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2.3
Temporal Clustering of Tropical Cyclone Occurrence

To examine the hypothesis that medium- and extended-range prediction of tropical cyclone activity is increased during periods when the large-scale forcing of tropical cyclone activity is favorable over such temporal scales, it is first necessary to objectively identify periods in which tropical cyclone activity is favored or reduced. While the MJO may impact the daily probability of tropical cyclone formation (Hall et al. 2001), there has not been a strict statistical examination of temporal variability as it relates to clustering of tropical cyclone activity in time due to the quasi-periodic nature of the MJO. In this context, a cluster is defined as a temporally bound group of tropical cyclone occurrences of sufficient size and concentration to be unlikely to have occurred by chance.

To examine temporal clustering, a Bernoulli random variable is used to identify all days when a tropical cyclone formed in any given basin. For each tropical cyclone season, a uniform multinomial probability distribution is estimated in which the distribution parameter is defined as the relative frequency of formation for that year. A cluster index (Tango 1984, 1990) is based on a measure of the temporal interval between each formation. The significance of the index is based on the degree of departure from the underlying uniform multinomial distribution. The procedure is applied to examine the clustering of tropical cyclone occurrences during 1979-2012 over several ocean basins during the tropical cyclone season in each basin. This clustering algorithm was chosen as it is capable of examining the cyclic occurrence of a phenomenon and is associated in an asymptotic sense to the chi-square distribution such that a level of significance may be identified.

To compare with previous studies of the impact on the probability of tropical cyclone formation due to the MJO, the probabilities due to the MJO are examined for all years, years with temporal clusters during MJO years, years with temporal clusters in non-MJO years, years without temporal clusters during MJO years, and years without temporal clusters in non-MJO years. Results for the western North Pacific indicate that the inactive convection portion of the MJO is more effective in producing temporal clusters via forcing of periods of anomalous low tropical cyclone activity. This differs from other basins in which there is a statistically significant increase in the probability during the enhanced convection portion of the MJO