2020-03-01_Australian_Geographic

Hot,drywes

terlywinds

across

theco

ntine

nt

MADDE

N-JU

LIAN

OSCI

LLAT

ION

IND

IAN

OCE

AN

DIP

OLE

SO

UT

HER

NA

NNU

LAR M

ODE

WEAKER

THAN

USUAL

STRONG

POSITIVE

PHASE

STRONG

NEGATIVE

PHASE

NEUTRAL

PHASE

March. April 49

NEGATIVE

SAM

SUMMER

Drier than average

in south-eastern Australia

WEAK

MJO

POSITIVE

IOD

Drier than average in central

and southern Australia

A

USTRALIA’S HIGHLY VARIABLE climate is infl uenced by

a range of oceanic climate systems, in particular the

El Niño-Southern Oscillation Index (ENSO) in the Pacifi c

Ocean, the Indian Ocean Dipole (IOD) and the Southern Annular

Mode (SAM) over the Southern Ocean.

While 2019 saw a neutral El Niño mode, a strong positive IOD,

characterised by cooler waters in the Indian Ocean off northern

Australia and warmer waters off Africa, contributed to the lack of

rainfall across southern Australia. Long-term observations indicate

that positive IODs have increased in frequency and strength since the

1960s. This is likely to continue with climate change, which is also

causing the SAM to become more positive. This moves storm tracks

south, reducing winter rain over southern regions as part of a longer-

term drying trend. In spring 2019 an unexpected warming event over

the South Pole – a Sudden Stratospheric Warming – shift ed the SAM

into temporary negative mode. This drew hot air from the centre

across southern Queensland and into the south-east regions.

This increased temperatures, decreased cloud cover, exacerbated ex-

isting drought conditions and lowered soil moisture, making fuels dri-

er and more prone to burning, heightening the risk of large bushfi res.

Fire weather is largely monitored by the Forest Fire Danger Index

(FFDI), which estimates the potential fi re danger based on observa-

tions of temperature, humidity, wind speed and a drought factor to

represent fuel dryness. Daily FFDI values can be accumulated during

longer periods of time and these values for spring 2019 were the

highest on record for 60 per cent of Australia, signifi cantly higher

than the previous highest values in 2002. By the start of Septem-

ber 2019, much of south-eastern Australia was already primed for

high fi re danger ratings, heralding the start of what would become

a catastrophic fi re season that would kill 33 people, including nine

fi refi ghters, destroy 3000 homes and burn more than 16 million

hectares of land with a devastating impact on native biodiversity.

3

The delayed positive

IOD phase then has its

strongest peak since 1997,

on 7 October 2019 (+2°C),

contributing to hotter and

drier-than-average conditions

across central and southern

Australia as summer

approaches.

2

Spring 2019

A weak Madden-

Julian Oscillation

event means the

monsoon arrives

later than usual in the

Southern Hemisphere.

It is the latest start

to the wet season

since records began

in 1957-58 and delays

the breakdown of the

IOD positive phase.

An unusual negative

SAM period in spring

2019, caused by a

Sudden stratospheric

warming event over

Antarctica, compounds

the already dry

conditions in southern

and eastern Australia

and elevates the risk of

fi re weather.

5

The fi rst fi res start

burning in September

2019. Hot and dry
      conditions continue
      as the 2019-2020
      summer begins.
      Fire ignitions are
      largely due to
      lightning strikes.

6

Summer

2019-2020

Catastrophic fi re

conditions in many

parts of Australia,

with dry westerly

winds and very dry

fuel loads in eastern

and southern forests

spreads fi res faster

than they can be

contained.

1

2017–present

Record low rainfall

since early 2017 leaves

the Murray-Darling

Basin in a long-running

drought. Almost all of

NSW, and large parts

of WA, NT, QLD, SA,

the ACT and VIC are

aff ected by drought.

Apr 2018–Sep 2019

Areas of severe

rainfall defi ciency

4

2019 WE ATHER

DRIVERS