EAST AFRICAN METEOROLOGICAL DEPARTMENT

HERMANN FLOHN

ON THE CAÜSES OF THE ARIDITY OF NORTH EASTERN AFRICA

Text From

Würzburger Geographische Arbeiten, Vol. 12 (1964)

Translated from the German by

H.T. MÖRTH

(E.A. Me teorologi cai Department)

Price Shs. 5/-

Issued by

E.A. Me t e o r o l o g i c a l Department,E.A.C.S.O.,

Nairobi.

1966.

ON THE CAÜSES OF THE ARIDITY OF NORTH E ASTERN AFRICA

by Hermann Flohn (Bonn)

,- f o O(Ge rm an o r i g i n a l text in W u r z b u r g e r G e o g r a p h i s c h e Arbe i ten , 12 (1964) -^ \.\- •t r a n s l a t e d in to Eng l i sh by H.T. Mo'rth, Eas t A f r i c a n M e t e o r o l o g i c a l

Depar tmen t ) .

INTRODUCTION

In his book "The Ea r th 1 s P rob lem C l i m a t e s " , TREWARTHA ( 2 6 )d i s cus se s the ques t i on of c l i m a t i c causes - or, to be more exact , theques t ion of the spec ia l d i s t r i b u t i o n pa t t e rns and the seasonal changes inc l i m a t i c e l emen t s . In do ing so, he c o n t r i b u t e s s u b s t a n t i a l l y to thedeve lopment of " c l ima to logy" , a concept whi ch for reasons of p r a c t i c a la p p l i c a t i o n i s o f t e n cons idered synonymous wi th c l i m a t o g r a p h y - thed e s c r i p t i o n of p a t t e r n s of c l i m a t i c e l e m e n t s . Fu r the r development s ofr e g i o n a l c l i m a t o l o g y at p resen t take two d i s t i n c t i v e courses - local ort o p o c l i m a t o l o g y (THORNTHWAITE) whose method was r e c e n t l y expounded byKNOCH (21) , and w o r l d - c l i m a t o l o g y (19) whose Problems have on lynowadaysbeen r e c o g n i z e d äs such. In bo th these approaches we can today no longerbe s a t i s f i e d w i t h m e r e l y d e s c r i p t i v e methods, a l though they c o n s t i t u t e t heb a s i s of any sys t ema t i c r e s e a r c h ; phys ica l ques t ions - such äs r a d i a t i o n andheat b a l a n c e , t u rbu l ence and f r i c t i o n - and t h e o r e t i c a l c o n s i d e r a t i o n smust be inc luded . G e n e r a l c l i m a t o l o g y is s t i l l in i t s i n f a n c y äs far äs"physical c l i m a t o l o g y " ard " t h e o r e t i c a l c l i m a t o l o g y " are concerned (11).

The author haa po in ted out in a d i scuss ion (15) tha t some problemc l i m a t e s can be ra the r more s a t i s f a c t o r i ly i n t e r p r e t e d th rough dynamicalc l i m a t o l o g y r a the r than by synopt ic c l i m a t o l o g y w h i c h f o r m s the bas is ofTREWARTHA's method. I t must not be f o r g o t t e n that synopsis is, in thef i r s t p lace , a c o n d e n s a t i o n of i nd iv idua l d e s c r i p t i o n and s t a t i s t i c s o fw e a t h e r s i t u a t i o n s , w i t h a l l the we l l - known s i m p l i f i e d model concepts l ikea i rmasses , fronts und wea the r types ( " t y p e s d e temps" of the French School ) .

