The African Monsoon Recent Evolution and Current Status Update prepared by Climate Prediction Center / NCEP 11 August 2008 For more information, visit:

Outline Highlights Recent Evolution and Current Conditions NCEP GEFS Forecasts Experimental Week-1 and Week-2 Outlooks Summary

Highlights: Last 7 Days Moderate to heavy rains sustained moisture surpluses over much of the western part of the Sahel, the Gulf of Guinea region, and western Ethiopia. Light rains, sustained moisture deficits in the central and eastern parts of the Sahel Below average rainfall persisted over Sudan and Eritrea.

Rainfall Patterns: Last 180 Days Over the past 180 days, rainfall was above average over the western part of the Sahel and in most areas in the Gulf of Guinea region. Rainfall was also above average over central Africa, and western Ethiopia. Rainfall was below average over eastern southern Africa and portions of the Greater Horn of Africa.

Rainfall Patterns: Last 90 Days Over the past 90 days, rainfall was above average over most areas in western and central Sahel, as well as the Gulf of Guinea region. Rainfall was above average over parts of central Africa, and western Ethiopia.

Rainfall Patterns: Last 30 Days Over the last 30 days, rainfall was above average over portions of the western and central Sahel, most areas in the Gulf of Guinea region, parts of central Africa, and western Ethiopia. Rainfall was below average over parts of central and eastern Sahel, and most areas in central and eastern Sudan.

Rainfall Patterns: Last 7 Days During the past 7 days, rainfall was above average over western Sahel and most areas in the Gulf of Guinea region, and western Ethiopia. Rainfall was below average from central Sahel eastward to Sudan and Eretria.

Recent Rainfall Evolution Over the past 30 days, rainfall has been above average over the western part of the Sahel (bottom panel – left) and over many areas in the Gulf of Guinea region (bottom panel – right). However, a few dry spells resulted in near average conditions in the central part of the Sahel (top panel – right).

SST: Last 7 Days 3-9 August 2008 The mean SST map (left panel) features a westward extension of the Atlantic equatorial cold tongue. However, SSTs remain warmer than normal over the Gulf of Guinea region and over most area sin the tropical north Atlantic. SST anomalies over the IO off the coast of Africa are weak. MeanAnomalies

Atmospheric Circulation: Last 7 Days Over the past 7 days, the 850 hPa wind anomaly (left panel) featured westerly wind anomalies over the Atlantic across the equator, while the tropical easterly jet (TEJ) extended well into West Africa and the eastern Atlantic as evidenced by the equatorial easterly wind anomalies over Africa (right panel).

NCEP GEFS Model Forecasts Non-Bias Corrected Probability of precipitation exceedance Week-1: Valid 12 – 18 August, 2008 Week-2: Valid August, 2008 NCEP global ensemble forecast system (GEFS) suggests a 90% chance or more for precipitation to exceed 50 mm over most areas in the Gulf of Guinea during week1. There is a better chance for precipitation to extend north into the Sahel during week-2. Note: The GFS tends to suppress (increase) precipitation in the Sahel (Gulf of Guinea region).

Experimental Week-1 Precipitation Outlook Valid August, An increased chance for above average rainfall over the Sahel and the Gulf of Guinea region. Favorable low level westerly wind anomalies over the equatorial western Atlantic and a warm Gulf of Guinea associated with an enhance west African monsoon from the Atlantic Ocean is expected to increase precipitation over the area. Confidence: Moderate

Summary Over the past 7 days, moderate to heavy rains sustained moisture surpluses across many areas in western Sahel, including Senegal, southern Mauritania, and western Mali. Beneficial rains also continued to fall over most areas in the Gulf of Guinea region Guinea eastward to Cote d’Ivoire, Ghana, Togo, Benin, and southern Nigeria. However, light rains resulted in below average rainfall over the central and eastern parts of the Sahel, including local areas in central Mali and Burkina Faso, most of Niger, northern Nigeria, and Chad. Rainfall was also below average over central Sudan and western Eritrea. In contrast, moderate to heavy rains continued to fall over the western Ethiopian highlands. Over the past 30 days, persistent rains resulted in moisture surpluses over the western part of the Sahel from Senegal to Burkina Faso. The rains also fell steadily over western Ethiopia resulting in above average conditions in this region. The Gulf of Guinea region also continued to receive beneficial rains. In contrast rainfall amounts over the central and eastern parts of the Sahel have been short over the past several weeks resulting in moisture deficits in the areas from Niger to Sudan and western Eritrea. For the week1 ending 18 August, 2008, there is an increased chance for above average rainfall over much the Sahel and the Gulf of Guinea region.