Climate Update 62 - August 2004

30 March 2007 - 10:42am

Science Centres: Climate

August

Climate

River flows

Soil moisture

Three-month outlook

Checkpoint

Backgrounder

August
Climate Update is a summary each month of New Zealand’s climate, including soil moisture and river flows. August 2004 – Number 62 July’s climate: Extremely wet in Bay of Plenty with severe floods, but dry in the south. High river flows in the central and eastern North Island and below normal elsewhere.

Backgrounder
Backgrounder Is New Zealand climate becoming more volatile? Severe floods this year in Manawatu and Bay of Plenty are leading many to ask whether New Zealand climate is becoming more variable. MAF Policy recently asked NIWA to undertake a preliminary study of New Zealand climate ‘volatility’. We use the term ‘volatility’ here to refer to the variance about the mean climate – for example, the day-to-day variation within a month, or the month-to-month variation within a year.

Checkpoint
Outlook and outcome – May 2004 to July 2004 As expected rainfall was normal over much of New Zealand, except for above normal rainfall in Bay of Plenty and Gisborne. Higher than normal rainfall occurred in the west of the northern South Island with drier than normal conditions in the east. Many districts recorded higher temperatures than forecast, due to a higher than normal frequency of northwesterlies, particularly in June.

Climate
New Zealand Climate Devastating floods in eastern Bay of Plenty Rainfall was at least 300 percent of average in eastern Bay of Plenty, and also above average in western Bay of Plenty, Gisborne, and in and around Blenheim. However, below average rainfall occurred in many northern and western North Island regions and throughout much of the South Island. July was relatively cool – the national average temperature of 7.3 °C was 0.6 °C below normal. Mean temperatures were as much as 1.5 °C below average in parts of Buller, Canterbury, and Otago.

Three-month outlook
The outlook for August 2004 to October 2004 Lower than normal mean sea-level pressures are expected in the Tasman Sea for early spring, with more northwesterly winds than usual over northern and central parts of New Zealand. Sea surface temperatures are likely to remain close to average around the country. Air temperatures are expected to be average or above average for the North Island, and average for the South Island. Rainfalls are expected to be near normal in all regions, except for above normal in Nelson.

River flows
River flows High river flows in the eastern North Island Streamflows were above normal in the central North Island, eastern Bay of Plenty, and East Cape. Streamflows were normal in the Hawke’s Bay region, and mostly below normal elsewhere.  

Soil moisture
East coast South Island below field capacity Soils in parts of the east coast of the South Island remained below field capacity at the end of July, following lower than normal rainfalls during the month. Typically, soil moisture levels would be at or close to field capacity at this time.   Soil moisture deficit in the pasture root zone at the end of July (right) compared with the deficit at the same time last year (centre) and the long-term end of July average (left). The analysis is done for an average soil type where the available water capacity is taken to be 150 mm.