-2-

Thus s y n o p t i c c l i m a t o l o g y i s j u s t an advanced d e s c r i p t i o n , however p r o g r e s s i v eand u s e f u l . But it cannot by i t s e l f reveal the in te r re la t ion of causea whichc h a r a c t e r i s e s so many p r o b l e m c l i m a t e s . Ins tead we must apply p r i n c i p l e sof dycamica l me teo ro logy t such äs d i v e r g e n c e in the hydro -dynamica l sensew h i c h can be s p l i t i n t o the terms "curvature" and "veloci ty" d ivergence( s e e append ix ) . A p p l i e d to a three d i m e n s i o n a l wind di s t r i b u t i o n , i t y i e l d s -in consequence of the equation of cont inui ty - the vcr t ica l windcomponent w h i c h cannot be measured d i r e c t l y . The d i r e c t i o n and i n t e n s i t yof such v e r t i c a l m o t i o n d e t e r m i n e the wea ther sequence , the s t a b i l i t y ori n s t a b i l i t y of an a i r m a s s äs w e l l äs the d i s s o l u t i o n or f o r m a t i o n of c loudand p r e e i p i t a t i o n . A I t h o u g h we a re seidorn a b l e , under s i m p l i f i e dsupposi t ions , to ca lcu la te this mot ion quant i ta t ively, at least itsq u a l i t a t i v e e s t i m a t e leadi to an i n s i g h t i n t o the phys i ca l p roces se s whichgovern a i r m a s s e s , f r o n t s » w e a t h e r and c l i m a t e . The reasons fo r a f a d i n gof the a i r m a s s concept in tbe eontemporary dcve lopment of m e t c o r o l o g yhave a l r e a d y been m e n t i o n e d ( 1 0 ) . A t p r e s e n t the b ig t r a n s i t i o n to n u m e r i c a lweather predic t ion takes p lace in the "synopsis", and apparentlyu n r e a l i s t i c a l l y s i m p l i f i e d m a t h e m a t i e a l models a l r e a d y produce, i n mid-l a t i t u d e S f p r o g n o s t i c cha r t s w h i c h a re b e t t e r than those p repared by the olde m p i r i c a l methods. A t i m i l a r t r a n s i t i o n in c l i m a t o l o g y , whose tasks aremore d e t a i l e d and c o m p l i c a t e d , ii l ead ing to a t h e o r e t i c a l l y baseddevelopment (11) which is a l ready taking shape.

The Problem

The au thor had the o p p o r t u n i t y du r ing Deeember 1959 to p a r t i c i p a t ein a Symposium on Trop ica l M e t e o r o l o g y in N a i r o b i C l ) when numerousaspects of synopt ie m e t e o r o l o g y , c l i m a t o l o g y and hydrology in A f r i c a werediseussed. I t cmerged tha t the concept of I n t e r t r o p i c a l Convergence Zone( ITC) , which is d i f f e r e n t in West A f r i c a or India aad in the P a c i f i c ,c a n n o t be v e r i f i e d on the d a i l y w e a t h e r cha r t s in K a a t A f r i c a . The sameholds for S o m a l i a , A r a b i a and West P a k i s t a n where the e q u a t o r i a l troughl i e s in a zone of maximum drynes i d u r i n g lummer . S t a r t i n g f r o m a remarkmade at N a i r o b i (14) I propose now te c o n t r i b u t e to the dynamic c l i m a t o l o -gy of the Somali peninsula; however, thii ttudy, wi thdut personal knowledgeof the r e g i o n , muat be.. l i m i t e d to t a rge ica le problem*^ Looking at a pre-c i p i t a t i o n map o f A f r i c a ( 2 ) , i t i s s t r i k i n g tha t the grca t g u b t r o p i c a l ar idbel t of the n o r t h e r n hemispbere extends to the equator in e a s t e r n par t s ofA f r i c a and indeed extends far south of the equator in the dry steppe ofeas te rn Kenya and Tanganyika. Aao-ther branch of this dry reg ion extendsacrots Lake Rudolf to the Sudan..

-3-

F l y i n g f r o m N a i r o b i to A d d i s Ababa beyond Mount K e n y a , whose t r o p i c a l

r a i n f o r e s t and g l a c i e r s f o r m a l andmark j u s t south of the e q u a t o r , one

t r a v e r s e s th i s a r i d , b a r r e n and h a r d l y usab le s e m i - d e s e r t a r e a , w i t h sparse

V e g e t a t i o n in the g e n e r a l l y d r i e d - u p w a d i s , b e f o r e coming to the dense ly

p o p u l a t e d a g r i c u l t u r a l m o u n t a i n r e g i o n s o f southern E t h i o p i a . A s a l r e a d y

men t ioned ( 6 ) , the Somal i p e n i n s u l a and i ts s u r r o u n d i n g ocean area r e p r e s e n t s

the on ly l a r g e r e g i o n on the g lobe b e t w e e n the equa to r and 11UN w h i c h is

r e a l l y a r i d . P r e c i p i t a t i o n amounts in the moun ta ins h a r d l y reach 500 mm

and g e n e r a l l y r e m a i n be low 200 mm or even 100 mm ( f i g . 1). TREWARTHA

( 2 6 ) c a l l s th is dryness in East A f r i c a the most impress ive cl i m a t i c anomaly

in the whole of A f r i c a . Only the h i g h l a n d s in e q u a t o r i a l East A f r i c a a rewe l l wa te red .

