The East Australian Current: anatomy of low frequency variability and its drivers.

1. The East Australian Current: anatomy of low frequency variability and its drivers. K. Hill1, S. Rintoul2, R.Coleman1, K. Ridgway2. 1CSIRO/UTas Quantitative Marine Sciences program. 2CSIRO Marine and Atmospheric Research and the Wealth from the Oceans Flagship.

2. Outline • Evidence of marine ecosystem changes in SE. Australia • Ocean currents relevant to Australia: • Focus on the East Australian Current • Maria Island coast station (Tasmania) • Relate Maria observations to: • the East Australian Current (EAC). • wider wind field/ gyre scale changes. • ENSO and Southern Annular Mode (SAM). • Conclusions.

3. Changes in S.E. Australia’s Marine Environment. Invasion of Sea Urchins native to NSW coast causing loss of Kelp forests in off eastern Tasmania (N. Bax) Changing composition of Phytoplankton blooms off Tasmania– increased tropical species and occurrences of red tides (S. Blackburn) Rock lobster catch and distribution correlated with regional SST changes around Tasman Sea (Harris et al 1988) In last decade 34 fish species have exhibited major distributional changes: either newly established south of Bass strait, or show significant range extensions. (P. Last) (CSIRO Workshop 2005)

4. Ocean currents and Australia’s Climate Maria Island Coast Station (42.36S, 148.14E). x

5. Maria Island Coast Station • Location: 42.36S, 148.14E. • Longest high quality ocean time series in the Southern Hemisphere. • Temperature, Salinity, Nutrients 1944 – present. • Surface to 100m @ 10m intervals. • Sensitive to large scale changes - on Equatorial side of the subtropical convergence zone (boundary between subtropical and subantarctic waters). • Sensitive to shifts in Zonal Westerly Winds • Potentially influenced by ENSO and Southern Annular Mode.

6. Time series of Maria Island Temperature

7. Changes in the properties at Maria Temperature (C) Salinity (psu)

8. Comparing Streamfunctions for 1950’s and 1990’s Using NCEP-1 winds and Stuart Godfrey’s Island Rule (Godfrey 1989) 1950’s 1990’s

9. Comparing Maria with South Pacific trends • Trend and decadal signal in Maria SST and Salinity • Correlates with Wind stress curl over South Pacific (at a lag) • And hence transport through Tasman sea (using Godfrey’s Island Rule). • NOTE: Trend in winds and transport not as evident in ERA-40 as NCEP-1.

10. Relationship between Maria SST and South Pacific wind stress curl. Correlating 5 year running means of temperature at Maria with Wind stress curl over South Pacific Time lag of response at Maria (months) Maria lagging Wind by 2 years

11. SODA shows some decadal variability in the strength of the EAC. Some relationship with winds – not as tightly correlated as Island Rule. Demonstration of winds/EAC relationship from SODA?

12. Trend: EAC/SAM relationship. Zonal Wind Difference: 1981:2000 minus 1948-1967 NCEP SAM (1st EOF of MSLP). Cai et al, 2003, 2005 • Evidence of +ve trend in westerlies over last 50 years – changed wind stress curl field. • Related to +ve trend in SAM. • Cai et al (2005) demonstrated relationship between SAM and increased strength of South Pacific Gyre and East Australia Current using CSIRO Mk3 GCM.

13. Anatomy of Decadal variability in the South Pacific (SODA). • First EOF of SST can be separated into “ENSO related” and a linearly independent “residual” comprising interdecadal variability. (Zhang et al,1997, Garreaud et al 1999) • No evidence of significant decadal timescale variability in lower EOF modes

14. Maria/ENSO relationship?

15. Conclusions (1) Clear trend found in Maria Island temperature and salinity. • 2.24C and 0.36psu per century over 60 years. • Ecosystem changes broadly consistent with shift south of the EAC extension. • Demonstrated relationship between strength of EAC and wind stress curl using sverdrup relationship and Godfrey’s Island Rule. • Long term changes at Maria correlate with wind stress curl changes in the South Pacific • Lagged by 2-3 years, Maximum R = ~0.7. • SODA does not recreate strengthened EAC (ERA-40 shows no trend in winds). • Trend at Maria represents changes in strength of EAC, and hence related to trend in SAM.

16. Conclusions (2) 2. Decadal Signal at Maria Island in T and S.. • Also evident in: • South Pacific wind stress curl field, • South Pacific gyre strength • Tasman sea transports (both island rule and SODA). • South Pacific decadal variability related to Decadal ENSO like variability (Zhang et al 1997, Garreaud et al 1999) • Relationship between SOI and Maria SST. • Decadal signal correlated (strong visual correspondance) • R = 0.4 at 20 months lag. • Decadal variability related to low frequency ENSO-like variability

17. Thank you. Katy Hill M.Sc B.Sc(Hons) katy.hill@csiro.au +61 3 6232 535

18. Xtra Slides…..

19. x x Influence of Seasonal current variability Maria Island. Summer: East Australia Current dominant influence Winter: Influence of the Zeehan Current SST Anomaly (regional mean removed). (Cresswell 2000)

20. Relationship between Maria and wider variability x

21. Simple Ocean Data Assimilation (SODA) • See Carton & Geise (in press), 2005. • 44 year reanalysis 1958-2001 • Forced by ECMWF ERA-40 winds • Data: • Hydrographic Profiles (WOD 2001) • Ship intake measurements • Moored hydrographic observations • Remotely sensed SST and sea level. • POP model 0.25*0.4 degrees, 40 levels • 10 day updating cycle.

22. Changes in Wind Stress Curl.. Wind Stress Curl: 1948:1967 Wind Stress Curl: 1981:2000 *1E-6 *1E-6 *1E-6 Zonal Average Greater Curl

23. Comparing variance in SODA with ERSST

24. Maria SST and the SOI.

25. Trend in the Southern Annular Mode Thompson and Solomon 2002 Visbeck