September signs of a looming El Nino

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September’s mix of hot and cold, wet and windy was typically capricious spring weather for Gisborne . . . but might also be the first sign of an El Nino gathering strength.

While earlier forecasts were for a weak El Nino, the past few weeks have seen above-average sea temperatures develop in the eastern Pacific. Weather scientists in the UK are now expecting a more powerful effect, with the possibility of severe winter storms there.

For Gisborne and the East Coast, a strong El Nino brings cooler, dry weather over summer and autumn. The last severe one was in 2015-16.

The New Zealand Hydrological Society’s reference on floods and droughts says during an El Nino the westerly wind belt is displaced northwards and cold fronts are able to penetrate further north and east.

This is what happened last month, with 18 days of cold winds from the east and southerly quarters, and just 12 from the north and westerly quarters.

The same pattern was recorded in 2016, while last year north-westerly winds dominated over September.

Temperatures varied wildly over last month, from a near-record 25.1 degrees on the 13th, to a chill air temperature of 1.2 degrees on the morning of the 10th, and a grass minimum of minus 4.3.

There were only four days where the temperature rose to 20 degrees or above but nine mornings where the grass minimum dropped below zero. Officially there were five ground frosts.

Despite these big fluctuations, the mean daily temperature in Gisborne was a fraction above normal at 12.2 degrees.

While daytime temperatures were on the 30-year average, the nights and early mornings were not as cold as it may have seemed.

The mean minimum was 7.4 degrees — 0.4 warmer than usual.

The city area had more rain than usual, almost 117mm against the 30-year average of 74mm, but this was compensated by quite a lot more sunshine than usual.

Almost 202 hours of sun were recorded in September — 34 percent more than the 30-year average of 140 hours.

The climate charts of the National Institute of Water and Atmospheric Research (Niwa) show the Gisborne-East Coast region is still wet.

The coast from the Poverty Bay Flats to Tokomaru Bay is 20mm to 40mm wetter than normal for the time of year. Most of the district is close to field capacity in soil moisture, slowing down planting and field activities.

The 900mm of rainfall to the end of September is about 100mm ahead of the 30-year average — but almost 200mm behind last year which had recorded 1086mm at the same date.

District Council soil thermometers at Puketawa and Tauwhare stations are still just below the 10-degree mark, while at Tatapouri the soil temperature has climbed to 14 in the past week.

While the coastal strip has warmed, Niwa’s figures show the rest of the district inland has generally been about half a degree cooler than usual, despite having up to 115 percent more sun over the month.