Spring climate report 2017

18 Dec 2017, 11:47 AM

Current situation

Parts of the Northland region is experiencing dry weather, in particular the southern and western catchments. The Kaipara Harbour rain gauge recorded only 10mm over the last twenty-eight days.  Some rivers in these catchments will be reaching mean annual low flows in December, which normally is not observed until February. Due to the consistent rain over winter the groundwater systems have recharged to good levels heading into summer.

The daily mean soil temperatures recorded at the NIWA climate stations in Dargaville and Warkworth are running 3-4 degrees above average, combined with the low rainfall, the hot temperatures are driving the soil moisture deficits down to 110-125mm in these areas.

Although rain is forecasted this summer, the timing of its arrival will be critical, if the region continues to run at hot temperatures, by January 2018 rivers will be down to very low flows in the Kaihu, Pouto, Bream Bay and Mangawhai catchments, the situation needs to be monitored closely until significant rain is received.

Dry weather through spring and early summer is not uncommon, due to a process called anticyclone replacement. The high-pressure systems stall east of Australia and sit over New Zealand, eventually they slowly break up with a cold front moving along the southern edge, often followed with minimal rain.  The warm ocean between Australia and New Zealand is up to six degrees warmer than normal and is very evident on the global maps, it’s the warm water that potentially can fuel high impact rain events that have been mentioned this summer.

EarthWindMap/earth map: Tasman ocean temperatures observed 7 December 2017

Earth map of Tasman ocean temperatures.

EarthWindMap/earth map: Global Ocean temperatures observed 7 December 2017

Earth map of global ocean temperatures.

Outlook

The outlook for December 2017 is for continued dry weather, the MetService models show a continuation of the high-pressure systems locking in over Northland for most of December 2017 with prevailing warm easterly winds. Towards the end of the month the winds may switch to a more northerly aspect, which may increase the chance of rain, but the current models do not signal any significant rain for Northland during December 2017.

The NIWA outlook for Northland for the period December 2017, January and February 2018 indicate it is most likely for average to above average temperatures, rainfall, soil moisture and flow, see the summary table below:

Seasonal climate outlook December 2017 - February 2018

 

  Temperature Rainfall Soil moisture River flows
Above average 60 45 40 40
Near average 30 30 30 30
Below average 10 25 30 30

The NIWA outlook for the tropical cyclone (TC) season for New Zealand indicates normal to above normal risk levels, highlighted in the map below. As mentioned above, the warmer than normal sea temperatures will increase the intensity of a significant rainfall event. Typically, when a tropical cyclone forms, the longer the system sits in the warm water over the tropics, the more energy it gathers before heading down towards the lower latitudes. As the TC moves down it goes through a process known as extra-tropical transition, as the TC hits the mid latitudes, the westerlies increase in strength with height, effectively cutting the top off the TC system, combined with the cooler ocean temperatures the TC then loses energy and is classed as a depression.

Typically, during a La Nina event the westerly winds are not as prominent, which means the TCs this year may take a more direct route when traveling down. The NIWA models look at similar pre-season conditions, these are called analogue seasons, this year they are: (1970/71; 1978/79; 1995/96; 2005/06; 2007/08), the models shown an equal possibility of an ex TC tracking down the west or east coast. Typically, a significant event that stalls off the west coast will deliver more rain from the north causing region-wide flooding.

NIWA Tropical cyclone outlook

Map of tropical cyclone risk for Pacific countries.

Southwest Pacific Tropical Cyclone Outlook

Southwest Pacific Tropical Cyclone Outlook - October 2017

Rainfall

The spring rainfall map for 2017 paints a picture of two halves. The northern catchments excluding the Bay of Islands had good spring rainfall totals, totalling above the expected amount, in contrast the lower half of Northland had a drier than normal spring. Comparing the 2016 and 2017 spring rainfall maps, the previous spring was plentiful for the entire region.

The monthly rain maps show rainfall totals dropping below normal from October 2017 onwards (excluding the mid-north) and continuing to decline through November 2017, which is typical of the Northland climate.  Statistically November is the driest month for a lot of the catchments across the region. What does this all mean, the winter rain this year replenished most groundwater systems to good levels, which is a positive leading into summer, but the dry spring will bring the soil moisture levels down earlier than normal.

Median Rainfall Map – Spring 2016 & 2017

Median rainfall map - Spring 2016.  Median rainfall map - Spring 2017.

Monthly Maps

Median Rainfall Maps September, October and November 2017

Median Rainfall Map September, October and November 2017.

Flows

The flow maps depict the higher than normal flows across the western catchments in early spring, over September 2017 flows begin to decline from the mid-north to the southern regions as the dry weather pattern sets in. Below is the predicted flow for some of the major rivers in Northland, without any rain, the Kaihu River at Gorge River gauge could reach the design drought flow (DDF) by the end of December 2017. It is very unusual for rivers to be running close to MALF and DDF this early in summer.

Predicted flows

Water level recorder Days to reach
Mean Annual Low Flow (MALF)
Days to reach
1-in-5 year low flow
Awanui at School cut 36 49
Kaihu at Gorge 12 22
Maungaparerua at Tyrees Ford 20 31
Ngunguru at Dugmores Rock 42 65
Manganui at Permanent Station 9 28

Northland monthly flow maps for spring 2017

Northland monthly flow maps for spring 2017.

Groundwater

The water level at most aquifers is at normal levels for this time of the year. The Whangārei area has nine groundwater monitoring stations, water level at six of the stations is below average. This may be attributed to low rainfall, the Whangārei area is carrying a 154mm rainfall deficit for the 12- month period from November 2016 to 2017.

Groundwater levels

Groundwater systems Status for November 2017
Aupouri OK
Taipa OK
Russell OK
Kaikohe OK
Whangārei BELOW AVERAGE
Mangawhai OK
Ruawai OK

The Poroti system shown in the graph below is close the average water level expected for December, the winter recharge is significantly higher than the previous two years.

Poroti groundwater system (water level)

Poroti groundwater system (water level).

Soil moisture deficits

All NIWA stations from the mid-north down are recording soil moisture deficits from 108-125mm, well below the average for this time of the year. The daily mean soil temperatures at Dargaville and Warkworth have spiked up late November 2017, running at 4-5 degrees warmer than expected. The soil moisture deficts will continue to decline, possibly at a faster rate as the forecasted hot temperatures take hold.

Regional soil moisture deficits as of 8 December 2017 - NIWA

NIWA Climate station Soil moisture deficit (mm)
Kaitāia 75
Kaikohe 75
Kerikeri 110
Whangārei 108
Dargaville 120
Warkworth 125

Soil Moisture deficit at Dargaville climate station - NIWA as of 8 December 2017

Soil Moisture deficit at Dargaville climate station.

Soil temperature - Warkworth.

Soil temperature - Dargaville.

National Soil Moisture deficit NIWA as of 8 December 2017

National Soil Moisture deficit NIWA as of 08 December 2017.