Changing South Pacific rainfall bands in a warming climate?

Changing

South Pacific

rainfall bands in

a warming

climate?

Image from 8 February 2012MTSAT-2 visible channel, Digital Typhoon, National Institute of Informatics

University of Hawaii colleagues:Axel Timmermann, Niklas Schneider, andKarl Stein

Collaborators:Shayne McGregor1, Matthew H. England1, Matthieu Lengaigne2, and Wenju Cai3

1Climate Change Research Centre, University of New South Wales2LOCEAN, France3CSIRO Marine and Atmospheric Research, Australia

Spotlight on the South Pacific Convergence Zone:

Matthew J. WidlanskyInternational Pacific Research Center

How will Pacific rainfall bands respond to a warming climate?

Southern Hemisphere Convergence ZonesAustral summer (DJF) climatology of satellite observed rainfall

(GPCPv2.1)

5 mm day-1 contour indicated by blue line

South Pacific Convergence Zone

(SPCZ)

South Atlantic Convergence Zone

(SACZ)

South Indian Convergence Zone

(SICZ)

SPCZ is the largest rainband in the Southern Hemisphere and provides most of the rainfall for

South Pacific island nations

Captain Fitz-Roy,Narrative of the surveying voyages of His Majesty’s Ships Adventure and

Beagle between the years 1826 and 1836

“I was struck by the precise similarity of the clouds, sky, peculiarities of wind, and weather, to what we had been accustomed to meet with off the coast of Patagonia: and I may here remark

that, throughout the southern hemisphere, the weather, and the turn or succession of winds, as

well as their nature and prognostications, are remarkably uniform.”

Historical perspective: Very early ship observation

Historical perspective: Early satellite observations

(Streten 1973, Mon. Wea. Rev.)

Cloud Cover Percentage (DJF)

Satellite cloud brightness1968-1971 composite of 5 day averages

S. Atlantic~30%S. Indian

~20%

S. Pacific~30%

Quasi-stationary Southern Hemisphere cloud band locations are

“related closely to that of the long-wave hemispheric pattern.”

Historical perspective: Literature review

Why does the SPCZ extend diagonally away from the equator in the Southern Hemisphere?

SACZ

SICZ

SPCZ

SPCZ

• SPCZ is a region of widespread cloud cover and rainfall extending southeastward from New Guinea into Southern Hemisphere mid-latitudes. (Streten 1973; Trenberth 1976)

• Tropical convection is oriented zonally and collocated with warmest SST. (Vincent 1994)

• Baroclinic-type disturbances influence the diagonal region. (Kiladis et al. 1989)

• Orientation changes during different phases of the El Niño-Southern Oscillation (ENSO). (Trenberth 1976; Streten and Zillman 1984; Karoly and Vincent 1999; Folland et al. 2002)

1) Historical perspective

2) Snapshot of SPCZ science, circa 2010

3) Recent advancements in understanding:

• Why is there a diagonal rainband?

• How will the rainband respond to climate change?

• Will frequency of future extreme SPCZ events change?

Answers to these questions are based on the underlying sea surface temperature (SST)

distribution and its projected change

Outline

August 2010: State of the science

Secretariat of the Pacific Regional Environment

Programme

Workshop on the SPCZApia, Samoa (Aug. 2010)

The Pacific climate change Science Program

1) Hypothesis for dynamics of the SPCZ

2) SPCZ related extreme events on interannual timescales such as droughts, floods, and tropical cyclones

3) Projections of the SPCZ response to climate changeM

eeting topics

(mm day-1)

28°C26°C

(2011, Clim. Dynam.)

Observed rainfall and SST climatology during DJF

~ GPCP rainfall~ NOAA SST

• Tropical SPCZ adjacent the meridional SST gradient (equatorial)

• Subtropical SPCZ transects the meridional SST gradient (mid-latitudes) and is west of maximum zonal SST gradient

(e.g., Lindzen and Nigam 1987)

1) Dynamics of the SPCZ

2) Interannual variability of the SPCZ

Tropical Cyclone Genesis

Neutral (mean)

Adapted from Figure 12 (Vincent et al. 2011, Clim. Dynam.)

