West African Monsoon 2010 Large-Scale Overview Ros Cornforth Department of Meteorology, University of Reading

Above average rainfall over much of W. Africa, Chad and Central African Republic, and parts of Nn Ethiopia. Highlights

Precipitation and Convection Monthly Atmospheric Fields over Africa Interannual Variability

Precipitation and Convection Above average over much of W. Africa, Chad and CAR and parts of Nn Ethiopia. Below average for local areas eg. groundnut basin of Senegal, Gambia, Sierra Leone, Sn Ghana. Interannual Variability Fig.1: Rainfall anomalies based on NOAA/CPC Rainfall Climatology

Interannual Variability Fig.2: Presao 13 seasonal rainfall forecast issued by ACMAD in May 2010 for JAS High probability for a deficit in precipitations (>0.30) along the coastal areas of the Gulf of Guinea. Low probability of rainfall deficit (Probability <0.20) over a large area which includes the Sahel, the north part of the Gulf of Guinea countries and the North East Cameroon

Interannual Variability Fig.3: OLR anomalies 2010

Interannual Variability Fig.3: OLR anomalies 2010: 15/6 – 15/9

Interannual Variability Fig.4: Sahelian Rainfall Index

“Apart from July where most places in Ghana experienced reduced rainfall, almost everywhere in Ghana was extremely wet during the period. The lake Volta which supplies water for hydroelectricity generation was so full and the water managers had no option but to open the spill way after 20 years that it was kept closed.” Charles Yorke, GMET, Ghana (pers comms) Interannual Variability

Over people in the Gonja area have been displaced by flood waters. The Interior Minister, Mr Martin Amidu, described the flood situation as a national disaster which calls for international assistance.

Interannual Variability Fig 5: Zonal wind anomalies at 850 hPa based on ref period October 2010 (Source : IRI/NOAA/NCEP) Westerly anomalies persisted through the season July 2010August 2010 Somali Jet stronger than normal in western Indian Ocean, weaker on the continent - drier conditions in East Africa

Interannual Variability Fig.6: AEJ at 700 hPa August 2010July 2010

Annual cycle and Monsoon onset Sudden early migration of the ITCZ due to mid-lat trough strengthening Malian low. Also responsible for the cyclonic wind flow over much of West Africa. Intensification of Mascarene high also pumped huge amounts of moisture into the source region of the convective systems tracking over West Africa (from the Congo basin up to Ethiopia).

Darfur rain crisis

Intraseasonal Variability ITCZ located polewards of mean position through the JJAS

Early May: Enhanced convection partly associated with MJO developed across Indian ocean and moved eastward. Suppressed convection developed over much of Africa in its wake. Since mid-July: Enhanced convection across maritime continent, suppressed over date line Aug-Sept: Arrival of a Kelvin wave across the Atlantic in early Sept Eastward propagation associated with higher frequency tropical variability eg. Kelvin wave. Arrival of a Kelvin wave across the Atlantic in early Sept enhanced conditions above the previous hurricane downward trend, extending the active period. Coincided with development of Hermine. Intraseasonal Variability

Early May: Enhanced convection partly associated with MJO developed across Indian ocean and moved eastward. Suppressed convection developed over much of Africa in its wake. Since mid-July: Enhanced convection across maritime continent, suppressed over date line Aug-Sept: Eastward propagation associated with higher frequency tropical variability eg. Kelvin wave. Intraseasonal Variability

Since mid-July: Enhanced convection across maritime continent, suppressed over date line Aug-Sept: Eastward propagation associated with higher frequency tropical variability eg. Kelvin wave Early May: Enhanced convection partly associated with MJO developed across Indian ocean and moved eastward. Suppressed convection developed over much of Africa in its wake. Since mid-July: More stationary anomalies with UL convergence across central Pacific and divergence from across Atlantic to Indian ocean. green=favourable for precip, Aug-Sept: Eastward propagation associated with higher frequency tropical variability eg. Kelvin wave UL div has become more organised in the last few days from Africa to eastern Asia - strenghtening MJO?