This note does no t d e a l w i t h the d ry c o n d i t i o n s o f e q u a t o r i a l Eas t

A f r i c a where an ex tens ive zone w i t h nea r or be low 500 mm p r e c i p i t a l i o n extends

to a p r o x i m a t e l y 8° South . This has becn t h o r o u g h l y i n v e s t i g a t e d by

D.H. Johnson ( A i r M i n i s t r y M e t e o r o l o g i c a l O f f i c e , B r a c k n e l D a n d H . T . M o r t h

( E a s t A f r i c a n M e t e o r o l o g i c a l D e p a r t m e n t , N a i r o b i ) us ing s imple t h e o r e t i c a l

c o n s i d e r a t i o n s (18) , w e a t h e r - c l i m a t o l o g i c a l s t ud i e s and ex t ens ive synopt ic

e x p e r i e n c e (17 ) . The presen t study i s l i m i t e d to the h i t h e r t o l i t t l e known

a r e a of Somal i p e n i n s u l a , i . e . the p r e s e n t Somal ia i n c l u d i n g French S o m a l i a ,

(a round D j i b o u t i ) , i n c l u d i n g a l so E t h i o p i a e a a t o f 40° East and the dry

a r e a s of n o r t h e r n Kenya n o r t h of 2° N. Recen t c l i m a t i c da ta for the

Somal i p e n i n i u l a are found in (8, 8a, 30, 31).

Annual March of P r e c i p i t a t i o n

TREWARTHA ( 2 6 ) q u i t e r i g h t l y observes that the annual march of the

r a i n s - v i z . t ab le l - d e p a r t s s t r o n g l y f r o m the no rma l for th is l a t i t u d e .

W i t h FANTOLI (8 , 8 a ) , we can d i s t i n g u i s h f o u r seasons which we sha l l , to

avoid mi sunders t a n d i n g , d e f i n e in the n o r t h e r n hemi sphere sense , con t r a ry to

FANTOLI' s n o m e n c l a t u r e w h i c h co r re sponds to the t e m p e r a t u r e v a r i a t i o n s

of the southern h e m i s p h e r e .

Win t e r (December - M a r c h ) = G i l a l

Spring ( A p r i l - June) = GuSummer (July - August) - Hagai or Harat

Autumn (September - November) = Der

It i s s i g n i f i c a n t that the spa rae p r e c i p i t a t i o n i s f r e q u e n t l y concen-

t r a t e d on the t r a n s i t i o n a l seasons ( F i g . 2) (Gu and D e r ) - these prov ide

maximum amounts a l so in e q u a t o r i a l East A f r i c a 1 s n o r t h e a s t e r n par t s to

a l ine between Lake Rudolf and the Usambaras ( P a n g a n i ) - w h i l e the w i n t e r

and a s t o n i s h i n g l y also the summer are f r e q u e n t l y comple te ly dry .

oo H a«

*)-"

M **

labte 1 Mean Rainfatt in Northeast Africa and frequency of occurrence of preci

Stati on

Sokotra

Zei t a

Erigavo

Scusci uban

Burao

Las Anod

Do 'Mo

Ga tkayu

Gorrahei

NeghetLi

Obbia

Betet Uen

Mandera

Moyate

Bardera

MarsaMt

Waji r

Afmadu

Garissa

Area10-13°N,Area 10-12%Area 8-1 0°N,

CIO ("E)

12,6° 53,9°

11,6 43,1

10,7 47,4

10,3 50,2

9,5 45,6

8,5 47,4

7,9 46,8

6,7 47,6

6,6 44,3

5,3 39,6

5,3 48,5

4,7 45,2

4,0 41,9

3,5 39,1

2,3 „42,3

2 , 3 - 38,0

1,8 40,1

0,4 42,1.

