Intraseasonal variability of the west African monsoon and Atlantic ITCZ

Intraseasonal variability of boreal summer rainfall and winds in tropical west Africa and the east Atlantic is examined using daily TRMM precipitation and NCEP/NCAR reanalysis during 1998-2006. Intraseasonal precipitation variability is dominated by two significant spectral peaks at timescales near 15 and 50 days, accompanied by corresponding peaks in eddy kinetic energy (EKE) and eddy enstrophy. Regional precipitation variability on 30-90 day timescales is significantly correlated (+0.6) with a global Madden-Julian oscillation time series based on equatorial zonal winds, supporting the results of Matthews. The overall amplitude of 30-90 day west African monsoon precipitation variability during a given summer does not appear to be strongly regulated by interannual variability in MJO amplitude, however.

Composite analysis and complex empirical orthogonal function analysis shows that 30-90 day precipitation anomalies are generally zonally-elongated, grow and decay in place, and have maximum amplitude near the Gulf of Guinea and in the Atlantic ITCZ. Composite 30-90 day enhanced precipitation events are accompanied by a significant suppression of eastern north Atlantic trades winds. Suppressed 30-90 day precipitation events are associated with an enhancement of the Atlantic trade winds. Enhanced (suppressed) EKE occurs just to the north of the east Atlantic ITCZ during positive (negative) 30-90 day precipitation events, with the maximum EKE magnitude lagging precipitation events by about 5 days.

East Atlantic tropical cyclone activity is significantly modulated on intraseasonal timescales. The number of tropical cyclones that occur in the Atlantic main development region to the east of 60°W is suppressed about 5-10 days before maxima in a regional intraseasonal precipitation time series, and enhanced about 5-10 days after time series maxima. An analysis of east Atlantic tropical cyclone activity based on an equatorial MJO index produces similar results. Consistent with the results of Mo, variations in vertical shear may help explain this modulation of tropical cyclone activity.