Bulletin November 2011 En

8/2/2019 Bulletin November 2011 En

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5, Avenue des Ministres BP : 13 18 4 Niamey- Niger

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SUMMARY

1. Months Synoptic Situation2. Months climatological

Situation / Impacts3. Outlook

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HIGHLIGHTS:The St. Helena high strengthened significantly. The Sahel, the northern Africa countries, thecentral Africa and the Gulf of Guinea countries had decrease in rainfall activities while GHA and southernAfrica countries had some increase. Increase in temperatures was observed in the northern and westernAfrica while decrease was observed in north-eastern and southern Africa.

1. SYNOPTIC SITUATION DURING THE MONTH OF NOVEMBER 2011

This section provides the strengths of the surface pressure systems; the 850hPa general circulation anomalies;upper troposphere thermal regimes; relative humidity; sea surface temperature (SST) and El Nino/Southern

Oscillation (ENSO).

1.1 Centres of Surface Pressure Systems

The Figure 1 shows surface pressure systems compared to the past month as described below:

The Azores high of 1021hPa strengthened slightlyby 1hPa and shifted west. Its centre was located overnorth Atlantic ocean at about 37N/48W extending aridge over northern Africa.

The St. Helena high of 1028hPa strengthenedsignificantly by 6hPa and shifted south-east. Itscentre was located at about 35S/12W over southAtlantic Ocean.

The Saharan thermal low of 1010hPa filled up by1hPa compared to the past dekad. Its centre waslocated at about 10N/20E over southern Chad.

The Mascarene high of 1019hPa weakened slightlyby 1hPa and shifted north-west. Its centre waslocated at about 30S/82E with an extended ridgeover Madagascar.

Figure 1 : Mean Surface Pressure during November 2011

(Source: NOAA/NCEP)

1.2 The 850hPa wind anomaly

The Figure 2 shows wind anomalies at 850hPa derived from reference period 1971-2000.

Strong westerly wind anomalies from equatorialAtlantic Ocean were observed over south-western part of the Gulf of Guinea.

In north-eastern Algeria and northern Tunisiastrong easterly/south-easterly wind anomaliesfrom Mediterranean sea prevailed.

Over the continent strong westerly/south-westerly wind anomalies were observed overnorthern Mali and southern Algeria while overSouth Sudan strong northerlies anomalieswere observed.

In the extreme north-east Sudan, Eritrea,Djibouti and north Ethiopia strong northerlywind anomalies from Red sea prevailed whileover northern Somalia strong south-westerlywind from Indian Ocean prevailed.

The average wind anomaly speed (shaded)

was observed at about 03m/s and above. Figure 2 : November 2011, Wind Anomalies at 850hPa(Source : IRI/NOAA/NCEP)

1.3 Thermal index


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In the month of November 2011, the ThermalIndex (TI) regime at 300hPa, Figure 3, had anisotherm value of 242K covering extremeeastern part of the Gulf of Guinea countries,most of central Africa, most of the GHA andextreme northern part of southern Africa. Thehigh TI (242K) triggered heavy rainfall withfloods over the areas characterized by highrelative humidity (>60%) as shown in Figure 6

while the low TI regime 241K values wereassociated with suppressed convection overthe rest of Africa.

Figure 3: Thermal Regime at 300hpa

(Source: NOAA/NCEP)

1.4 Relative Humidity at 850hPa

The 850hPa (Figure 4) shows high RH (>60%)in November 2011, over Gulf of Guinea

countries, Central Africa, most part of GHA,extreme northern part of southern Africa andmost of Madagascar. However, the northernpart of the continent above 12N and south-western part of the continent experienced dryconditions characterized by the lowest RH (40%).

Figure 4:RH at 850 hPa (Source : NOAA/NCEP)

1.5 Sea Surface Temperature (SST) and El Nino/Southern Oscillation (ENSO)

Warming conditions persisted in most of western part of the Pacific Ocean while in the eastern part coolingconditions continued. Neutral to warming conditions continued in most of the Atlantic Ocean except in thesouthern and north-central part where cooling conditions continued. Neutral to warming conditionspersisted in most of Indian Ocean and Mozambique Channel.

Figure 5: Sea Surface Temperature (Source IRI).

2. CLIMATOLOGICAL SITUATION AND IMPACTS DURING NOVEMBER


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The section provides the general climatological situation covering two major parameters, the rainfall andtemperature.

2.1 RainfallThe estimated rainfall for November, 2011 in Figure 6, shows increase in rainfall activities over GHA andSouthern Africa countries. However, decrease in rainfall was observed over the Sahel, northern Africa,the Gulf of Guinea and central Africa countries. In detail:

North Africa: had decrease in rainfall distribution with amounts ranging from 10mm to 80mm overnorthern Algeria, Tunisia with the highest amounts of about 200mm over extreme northern Morocco.

