Post on 04-Jul-2020
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
**
*
**
* *
*
**
*
*****
**
****
*
***
**
* * *
*
*
**
**
*
*
*
++
+ + + +
+
+++
+++
++
*
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.