NIWA Outlook: September – November 2019
Outlook Summary
• The central Pacific El Niño event
that arrived in March 2019 has ended, giving way to ENSO
neutral conditions, owing to cooling sea surface
temperatures (SSTs) in the tropical Pacific and a neutral
Southern Oscillation Index (SOI) during August.
• SSTs
in the central equatorial Pacific were 0.2C above average
and the SOI was -0.2 for the month of August, both within
the neutral range.
• Oceanic ENSO neutral will most
likely continue (60% chance) over the next three
months.
• An area of warmer than average seas in the
west-central tropical Pacific is expected to occasionally
influence New Zealand’s weather patterns, contributing to
sub-tropical low pressure systems that can bring heavy
rainfall.
• For September to November, air pressure is
forecast to be lower than normal in, and particularly south
of, the New Zealand region. This is forecast to bring more
southwest quarter winds than normal for the season as a
whole, although periodic easterly quarter winds are
probable.
• September and at least the start of October
are forecast to be particularly unsettled, along with an
elevated risk for sharp cold snaps. Those with interests in
New Zealand’s primary sectors (e.g. agriculture) should
pay close attention to weather forecasts to assist with
mitigating potential impacts.
• Temperatures for the
coming three month period are forecast to be near average
for most of the country except the north and west of the
South Islnd, where near average or below average
temperatures are about equally likely.
• Rainfall is
forecast to near or above normal for most of New Zealand
except for the west of the South Island where near normal
rainfall is most likely.
September – November 2019
temperatures have about equal chances of being near average
(45% chance) or below average (40% chance) for the north and
west of the South Island. All other regions of New Zealand
will most likely experience near average temperatures (50%
chance). There is an elevated risk for sharp cold snaps,
especially for the South Island, during the first half of
the season.
September – November 2019 rainfall is about
equally likely to be near normal (40-45% chance) or above
normal (35-40% chance) for much of New Zealand except for
the west of the South Island where near normal rainfall (45%
chance) is most likely.
September – November 2019 soil moisture levels and river flows are most likely to be near normal (40-45% chance) for all regions of New Zealand.
Regional predictions for the September – November 2019 season
Northland, Auckland, Waikato,
Bay of Plenty
The table below shows the
probabilities (or percent chances) for each of three
categories: above average, near average, and below average.
In the absence of any forecast guidance there would be an
equal likelihood (33% chance) of the outcome being in any
one of the three categories. Forecast information from local
and global guidance models is used to indicate the deviation
from equal chance expected for the coming three-month
period, with the following outcomes the most likely (but not
certain) for this region:
• Temperatures are most
likely to be near average (50% chance).
• Rainfall
totals are about equally likely to be near normal (40%
chance) or above normal (35% chance).
• Soil moisture
levels and river flows are most likely to be near normal
(40% chance).
The full probability breakdown
is:
Temperature Rainfall Soil moisture River
flows
Above average 15 35 30 30
Near
average 50 40 40 40
Below
average 35 25 30 30
Central
North Island, Taranaki, Whanganui, Manawatu,
Wellington
Probabilities are assigned in three
categories: above average, near average, and below average.
• Temperatures are most likely to be near average (50%
chance).
• Rainfall totals are about equally likely to
be near normal (45% chance) or above normal (40%
chance).
• Soil moisture levels and river flows are
most likely to be near normal (45% chance).
The full
probability breakdown is:
Temperature Rainfall Soil
moisture River flows
Above average 15 40 35 35
Near
average 50 45 45 45
Below
average 35 15 20 20
Gisborne,
Hawke’s Bay, Wairarapa
Probabilities are
assigned in three categories: above average, near average,
and below average.
• Temperatures are most likely to
be near average (50% chance).
• Rainfall totals are
about equally likely to be near normal (40% chance) or above
normal (35% chance).
• Soil moisture levels and river
flows are most likely to be near normal (40% chance).
