the “NZ box” (160°E-170°W, 30-45°S) is currently nearing +0.6oC. Ocean waters are still much warmer than average across the Tasman Sea, particularly in the south and west.
The dynamical models’ forecasts indicate that warmer than average SSTs around New Zealand are likely to persist over the September – November 2017 period.