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Tropical Cyclones in the Southwest Pacific: Spatial Patterns and Relationships to Southern Oscillation and Sea Surface Temperature

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  • 1 National Institute of Water and Atmospheric Research, Wellington, New Zealand
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Abstract

An analysis of a 20-yr dataset of tropical cyclones in the southwest Pacific has been made to determine the spatial patterns of cyclone occurrence and how these depend on the Southern Oscillation (SO) and sea surface temperature. A local measure of cyclone incidence is defined as the number of cyclones that enter into a 2° lat-long square during some period (eg., within one season) and this quantity is mapped for four Southern Oscillation index (SOI) categories. The maps show that the geographical distribution of cyclone incidence shifts eastward and northward during negative SOI phases and vice versa. The mean annual incidence for the whole region is 28% higher than average for our strongly negative SO category and 16% below average for our near-zero category.

To explore the roles of the SO and local SST, correlation analysis was applied to mean values of cyclone incidence for four subregions of 20° longitude width between 10° and 22°S. Markedly different SOI/SST responses were found on either side of 170°E. In the Coral Sea subregion (150°–170°E), seasons with a relatively high tropical cyclone incidence were preceded by significant positive anomalies in SST and weak positive anomalies of the SOI. The behaviors of the three eastern subregions (lying between 170°E and 130°W) were similar to each other, but were very different to that of the Coral Sea subregion. Their seasons of higher cyclone incidence were preceded by significant negative anomalies of SOI and Tahiti pressure, which continued into the cyclone season, but with only weak or nil SST relationships. Analysis of variance was used to determine the relative contributions of the main variables for each subregion. Altogether, SOI/SST variables before the cyclone season can explain from 20% to 50% of the variance for the individual subregions. Concurrent variables can explain 70% of the variance for the three eastern subregions taken as a single unit.

On the basis of these statistical results, the authors proposed that the primary influence on tropical cyclone incidence west of 170°E is the local oceanic conditions, as represented by SST, while to the east of 170°E it is the eastward extent of favorable atmospheric conditions, as indicated by the SOI or Tahiti pressure. The results have application to seasonal cyclone forecasting and natural hazard assessment and mitigation activities within the region.

Abstract

An analysis of a 20-yr dataset of tropical cyclones in the southwest Pacific has been made to determine the spatial patterns of cyclone occurrence and how these depend on the Southern Oscillation (SO) and sea surface temperature. A local measure of cyclone incidence is defined as the number of cyclones that enter into a 2° lat-long square during some period (eg., within one season) and this quantity is mapped for four Southern Oscillation index (SOI) categories. The maps show that the geographical distribution of cyclone incidence shifts eastward and northward during negative SOI phases and vice versa. The mean annual incidence for the whole region is 28% higher than average for our strongly negative SO category and 16% below average for our near-zero category.

To explore the roles of the SO and local SST, correlation analysis was applied to mean values of cyclone incidence for four subregions of 20° longitude width between 10° and 22°S. Markedly different SOI/SST responses were found on either side of 170°E. In the Coral Sea subregion (150°–170°E), seasons with a relatively high tropical cyclone incidence were preceded by significant positive anomalies in SST and weak positive anomalies of the SOI. The behaviors of the three eastern subregions (lying between 170°E and 130°W) were similar to each other, but were very different to that of the Coral Sea subregion. Their seasons of higher cyclone incidence were preceded by significant negative anomalies of SOI and Tahiti pressure, which continued into the cyclone season, but with only weak or nil SST relationships. Analysis of variance was used to determine the relative contributions of the main variables for each subregion. Altogether, SOI/SST variables before the cyclone season can explain from 20% to 50% of the variance for the individual subregions. Concurrent variables can explain 70% of the variance for the three eastern subregions taken as a single unit.

On the basis of these statistical results, the authors proposed that the primary influence on tropical cyclone incidence west of 170°E is the local oceanic conditions, as represented by SST, while to the east of 170°E it is the eastward extent of favorable atmospheric conditions, as indicated by the SOI or Tahiti pressure. The results have application to seasonal cyclone forecasting and natural hazard assessment and mitigation activities within the region.

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