Extreme El Niño (anomaly)

(mm day-1)

28°C26°C



La Niña

El Niño

Extreme El Niño

3) Uncertain rainfall projection in IPCC AR4DJF (2080 to 2099)

Rain

fall

proj

ectio

n (%

)N

umbe

r of m

odel

s >

0

Adapted from Figure 11.25 (IPCC AR4, Chapter 11)

%

CMIP3 (A1B, 21 models)

IPCC Fourth Assessment Report “Regional Climate Projections- Small Islands”:

1) Rainfall is likely to increase along equator and decrease in the Southeast Pacific (where it is already dry)

2) Multi-model mean trend is small in the SPCZ and inter-model uncertainty is large

3) Impact of coupled model biases on future rainfall projections not addressed

Why is there a diagonal rainband in the Southern Hemisphere, but not in the Northern Hemisphere?

Why is the tropical Pacific rainfall response to greenhouse warming so uncertain?

How will extreme events, such as strong El Niño occurrences and zonally oriented SPCZ events, respond to climate change?

Fundamental questions unanswered in Samoa

Today, I will present three papers (2012) addressing each question individually

Question #1




(Q. J. Roy. Meteor. Soc., 2012)

Influence of SST forcing on basic-stateSST Climatology

240 W m-2 OLR (rainfall proxy) climatology indicated by blue line

SPCZ (A) is west of the maximum zonal gradient (B-C)

The background quasi-stationary 200 hPa flow is partially dictated by the SST distribution (e.g., Gill 1980)

200 hPa Zonal Winds & Negative Zonal Stretching Deformation

Upper-troposphere zonal flow

A decelerating jet stream creates a band of upper-tropospheric negative zonal stretching deformation (s-1, 200 hPa) near the subtropical SPCZ:

0x

U

200 hPa Zonal Winds

Distribution of mean zonal winds acts to refract Rossby waves(e.g., Hoskins and Ambrizzi 1993, J. Atmos. Sci.)

SPCZ acts as a synoptic ‘graveyard’

TRANSIENT WAVES∂U/∂x < 0

From Matthews (2012, Q. J. Roy. Meteor. Soc.):

“The propagation of Rossby waves in a spatially varying mean flow can also be interpreted in terms of accumulation of wave energy (Webster and Holton, 1982). In particular, in jet-exit regions where the mean westerly wind u decreases eastward (∂u/∂x < 0), the zonal wavenumber will increase along a ray path. This leads to a decrease in the wave group speed and an increase in the wave energy density (Webster and Chang, 1998). When applied in the region of the SPCZ (Widlansky et al., 2011), this can explain the observation that the SPCZ acts as a synoptic ‘graveyard’ (Trenberth, 1976).”

Modes of SPCZ variability

Observed rainfall and 200 hPa zonal wind (DJF) ‘Shifted SPCZ’ mode 1 (12%)

~ TRMM rainfall~ NCEP Reanalysis U

Later, we will look at mode 2SPCZ position and intensity varies on multiple timescales:

• Synoptic, Rossby waves• Intraseasonal, MJO• Interannual, ENSO

Adapted from Figures 1 and 3 (Matthews 2012, Q. J. Roy. Meteor. Soc.)

(e.g., Widlansky et al. 2011, Clim. Dynam.)

Synoptic disturbances from higher latitudes‘Shifted SPCZ’ (mode 1) composite: OLR (rainfall proxy) and 200 hPa vorticity anomalies

Path of wave propagation

Mean diagonal SPCZ is the sum of equatorward propagating synoptic waves from the subtropical jet

towards the equatorial westerly wind duct

Adapted from Figure 5 (Matthews 2012, Q. J. Roy. Meteor. Soc.)

Change in basic-state during ENSOLa Niña minus El Niño: SST anomaly (shading) ULa Niña = 0 UEl Niño = 0

Path of wave propagation

Westerly wind duct constricts during El Niño, hence synoptic waves refract equatorward further east,

shifting the diagonal SPCZ northeastward

Adapted from Figure 11 (Matthews 2012, Q. J. Roy. Meteor. Soc.)

Why no diagonal rainband in North Pacific?