The
full probability breakdown is:
Temperature Rainfall Soil
moisture River flows
Above average 15 35 30 30
Near
average 50 40 40 40
Below
average 35 25 30 30
Tasman, Nelson, Marlborough, Buller
Probabilities are assigned in three categories: above average, near average, and below average.
• Temperatures are about equally
likely to be near average (45% chance) or below average (40%
chance).
• Rainfall totals are about equally likely to
be near normal (40% chance) or above normal (35%
chance).
• Soil moisture levels and river flows are
most likely to be near normal (40% chance).
The full probability breakdown is:
Temperature Rainfall Soil
moisture River flows
Above average 15 35 30 30
Near
average 45 40 40 40
Below
average 40 25 30 30
West
Coast, Alps and foothills, inland Otago,
Southland
Probabilities are assigned in three
categories: above average, near average, and below average.
• Temperatures are about equally likely to be near
average (45% chance) or below average (40%
chance).
• Rainfall totals are most likely to be in the
near normal range (45% chance).
• Soil moisture levels
and river flows are most likely to be near normal (45%
chance).
The full probability breakdown
is:
Temperature Rainfall Soil moisture River
flows
Above average 15 35 30 30
Near
average 45 45 45 45
Below
average 40 20 25 25
Coastal
Canterbury, east Otago
Probabilities are assigned
in three categories: above average, near average, and below
average.
• Temperatures are most likely to be near
average (50% chance).
• Rainfall totals are about
equally likely to be near normal (40% chance) or above
normal (35% chance).
• Soil moisture levels and river
flows are most likely to be near normal (45% chance).
The
full probability breakdown is:
Temperature Rainfall Soil
moisture River flows
Above average 15 35 30 30
Near
average 50 40 45 45
Below
average 35 25 25 25
Background
The NINO3.4
Index anomaly (in the central Pacific) for the month of
August (to the 25th) was +0.20C, suggesting that oceanic
ENSO neutral conditions have arrived. Warmth was steadfast
in the NINO4 region (west-central Pacific) with a monthly
value of +0.75C.
Upper-oceanic heat content anomalies continued to decrease, but remained slightly above average in the west-central equatorial Pacific. In the eastern part of the basin, heat content was below normal for the time of year. Overall, this distribution of heat content across the Pacific reflects ENSO neutral conditions, but with a slight lean toward El Niño-Modoki.
During August, above normal rainfall occurred from the eastern Maritime Continent into the western Pacific, with below normal rainfall just north of the equator in the central Pacific. This remained broadly consistent with El Niño Modoki conditions.
Trade winds were slightly weaker than normal in the eastern Pacific and slightly stronger than normal in the west-central Pacific. During September, reduced trades are forecast to west of the International Dateline, which may help to sustain the warm pool of water in the NINO4 region.
On the other hand,
stronger than normal trade winds are expected in the
east-central tropical Pacific, which may lead to additional
cooling of SSTs.
According to the consensus from
international models, oceanic ENSO-neutral conditions are
most likely at 59% chance for the September – November
period with El Niño at 33% chance. For the December 2019
– February 2020 period, the probability for neutral
conditions is 49% with the chance for El Niño increasing to
41%. For the March – May 2020 period, the probability for
neutral conditions and El Niño is 55% and 43%,
respectively.
New Zealand’s coastal water temperatures
for August 2019 cooled compared to July for all regions
except the east of the North Island, where a slight increase
occurred. The east of the South Island remains the most
anomalously warm coastal region with the rest of New Zealand
experiencing near or slightly above average coastal SSTs.
Elsewhere, SSTs in the Southwest Pacific remain above
average for the time of year and near or above average in
the Tasman Sea.
In other words, the warm seas that
helped to modify sub-Antarctic air masses earlier in the
season have now cooled.
New Zealand Coastal Sea Surface
Temperatures during August 2019
North
NI West NI East
NI North SI West
SI East SI
+0.20C +0.32C
+0.11C +0.31C +0.22C +0.76C
The Southern Annular
Mode (SAM) was negative for most of August. For September,
the SAM is forecast to become strongly negative during the
first week. This supports unsettled weather around New
Zealand.
