Following on from a record hot summer, the first half of 2018 has delivered one of the wettest January to June periods in recent history. Some of the key growing areas have seen record-breaking rainfall for the first half of the year. We take a look at why, and discuss what we expect going further forward into spring. A STORMY START TO THE YEAR
Northern areas of both Islands started the year with a deluge, with an intense sub-tropical low traversing the country on January 4, and another on January 18, dropping 333m of rain in 24 hours over Golden Bay in Nelson. Otago and Southland had to wait until February for significant rainfall. This came in the form of ex-tropical cyclone Fehi on February 1 which dropped 78mm at Cromwell, spiking the accumulations well above average for the year. Ex-tropical cyclone Gita then arrived on February 20-21, targeting the northern South Island and helped Nelson and Blenheim set the wettest February on record for both locations. The rainfall was 370% and 400% (i.e. four times) of the February average, respectively.
On March 12 the final ex-tropical cyclone, Hola, of the season passed over the New Zealand region offering good totals to the northeast of the country. Climatologically, about one ex-tropical cyclone comes within 550km of the country per season. The influence of a weak La Figure 1. Rainfall accumulation plots for Pukekohe, Blenheim and Cromwell for the year-to-date (red line), tracking well above average.
Nina over this cyclone season was clear, with three ex-tropical cyclones affecting New Zealand.
AUTUMN: A BREAKDOWN OF THE HEAT, AND A REGIME SHIFT
In late March, the Southern Annular Mode (SAM) returned to a sustained negative phase for the first time since October
2017. The SAM describes the westerly wind belt that circles Antarctica and the strength and extent of these westerly winds dominate our weather maps. In a negative phase, westerly gales and active cold fronts that lie over the Southern Ocean are displaced to the north, and wash up and over New Zealand. In the tropics, La Nina weakened to neutral conditions.
Cooler southwesterlies helped break down the heat in the oceans around New Zealand and sea surface temperatures cooled significantly as a result over the autumn months.
Stormy westerlies in autumn continued to see rainfall rates tick along at near average rates, with the most notable storm affecting the entire country from April 8 to 10.A total of 70mm in 12 hours was recorded in gauges across the Wellington region whilst Auckland was battered by highly damaging winds.
WINTER: A GLOOMY START FOR THE EAST
High pressure prevailed over the southern half of the South Island during June heralding the start of winter as the SAM swung back to positive. Periodic wet and cloudy easterlies affected the North Island and upper South Island, producing an extremely wet month for all eastern regions between Christchurch and Northland.
In contrast, this pattern produced an abnormally dry June for the western and southern regions of the South Island.
WHERE ARE WE NOW?
Unsurprisingly, rainfall accumulations for the first half of the year in the key growing regions are well above average, with some regions breaking records. Pukekohe saw its wettest January to June period on record – 858.4mm, breaking the 1996 record – whilst Blenheim recorded its third wettest January to June period (578.6mm) since records began in 1941. Cromwell was also sitting some 70mm above its average January to June accumulation. The constant, and sometimes extreme, rainfall has been wide reaching and soil moisture deficits are now nationally at or below average compared with the historical average deficit at this time of year, apart from the far south of the country; quite a drastic turn-around from late last year.
Confidence is growing amongst the models that El Nino will form by spring. During El Nino springs, New Zealand generally sees enhanced southwesterly winds. These contribute to our El Nino spring trademark: below average temperatures right across the country.
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