The Sahel: The region was under the influence of the Harmattan characterised by cool, dry and dustywind from the Sahara.

Gulf of Guinea countries: had significant decrease in rainfall distribution and amounts. Only thecoastal zones had rainfall amounts ranging from 10mm to 150mm intensifying from 150mm to 300mmover Southern Cte dIvoire, Liberia and Guinea Bissau.

Central Africa: had decrease in rainfall distribution mostly over the northern part. The southern parthad amounts ranging from 10mm to 400mm increasing to the maximum amount of about 500mm overCongo and Gabon.

GHA: countries had increase rainfall distribution and amounts ranging from 10mm to 400mm overmost countries with localized high amounts between 400mm to 500mm over southern Ethiopia.

Southern Africa had increase in rainfall distribution and amounts recording 10mm to 250mm withmaximum of about 300mm over Angola.

In November, 2011 the rainfall anomalies compared to the reference period 1971-2000, Figure 7showed rainfall deficit in western part of the Gulf of Guinea countries, northern part of central Africacountries and south-eastern part of southern Africa countries. Excessive rainfall was observed inextreme northern part of northern Africa, southern part of central Africa most of GHA and northern partof southern Africa countries.

2.2 Surface Temperature Anomalies

Figure 6: Monthly cumulative rainfall(Data Source: NOAA/NCEP)

Figure 7: Monthly Precipitations Anomalies(Data Source: NOAA/NCEP)


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In November, 2011, the temperatureanomalies (Figure 8) compared to 1971-2000base period, were generally normal over thecontinent. Warmer temperatures by 1.5Cwere observed over extreme northern part ofnorthern Africa, western part of west Africaand north-eastern part of southern Africa withthe maximum temperature anomalies above2.5C over northern Algeria. However, lowtemperature anomalies of 1C were observed

over north-eastern part of northern Africa andcentral part of Southern Africa coveringsouthern Namibia, southern Botswana andnorthern western South Africa.

Figure 8 : Monthly Temperatures Anomalies

(Data Source: NOAA/NCEP)3. OUTLOOK

The subsections provide the expected SSTs and ENSO characteristics and evolution of events based onFigures 9 and 10 respectively and expected rainfall outlook.

3.1 Forecast Sea Surface Temperature (SST)

The figure 9 shows the forecast Sea Surface Temperature Anomalies from November for the period ofNovember-December-January 2011.

3.2 El Ni Nio/La Nia

The set of dynamical and statistical model forecasts ofENSO over Nino 3.4 domain (5N 5S, 120W 170W) are shown in Figure 10.

About two-thirds of the set of dynamical and statisticalmodel predictions issued during late October and early

November 2011 predict La Nina conditions from theNovember 2011 to January 2012 season currently inprogress while with most of the other one-third predictingneutral ENSO conditions.Taking into account the latest observations of SST andothers parameters, the probabilities for La Nina,conditions is estimated at 74%. At the time of preparingthis, the SST observations in the NINO3.4 region were inthe weak La Nina range, with an area-averaged weeklyanomaly of -0.9C in the most recent week. Currentpredictions and observations indicate probabilities of69% and 62% for La Nina conditions during theDecember-February and January-March periods,

respectively.

Figure 10 : Multi-model ENSO Forecast (source IRI)

3.3. Rainfall

Pacific Ocean: warming conditions will persistover most of western part while over most ofeastern and equatorial parts the cooling willcontinue.

Atlantic Ocean: Neutral to warming conditionwill persist over most of the Ocean except oversouth-central part where some cooling isexpected.

Indian Ocean and Mozambique Channel:Neutral towarming conditions are expected topersist in most of the Ocean and MozambiqueChannel. Figure 9 : Forecast Sea Surface Temperatures Anomalies

(source IRI)


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The prevailing high relative humidity coupled with high conditional instability manifested by TI regimes at300hPa will continue to maintain heavy rainfall with localised highest probability of flooding over southernparts of central Africa countries, parts of GHA countries and north eastern part of Southern Africa countries.The ITD will continue its southwards migration, leading to the reduction of rainfall activities over the Gulf ofGuinea countries and the intensification of Harmattan in the Sahel, the Sahara and northern part of the Gulfof Guinea countries. In detail:

North Africa countries: will have no significant change in rainfall distribution and amounts. However, somelocalised light amounts ranging from 10mm to 80mm will be observed in the extreme northern part.

The Sahel: will continue to be dry. The region will be under the influence of the Harmattan, characterized bycold, dry and dusty conditions.

Gulf of Guinea countries: The northern part will be under the influence of the Harmattan while the extremesouthern part will experience rainfall amounts ranging from 10mm to 100 mm.