0,5S 39,6

57-60°E 162-68 OE 262-680E L

Attitüde

43

10

1.737

344

1050

705

555

300

362

1444

15

173

331

1113

118

1345

244

100

193

JAN FEB

2 3

12 5

18 13

0 0

3 1

1 0

1. 0

1 5

0 0

6 25

10 2

0 1

1 5

11 19

6 6

32 17

5 5

5 12

10 5

1 1

1,1 1,2,2 0,

1 Source (28) about 18000 observati ons2 Materiat from Seewetteramt Hamburg- armind 19,500

^

MÄR APR

10 0

5 18

33 38

3 20

5 30

2 15

3 23

1 25

13 51

9l 158

26 21

5 48

18 82

53 180

19 85

75 209

20 65

32 102

26 55

1 00 0,4 0,69 0,6 0,3

observati ons.

A - '<yi

MAY

2

8

81

14

58

41

81

92

10

190

47

59

25

117

56

100

35

81

17

33,03,7

JUN

30

1

64

9

18

2

5

2

0

5

0

6

0

17

14

7

1

23

5

38,314,1

pitation over the Indian Ocean (based on at

JUL

0

2

10

0

10

0

111

9

0

3

0

17

26

17

3

30

2

12,6

•6,3

ÄUG

0

5

41

1

13

0

0

5

0

2

0

2

1

15

7

16

2

13

6

01,32,9

SEP OCT

2 1.0

1 ' 8

114 8

8 2

25 20

20 30

10 43

2 53

0 10

26 105

1 25

8 51

3 45

26 91

7 64

14 116

7 24

18 84

5 20

2 33,7 4 ,47,6 5,1

NOV

51

25

13

4

13

8

8

12

1

48

49

16

27

83

57

135

46

97

64

55,97,9

DEC

81

13

2

0

3

2

1

1

0

6

22

5

9

40

28

73

22

53

65

2

6|1

L»

YEAR

193

102

434

60

198

119

173

198

86

671

203

204

216

669

375

81t

235

549

280

4*

1

W*

l

-6-

The l a t t e r anomaly must be emphasised; nowhere eise do we know of sucha comparable dry season July - Augus t w i t h i n the b e l t of t r o p i c a l z e n i t h a lr a i n s in the n o r t h e r n hemisphe re . For i n s t a n c e , one such anomaly in theGulf of Guinea extends f r o m the sou thern hemisphere to 1° N; at the coastto the n o r t h of it, even in the d r i e s t par ts around 5°N (Accra) , we onlyf i n d a r e l a t i v e min imum w i t h 20-50 mm ra in per mensem. In Somal ia ,h o w e v e r , less than 2% of the annua l to ta l r a i n f a l l in the area f r o m 1° Nto 12°N occurs in the summer months July - August ( F i g . 3) and the areamust t h e r e f o r e be c lassed äs c o m p l e t e l y dry . In the same a r e a , 80%, and atsome s t a t i o n s over 95%, of the annua l t o t a l are r e c e i v e d du r ing the 6monthsof the spr ing and autumn.

Where do these spr ing and autumn t a i n s come f r o m ? The m e t e o r o l o g i s ti s no t s a t i s f i e d w i t h an e x p l a n a t i o n f r o m the sur f ace p re s su re d i s t r i b u t i on ,which does not a l low di f f e r e n t i a t i o n between the shal low, thermally stableheat l ows w h i c h are topped by a n t i c y c l o n e s and deep d e p r e s s i o n s wi th i n s t a -b i l i t y a l o f t . Nor does i t g ive an i n d i c a t i o n of the three d imens iona ls t r u c t u r e of the wind f i e l d , the a s soc ia ted a i rmasses and the course of theweather . T o d o this we should look at the upper wind charts . Ins p r i n g they f r e q u e n t l y show a marked upper t rough ( 1 6 ) w h i c h extends f r o mthe eas t e rn Medi t e r r a n e a n to t h e R e d S e a . For dynamica l reasons w h i c h a renot described he fe , the upper divergent winds and the lower convergentwinds ahead of the t rough - äs w i t h all upper t roughs - lead to convec t ivechanges wh ich can best be d e s c r i b e d äs a sor t of "upper cold f r o n t " . Theyare s t renges t where the löwest s t r a t a consis t of warm and humid a i rmasseswi th high condit ional ins tab i l i ty which are available in the tropicalo c e a n i c r eg i ons . Indeed we f i n d occas iona l r a i n showers in spr ing overE t h i o p i a and A r a b i a ( a l s o over areas in the no r the rn S a h a r a ) . The dreadedspring "Khamsin" s i tuat ions in the Middle East belong to this group sincethe l i f t i n g of aand ahead of the upper t rough is a lso a convec t ive mecha-nism sponsored by thermal i n s t a b i l i t y .

