October climate report 2018
23 Nov 2018, 9:30 AM
The high-pressure systems continued to hang over Northland for the month of October 2018, catchments will continue to feel the effect of the persistent dry conditions. The rainfall totals are below average for most of the region with rivers running 40-60% of normal flows. The National Institute of Water and Atmospheric Research (NIWA) climate stations are measuring soil moisture deficits extremely low for this time of the year.
What’s driving these big highs over New Zealand, is “SAM”, the Southern Annular Mode. SAM is essentially a measure of the windiness and storminess in the mid and high latitudes, from New Zealand down to the Antarctic seas. In September and October 2018, SAM switched to a positive mode, which is associated with relatively light winds and more settled weather over New Zealand. The absence of wind and wave action prevented mixing of the water column over the ocean which elevated sea temperatures, clearly shown in the map below for late October 2018. Sea temperatures in the Tasman have reduced for the beginning of November but are forecast to rise again through December 2018.
Sea temperatures for 22 October 2018
Current climate models are predicting an 88% chance of El Nino occurring in the coming weeks. The equatorial sea temperature has aligned with El Nino conditions, but the atmospheric conditions are not quite there yet. A typical El Nino event peaks in December then normally fades over the February/ March period. But this El Nino maybe a protracted event, which means it could continue into the April-May period, possibly extending out the end of the tropical cyclone season. Below is the NIWA’s predicted forecast for Northland for the three-month period from November 2018 – January 2019:
|Temperature||Rainfall||Soil moisture||River flows|
The rainfall totals were well below the expected amount for Bay of Islands, the mid-north and Whangārei catchments. The SPI map indicates moderately dry catchments, in the same areas mentioned above. The dots with yellow around them indicate they are close to the next level of dryness. The top of the Mangakahia and Whangārei catchments are close to the SPI classification of “Severely dry”.
Median Rainfall Map - October 2018 NRC SPI Maps - October 2018
The dry weather is driving the river flows down. The Waipapa, Maungaparerua and Manganui running at below 40% of the expected flows with other rivers running at 40-60% of normal flows expected for October, refer to the map below:
Flow Map - October 2018
The antecedent conditions leading into a drought determine the severity. To put some context on the current dry conditions, comparing October 2018 flows to October 2009 flows (2009/10 significant drought), over half of the river recorders are 50-70% of the flows recorded in October 2009. If there is no significant rain over December 2018, river flows will be reaching critical levels from mid-December onwards, the table below estimates when the rivers at long term stations will reach Mean Annual Low Flow (MALF) and Design Drought Flows (DDF).
Predicted River flows for Northland from 21 November 2018 (assuming no rainfall from this point onwards):
|Water level recorder||Days to reach
Mean Annual Low Flow (MALF)
|Days to reach
1-in-5 year low flow
|Awanui at School cut||28||42|
|Kaihu at Gorge||15||27|
|Maungaparerua at Tyrees Ford||13||24|
|Ngunguru at Dugmores Rock||48||61|
Water level reading indicate most of Northland's aquifers are below average for October 2018, except for the systems in the Aupouri region, see the table below for more details:
Northland aquifer status
|Groundwater systems||Status for November 2017|
Below is the water level at the Poroti Spring ground water station. It details the current water level in comparison to the expected water level for October (red dotted line).
Poroti Springs water level
Soil moisture deficits
NIWA water balance modelling indicates soil moisture deficit is well below average at Kaitaia, Kerikeri and Dargaville, see below:
|NIWA Climate station||Soil moisture deficit (mm) as of 20/11/2018||Normal soil moisture deficit (mm)|
Northland NIWA Climate Stations soil moisture deficits