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Central
and eastern Pacific sea surface temperatures (SSTs) currently exceed
El Niño thresholds. International climate models suggest some
further warming of the central and eastern tropical Pacific Ocean is
likely. All surveyed models indicate SSTs will remain above El Niño
thresholds until at least the end of the 2023–24 southern
hemisphere summer.
Bureau
long-range forecasts are for SSTs up to 2.5 °C warmer than average
off eastern Tasmania and in the eastern Tasman Sea from October to
the end of 2023.
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Australian
Bureau of Meteorology,
19 September 2023:
ElNiño and positive Indian Ocean Dipole underway
An
El Niño and a positive IOD are underway.
The
declaration of these events, and their concurrence over spring,
reinforces the Bureau's long-range rainfall and temperature
forecasts, which continue to predict warmer and drier conditions for
much of Australia over the next three months. The confirmation of an
established El Niño increases the likelihood that the event will be
sustained through the summer period.
Oceanic
indicators firmly exhibit an El Niño state. Central and eastern
Pacific sea surface temperatures (SSTs) continue to exceed El Niño
thresholds. Models indicate further warming of the central to eastern
Pacific is likely.
Broadscale
pressure patterns over the tropical Pacific reflect El Niño, with
the 90-day Southern Oscillation Index (SOI) at −7.7. Recent trade
wind strength has been generally close to average, but was slightly
weaker than average across the tropical Pacific in August 2023 for
the first time since January 2020.
Overall,
there are signs that the atmosphere is responding to the pattern of
SSTs in the tropical Pacific and coupling of the ocean and atmosphere
has started to occur. This coupling is a characteristic of an El Niño
event and is what strengthens and sustains an event for an extended
period. Climate models indicate this El Niño is likely to persist
until at least the end of February. El Niño typically leads to
reduced spring and early summer rainfall for eastern Australia, and
warmer days for the southern two-thirds of the country.
A
positive Indian Ocean Dipole is underway. The Indian Ocean Dipole
(IOD) index is +1.25 °C for week ending 17 September. This is its
fifth week above the positive IOD threshold (+0.40 °C). The
longevity of this trend, combined with the strength of the dipole
being observed and forecast, indicate a positive IOD event is
underway. All models predict this positive IOD will persist to at
least the end of spring. A positive IOD typically leads to reduced
spring rainfall for central and south-east Australia.
When
a positive IOD and El Niño occur together, their drying effect is
typically stronger and more widespread across Australia.
The
Madden–Julian Oscillation (MJO) is currently weak and is forecast
to remain weak over the coming week.
The
Southern Annular Mode (SAM) index is currently negative and is
expected to remain negative for at least the coming week, before a
possible return to neutral late in September. During spring, a
negative SAM is associated with decreased rainfall across parts of
the east in both NSW and Victoria, and increased rainfall over
western Tasmania.
The
long-rangeforecast for Australia indicates warmer and drier than average
conditions are likely across most of southern and eastern Australia
from October to December. The Bureau's climate model takes into
account all influences from the oceans and atmosphere when generating
its long-range forecasts.
Global
warming
Global
warming continues to influence Australian and global climate. Global
sea surface temperatures (SSTs) were warmest on record for their
respective months during April to August 2023. August 2023 SSTs were
also the warmest globally for any month since observational records
began in 1850. July and August 2023 were also respectively the
hottest and second-hottest months globally in terms of 2-metre air
temperature.
Australia’s
climate has warmed by an average of 1.48 ± 0.23 °C since national
records began in 1910. There has also been a trend towards a greater
proportion of rainfall from high intensity, short duration rainfall
events, especially across northern Australia. Southern Australia has
seen a reduction, by 10 to 20%, in cool season (April to October)
rainfall in recent decades. This is due to a combination of natural
variability on decadal timescales and changes in large-scale
circulation caused by an increase in greenhouse gas emissions.
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