Why do the upper troughs more frequent ly extend fur ther south inspr ing than in w i n t e r ? This pa r t of a h i t h e r t o unsolved problem w h i c h isconnec ted w i t h the appearance of sp r ing and autumn ra ins that occur inwidely separated areas on the f r i n g e of the subtropics , such äs inMedi t e r r a n e a n Europe , in U tah , in New M e x i c o , and at the f r i n g e of thesou thern hemisphere w i n t e r r a in r e g i o n s . A comprehens ive p r e s e n t a t i o n ofthis problem wi l l be /^ e l s ewhere . Here, only two associa ted phenomenat v<*vsbould be ment ioned, the semi-annual pressure Va r i a t i on and the charac te r i s t icwarm spr ing h igh over the a r c t i c . I t i s most u n c e r t a i n whe ther the spr ing-autumn r a i n s over Nor th A f r i c a south of 10° N are connec ted w i t h these uppertroughs although there can be no doubt about the connection wi th the springr a i n s in A r a b i a , E r i t r e a and Yemen. These upper t roughs are c e r t a i n l y notf r e q u e n t in sp r ing and they are not a t a l l c o n f i r m e d in autumn.

-7-

2« Srtenaiem of the «quinootlal rminy

^ ehÄre of spring (IV-VI) and autuinE (IX-XI)

in annuftl raiafall aoount.

Limit* of- Summer Rains VII-VII!

bmih of UinVer Rain* XII" HI

Pig. 3- Proportion of summer (VII-VIIl) of total

preoipitation in %; Limit of aummer

(VII-VIII 17?̂) and southern limit of

winter raina (XII-III 33#) « arrows

rflflultant winds in summer (st'eadiness

88-96̂).l

-9-

The main c h a r a c t e r i s t i c o f the s e a s o n a l d i s t r i b u t i o B o f p r e c i p i t a t i o n

in Nor th Eas t A f r i c a is no t r e a l l y the p r e p o n d e r a n c e o f s p r i n g and autumn

r a i n s bu t the absence o f the t r o p l c a l aummer r a i n s w h i c h a re so c h a r a c t e r -

i s t i c f o r these l a t i tudes . In the immedi a te ly n e i g h b o u r i n g h i g h l a n d s o f

E t h i o p i a the l a t t e r b r i n g the ma j o r i ty o f the a n n u a l r a i n A l l over

s o u t h e r n E t h i o p i a , v i z . s t a t i on s Add i s Ababa , J imma, G o r e , Lekemt i ,(Y° -9° N ) , the summer r a i n y season (.May-September) r eaches i ts maximum

in July/August . In the N o r t h , v i z . M a c a l l e , D e s s i e , 55-66% of the annualr a i n f a l l t o t a l s f a l l s d u r i n g the two midsummer months and even more on the

h i g h l a n d s of E r i t r e a ( A g o r d a t , 15.6° N, 77%). T a b l e s 2 and 3 r e p r e s e n t

mean c o n d i t i o n s in two m e r i d i o n a l s e c t i o n s . The dry a r e a s of Somal ia wou ld

undoub ted ly b e l ess e x t r e m e , i f s u f f i c i e n t r a i n were t o f a l l d u r i n g t h e

n o r t h e r n summer ( m i d d l e o f June to b e g i n n i n g o f S e p t e m b e r ) . Check ing

a c c e s s i b l e map and d a t a m a t e r i a l i t can be shown tha t t h i s anomaly in the

l a t i t u d e b e l f c 0 - 12° N is indeed unique. Cpmparab l e w i t h it is only thedry a rea on the n o r t h e r n coas t of South Amer ica (10°-15° N) which i s ,

a c c o r d i n g to LAHEY ( 2 2 ) , connec ted w i t h d i v e r g e n c e in the t rade wind

r e g i m e . There a r e , however , t r a d e wind d e s e r t s n o r t h of 12° N in many

p l a c e s on e a r t h . The p r o b l e m of a r i d i t y in the Somali p e n i n s u l a r evo lves

t h e r e f o r e about the q u e s t i o n : why are the summer r a i n s m i s s i n g in these

a r e a s ? We must look i n t o the wind d i s t r i b u t i o n to d i s c u s s th is q u e s t i o n .