1) Subtropical jet is strong and narrow (topography)

2) Equatorial westerly wind duct is absent during Northern Hemisphere summer (weaker Walker circulation)

3) NH warm pool is confined near equator during winter

SPCZ orientation determined by warm pool configuration and its projected change

(Q. J. Roy. Meteor. Soc., 2012)

A diagonal rainband is the default, triggered by equatorward refraction of synoptic waves, but in the North Pacific:

Question #2




(2012, in press)

Inter-model standard deviation

21st century projection (shading)20th century control (black lines)

Blue lines enclose simulated 20th century rainfall > 5 mm day-1

Uncertainty remains in CMIP5

Inter-model uncertainty is larger than ensemble mean projected rainfall

trend

Rainfall trend (RCP 4.5, 21 models)


Regional rainfall trend

Rain

fall

chan

ge (%

Con

trol

)

Equatorial islands SPCZ islands

Rainfall bias(% observed climatology)

Model bias and projected rainfall changeRa

infa

ll pr

ojec

tion

(% 2

0th c

entu

ry c

ontr

ol)

scal

ed b

y w

arm

ing

at e

quat

or, K

-1

r2 = 0.27 (n = 74)

Shifted South Pacific rainfall bands in a warming climate?

Tropical SPCZ(10°S-20°S,

150°E-150°W)RainfallOBS >5 mm day-1

SST gradients influence the observed location and strength of the SPCZ

Coupled GCMs yield uncertain 21st century rainfall projections, especially in Southwest Pacific

1) Removing SST bias improves simulated diagonal rainband

2) Bias-corrected climate experiments suggest future drying as regional SST gradients weaken

3) Net rainfall change depends on balance of two mechanisms (of opposite sign)

Goal is to explain inter-model uncertainty

Procedure: Uncertain rainfall projection?

SST bias (CMIP5, 20 models)

Removing SST bias improves climatology

Rainfall climatology (CMIP5, 21 models)

Rainfall bias (CMIP5, 21 models)

Rainfall climatology (AMIP, 5 models)

Rainfall bias (AMIP, 5 models)

Equatorial Pacific is too cold and Southeast Pacific is too warm

• Double-ITCZ bias partly related to SST biases (e.g., Wittenberg et al. 2006, J. Clim.)

• Atmosphere GCMs (observed SST) simulate a more diagonal SPCZ

AMIP rainfall is too heavy

Warm Pool

21st century projection (RCP 4.5 W m-2, 20 models)

Green lines enclose simulated 20th century Warm Pool (27.5 °C)

Robust SST warming pattern

SST trend (tropical mean removed)


Maximum equatorial warming is a robust response to

greenhouse warming(e.g., Xie et al. 2010, J. Clim.)

Biased SST climatology does not affect SST projection

No flux correction Radiative flux correction

Coupled GCM (CCSM3) response to 2xCO2

CO2 increased 10% per year to 710 ppmProjections from last 20 years of 90 year simulations

Removing SST bias does not change the warming pattern and improves rainfall climatology

Each experiment projects more rain along equator and drying in the South Pacific, but drying in SST bias-corrected experiment occurs in Southwest Pacific

collocated with observed SPCZ

Warm Pool (27.5°C), climatologyShading, warming trend

Bias-corrected island rainfall projectionsCoupled GCM (CCSM3) response to 2xCO2

CCSM3 experiment with no flux correction shows no consistent rainfall projection for

the SPCZ islands

SST bias-correction experiment projects drying for SPCZ islands (typically 5-10%) and more rain along some parts of the

equator

Equatorial islands SPCZ islandsEquatorial islands SPCZ islands

Rainfall response to changing SST gradients

Green lines enclose observed Warm Pool (27.5°C) and the changing threshold for deep convection (dashed) (Graham and Barnett 1987, Science; Johnson and Xie 2010, Nature Geosci.)

21st century projection (shading)20th century control (blue & black lines)

Increasing model com

plexity

21st century trend (tropical mean removed) 2 and ½ Layer Atmospheric Model

Idealized Atmospheric GCM (ICTP)

Full Atmospheric GCM (CAM3)

Tropical Channel Run

• SST bias-corrected experiments have a more realistic SPCZ climatology than coupled GCMs. • In response to 21st century SST gradient pattern, rainfall increases where SST warms the most and decreases elsewhere.

• SPCZ drying is a robust response regardless of model resolution or convection parameters.

CMIP3 A1B scenario

Rainfall response to tropical mean SST increase

Wet regions tend to get wetter

Rainfall response(Total SST trend)

Rainfall response(SST gradient pattern)=

Rainfall response(Uniform SST warming, 2.2°C)

?

(Held and Soden 2006, J. Clim.)

+Warmest regions tend to get wetter

(Ma et al. 2012, J. Clim.)

Mean specific humidity increases over entire tropical Pacific supporting an enhanced hydrological cycle.

