March 2018 - Climate report

23 Mar 2018, 12:33 PM

Current situation

Summer kicked off with hot temperatures, the hottest on record. The nation-wide average temperature for summer 2017-18 was 18.8°C (2.1°C above the 1981-2010 from NIWA’s seven station temperature series which began in 1909).

NIWA's seven station temperature series

NIWA’s seven station temperature series graph.

Cape Reinga, Kerikeri and Whangārei all recorded the highest mean air temperatures for summer since records begun. Also notable were the soil temperatures in December, running 3-4 degrees hotter than normal. The pattern of blocking highs (anticyclone replacement process) continued through December 2017 coupled with the warm seas in the Tasman pumped the temperatures well above normal, this all ended abruptly with the first deep low tracking down from the Tasman on 05 January 2018, although Northland dodged the rain, the storm skimmed along the North Island west coast whilst spring high tides were present, this had a significant effect on the tidal levels, resulting in minor flooding North of Kaeo and the Dargaville central business district. The tide gauge at Pouto recorded the highest water level since July 2009. The atmospheric door was now wide open for the tropics, there was shift to very humid conditions and storms forming in the tropics and heading south, multiple low pressure systems passing down the west coasts of New Zealand mid-January and over February, NIWA noted:

On 13 February, heavy rain across the Far North District in Northland led to flooding and slips, particularly around the Mangamuka Ranges. A slip blocked State Highway 1 near Rangiahua, making the roadway impassable. State Highway 11 in the Bay of Islands was also closed. In addition, the Ministry of Education said 16 schools in Northland and one early childhood centre had closed due to the inclement weather.

The ex-tropical cyclones Fehi (02 February 2018) and Gita (20 February 2018) missed Northland and delivered significant amounts of rainfall to the South Island.

Rainfall

Rainfall totals for the 2017 year were close to average for most of the region. The 2017 year started off with a drought, then moved onto to a stormy autumn with ex TC Debbie delivering some high intensity punches of rain, winter was wet due to steady rain, then a dry spring and start to summer, which all balanced out to an average year for rainfall for most of the region.

Parts of the east coast were slightly below the normal amount, such as the Whakapara and Waiotu catchments, the eastern hills typically pick up a lot of rain during storm events, which in turn bumps up the annual rainfall average. There has not been a significant flood event since July 2014, which may explain the below average rainfall in these areas.

Annual rainfall map - 2017                                    Summer Rainfall Map 2017-18

Northland Annual Rainfall Map 2017.  Summer Rainfall Map 2017-18.

Moving onto the monthly rainfall maps over summer, Northland had a dry December 2017 then two wet months over January and February 2018. The Waitangi at McDonalds Road rain gauge logged a dry December 2017, but still recorded the second wettest summer since records began in 1986.

Monthly maps

Median rainfall maps - December 2017, January and February 2018

Median rainfall maps - December 2017-February 2018.

Flows

The flow maps mirror the rainfall maps, the impact of the two ex-tropical cyclones in February 2018 is evident in the Bay of Islands catchments, north through to Kaeo. Provided Northland receives the winter rains, the groundwater systems should be at a good level heading into the next summer which should sustain the base flows for longer during November and December 2018.

River flow maps - December 2017-February 2018.

Groundwater

The water level at all aquifers are currently at normal to above normal levels for this time of the year.

Groundwater levels

Groundwater systems Status for February 2018
Aupouri OK
Taipa OK
Russell OK
Kaikohe OK
Whangārei OK
Mangawhai OK
Ruawai OK

The Poroti system shown in the graph below is above average water level expected for February. The system started to lift early this year, over February 2018, normally the levels don't start to rise until autumn or even winter.

Poroti groundwater system (water level)

Poroti groundwater system (water level) graph.

Soil moisture deficits

The NIWA stations from the mid-north down to Whangārei are recording soil moisture deficits from 0-5mm, well above the average for this time of the year, which normally is about 100-120mm, essentailly any rain will run straight off the soil, as it is at field capacity.

The daily mean soil temperatures at Dargaville and Warkworth are running at 2-3 degrees warmer than expected.

Regional soil moisture deficits as of 16 Feburary 2018 - NIWA

NIWA Climate station Soil moisture deficit (mm)
Kaitāia 40
Kaikohe 0   (at field capacity)
Kerikeri 5
Whangārei 5
Dargaville 70
Warkworth 20

Soil Moisture deficit at Kerikeri climate station - NIWA as of 16 March 2018

Soil Moisture deficit at Kerikeri climate station - NIWA as of 16 March 2018 

Soil Temperature at Dargaville climate station - NIWA as of 16 March 2018

Soil Temperature at Dargaville climate station - NIWA as of 16 March 2018 (graph).