87th AMS Annual Meeting

Wednesday, 17 January 2007
Observations and predictability of tropical cyclones in the Southwest Pacific Ocean
Exhibit Hall C (Henry B. Gonzalez Convention Center)
Kevin H. Goebbert, CIMMS/Univ. of Oklahoma, Norman, OK; and H. A. Ramsay, L. M. Leslie, and M. Leplastrier
Tropical cyclones have been studied in many basins across the world; however the Australian regions have garnered less attention than the North Atlantic and Northwest Pacific Ocean basins. The Australian tropical cyclone (TC) database extends from 1906 to present. Holland (1981) summarized the confidence in data quality for cyclones in the Australian region between 1909 and 1979. Prior to 1970, satellite coverage of the Southwest Pacific Ocean Basin (SWPB) was minimal and many storms may have been missed or not properly diagnosed. He found that the data quality for the decade 19691979 was superior to any other decade and attributed that to the availability of reliable satellite measurements throughout the Australian TC region. Therefore, the data from 1970 to present is used in this study.

Even though this is the most reliable Australian dataset, there are still limitations that must be considered. One factor to be considered is that many local weather bureau offices had differing definitions of TC intensity, as well as local definitions changing with time. In recent years improvements in remote sensing techniques have improved since 1970 increase our confidence in TC intensity observations. Another factor to consider is the 35-year dataset may not be capturing a full multi-decadal cycle in TC activity and may affect the rather short dataset reliability. While keeping these and other limitations in mind, a general overview of the SWPB can be achieved.

In the SWPB there has been an average of 5 tropical cyclones a year between 1970 and 2004, but linear trends indicate that the number of tropical cyclones is decreasing. For example, running five-year TC averages have dropped from 8 in 197074 to 4 in 200004. Separating the TC dataset based on storm intensity indicates a decrease in TC frequency of less than one storm in the past 35 years for moderate and intense tropical cyclones. TC activity separated by pentads indicates similar results.

Correlations maps of TC activity with atmospheric and oceanic variables are computed for the TC season. Results imply that there are multiple parameters that explain the variability in TC activity. Many of the typical TC parameters have moderately strong correlations with TC activity including: sea surface temperatures and 850200 hPa zonal vertical wind shear. Other parameters including ENSO indices and an Australian blocking index show moderate correlations to TC activity. Correlation maps of different parameters are also computed for a season prior to the Australian TC season. Moderately strong correlations are present for some of the atmosphere/ocean parameters, especially sea surface temperatures. From the set of parameters important to TC activity, a suitable set of predictors for the SWPB is be obtained and discussed.

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