The Impact of Subtropical Indian Ocean Anticyclone over Tasmania Precipitation

Aims: To explore the association between observed frequency of anticyclones over subtropical Indian Ocean (10°S-50°S, 102.5°E-142.5°E) and average autumn rainfall over the districts of Tasmania.

Methodology: We have divided the rainfall amounts over nine districts of Tasmania into three categories (Above average, average and below average rainfall) by using z-scores technique for each autumn (March-May) month for the period of 1951-2016. We have also categorized the calculated frequency of anticyclone over the subtropical Indian Ocean in a similar fashion.

Results: Our analysis suggests inverse association between each category of average rainfall and the corresponding frequency of anticyclones for each month of autumn. This association revealed that during below average rainfall, the high frequencies of anticyclones were occupied over the most of the area of subtropical Indian Ocean that resulted less rainfall in the districts while during above average autumn rainfall, the high frequencies of anticyclones observed in the west of 110°E this enabled more autumn rainfall over the districts. We have also found the negative correlation coefficients between mean sea level pressure over the same domain of Indian Ocean and average rainfall over each district in each month of the autumn (March-May).

Conclusion: The slightly shift of Mean Central Pressure (MCP) to the north from the west coast of Tasmania from March-May enabled the extra-tropical activity of rainfall systems. This causes enhanced rainfall during average and above average rainfall conditions than March-April over nine districts of Tasmania.