The Madden Julian Oscillation’s weak
signature during August is expected to become more active as
the month of September progresses. Historically, phases 6
through 1 have brought cooler than average temperatures to
New Zealand during the month of September – phase 6 is
expected to be reached by mid-month.
During late August
2019, a Sudden Stratospheric Warming (SSW) event started to
unfold in the polar stratosphere, about 30 km above
Antarctica. SSWs represent a dramatic warming of the air at
the stratopsheric level, where a minor event is defined as a
warming of at least 25C in less than a week’s time and a
major event involves both a substantial warming as well as a
split of the stratospheric polar vortex.
The flow-on impacts to sensible weather can take several days to weeks as the SSW can encourage a weaker polar vortex (ring of cold, stormy conditions typically locked up over the pole), allowing ‘streamers’ or pockets of polar air to break off into the mid-latitudes.
Historically speaking, there have only been two SSWs on record in the Southern Hemisphere, one in September 2002 (major) and another in September 2010 (minor). SSWs are more common in the Northern Hemisphere. For New Zealand, the 2002 SSW event resulted in a significant cold outbreak in October while the 2010 event was associated with record rainfall during September.
Consequently, there is an elevated risk for periods of unusually active weather during spring 2019, including sharp cold snaps, especially in the first half of the season.
Forecast
Confidence
Temperature
Forecast
confidence for temperature is above normal, due to in large
part to good agreement among the dynamical models.
Additionally, a distinct climate driver (SSW) is present,
which will likely influence New Zealand’s weather patterns
into October.
Rainfall
Forecast confidence for rainfall is near normal. While there is good agreement for normal or above normal rainfall for much of the country, this is offset by several models indicating an opposing outcome (normal or below normal rainfall).
For
comment, please contact
Chris Brandolino, Principal
Scientist – Forecasting and Media, NIWA
National Climate Centre
Notes to reporters and
editors
1. NIWA’s outlooks indicate the likelihood of
climate conditions being at, above, or below average for the
season as a whole. They are not ‘weather forecasts’ as
it is not possible to forecast precise weather conditions
three months in advance.
2. The outlooks are the result
of the expert judgment of NIWA’s climate scientists. They
take into account observations of atmospheric and ocean
conditions and output from global and local climate models.
The presence of El Niño or La Niña conditions and the sea
surface temperatures around New Zealand can be a useful
indicator of likely overall climate conditions for a
season.
3. The outlooks state the probability for above
average conditions, near average conditions, and below
average conditions for rainfall, temperature, soil moisture,
and river flows. When a particular probability reaches or
exceeds 60%, we conclude it is “very likely”.
4. This
three-way probability means that a random choice would be
correct only 33 per cent (or one-third) of the time. It
would be like randomly throwing a dart at a board divided
into three equal parts, or throwing a dice with three
numbers on it.
5. Each month, NIWA publishes an analysis
of how well its outlooks perform. This is available online.
See www.niwa.co.nz/our-science/climate/publications/all/cu
6. All
outlooks are for the three months as a whole. There will
inevitably be wet and dry days, and hot and cold days,
within a season. The exact range in temperature and rainfall
within each of the three categories varies with location and
season. However, as a guide, the “near average” or
middle category for the temperature predictions includes
deviations up to ±0.5°C for the long-term mean, whereas
for rainfall the “near normal” category lies between
approximately 80 per cent and 119 per cent of the long-term
mean.
7. The seasonal climate outlooks are an output of a
scientific research programme.
8. Where probabilities are
within 5% of one another, the term “about equally” is
used.
9. The forecast confidence meter for temperature
and rainfall represents the expert judgement of NIWA’s
climate scientists. It aims to synthesize various forecast
elements, such as global and local climate drivers, in order
to clearly communicate forecaster confidence.
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media centre at: www.niwa.co.nz/news-publications/media-centre
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