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Decadal Climate Events in the Seas North of Australia

 

Gary Meyers

CSIRO Marine Research

 

Starting in 1983, the CSIRO XBT Network established frequently repeated, cross-equatorial XBT lines in the seas north of Australia. The sampling rate on these lines was nominally at least 18 times per year with an XBT-drop every 100km. With large scale, low frequency variability in mind, the rate provided what we thought was the maximum, practicable reduction in eddy-noise that could be obtained with volunteer observing ships. The seas north of Australia were monitored because SST anomalies there have an important impact on rainfall over Australia. Thermal analysis on the frequently repeated lines gives an accurate description of seasonal to inter-annual variations. This talk presents analyses on lines IX1 (Fremantle, Australia to Sunda Strait, Indonesia) and IX12 (Fremantle to Red Sea). Inter-annual and decadal variations of surface and subsurface temperature and dynamic height are described, and compared to basin-wide analyses of sea level from the Topex-Poseidon altimeter. Significant interannual variations are found within the key areas of the so called “Nicholls'“ Indonesian/Indian Ocean SST-anomaly dipole, which has been related to rainfall in western and southern Australia. The extent to which SST anomalies are controlled by ocean dynamical structure has not yet been determined, but is an important issue to understand predictability of the dipole. Decadal variation was estimated as the linear trend during 15 years (1983 to 1998). Significant variations (exceeding 95% confidence intervals) are found over large portions of the water column on both tracks. Time permitting the trends will be compared to trends in the wind field, and to results from a wind driven ocean model developed by A. Schiller and colleagues.

 

 

 

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