The non-stationary correlations between West African precipitation and Atlantic hurricane activity

For years, various indices of seasonal West African precipitation have served as useful predictors of the overall tropical cyclone activity in the Atlantic Ocean; however, since the mid-1990s, the correlation has deteriorated. In the present study, statistical techniques are developed to describe the non-stationary nature of the correlations between annual measures of Atlantic tropical cyclone activity and three selected West African precipitation indices. The correlations between these parameters are found to vary over the period from 1921 to 2007 on a variety of time scales. Broadly, in years when the environment in the Main Development Region is generally favorable for enhanced tropical cyclogenesis (e.g., when sea surface temperatures are high, when there is relatively little wind shear through the depth of the troposphere, or when the relative vorticity in the mid-troposphere is anomalously high), the correlations between indices of West African monsoon precipitation and Atlantic tropical cyclone activity are considerably weaker than in years with the overall conditions in the region are less conducive. Other more-remote climate parameters, such as the phase of the Southern Oscillation, are less effective at modulating the nature of these interactions.