This work describes an evaluation of tropical cyclones (TCs) and depressions in order to determine if the El Niño Southern Oscillation (ENSO) may related to the recent rise of TC remnants affecting Missouri or if the variability is more sensitive to a long term Pacific Decadal Cycle. Sea surface temperatures (SST), mean sea level pressure (MSLP), the Pacific Decadal Oscillation (PDO), the Atlantic Multi-decadal Oscillation (AMO), the Quasi-Biennial Oscillation (QBO), and the (ENSO) were studied to determine possible correlations with the frequency of tropical remnants affecting Missouri. The study found a significant positive correlation between the frequencies of Missouri impacts per year to the frequency of Atlantic Ocean TCs. The more active the Atlantic Ocean basin is, the more times Missouri can expect to be impacted. TC paths were classified based on their direction of travel. TC remnants interacting with frontal boundaries took a more southwest to northeast track. Whereas TC remnants that entered a more zonal weather pattern traveled along a south to north path. Results found that the positive PDO (PDO one) 1938–1946 and 1977–1998 involved a total of 10 TCs affecting Missouri, an average of 0.32 events per year. The negative PDO (PDO two) 1947–1976 and 1999–present involved a combined result of 25 TCs affecting Missouri, an average of 0.57 events per year. A similar result is found for the AMO.

A 2005 case study shows how the rare combination of elevated SSTs in the Gulf of Mexico, anomalously low MSLP, and the negative phase QBO led to increased TC activity in the tropical Atlantic Ocean. Also, the frequency of TC affecting Missouri since 1938 was compared to the type of ENSO cycle. La Niña periods produced an average of 0.37, El Niño produced 0.31, and Neutral periods produced 0.58 TC per year. The frequency of Missouri impacts was separated by month during each respective ENSO cycle. Chi-squared tests show - with four degrees of freedom and a value of 0.99 - that the distributions of TC per month versus ENSO cycle are not significantly different. Thus, Missouri is impacted more often by TCs during August and September regardless of ENSO phase. The conclusions suggest that Missouri TC climatology is more sensitive to long term PDO cycle fluctuations, and the resulting frequency of TC in the Atlantic Ocean, than to short term ENSO variability.

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