Faculty of Science and Technology

Projected increase in El Niño-driven tropical

cyclone frequency in the Pacific

Dr Savin ChandCentre for Informatics and Applied Optimization

Federation University Australia

s.chand@federation.edu.au

Hurricane Katarina (Aug. 2005)

• Most costliest Atlantic hurricane (damage

estimate ~$US125 B);

• One of the five deadliest.

Florida

The Bahamas

Louisiana

Source: CIMSS

Tropical cyclone Yasi (Jan. 2011)

One of the most powerful tropical cyclones to affect Queensland since

records commenced (Average winds > 252 km/h at landfall).

Source: Bureau of Meteorology

Hurricane Sandy (Oct. 2012)

Most intense system to make

landfall along US east coast

north of North Carolina.

Usually as storms move further

north, they encounter cooler

SSTs, which reduces energy

uptake... in Sandy’s case, it

was feeding on exceptionally

warm SSTs (~3-4°C warmer) in

the Atlantic Ocean. Source: CIMSS

Tropical cyclone PAM (March 2015)

Regarded as the most intense

tropical cyclone to form in

the Southern Hemisphere

And one of the worst natural

disasters in the history of

Vanuatu

Source: CIMSS

Hurricane Patricia (October 2015)

Most intense tropical cyclone ever

recorded in Western

Hemisphere in terms of

barometric pressure (879-hPa)

Strongest globally in terms of

maximum sustained wind speed

(325 km/h)Source: CIMSS

Winston (Fiji, February 2016)

Strongest tropical cyclone

to make landfall in Fiji

and the South Pacific

(Cat 5; >255 km/h)

Source: John and Michelle Philp

Perhaps we do see a pattern toward more intense

tropical cyclones over the past decade?

• Is it because we now have better quality data then we used to have,

and this is giving us an impression that tropical cyclones are getting

more intense?

• Or is it that our climate is changing, and therefore impacting tropical

cyclone intensity?

Tropical cyclone monitoring and recordsIn some regions, tropical cyclone records extend back to 1800s...

But are these records reliable?

Source: Velden et al. 2006, BAMS,

Metrics related to TC

frequency have improved

considerably after 1960s...

TC monitoring has since

evolved, particularly after the

introduction of geostationary

satellites in late 1970s.

Methods for estimating TC intensity

also changed accordingly, and it is after

mid-1980s, TC intensity data is

considered reliable.

TC trends in observational records

Source: Webster (2005), Science

Saffir-Simpson scale:

Cat. 4 – 58-70 m/s (209-251 km/h)

Cat. 5 – > 70m/s (>252km/h)

Webster et al (2005) – Also indicated

substantial global increase (~100%) in

category 4 and 5 cyclones on the Saffir-

Simpson scale.

Study objecting

Webster et al (2005)…

Latitude of Lifetime Maximum Intensity

Source: Kossin et al.

(2014), Nature

Poleward migration of the

latitude of tropical cyclone

Lifetime Maximum

Intensity(LMI) away from

the tropics.

Source: Moon et al. (2014), Environ. Res. Lett.

Poleward migration of LMI is greatly influenced by

basin-to-basin changes in TC frequency associated

with multi-decadal variability, particularly for the

Northern Hemisphere (NH).

Study contradicting Kossin et al (2014)…

But the fact remains… the tropical

cyclones that do form now form in

a different climatic conditions than

it was decades ago.

Source: Trenberth et al. 2007, IPCC-AR4

Global Mean SSTs

Source: Santer et al. 2007, PNAS, Vol. 104, 15248-15253.

Total Atmospheric Moisture (Near- Global Oceans)

Even though there is a general consensus that

our climate is changing, and therefore it is

highly likely that tropical cyclone characteristic

will change as a result, we need longer and

more reliable tropical cyclone data records to

more confidently ascribe an anthropogenic

signal to the change.

Climate models What are they?

100-400 km

40 layers

Temp, wind,

rain

Laws of

physics

• The climate system is very complex

Can’t just assume that current trends will continue

• Global Climate Models (GCMs)

Simulate the fundamental processes driving the weather and climate, on powerful computers

Future Projections

Source: Tory, Chand et al. 2014, J. Climate

TC frequency change (late 21st century/ 20th century based

on CMIP5 simulations using RCP8.5 radiative forcing)

Global NH SH SI SP NI WNP ENP NA-50

0

50

Perc

enta

ge C

hange

CNRM-CM5 CCSM4 CSIRO-Mk3.6.0 GFDL-CM3 GFDL-ESM2M GFDL-ESM2G BCC-CSM1.1 MIROC5

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How tropical cyclone numbers will change

at a regional scale is not yet known?

