During the autumn of 2016 New Zealand was subject in many areas to an “Indian Summer”, where for a prolonged period, long after the official autumn period had started, summer like conditions remained in a gradually decaying state. Even in mid May there were still temperatures in the low-mid 20ºC range. A year later and things could not be more different.
After a Summer that spent most of the time in neutral – where the Southern Oscillation is in neither negative (La Nina) or positive (El Nino)territory, the E.N.S.O. index is beginning to drop again. This suggests an El Nino revival may be going to occur in the latter half of this year. At this stage an El Nino watch is in force, suggesting that there is a potential 50% chance of the index entering positive territory later in 2017. It becomes an alert when the probability is considered to be 70% or greater. The University Corporation for Atmospheric Research has shown the strength in the El Nino and La Nina phases in the chart below.
New Zealand has had a chequered record with El Nino phases. In the 1972-73, 1982-83 and 1997-98 the strong El Nino phases in those periods caused severe drought in the south and east as well as abnormally high rainfall in the west. During the 1997-98 event over a 13 month period, there was no more than two consecutive days without a rainfall event of some sort on the West Coast.
La Nina phases can lead to drought in Canterbury and other locations in the south and southwest with lesser rain. This was well demonstrated in the phase that started in late 1998 and persisted through to late 2001. During the summer of 1998-99 river levels in Canterbury dropped to as low as 29m³s-¹ in the Waimakariri River, which has a minimum flow of 37m³s-¹, below which irrigation is not permitted.
I find the neutral phases, which seem to be somewhat rarer to be the most interesting. Neutral phases generally determine no particular preference for wind directions, meaning all quarter winds may occur at some point or another. Around the Pacific basin, E.N.S.O. neutral phases generally suggest average rainfall and temperatures will exist. But they are generally transit phases between El Nino and La Nina patterns.