Aug-Sept: Eastward propagation associated with higher frequency tropical variability eg. Kelvin wave. Since mid-July: More stationary anomalies with UL convergence across central Pacific and divergence from across Atlantic to Indian ocean. Arrival of a Kelvin wave across the Atlantic in early Sept Arrival of a Kelvin wave across the Atlantic in early Sept enhanced conditions above the previous downward trend, extending the active period. Coincided with development of Hermine. Intraseasonal Variability

Synoptic Variability Atlantic Hurricanes come thick and fast after a quiet start AEWs 2010

Synoptic Variability AEWs 2006 Compare 2006 AEWs with 2010

Easterly anomalies persisted over W. Pac during last 5-10 days Westerly anomalies persisted over E. Pac but weakened in the Atlantic Atmospheric circulation Westerly (RED) anomalies which persisted through September have weakened Changing general circulation

Easterly anomalies persisted over W. Pac Westerly anomalies persisted over E. Pac but weakened in the Atlantic Westerly (RED) anomalies which persisted through September > enhanced convection, have now weakened > suppressed convection over West Africa now Atmospheric circulation

Westerly anomalies over maritime continent. Easterlies persist over Africa Atmospheric circulation

Atlantic Activity

2010 ATLANTIC HURRICANE SEASON SourceDateNamed StormsHurricanesMajor Hurricanes CSU10 th December, CSU7 th April, NCSU26 th April, N/A NOAA27 th May CSU2 nd June, UKMO17 th June, (Jul-Nov)N/A NOAA5 th August, TOTAL ACTIVITY (22 nd November)19125 AVERAGE ACTIVITY ( ) RECORD HIGH ACTIVITY28158 RECORD LOW ACTIVITY420 Ricardo Fonseca

WHY WAS THE SEASON SO ACTIVE? THREE MAIN FACTORS ACCORDING TO NOAA: La Nina  Reduced vertical wind shear in the tropical Atlantic and Caribbean Sea; Above average SSTs  SSTs in the tropical Atlantic and Caribbean Sea were in general 1-2C above average; Tropical multi-decadal signal  reduced vertical wind shear + weaker easterly trade winds + an extensive area of cyclonic shear at 700hPa along the equatorward flank of the African Easterly Jet also conductive for hurricane formation. 17 th June nd November 2010 very warm SSTs La Nina strengthened throughout the season Ricardo Fonseca/Ros

Atlantic Activity Enhanced WAM and suppressed Amazon = same as active period between La Nina contributes further to warm SSTs but still at record levels.

From August 22 - September 29, we had 11 named storm formations (Danielle - Nicole) but no cat 3 or above landfalling hurricanes.... This is the most storms that have formed in the Atlantic during this period. In a tie for second place over the August 22 - September 29 period with 9 named storm formations are: 1933, 1949, 1984 and Atlantic Activity

12 HURRICANES AND NO US LANDFALLS? IT NEVER HAPPENED BEFORE! ONLY ONE TROPICAL STORM LANDFALL (BONNIE) OUT OF 19 STORMS? Southerly flow in the “weakness” between the two ridges shoved approaching storms towards the north and back out to sea Northerly flow blocked storms that formed in the Caribbean from turning northwards in time to cross the U.S. Gulf Coast. Ricardo Fonseca Z500 anomaly

Highlights: Triple Hurricanes... Same latitude band.

Interesting range of scales. Highlights: Triple Hurricanes...

Hurricane Trivia Other years with triple hurricanes in 1 time-period: August 17 – August 24, 1893 September 12 - September 14, 1926 August 31 – August 31, 1950 September 2 – September 4, 1950 September 7 – September 12, 1961 September , 1967 September 8 – September 8, 1980 August 30 – August 31, 1995 September 23 – September 27, 1998