Central Africa countries: will continue to experience rainfall amounts ranging from 10mm to 400mm oversouthern part, intensifying to maxima of about 600mm over some parts.

GHA countries: will have amounts ranging from 10mm to 300mm over some parts, intensifying over GreatLake countries with localized peaks ranging from about 300mm to 500mm.

Southern Africa countries: most of the region will observe some rainfall amounts ranging from 10mm to

150mm over most part with maximum of about 400mm over the north-eastern part.

3.2 IRI seasonal Rainfall outlook for Africa issued in November 2011 for DJF

The IRI seasonal rainfall forecast issued inNovember for the period of December-January-February 2011 shows:

High probability of above normal tonormal rainfall over most of southernAfrica, south-western part of CentralAfrica and central part of the Gulf ofGuinea countries.

Elsewhere, the climatology will prevail.

Figure 11: IRI forecast

3.3 Seasonal Rainfall Outlook of central Africa for OND 2011 by ACMAD issued inSeptember

Zone I covering Equatorial Guinea, Sao Tome etPrincipe, the coastal area of Cameroun, most part ofGabon and coastal part of Congo, will have abovenormal rainfall.

Zone II including south-east Cameroun, south-west ofCentral African Republic, east Gabon, central andnorth of Congo and part of western DemocraticRepublic of Congo will be characterized by normal tobelow normal rainfall.

Zone III, covering the extreme south of Chad, centralpart of Cameroon, most part of Central Africa Republicand democratic Republic of Congo will becharacterized by normal rainfall.

Figure 12: Precipitation Outlooks Valid for October-November-December 2011 over Central Africa.

3.4 Seasonal Rainfall Outlook for west Greater Horn of Africa for SOND 2011 by ICPACissued in September


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Zone I: Increased likelihood of near to belownormal rainfall over western Tanzania; southernBurundi and southwestern Rwanda.

Zone II: Increased likelihood of near normal toabove normal rainfall over much of Tanzania;northern Burundi; much of Rwanda; westernKenya as well as southern and central Uganda.

Zone III: Increased likelihood of above to nearnormal over northeastern and northern coast ofTanzania; coastal, central and northeastern

Kenya; extreme southern Ethiopia as well assouthern and central Somalia.

Zone IV: Increased likelihood of to near normal tobelow normal rainfall over northern Somalia;eastern and southern Ethiopia; northwesternKenya and northern Uganda.

Zone V: Increased likelihood of near normal toabove normal rainfall over much of South Sudan;western, central and northern Ethiopia andDjibouti.

Zone VI: Increased likelihood of generally dryconditions over Sudan; northern Ethiopia andmuch of Eritrea. Figure 13 : GHA Climate Outlook for Septemberto December 2011

3.5 Southern Africa Rainfall Outlook (SARCOF-15) for OND 2011 issued in September

Zone 1 (The extreme north of the DRC): Increasedchances of normal to below-normal rainfall

Zone 2 (North-eastern half of Tanzania): Increasedchances of above-normal rainfall

Zone 3 (North-western half of Angola, the bulk of DRC,south-western half of Tanzania, extreme north-easternparts of Zambia, northern half of Malawi and north

eastern Mozambique): Increased chances of normal toabove-normal rainfall

Zone 4 (Central South Africa, south-western half ofLesotho, western half of Botswana, most ofNamibia, south-eastern half of Angola, extreme southof DRC, central parts of Zambia, southernhalf of Malawi, north-eastern half Zimbabwe andcentral Mozambique): Increased chances of normal tobelow-normal rainfall

Zone 5 (North-eastern half of Lesotho, north-easternparts of South Africa, Swaziland, southern

parts of Mozambique, south-western half of Zimbabwe,eastern half Botswana and southwesternZambia): Increased chances of normal to below-normal rainfall

Zone 6 (The west coastal areas of South Africa,Namibia and the extreme south-western Angola):Increased chances of normal to below-normal rainfall

Zone 7 (Western Madagascar): Increasedchances of normal to below-normal rainfall

Zone 8 (Eastern Madagascar): Increasedchances of normal to above-normal rainfall

Zone 9 (Southern Madagascar): Increasedchances of above- normal to normal rainfall

Zone 10 (Mauritius): Increased chancesof normal to above-normal rainfall

Figure14: Rainfall forecast for October-December 2011

ADVICES: The high rainfall variability in the region may cause risks with adverse effects throughout the season, particularly on goods

and persons (flooding) on plants (locust invasion) and Public Health (malaria epidemics and other waterborne diseasessuch as cholera).

Users of this product are encouraged to contact NMHSs for more detailed advices at country level.

Bulletin November 2011 En

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