Spatial and Transient Behavior of the South Pacific Convergence Zone

Presented by,

Matthew Widlansky

Peter J. Webster, Advisor

Hai-Ru Chang

Carlos Hoyos

School of Earth and Atmospheric Sciences,

Georgia Institute of Technology

November 14, 2008

ITCZ

SPCZ

SACZ

SICZ

SPCZ

Literary Review

Focus of Study

Why does the SPCZ veer southward away from the ITCZ?

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

• Tropical convection is oriented zonally and highly correlated with the warmest SSTs. (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 1997; Karoly and Vincent 1999)

Cloud Cover

Motivation

“While basin-scale climate studies point to the southwest Pacific as a region pivotal to decadal climate variability, neither its oceanic or atmospheric features [specifically the South Pacific Convergence Zone] have been properly depicted by models or observations…” (Ganachaud et al. 2007, CLIVAR SPICE Project)

Stated Goals of SPICE:1. Improve dynamical explanations for why the SPCZ forms.

2. Understand why General Circulation Models (GCMs) misdiagnose the southward veering of the SPCZ. (Double ITCZ problem)

3. Determine where new meteorological observations are necessary to better constrain atmospheric GCMs.

Variable Source Resolution Uses Caveats

OLR(Out-going Longwave Radiation)

NOAA (OI)(Liebmann and Smith 1996)

2.5˚ by 2.5˚

Daily average

•Climatology•Wave tracking

•Sat & Met Interference•Tropics/sub-tropics

Winds NCEP/NCAR Reanalysis(Kistler et al.

2001)

2.5˚ by 2.5˚

Daily average

•Upper-troposphere dynamics

•Data poor South Pacific

SST(Sea Surface Temperature)

NOAA (ER)(Smith and

Reynolds 2004)

2.0˚ by 2.0˚

Daily average

•Basin-scale gradients

•Sat & Met Interference

Data and Methods

Upper-troposphere (200 hPa) zonal winds (u) diagnose regions of negative zonal stretching deformation (s-1):

Nomenclature

0x

u

• Regions with OLR values less than 240 W m-2 (Vincent

1994) are experiencing deep atmospheric convection.

• Seasonal convection patterns are driven by the meridional shift of the West Pacific Warm Pool.

• SPCZ reaches strongest intensity during the austral summer months (DJF).

SPCZ Seasonal Cycle

DJF Climatology: OLR and Zonal Stretching Deformation (200 hPa)

JJA Climatology: OLR and Zonal Stretching Deformation (200 hPa)

Contours: Negative Zonal Stretching Deformation (200 hPa)

Fundamental Questions

• What dynamical processes amplify convection in the mid-latitude (diagonal) SPCZ?

• Why do many GCMs fail to simulate convection in the diagonal portion of the SPCZ?

• Why does the SPCZ veer southward away from the ITCZ?

NCEP Reanalysis

2-8 day bandpass filtered OLR linear regression: Base Point = 35˚S, 195˚E

OLR (unfiltered)- Shaded contours∂u/∂x (unfiltered)- Values greater (less) than 4x10-7 s-1 (-4x10-7 s-1) are shown. Solid contours depict negative anomalies, 2x10-7 s-1 interval.

Methods adapted from (Serra, Kiladis, Cronin 2008)

Mid-latitude Wave Trains

• Mean zonal winds create a band of upper-tropospheric negative stretching deformation near the subtropical and mid-latitude SPCZ.

0x

u

• Group velocities (Cgd) of Rossby waves slow down in these regions leading to an accumulation of wave energy (ε). (Webster and Chang 1997)

Wave Energy Accumulation

dx

ud

xC

t gd

Upper Troposphere

NCEP Reanalysis:

Baroclinic Instability

22 ''2

1vuPKE

Do mid-latitude cyclones influence the SPCZ?

• Disturbance activity measured by eddy perturbation kinetic energy (PKE):

• Amplification near sub-tropical jet stream exit region

• Maximum PKE from east coast of Australia to mid-latitude SPCZ

(e.g., Webster 1985; Webster 1989;

Matthews and Kiladis 1999)

NCEP Reanalysis:

Baroclinic Instability

• Bursts of PKE in sub-tropical and mid-latitude SPCZ.

• No clear latitudinal propagation into tropical SPCZ.

Do disturbances accumulate near the SPCZ?

Latitude-Time PKE Chart:

22 ''2

1vuPKE

NCEP Reanalysis:

NCEP Reanalysis:

2006 Case Study

Mid-latitude Wave Accumulation

Hovmoller (Longitude- Time) Diagram

OLR and Zonal Stretching DeformationA B C

A B C

• Case Study (DJF 2006)

• Meridional average (20˚S-35˚S)

Three regions of enhanced convection:

A) South Indian Convergence Zone

B) SPCZ

C) South Atlantic Convergence Zone

Pronounced eastern boundary of the SPCZ.

Tim

e (d

ays)

OL

R (

W m

-2)

Zo

nal

Str

etch

ing

D

efo

rmat

ion

(s-1

)

2006 Case Study

Mid-latitude Wave Accumulation

Case Study Observations:

• Many disturbances propagate slowly through the SPCZ.

• Convective anomalies increase.

2-8 day Filtered OLR and Zonal Stretching Deformation

Two Propagation “Regimes”

1) Fast: 1,500 km day-1

2) Slow: 600 km day-1

Tim

e (d

ays)

2006 Case Study

(e.g., Webster and Chang 1988)

Wave Energy Accumulation

Boundary Layer (Previous Work)

Upper Troposphere (Current Focus)

(Schematic based on concepts in Webster and Chang 1997)

Tropical modes may accumulate in lower troposphere bands of:

Mid-latitude Rossby waves may accumulate in upper troposphere region of:

Review of hypothesis for wave accumulation near the SPCZ:

0x

u

0x

u

SST Forcing on Zonal Winds

Zonal SST Gradient

Correlations: Zonal Stretching and OLR

90% Confidence Bounds

90% Confidence Bounds

• Strong basin-scale zonal SST gradient.

• Greatest SST gradient found near SPCZ eastern boundary.

EPACCPAC SSTSSTSST

Zo

nal

Str

etch

ing

D

efo

rmat

ion

(s-1

)

OL

R (

W m

-2)

SST Gradient (˚C)

SST OLR∂u/∂x

∂u/∂x OLR

-0.4 -0.6

∂u/∂x 0.5

SST

Increased Convection

More Negative

Intra-seasonal Forcing

Intensity Changes

Correlation of Seasonal Averages: OLR and Zonal Stretching Deformation

Correlations statistically significant (95% level) for r >0.4

El Niño(NE Shift)

La Niña(SW Shift)

El Niño events (e.g., 1998):

SST OLR∂u/∂x > 0

Convection decreases in mid-latitude SPCZ (black box)

Spatial Behavior(Standardized Indices)

Review of Dynamical Processes

• Sub-tropical jet stream enhances baroclinic instability in diagonal SPCZ. (Kiladis et al. 1989)

• Negative zonal stretching deformation cause synoptic disturbances to slow down and accumulate in the mid-latitudes. (Webster and Chang: 1988 and 1997)

• Wave energy accumulation may enhance convection in the diagonal SPCZ. (Current Work)

• Zonal stretching deformation may be forced by basin-scale SST gradients which are influenced by the phase of ENSO. (Current Work)

Remaining Questions and Future Work

• Do correlations between wave energy accumulation and convection exist on synoptic timescales?

• What causal relationships exist between SST, ∂u/∂x, and convection?

Large-scale and high resolution modeling experiments

6hr WRF simulation of the SPCZ