“Wet gets wetter” Thermodynamic mechanism

Contours depict projected moisture increase (lower troposphere) as simulated by AGCM forced with 21st century SST trend (A1B)

Rainfall response to tropical mean SST increase (2.2°C):

(Held and Soden 2006, J. Clim. and Seager et al. 2010, J. Clim.)

“Warmest gets wetter” Dynamic mechanism

Red contours depict warming more than tropical mean 21st century multi-model trend (CMIP3 A1B emissions)

Rainfall and wind response to prescribed SST gradient: • Anomalous divergence of moisture away

from minor warming regions, such as SPCZ. • Moisture convergence towards warmest waters accounts for increased rainfall at equator. (Ma et al. 2012, J. Clim.)

Delicate balance of opposing rainfall mechanisms

Warmest gets wetter

Wet gets wetter

~2 x CO2 scenario

How does this balance change for more extreme greenhouse-warming?

Rainfall response to tropical mean SST increase for 4 x CO2 (4.4°C):

AMIP-future ensemble (4 x CO2 SST) projects rainfall increase for parts of SPCZ

For 4 x CO2 conditions, wet gets wetter mechanism almost completely offsets SPCZ drying associated with

diminished SST gradient between SPCZ and Equator

Warmest gets wetter Wet gets

wetter4 x CO2 scenario

Moisture convergence in the SPCZSPCZ rainfall response to greenhouse warming influenced by two opposing mechanisms:

1) Increasing moisture convergence in lower troposphere (Thermodynamic mechanism)

2) Divergence of moisture away from the rainband towards equatorial regions of greater warming (Dynamic mechanism)

Projected SST trend (°C) in the SPCZ

Mo

istu

re c

on

verg

ence

(g

kg

-1 s

-1)

in t

he

SP

CZ

% 20

th centu

ry ob

servation

s

Net drying Net moisture increase

Large inter-model spread

Robust response

76 experiments

SST gradients influence the observed location and strength of the SPCZ

Coupled GCMs yield uncertain 21st century rainfall projections, especially in Southwest Pacific

1) Removing SST bias improves simulated diagonal rainband, but rainfall intensity is prone to errors

2) According to bias-corrected experiments, summer rainfall may decrease by 10-20% for some South Pacific islands, assuming moderate warming

3) Net rainfall change depends on delicate balance of opposing thermodynamic and dynamic mechanisms

Multi-model scatter of net moisture convergence helps explain inter-model variance in CMIP5 rainfall projections

Answers: Uncertain rainfall projection?

Question #3




(Nature, 2012)

Defining a “zonal SPCZ event”

GPCP rainfall

Moderate El Niño

La Niña

Neutral

Zonal SPCZ

: PC1 > 1 and PC2 > 0

Nonlinear behavior of 2nd principal component

1997/98El Niño

1997/98El Niño

PC1 PC2

How will “zonal SPCZ events” respond to climate change?

201 zonal SPCZ events95 zonal SPCZ events

CMIP5 experimentsConsidering only models able to simulate the nonlinear

behavior of the SPCZ (12 out of 20 models)

20th century 21st century RCP 8.5 W m-2

Correcting SST errors

179 zonal SPCZ events57 zonal SPCZ events

Flux adjusted perturbed physics experiments with HadCM3 CGCM (12 out of 17 experiments considered)

20th century 21st century CO2 increased 1% per year

(mm day-1)

28°C26°C


Meridional SST gradient & zonal SPCZ events

= [Box 1 SST – Box 2 SST]

Box 1

Box 2

1997/98El Niño


21st century projection (RCP 4.5 W m-2, 20 models)

SST trend

Smaller future meridional SST gradient

(departure from tropical mean)

Box 1

Box 2

Maximum equatorial warming is a robust response to greenhouse warming

(e.g., Xie et al. 2010, J. Clim.)

2) Increased frequency of zonal SPCZ events

Increased number of zonal SPCZ eventsFlux adjusted perturbed physics experiments with HadCM3 model

(12 out of 17 experiments considered)

1 2

Greenhouse warming is likely to cause:1) More summers with small meridional SST gradients

Pacific island communities experience extreme weather –droughts, floods, & tropical cyclones–

during zonal SPCZ events

Increased frequency of

extreme events is

consistent with projected SST warming

pattern

Extreme zonally oriented SPCZ event:4 January 1998GMS-5IR water vapor6.70-7.16 μm

Katrina(28 days)

Susan(125 kts)

Ron(Tonga: 67% damaged)

Thank you

Documents

Changing South Pacific rainfall bands in a warming climate?