WIND DISTRIBUTION

An e v a l u a t i o n o f the a v a i l a b l e w i n d s t a t i s t i c s fo r Kenya ( 5 ) , even

by the use of s u r f a c e d a t a , can lead to s i g n i f i c a n t r e s u l t s , e s p e c i a l l y when

we r e s t r i c t them to the hours w i t h h igh v e r t i c a l mix ing around noon (12 -15h) and i f we take i n t o a c c o u n t the p r e v a l e n t d i u r n a l . c i r c u l a t i o n s of

c e r t a i n s t a t i o n s . The s p r i n g - aad - autumn r a i n s c o i n c i d e at l a t i t u d e s 2

to 5° w i t h the m o n s o o n a l r e v e r s a l of the p r e v a l e n t winds wh ich occu r dur ing

the p e r i o d s M a r c h / A p r i l and October /November . In t h i s a r e a , cer t a in ly eas t

of 39° E, one can speak of an I n t e r T r o p i c a l Gonvergence zone ( I T C ) ; butth i s does no t so lve the p r o b l e m of the s p r i n g r a i n s which extend a c r o s sE t h i o p i a to the Yemen and the b o r d e r i n g Saudi A r a b i a ( v i z . Er Riyad at24 .6° N, 90% of annua l r a i n d u r i n g February - May) and a l s o extend i n t o the

Ki l i m a n j a r o a r e a ' t o 4° S, i . e . in a m e r i d i o n a l d i r e c t i o n over some 3 , 0 0 0 kmw h i l e the c o r r e s p o n d i n g conve rgence in w i n d s is l i m i t e d to a few degrees of

l a t i t u d e . The P r o g r e s s i o n of the l a t t e r between the equa to r and 5C N has_ been d e s c r i b e d by BOSSOLASCO (2) to whom are a l s o due some i m p o r t a n t> i n v e s t i g a t i o n s of the upper winds over Mogad i shu ( 3 ) .X "

Fable 2. Hean Ralnfall (m) along 8°E

Stati on

Agades

Zinder

K a n o

Kaduna

Jos

Wamba

Makurdi

Og o j a

Calabar

l VictoriarH

1

Atbara

Shendi

Kassala

Sennar

R os ei r es

M a l a k a L

Gambela

Halek

To ri t

Ki tgum

Lokitaung

Lodwar

Lat (°N)

17,0

13,8

12,0

10,6

9,9

8,9

7,7

6,6

5,0

4,0

17,7°

16,7

15,5

13,6

11,8

9,6

8,2

6,1

4,4

3,3

4 ,2

3,1

Long (°E)

8,0

9,0

8,5

7,5

8,9

8,6

8,5

8,7

8,3

9,2

Table

34,0°

33,5

36,4

33,6

34 ,4

31,6

34,6

31,6

32,5

32,9

35,8

35,6

Alti tude

500

511

467

645

1222

305

112

225

23

3

3. Hean

345

360

500

420

465

390

649

420

624

914

762

506

JAN

0

0

0

0

2

0

8

15

43

43

R a i n f a l l

0

0

0

10

0

5

6

5

8

10

7

FEB

0

0

1

2

3

5

10

20

76

69

( m m )

0

0

0

0

12

10

7

20

33

18

5

MÄR

0

0

2

13

28

30

33

53

152

155

along 34

0

0

12

2

5

36

34

46

81

50

16

APR MAY

T 6

3 27

8 69

69 147

86 203

74 152

107 201

127 231

213 312

211 338

JUN

8

55

114

180

226

193

206

297

406

638

E ( S u d a n / E t h i o p i a

1 3

1 3

3 12

2 18

15 61

30 79

81 150

84 12t

102 132

1 24 1 85

129 46

41 25

2

2

30

66

127

132

170

115

122

150

23

9

JUl

49

153

203

218

330

335

191

203

450

985

ÄUG SEP

78 20

232 71

315 140

31 2 279

292 .ZI 3

399 330

216 241

254 295

406 427

805 429

OCT

1

7

13

76

41

132

150.:

295

310

259

N O V

0

0

15

3

18

10

56

190

104

DEC

0.

