TECHNICAL
DRYNESS IN THE EASTERN NORTH ISLAND Words by Georgina Griffiths : MetService Meteorologist
2021 year-to-date rainfall Soil moisture deficits continue across the eastern North Island (Gisborne, Hawke’s Bay and the Wairarapa), even at the halfway point of autumn. This in part reflects a much drier than normal, and much warmer than normal autumn so far (at the time of writing, halfway through April). And in part reflects well below normal rainfall across both January and February 2021 in the east of the North Island. Year-to-date rainfall accumulations in the Hawke’s Bay so far in 2021 are currently sitting under half of the long-term average. At Napier Airport (Figure 1), year-to-date rainfall totals are only 42% of year-to-date normal. In Gisborne, year-to-date rainfall totals so far in 2021 are 57% of year-to-date normal (not shown). Year-to-date rainfall accumulations in Masterton at the time of writing (mid-April) are currently sitting at 70% of year-to-date normal (Figure 3).
2021 year-to-date temperatures Daily temperature anomalies for Napier and Gisborne are shown in Figures 3 and 4 respectively, across the first quarter of 2021. These plots show a considerably warmer than usual January in the eastern North Island, as well as a run of unusual heat to start and end the month of March. Temperatures at both Napier and Gisborne were more of a ‘mixed bag’ during February 2021. Temperatures across the first half of April at both Napier and Gisborne (note: not shown in Figure 3 or 4), have run almost 2 degrees above the average for the time of year. This is a very significant deviation from normal – in some locations, this will be close to record-breaking for April if the unusual warmth continues.
Soil Moisture Status The combination of drier and warmer than normal weather across much of the eastern North Island has resulted in reduced soil moisture, compared to typical autumn levels, at the time of writing. The Napier soil moisture levels are shown in Figure 5. Extreme soil moisture deficit is shown with the orange shading, when soil moisture levels are between 130 and 150mm of deficit. 50
NZGROWER : MAY 2021
What is noticeable in 2021, is the prolonged period of extreme deficit that extends well into April, similar to the last two years (2019 and 2020). Normally, on average, some decent autumn rainfall during April would usually reduce soil moisture deficits rapidly at this time of year. (In Figure 3 the black line shows the longterm average soil moisture deficit.)
Looking ahead Due to the currently ‘neutral’ El Niño Southern Oscillation (ENSO) status in the Pacific Ocean, the primary drivers of our winter weather patterns will be the Tasman Sea and the Southern Ocean. Both have been very active lately, producing some heavy rain for western and northern areas of both Islands. The Tasman Sea has recently been producing wetter northerly flows, while the Southern Ocean has recently produced an unsettled westerly regime. However, the eastern North Island has largely missed out on April rainfall from both of these drivers so far. As always, you should keep up to date with the MetService long-range forecast at http://metservice. com/rural/monthly-outlook. Or sign up to the Monthly Outlook for delivery straight to your email inbox at www.metservice.com/emails
