Intraseasonal variability of the impacts of the Madden-Julian Oscillation in the Gulf of Guinea

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Monday, 3 February 2014
Hall C3 (The Georgia World Congress Center )
Fisseha Berhane, Johns Hopkins University, Baltimore, MD; and B. F. Zaitchik

Handout (1.9 MB)

In this study, we investigate the influence of the Madden-Julian Oscillation (MJO) on precipitation in the Gulf of Guinea for each month of the boreal summer using independent datasets. Within the June-September rainy season, the MJO influence in the region is found to be strongest in August. In August, precipitation significantly increases (decreases) when the MJO convection at 200 E (1600 E) is enhanced. Consistent with this pattern, relative humidity in the middle and upper troposphere significantly increase (decrease), outgoing long wave radiation (OLR) decreases (increases) and vertical velocity increases (decreases). The characteristics of the African easterly jet (AEJ) have long been implicated as important factors in controlling moisture available for convection and strength of uplift in western Africa. Associated with the increase (decrease) of MJO convection at 200 E (1600 E), it is found that the southern boundary of the AEJ moves northward. Surface temperature composites show significant anomalous warming (cooling) in the Gulf of Guinea and cooling (warming) in the Sahara, which weakens (strengthens) the thermally driven AEJ, when the MJO convection is enhanced at 200 E (1600 E). Composites of AEJ reveal significant increase (decrease) of the speed of the southern boundary of the AEJ when MJO convection is enhanced at 1600 E (200 E). Composites of vertical velocity based on the speed of the AEJ over its southern boundary show that deep upward vertical motion is enhanced in the Gulf of Guinea when the AEJ weakens and moves northwards. This presentation will detail these relationships, explore the causality of the observed associations between AEJ anomalies and precipitation variability, and evaluate mechanisms of influence of the MJO relative to other potential large-scale drivers of regional atmospheric circulations in western Africa.