0

0

0

2

1

0

13

43

38

Y E A R

164

549

861

1303

1430,

1669

1377

1859

3030

4074

b o r d e r reg ion j

18

43

91

118

185

175

2t 6

123

158

178

33

14

r

38 6

64 2

124 58

160 66

22t 155

201 1 45

241 185

131 120

142 112

175 130

13 8

9 4

2

3

8

18

30

79

89

87

99

112

11

6

0

0

0

2

5

13

46

18

41

74

16

7

0

0

0

0

2

2

10

9

15

30

27

9

70

118

327

444

800

861

1240

855

933

1280

384

152

-11-

In summer (May-Sep tember ) we observe in the zone 0-2° N a very

steady wind f low f rom S t o SSE w h i c h changes i n t o the w e l l - k n o w n

SW-monsoon at 3-5° N east of 40° E but becomes a steady SE f l o w in Moyale

C f u r t h e r w e s t ) . This s t r o n g d i v e r g e n c e can be best i n t e r p r e t e d by

m e n t i o n i n g the a d j o i n i n g E t h i o p i a n h igh l ands to the Nor th over w h i c h the

s t r o n g o r o g r a p h i c c o n v e c t i o n a l p r e c i p i t a t i o n c r e a t e s a r e g i o n a l c i r c u l a t i o n -

l ow level conve rgence b e l o w , d i v e r g e n t o u t f l o w in t he m i d d l e t r o p o s p h e r e .

U n f o r t u n a t e l y , t h e w i n d s t a t i s t i c s o f N e g h e l l i ( 7 ) a r e n o t separa ted f o r

t imes o f the day . The annual V a r i a t i o n sugges t s s o u t h e r l y winds d u r i n g

summer a round noon , r e p r e s e n t a t i v e fo r the g r a d i e n t w i n d , w i t h a secondary

N E d i r e c t i o n r e s u l t i n g f r o m n o c t u r n a l k a t a b a t i c f l o w ; t h e l a t t e r h a s n o tbeen taken i n t o account when c a l c u l a t i n g the r e s u l t a n t s in F i g . 3 .

The w i n d s of Moya le and N e g h e l l i d u r i n g summer are p a r t of the

b r o a d SW-monsoon f l o w bu t h e r e , a t the sou the rn edge o f E t h i o p i a , l o c a l l y

d e f l e c t e d by 30-60° . The same h o l d s , a c c o r d i n g to r e c e n t data by FANTOLI

( 9 ) , for A d d i s Ababa where a SSE f l o w p r e v a i l s a t 3000-3500 m in summer

w i t h on ly a s l i g h t d i u r n a l chaage . A somewhat d i f f e r e n t r e s u l t emerges

f r o m the more r e c e n t KAWIN s o u n d i n g s a t Add i s Ababa (1957-1961 w i t h

many i n t e r r u p t i o n s ) for w h i c h I am g r e a t l y indeb ted to D r . M. Hof f mann }

( A d d i s A b a b a ) . These d a t a i n d i c a t e a WNW d i r e c t i o n for the July/August

f l o w at 700 mb ( a p r o x i m a t e l y 3150 m ) , w h i c h cannot however be c o n s i d e r e dr e p r e s e n t a t i v e s i n c e the s t e a d i n e s s is äs low äs 15%. Here , ÖD the s teep

s lopes o f t h e E t h i o p i a n r i f t v a l l e y loca l c i r c u l a t i o n s i n t e r f e r e obv ious lyw i t h r -eg iona l ones, making an unambi^uous I n t e r p r e t a t i o n very d i f f i c u l t .

The s u r f a c e winds at Lodwar and L o k i t a u n g , nea r the w e s t e r n shore

of Lake R u d o l f , are obviously s t r o n g l y i n f l u e n c e d by the orography . The

f o r m e r , äs observed at 09 and 15h, shows secondary winds d i r e c t i o n s of

W-NW in a d d i t i o n to the p r e v a l e n t e a s t e r l i e s (NE-SE) . A s imi lar si t u a t i o noccurs a t M a r s a b i t . P resumably these are the f u r t h e s t ex t ens ions of the

e q u a t o r i a l w e s t e r l i e s w h i c h , d u r i n g the n o r t h e r n summer , . nea r the s u r f a c e

pene t r a t e to 38° E in 35% of the cases.

L i t t l e i s known about the c l i m a t i c cond i t i ons of Eas te rn E t h i o p i a ,

e s p e c i a l l y the Ogaden a r ea . SCHOTTENLOHER ( 2 7 ) comes to the conclus ion

that the d i s t r i b u t i o n of V e g e t a t i o n on the p l a t eau in the Har ra r a rea resu l tf r o m r a i n - b r i n g i n g s o u t h e a s t e r l i e s . This p robab ly app l i e s in summer when

most of the convec t ive ra ins f a l l on to the h igh lands .