Regional tropical cyclone activity is complicated by

model deficiencies and biases that limit adequate

simulation of regional changes in large-scale drivers

of TC variability.

The El Niño Southern Oscillation (ENSO) drives

substantial variability in tropical cyclone

activity around the world.El Niño CONDITION

“How will future changes in the El Niño

Southern Oscillation (ENSO) in a warmer

world impact regional tropical cyclone

activity?”

60 E 120 E 180 120 W 60 W

30 S

0

30 N

Observations - El Niño (1970-2000)

60 E 120 E 180 120 W 60 W

30 S

0

30 N

Observations - La Niña (1970-2000)

60 E 120 E 180 120 W 60 W

30 S

0

30 N

CMIP5 historical simulations - El Niño (1970-2000)

60 E 120 E 180 120 W 60 W

30 S

0

30 N

CMIP5 historical simulations - La Niña (1970-2000)

30 N

0

30 S

60 E 120 E 180 E 120 W 60 W

0 0.005 0.01 0.015 0.02 0.025 0.03 0.0350 1.0 2.0 3.00.5 1.5 2.5 3.5

(Gaussian TC density ( 10-2) per year and per 2.5 2.5 grid)

60 E 120 E 180 120 W 60 W

30 S

0

30 N

Observations (El Niño-La Niña)

a d

b e

c f

-0.2 0.2-0.6 0.6-1 1

(Gaussian TC density difference ( 10-2) per 2.5 2.5 grid)

0.4 0.8-0.4-0.8

60 E 120 E 180 120 W 60 W

30 S

0

30 N

CMIP5 historical simulations (El Niño-La Niña)

Ability of climate models to simulate ENSO-related

tropical cyclone variability

Projected ENSO-related tropical cyclone changes between late 20th century

(i.e. 1970-2000) and 21st century (2070-2100) simulations.

(Percentage future change of TCs per 2.5 2.5 grid)

-20 20 30 40 50-30-40-50 60-60

c

60 E 120 E 180 120 W 60 W

30 S

0

30 N

a

60 E 120 E 120 W 60 W

30 S

0

30 N

180

SW Pacific FrenchPolynesia

HawaiiMarshall Is.

b

60 E 120 E 180 120 W 60 W

30 S

0

30 N Horseshoe-shaped region

Projected changes between future and current-climate El Niño TCs

Projected changes between future and current-climate La Niña TCs

Projected changes between future and current-climate overall TCs

Changes in environmental conditions…

a

120 E 150 E 180 150 W 120 W 90 W

30 S

15 S

0

15 N

30 N

Environmental vertical wind shear

(m s-1)

0 3.5 7 10.5 14 17.5 21 24.5 28 31.5 35

120 E 150 E 180 150 W 120 W 90 W

30 S

15 S

0

15 N

30 N

Relative vorticity at 850 hPa

(10-6 s-1)

-10 -8 -6 -4 -2 0 2 4 6 8 10

120 E 150 E 180 W 150 W 120 W 90 W

30 S

15 S

0

15 N

30 N

Relative Humidity at 700 hPa

(%)

10 18 26 34 42 50 58 66 74 82 90

120 E 150 E 180 150 W 120 W 90 W

30 S

15 S

0

15 N

30 N

Vertical -velocity at 500 hPa

(x10-2 Pa s-1)

-10 -8 -6 -4 -2 0 2 4 6 8 10

120 E 150 E 180 150 W 120 W 90 W

30 S

15 S

0

15 N

30 N

Sea surface temperature

(oC)

15 17 19 21 23 25 27 29 31 33 35

b

c d

e f

120 E 150 E 180 150 W 120 W 90 W

30 S

15 S

0

15 N

30 N

Sign-agreement plot

Regions of unfavourable conditions Regions of favourable conditions

Marshall Is. Hawaii

SW

PacificFrench

Polynesia

Horseshoe-shaped

region

Schematic illustration of a mechanism for increased TC formation in the

‘horseshoe-shaped’ region in the Pacific during future-climate El Niño events

Strong SST-anomaly gradients:

barrier to shifts in convective zones

Present-climate El Niño (SST anomalies)

Weak SST-anomaly gradients:

facilitates convective zone movements

Future-climate El Niño (SST anomalies)

(oC)

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

120 E 150 E 180 W 150 W 120 W 90 W 30 S

15 S

0

15 N

30 N

a

b

SPCZ

ITCZ

SPCZ

ITCZ

Thank you

Acknowledgement

• Colleagues at Federation University Australia

• Collaborators at Australian Bureau of Meteorology and

the University of Melbourne.