Changing South Pacific rainfall bands in a warming climate? Image from 8 February 2012 MTSAT-2 visible channel, Digital Typhoon, National Institute of Informatics

University of Hawaii colleagues: Axel Timmermann, Niklas Schneider, and Karl Stein Collaborators: Shayne McGregor 1, Matthew H. England 1, Matthieu Lengaigne 2, and Wenju Cai 3 1 Climate Change Research Centre, University of New South Wales 2 LOCEAN, France 3 CSIRO Marine and Atmospheric Research, Australia Spotlight on the South Pacific Convergence Zone: Matthew J. Widlansky International Pacific Research Center How will Pacific rainfall bands respond to a warming climate?

Southern Hemisphere Convergence Zones Austral 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 Majestys 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 brightness 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 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 )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 SPCZ Apia, 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 change Meeting topics

(mm day -1 ) 28°C 26°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°C 26°C Observed rainfall and SST climatology during DJF ~ GPCP rainfall ~ NOAA SST La Niña El Niño Extreme El Niño

3) Uncertain rainfall projection in IPCC AR4 DJF (2080 to 2099) Rainfall projection (%) Number of models > 0 Adapted from Figure (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 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? (Q. J. Roy. Meteor. Soc., 2012)

Influence of SST forcing on basic-state SST 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: 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 2 SPCZ 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 ENSO La Niña minus El Niño: SST anomaly (shading) U La Niña = 0 U El 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 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? (2012, in press)

Inter-model standard deviation 21 st century projection (shading) 20 th century control (black lines) Blue lines enclose simulated 20 th 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) Inter-model standard deviation Regional rainfall trend Rainfall change (% Control) Equatorial islandsSPCZ islands

Rainfall bias (% observed climatology) Model bias and projected rainfall change Rainfall projection (% 20 th century control) scaled by warming at equator, K -1 r 2 = 0.27 (n = 74) Shifted South Pacific rainfall bands in a warming climate? Tropical SPCZ (10°S-20°S, 150°E-150°W) Rainfall OBS >5 mm day -1

SST gradients influence the observed location and strength of the SPCZ Coupled GCMs yield uncertain 21 st 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

21 st century projection (RCP 4.5 W m -2, 20 models) Green lines enclose simulated 20 th century Warm Pool (27.5 °C) Robust SST warming pattern SST trend(tropical mean removed) Inter-model standard deviation 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 2xCO 2 CO 2 increased 10% per year to 710 ppm Projections 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), climatology Shading, warming trend

Bias-corrected island rainfall projections Coupled GCM (CCSM3) response to 2xCO 2 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 islandsSPCZ islandsEquatorial islandsSPCZ 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.) 21 st century projection (shading) 20 th century control (blue & black lines) Increasing model complexity 21 st 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 21 st 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 21 st 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 21 st 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 CO 2 scenario How does this balance change for more extreme greenhouse-warming? Rainfall response to tropical mean SST increase for 4 x CO 2 (4.4°C): AMIP-future ensemble (4 x CO 2 SST) projects rainfall increase for parts of SPCZ For 4 x CO 2 conditions, wet gets wetter mechanism almost completely offsets SPCZ drying associated with diminished SST gradient between SPCZ and Equator Warmest gets wetter Wet gets wetter 4 x CO 2 scenario

Moisture convergence in the SPCZ SPCZ 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 Moisture convergence (g kg -1 s -1 ) in the SPCZ % 20 th century observations Net dryingNet 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 21 st 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 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? (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 2 nd principal component 1997/98 El Niño 1997/98 El Niño PC1 PC2

How will zonal SPCZ events respond to climate change?

201 zonal SPCZ events 95 zonal SPCZ events CMIP5 experiments Considering only models able to simulate the nonlinear behavior of the SPCZ (12 out of 20 models) 20 th century21 st century RCP 8.5 W m -2

Correcting SST errors 179 zonal SPCZ events 57 zonal SPCZ events Flux adjusted perturbed physics experiments with HadCM3 CGCM (12 out of 17 experiments considered) 20 th century21 st century CO 2 increased 1% per year

(mm day -1 ) 28°C 26°C Observed rainfall and SST climatology during DJF Meridional SST gradient & zonal SPCZ events = [Box 1 SST – Box 2 SST] Box 1 Box /98 El Niño ~ GPCP rainfall ~ NOAA SST

21 st 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 events Flux 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 1998 GMS-5 IR water vapor μm Katrina (28 days) Susan (125 kts) Ron (Tonga: 67% damaged) Thank you