The Orchardist

Metservice: Large-scale drought declared

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The Ministry for Primary Industries (MPI) declared a medium-scale adverse event (drought), in Northland and north Auckland in early February. This was extended later in February into south Auckland, Waikato and Hauraki-coromandel.

These regions were worst affected by this drought, due to both a record-dry summer experience­d in these regions (see below), and by the fact that the entire year of 2019 had run extremely dry, beforehand. (If you remember, in 2019 it was the driest January to July period on record, for northern New Zealand).

Summer of 2019–20 was the second driest summer on record for Whangarei (70mm rainfall) since 1937. Only the summer of 1945–46 was drier (55mm rainfall) for Whangarei. Auckland and Coromandel also experience­d the driest or second driest summer on record. Hamilton (Ruakura) had its driest summer since records began in 1905 (with 91mm rainfall). It was also the second driest summer on record for Tauranga (88mm rainfall) since observatio­ns began in 1898.

On 12 March, the Minister of Agricultur­e, Damien O’connor, announced that the drought had been reclassifi­ed as a largescale adverse event covering the entire North Island, as well as the north of the South Island (Tasman, Marlboroug­h and Kaikoura) and North Canterbury, as well as the Chatham Islands.

The last time a large-scale adverse event was declared was in 2013, and there are obvious similariti­es between the two droughts.

WHAT CAUSED THE 2020 DROUGHT?

Both the 2013 and the 2020 droughts were characteri­sed by persistent high pressure over the Tasman Sea (Figure 1). The Tasman Sea is the breeding ground for most of the North Island rain, so a lack of fronts and lows there, over multiple months, is the primary cause of widespread drought across the North Island.

In contrast, east coast droughts, where only eastern regions are affected, are typically caused by persistent westerly winds, which are often linked to strong El Niño phases.

RAINFALL ACCUMULATI­ON PLOTS

The rainfall accumulati­on plots for Te Puke, Napier and Blenheim are shown in Figures 2-4. They show the rainfall tallies for the 2020 year-to-date, as well as clearly highlighti­ng the extremely dry 2019.

The absolute dominance of the Tasman Sea high started to intermitte­ntly break down in mid-february (Figure 1). From that time on, although high pressure still appeared frequently on our weather map,weak troughs started to penetrate across the country, and were very important in delivering minor (and often ‘spotty’ or ‘hit and miss’) totals during the drought.

 ??  ?? Figure 1. The Tasman Sea Index (TSI). Positive values indicate higher than normal pressures over the Tasman Sea (35S to 42.5S, 160E to 170E), while negative values indicate lower than normal pressures. The TSI is derived from observed midday mean sea level pressure, averaged over the area as above, minus the monthly average to calculate a daily anomaly. The values for the period 20 March to 2 April 2020 are prediction­s (shown in opaque colours), based on the ECMWF (European Centre for Medium-range Weather Forecasts) weather model.
The green box shows the extended run of abnormally high pressure observed over the Tasman Sea between mid-december 2019 and mid-february 2020.
Figure 1. The Tasman Sea Index (TSI). Positive values indicate higher than normal pressures over the Tasman Sea (35S to 42.5S, 160E to 170E), while negative values indicate lower than normal pressures. The TSI is derived from observed midday mean sea level pressure, averaged over the area as above, minus the monthly average to calculate a daily anomaly. The values for the period 20 March to 2 April 2020 are prediction­s (shown in opaque colours), based on the ECMWF (European Centre for Medium-range Weather Forecasts) weather model. The green box shows the extended run of abnormally high pressure observed over the Tasman Sea between mid-december 2019 and mid-february 2020.
 ??  ?? Figure 2. Te Puke annual rainfall accumulati­on (mm) for the last five years (2016 to 2020). The annual average rainfall accumulati­on is shown in black.
Figure 2. Te Puke annual rainfall accumulati­on (mm) for the last five years (2016 to 2020). The annual average rainfall accumulati­on is shown in black.
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 ??  ?? Figure 3. Napier annual rainfall accumulati­on (mm) for the last five years (2016 to 2020). The annual average rainfall accumulati­on is shown in black.
Figure 4. Blenheim annual rainfall accumulati­on (mm) for the last five years (2016 to 2020). The annual average rainfall accumulati­on is shown in black.
Figure 3. Napier annual rainfall accumulati­on (mm) for the last five years (2016 to 2020). The annual average rainfall accumulati­on is shown in black. Figure 4. Blenheim annual rainfall accumulati­on (mm) for the last five years (2016 to 2020). The annual average rainfall accumulati­on is shown in black.

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