March climate summary

The Tasman Tempest causes a deluge in the upper North Island

Overview

March 2017 was characterised by higher than normal atmospheric pressure over and to the south and west of the South Island combined with lower than normal pressure to the northwest of New Zealand. This pressure pattern resulted in a predominant wind flow from the northwest over the North Island, bringing with it moist tropical air masses and record-high rainfall for the upper North Island. Anomalous southeasterly airflow was prevalent over the South Island for the month of March, bringing rainfall and cool temperatures to the east but sheltering the west.

Heavy rain was the flavour of the month for much of the North Island. Warm, humid air masses crossing the North Island brought heavy rain and flooding to much of the top half of the North Island, particularly for Northland, Auckland, and the Coromandel. The impacts of these significant rainfall events are covered in more detail in the Highlights and Extreme Events section. Many locations in these regions, as well as Bay of Plenty, Waikato, Hawke’s Bay and the Kapiti Coast received more than double the normal rainfall for March (>200% of March normal). Some locations in Auckland, Waikato and the Bay of Plenty recorded over three times the normal rainfall for March (>300% of normal), while Whangaparaoa (north of Auckland) received an incredible 497% of normal March rainfall (354 mm), or nearly five times the normal March total rainfall. Many of these monthly totals broke records for March and also for any month (see Rainfall section on Page 4). The remainder of the North Island had a relatively wet month, with most locations recording above normal rainfall or more (>120% of normal). In addition, Tasman District and parts of coastal Canterbury and Otago recorded above normal rainfall (120-149% of normal March rainfall) or well above normal (>149% of normal); the high rainfall in the east being due to the predominant southeasterly airflow. In contrast, due to the persistent high pressure and blocking of moist air masses from the southeast by mountain ranges, the south and west of the South Island recorded low rainfall totals. Some locations in Southland, Otago and around the Main Divide recorded well below normal rainfall (<50% of normal March rainfall) – a mere 14% of normal March rainfall (53 mm) was observed at Mt Cook Village. Other locations in the south and west of the South Island recorded below normal (50-79% of normal) or near normal (80-119% of normal) rainfall for March.

Due to the frequent tropical air masses travelling over the North Island, higher than average mean temperatures were experienced throughout the Island. Well above average (>1.20°C above average) temperatures were experienced in parts of Northland, Auckland, Waikato, Bay of Plenty, Hawke’s Bay, and Taranaki for March. Above average (0.51 to 1.20°C above average) temperatures were recorded in all other North Island regions, except for the Wairarapa which experienced near average (-0.50 to 0.50°C of average) or below average (-1.20 to -0.51°C below average) temperatures. The South Island was an island of two halves due to the predominant southeasterly airflow, with above or well above average temperatures recorded in the west and north, and average or below average temperatures recorded in the east and south. A few locations in coastal Canterbury recorded well below average temperatures (< -1.20°C below average). This distinct division in temperatures that followed the Main Divide was due to the foehn effect, as cool southeast air masses affected the south and east of the South Island but this air warmed as it travelled down the western (lee) side of the mountain ranges. The nationwide average temperature in March 2017 was 16.0°C (0.3°C above the 1981-2010 March average from NIWA’s seven station temperature series which begins in 1909).

Soil moisture levels across the country at the end of March approximately reflected the rainfall pattern experienced during the month. Soils in the upper half of the North Island were much wetter than normal for the time of year, as well as around Wellington, northern Tasman-Nelson and part of the coastal Ashburton District. Soils were much drier than normal for the time of year for Southland, Stewart Island, and Queenstown-Lakes District, as well as northern Canterbury and Marlborough. Soil moisture was near normal for the time of year for other regions.

Sunshine was lower than normal for some parts of the North Island and the upper South Island, due to the moist airmasses that brought cloudy and rainy conditions to those parts of the country. Locations in Northland, Bay of Plenty, Wellington, Taranaki, West Coast, Nelson and Marlborough recorded well below normal (<75% of normal March sunshine) or below normal (75-89% of normal) sunshine, while Auckland recorded near normal (90-109% of normal) sunshine for March. In contrast, parts of Otago and Southland recorded well above normal sunshine (>125% of normal), consistent with clear skies and low rainfall totals for the month.

Further Highlights:

• The highest temperature was 33.0°C, observed at Leeston (Canterbury) on 17 March.

• The lowest temperature was -1.7°C, observed at Lake Pukaki on 8 March.

• The highest 1-day rainfall was 231.8 mm, recorded at North Egmont on 11 March.

• The highest wind gust was 137 km/hr, observed at Castlepoint on 20 March.

• Of the six main centres in March 2017, Tauranga was the warmest and wettest, Dunedin was the coolest, driest, and sunniest, and Christchurch was the least sunny (although it was missing three days of data).

• Of the available, regularly reporting sunshine observation sites, the sunniest four locations in 2017 (1 January – 31 March) were Whakatane (753.6 hours), Lake Tekapo (751.3 hours), Blenheim (729.0 hours) and Gisborne (726.3 hours).

Climate_Summary_March_2017_FINAL.pdf

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