ECPC's 2001 Forecast of Australia Annual Rainfall Anomalies

contributed by Warren B. White

Experimental Climate Prediction Center Scripps Institution of Oceanography UCSD, 0224 La Jolla, CA 92093

1. ECPC's Southern Hemisphere Long-Lead Time Forecast System

White (2000) found year-to-year changes in Australia annual precipitation (APP) covarying with those in sea surface temperature (SST) and troposphere moisture flux (MF) over the three oceans surrounding Australia for 40 years from 1958 to 1997. Australia's wet (dry) years are associated with warm (cool) SST anomalies surrounding Australia and convergent (divergent) MF anomalies directly overhead. Differences in APP (SST) between wet and dry years can reach 0.75 m (1.2C) in northeast Australia (subtropical Indian Ocean). Wet (dry) years often occur during La Nina (El Nino) but White (2000) found significant differences in covarying SST, MF, and APP anomalies from one El Nino to the next, indicating that regional climate changes also influence APP. White (2000) found covarying SST and MF anomalies on basin space scales and interannual time scales taking 2 to 3 years to propagate eastward from Africa to Australia. This occurs in association with the Antarctic circumpolar wave (ACW) in the Southern Ocean (White and Peterson, 1996; White et al., 1998), the north branch of the ACW in the Indian Ocean (Peterson and White, 1998), and the global El Nino-Southern Oscillation (ENSO) wave in the tropical ocean (White and Cayan, 2000). White (2000) constructed a statistical climate prediction system based upon the slow eastward propagation of SST anomalies and their nearly one to one relationship with APP anomalies, similar to that constructed by White and Cherry (1999) for New Zealand.

2. Hindcast Skill Evaluation

This statistical climate prediction system of White (2000) yielded significant hindcast skill for predicting interannual APP anomalies at lead times of 1 and 2 years for the 50 years from 1950 to 1999. Best hindcast skill for the extratropical portion of Australia derives from the ACW south of Australia and the north branch of the ACW west of Australia. Eastward propagation of SST anomalies in these two oceanic domains is capable of predicting more than 50% of the total interannual variance in annual rainfall over Victoria and New South Wales and over Western Australia poleward of 20S over the 40 year record. This is much better than expected from chance or persistence, demonstrating the importance of the ACW upon year to year changes in APP at these latitudes.

3. Experimental Statistical Forecast of Annual Rainfall of Australia for 2001

Fig. 1 shows the hindcast of annual rainfall centered on December-January-February (DJF) and June-July-August (JJA) seasons from DJF 2000 to JJA 2001, based upon the evolution of interannual SST anomalies over the Indian and western Pacific Oceans for the two years prior. For the JJA 2000 which has just past, this forecast predicted greater than normal annual rainfall over eastern Australia and less than normal annual rainfall over western Australia, both of which are consistent with that observed. For JJA 2001 we predict the opposite situation, with greater than normal annual rainfall over western Australia and less than normal annual rainfall over eastern Australia.

4. Future Developments

This prediction system has also been applied to the winter monsoon rainfall over Southeast Asia (Tangang and White, 2001), which has been found to depend strongly upon the north branch of the ACW in the Indian Ocean. We are presently in the process of developing a full spectrum statistical climate prediction system for seasonal mean rainfall anomalies over Australia, similar to that produced for the all India summer monsoon rainfall from 1900 to 1999 (White, Annamalai, and Tourre, 20001). Other experimental forecast products can be found at http://acw.ucsd.edu/.

5. References

Peterson, R.G. and W.B. White, 1998. Slow oceanic teleconnections linking the Antarctic Circumpolar Wave with tropical ENSO. J. Geophys. Res. ,103, 24,573-24,583.

White, W.B., H. Annamalai, Y. Tourre, 2001. Influence of Global SST/SLP waves on India summer monsoon rainfall from 1900-1995. J. Clim., (in review).

White, W.B., S-C. Chen, R.J. Allan, R.C. Stone. 2001. Positive feedbacks between the Antarctic circumpolar wave and the El Nino-Southern Oscillation. J. Geophys. Res. (in review).

White, W.B., and F. Tangang, 2001. Influence of ENSO and the ACW on Southeast Asia Winter Monsoon Precipitation. J. Geophys. Res. (in review).

White, W.B., 2000. Influence of the Antarctic circumpolar wave on Australian precipitation from 1958 to 1997, J. Clim., 13, 2125-2141.

White, W.B. and D.R. Cayan, 2000. A global ENSO wave in surface temperature and pressure and its interdecadal modulation from 1900 to 1997. J. Geophys. Res., 105, 11,223-11,242.

White, W.B. and Neil J. Cherry, 1999. Influence of the Antarctic circumpolar wave upon New Zealand temperature and precipitation during Autumn-Winter. J. Clim., 12, 960-976

White, W.B., S.-C. Chen, and R. Peterson, 1998. The Antarctic circumpolar wave: A beta-effect in ocean-atmosphere coupling over the Southern Ocean. J. Phys. Oceanogr., 28, 2345-2361.

White, W.B. and R. Peterson, 1996. An Antarctic circumpolar wave in surface pressure, wind, temperature, and sea ice extent. Nature, 380, 699-702.