On the Potential Causes of the Nonstationary Correlations between West African Precipitation and Atlantic Hurricane Activity

Abstract

For years, various indices of seasonal West African precipitation have served as useful predictors of the overall tropical cyclone activity in the Atlantic Ocean. Since the mid-1990s, the correlation unexpectedly deteriorated. In the present study, statistical techniques are developed to describe the nonstationary nature of the correlations between annual measures of Atlantic tropical cyclone activity and three selected West African precipitation indices (namely, western Sahelian precipitation in June-September, central Sahelian precipitation in June-September, and Guinean coastal precipitation in the preceding year's August-November period). The correlations between these parameters are found to vary over the period from 1921 to 2007 on a range of time scales. Additionally, considerable year-to-year variability in the strength of these correlations is documented by selecting subsamples of years with respect to various meteorological factors. 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 midtroposphere is anomalously high), the correlations between indices of West African monsoon precipitation and Atlantic tropical cyclone activity are considerably weaker than in years when 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.

North-West African Hydrologic Changes in the Holocene: A Combined Isotopic Data and Model Approach

“…The Sahel drought in the late-1960s to mid-1990s can be traced in a+10 ‰ shift of the δD signal. Changes in δD closely correspond to a rainfall index for the western Sahel (Fink et al 2010). Comparison with precipitation-isotope data provides an estimate of a 30 % reduction in rainfall during the Sahel drought, in accordance with meteorological observations.…”

The “Year” of Tropical Convection (May 2008–April 2010): Climate Variability and Weather Highlights

“…The year-to-year variability of Sahel rainfall, and thus the West African monsoon, is of great interest due to the very strong north-south gradient in natural and anthropogenic conditions. Figure 13 shows that both 2008 and 2009 MaySeptember seasons were slightly wetter than the long-term average, consistent with a recent wet trend in this region (Fink et al 2010). Further scrutiny illustrates that the anomalously wet Sahel signal in both years was mainly exhibited in the western Sahel west of about 0°E.…”

Meteorological characteristics and potential causes of the 2007 flood in sub‐Saharan Africa

“…One further potential difference pertains to the strength, depth and northward incursion of (anomalous) westerlies in (Figure 14a). These years rank as the first, second, and fifth wettest years in the Central Sahel since 1961 (Fink et al, 2010). Since no significant zonal (and also meridional) wind anomalies with respect to the 1989-2008 mean were found in August 2007 at 925 and 850 hPa (not shown), this result suggests that the monsoon layer was anomalously deep in 2007.…”

“…A comparison of GPCP with Climate Prediction Center Merged Analysis of Precipitation (CMAP) on a monthly basis revealed a good performance of GPCP data in regions with lowgauge density such as West Africa (Yin et al, 2004). Finally, we used monthly rainfall data from the station network described by Fink et al (2010). This network covers the West and Central Sahel.…”

“…Although a slight recovery of precipitation amount has been observed during the last 10 years, rainfall totals are still under the level of the 1950s and 1960s (Nicholson, 2008;Lebel and Ali, 2009;Fink et al, 2010). However, in August and September 2007 media and aid organizations have brought a completely different meteorological phenomenon to the attention of the World's public: between Senegal and Kenya large parts of sub-Saharan Africa were hit by severe flooding caused by a sequence of anomously abundant rain events (e.g.…”