Old diary sheds light on weather trends

Thursday, 27 August 2015

Once upon a time I kept a weather diary. It was in the days when my love for the weather knew no bounds and my time was relatively unencumbered, before the arrival of children.

Eventually, as the demands of fatherhood grew, I gave up the diary. My wife also hinted I was becoming a little obsessed with the nightly writing ritual.

That proved the final nail in the coffin for my weather memoirs. Nevertheless, looking back, I kept that journal faithfully for about 15 years, between 1989 and 2004, and for wherever I happened to be living at the time.

Occasionally, just very occasionally, I sneak up into the loft and grab a volume to see what I can remember of that time and what I might still learn from the weather patterns preserved within. It would be spectacularly self-centred to call this piece 'El Niño and Me'.

But with all this talk of El Niño, and of how it is strong in magnitude and still strengthening, it seemed a good time to reminisce about what happened in the last significant event.

Put simply, El Niño is a periodic weather phenomenon driven by changes in sea temperature around the Pacific. The intensity of the atmospheric El Niño - and its opposite partner in crime, the La Niña - is measured by the Southern Oscillation Index, or SOI, which roughly speaking is the difference in air pressure between Tahiti and Darwin.

In an El Niño, average pressures are lower in Tahiti than in Darwin, resulting in a negative value of the index.

This year's event is already quite strong, but it has not reached the epic levels of the 1997-98 and 1982-83 El Niños. On the face of it you might think those two super-El Niños would have brought similar summer weather to Canterbury. Instead, they generated wildly different summers.

That makes picking what this approaching El Niño summer might be like rather a fraught exercise. Generally it can be said that an El Niño will bring cooler than normal winter weather to Canterbury, a spring that is much colder, wetter and stormier than usual, and then a dry summer.

Some time in late spring, the prevailing, 'average', westerly winds that accompany the event suddenly warm up and dry out.

Delving back into the dark corners of my mind, I recall the 82-83 El Niño was extremely capricious. Frequent squally southwesterly changes raced up the Canterbury Plains right through that spring and summer, with lots of thunderstorms, one spawning the highly damaging Halswell tornado in January 1983.

Large, pulverising hail accompanied these storms, with many afternoons where hail was lying quite thickly on the ground.

Don't laugh, but somewhere up in the loft I still have a cassette-tape recording I made of one of these thunderstorms - the distant cannon fire of thunder rumbling ever closer, the pinging onset of the hail, the deafening torrential rain that came later, the thunder receding into the distance.

The storminess of the 82-83 El Niño summer ensured it was a much colder than usual season. Yet the El Niño summer 15 years later was wildly different - it was extremely hot and dry. Could this summer go the same way? Cue entry of aforementioned dusty diaries.

Flicking through the 1997 one I came across this on September 10: 'Big El Niño event still predicted for next 12 months - as big as 1982-83 event, at least.' On September 28: 'Only 8mm of rain this month.' And on October 4: 'First significant rain in six weeks, and first cricket game cancelled.'

October looks to have been quite stormy with warm nor'west bursts bringing several days in the mid-20s. Two days into November the temperature reached 29.8 degrees Celsius and the first 30C-plus day of the summer was the 24th, with a high of 31.8C.

That marked the start of an incredibly hot Christchurch summer. Blisteringly hot and dry Australian heatwave air swamped the South Island almost every few days with little respite.

From the end of November 1997 to the end of March 1998 there were 23 days in the 30s and another 10 with maximums between 28C and 30C. Of the days in the 30s, five had highs above 34C, with January 3 the summer's hottest, peaking at 35.5C - the hottest day since 36.1C on January 29, 1990. The last 30C+ day that summer/ autumn was March 24, with a maximum of 35.2C, incredible for so late in the season.

So how could that El Niño have been so different from its earlier colleague? While westerly winds prevail in an El Niño, all Cantabrians know the huge difference it makes to our weather if the winds are slightly more northwest than west, or more southwest than west.

And that's the case here - the 82-83 event had a southwesterly component, which frequently dragged cold, showery air up from the sub-Antarctic, while the 97-98 El Niño came with a northwesterly bias from outback Australia.

Such variability makes predicting the way it might swing this summer extremely difficult. It will more likely than not be dry, and I'm hoping the Government is clever enough to already be taking advice from MetService and Niwa (the National Institute of Water and Atmospheric Research) on the possible widespread drought situation.

Personally I'm not willing to pin my colours to the mast just yet. Instead, I'm just happy to say some very interesting weather lies ahead.

Paul Gorman is science editor and senior writer at The Press.