A Climatological Study of Tropical Cyclones in the Southwest Pacific Ocean Basin

Abstract

The purpose of this research is to better characterise the specific behaviour of Tropical Cyclones (TCs) in the Southwest Pacific ocean basin in response to a number of key global and regional teleconnection patterns. While there have been many TC studies in other basins, there have been far fewer research efforts in this basin (essentially the area between the east coast of Australia and French Polynesia) for a number of reasons including: (1) the sheer vastness of the basin; (2) a limited amount of TC data from a temporal standpoint; and (3) the lack of a collated, comprehensive, and homogeneous database of TC information. The underlying basis for this work lies in its importance towards developing a more comprehensive climatology of TCs in the region for the benefit of the scientific community, as well as in being an aid to the various Pacific Island national meteorological services in the region to better prepare their citizens for TCs. In the formal sense of synoptic climatology, the research methodology employed follows the environment to circulation model whereby whereby synoptic processes are chosen because they are believed to have a distinctive effect on data, measurements, and indices as proxies of environmental phenomena such as TCs. The research provides an improved climatology of TCs in the basin (from 135°E - 120°W and 0°-40°S) from 1970 to 2011. This study is conducted under an environment of an interdecadal Pacific Decadal Oscillation which has remained in a fairly stable positive phase for the period from 1978-99 during most of the 42-year study period. In addition to the development of a new dataset of TCs, the research investigates the spatial patterns and characteristics of TCs for the period with respect to the following teleconnection patterns: (1) interannual El Niño Southern Oscillation (ENSO); (2) intraannual Southern Annular Mode (SAM); and (3) intraseasonal Madden-Julian Oscillation (MJO). By demonstrating subtle differences between comparisons of combinations of atmospheric- or oceanic-dominated ENSO phases, or neutral conditions, the importance of ENSO activity on regional TC behavior has been better defined, including documenting the spatial extent and frequency of TCs and how they undergo extratropical transition (ETT). Regarding the SAM, there is a definite relationship between the positive phase of SAM and TC behaviour, particularly with the positive phase of the Sothern Oscillation Index, when the frequency of TCs undergoing ETT near New Zealand is greatest. Finally, with respect to the MJO, the greatest proportion of TCs (36.7%) occurs during paired MJO phases 6-7; while the lowest proportion of TCs (17.0%) occurs during paired MJO phases 2-3. Additionally, during paired MJO phases 6-7, the largest proportion of TCs undergoing ETT (38.9%) were found, and major TCs (37.8%); and conversely those proportions are only 13.5% and 12.8% respectively for paired MJO phases 2-3. Also, a statistically significant relationship between the MJO and SAM during two MJO states (weak paired phases 4-5 and 6-7) that are coincident with the positive phase of the SAM. The anomalously greater frequency percentages of TCs undergoing ETT are +9.4% and +11.1% respectively for weak MJO paired phases 4-5 and 6-7. In summary, the research involves a thorough classification of a wide-range of synoptic variables, during the timeframe from 1970-2011, as they are modulated by the effects of three major modes of atmospheric/oceanic circulation associated with ENSO, SAM, and MJO. The study then ties the resultant values and anomalies of these synoptic variables to various aspects of the climatology of TCs involving their genesis, frequency, intensity, morphology of tracks, and ETT. The overarching result of this study is a climatology of TCs that contributes towards an enhanced knowledge base of TC behaviour for the benefit of communities across the southwest Pacific region.