Will we have a drought this summer?
A look at historical rainfall and Southern Oscillation Index (SOI) data might put things into perspective! The SOI tracks the atmospheric part of the El Niño Southern Oscillation (ENSO), which is a movement of warm equatorial water across the Pacific Ocean and the atmospheric response. When the index is negative, we have El Niño conditions, when it’s positive we have La Niña.
Daily rainfall data for the Lincoln area is available from 1881. Monthly SOI data is available from 1876. The driest calendar year on record was 1988, at 312.4mm. The second driest was not far behind, on 337.9mm, way back in 2015.
But what if we ignore winter rainfall and just focus on the September to May period – the growing season? It turns out that quite a different picture emerges. The season with the lowest September to May rainfall was 1997/98, closely followed by 2014/15, as Figure 1 shows.
To find out if drought years coincide with El Niño periods, we must first define what we mean by drought. For the purposes of this article, I have defined a drought season as one in which the September to May rainfall is less than or equal to the 10-percentile seasonal rainfall over the last 142 years. For Lincoln this is 321mm. In general terms, the September to May rainfall will be less than 321mm about one year in ten years, on average.
If we divide the period of record into thirds and count up the number of droughts in each, we find there were five in the first third, one in the second and nine in the most recent third. Furthermore, the severest droughts occurred in the most recent third. This suggests that droughts have become more frequent and more severe.
Figure 2 shows the month-to-month variation in the SOI from 1970 onwards. Negative SOI values indicate El Niño conditions.
How well do our droughts match up with El Niño conditions over this time period? Most of our droughts since 1970 occurred under El Niño conditions, but not all. The 1988/89 drought occurred under La Nina conditions.
However, El Niño conditions do not always result in droughts developing. For example, droughts did not occur in 1982/83, 1986/87, 1991/92 or 2004/05 despite strong El Niño conditions.
On balance, the odds of a drought developing under the current El Niño conditions are slightly positive. However, it is far from certain!