Spatial and Transient Behavior of the South Pacific Convergence Zone

1. 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

2. Literary Review SPCZ ITCZ SICZ SPCZ SACZ Cloud Cover • 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) Focus of Study Why does the SPCZ veer southward away from the ITCZ?

3. 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: • Improve dynamical explanations for why the SPCZ forms. • Understand why General Circulation Models (GCMs) misdiagnose the southward veering of the SPCZ. (Double ITCZ problem) • Determine where new meteorological observations are necessary to better constrain atmospheric GCMs.

4. Data and Methods Upper-troposphere (200 hPa) zonal winds (u) diagnose regions of negative zonal stretching deformation (s-1): Nomenclature

5. SPCZ Seasonal Cycle JJA Climatology: OLR and Zonal Stretching Deformation (200 hPa) • 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). DJF Climatology: OLR and Zonal Stretching Deformation (200 hPa) Contours: Negative Zonal Stretching Deformation (200 hPa)

6. 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

7. Mid-latitude Wave Trains 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)

8. Wave Energy Accumulation NCEP Reanalysis: Upper Troposphere • Mean zonal winds create a band of upper-tropospheric negative stretching deformation near the subtropical and mid-latitude SPCZ. • Group velocities (Cgd) of Rossby waves slow down in these regions leading to an accumulation of wave energy (ε). (Webster and Chang 1997)

9. Baroclinic Instability 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:

10. Baroclinic Instability 2006 Case Study NCEP Reanalysis: Latitude-Time PKE Chart: • Bursts of PKE in sub-tropical and mid-latitude SPCZ. • No clear latitudinal propagation into tropical SPCZ. NCEP Reanalysis: Do disturbances accumulate near the SPCZ?

11. Mid-latitude Wave Accumulation 2006 Case Study OLR and Zonal Stretching Deformation B A C Hovmoller (Longitude- Time) Diagram • Case Study (DJF 2006) • Meridional average (20˚S-35˚S) Time (days) • Three regions of enhanced convection: • South Indian Convergence Zone • SPCZ • South Atlantic Convergence Zone Pronounced eastern boundary of the SPCZ. Zonal Stretching Deformation (s-1) OLR (W m-2) B A C

12. Mid-latitude Wave Accumulation 2006 Case Study 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 Time (days) • Case Study Observations: • Many disturbances propagate slowly through the SPCZ. • Convective anomalies increase.

13. Wave Energy Accumulation Review of hypothesis for wave accumulation near the SPCZ: Boundary Layer (Previous Work) Tropical modes may accumulate in lower troposphere bands of: (e.g., Webster and Chang 1988) Upper Troposphere (Current Focus) Mid-latitude Rossby waves may accumulate in upper troposphere region of: (Schematic based on concepts in Webster and Chang 1997)

14. SST Forcing on Zonal Winds Zonal SST Gradient • Strong basin-scale zonal SST gradient. • Greatest SST gradient found near SPCZ eastern boundary. 90% Confidence Bounds Correlations: Zonal Stretching and OLR Zonal Stretching Deformation (s-1) OLR (W m-2) 90% Confidence Bounds ∂u/∂x OLR Increased Convection More Negative SST Gradient (˚C)

15. Intra-seasonal Forcing Intensity Changes El Niño events (e.g., 1998): ∂u/∂x > 0 OLR Convection decreases in mid-latitude SPCZ (black box) (Standardized Indices) Spatial Behavior Correlation of Seasonal Averages: OLR and Zonal Stretching Deformation El Niño (NE Shift) La Niña (SW Shift) Correlations statistically significant (95% level) for r >0.4

16. 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 deformationmay be forced by basin-scale SST gradients which are influenced by the phase of ENSO. (Current Work)

17. 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