-12-

Look ing a t the S i t u a t i o n on a l a r g e r s c a l e , i t i s a p p a r e n t that the

equa tor ia l wes te r l i e s which , d u r i n g the nor the rn summer can be traced f rom

the G u l f of Guinea to the N i l e (3) are d e f l e c t e d i n t o a S to SSE s t r e a m

by the t h e r m a l l y d r i v e n c i r c u l a t i o n s o f the E t h i o p i a n h i g h l a n d s . This

o c c u r s in the a rea be tween Lake R u d o l f and the Somal i b o r d e r , a p p r o x i m a t e l yf r o m 3° to 8° N l a t i t u d e and f r o m 35° to 41° l o n g i t u d e This d e f l e c t i o n

leads to convergence and increased p r e c i p i t a t i o n * on the southwestern

f r i n g e o f t he h i g h l a n d s , bu t t o d i v e r g e n c e , s u b s i d e n c e , l i t t l e c l o u d and

ra in in the S and SE, on ly 400-700 km away The wind a r r o w s in F i g . 3

i n d i c a t e th i s d i r e c t i o n a l d i v e r g e n c e ; t he cons t ancy i s 88-96%.

There i s a l s o d i v e r g e n c e in the broad o r o g r a p h i c d e p r e s s i o n tha t

l inks the Sudan via Lake Rudo l f bas in ( v i z . Lodwar and L o k i t a u n g in Table

3) w i th nor thern Kenya- This is because heavy convective rains f a l l on the

E t h i o p i a n h i g h l a n d s a n d o n e i t h e r s ide t h e East A f r i c a n r i f t v a l l e y (E lgon-

Moroto on the Kenya/Uganda b o r d e r , K e n y a - N g i r o on the e a s t e r n s i d e ) . An

e a s t e r l y f l o w near t h e s u r f a c e a p p a r e n t l y p r e v a i l s d u r i n g summer, w h i c h i s

only o c c a s i o n a l l y i n t e r r u p t e d b y e q u a t o r i a l w e s t e r l i e s .

There a r e two more d i v e r g e n c e e f f e c t s be s ide s t he d i v e r g e n t d e f l e c -

t ion of the SW monsoon in to a SSE f l o w in B o r a n a , Sida^o and Ogaden£ which

is produced by the E t h i o p i a n h i g h l a n d s . P robab ly the most i m p o r t a n t i s the

i n c r e a s e in the wind speed of the SW-monsoon n o r t h of the e q u a t o r w h i c h can

be d e m o n s t r a t e d over the ocean by the a v a i l a b l e s t a t i s t i c s . Data f r o m theB r i t i s h a t l a s fo r t he I n d i a n Ocean shows the f o l l o w i n g winds d u r i n g June-August in the three 5 degree squares along the A f r i c a n east coast which

f o l l o w c o n s e e u t i v e l y f r o m NE to SW.

Tabla 4 . Wind d i s t r i b u t i o n a l o n g the S o m a l i a c o a s t ( J u n e - A u g u s t )L a t . Long. P r o p o r t i o n of SW w i n d s . P r o p o r t i o n >7 B e a u f o r t Mean Speed(N) (E)

10-15°

5-10°

0-5°

55-60°

50-55°45-50°

68%-

57%38%

28%

19%1%

6.0 Beauf .

5.6

3.9

* This increase cannot be documented by the data ava i lab le . Bonga inK a f f a p r o v i n c e ( 7 . 2 ° N, 36 8° E, 1725 m, 1937-40) has according toFANTOLI (Riv. Meteor - Aeronaut . 20, 1960, No. 1) 233 mm rain in Julyand 218 mm in August but c o n s i d e r a b l y more in May (331 mm) and June (366

mm) , a l so the mon th ly number of ra indays in Apri l /May ( 2 0 ) is h i g h e r thanin July/August ( 1 6 ) . But Bonga a l r e a d y i s in the r a i n s h a d o w , s i m i l a r l yJimma (FANTOLI , I b i d , 19, 1959, No. 3) ; o b a e r v a t i o n s f r o m the lower pa r t s

of the Omo Bottego r iver are h i the r to miss ing .

-13-

Thus the mean wind speed i n c r e a s e s away f r o m the equa tor ( e s p e c i a l l y

in the sea a rea south of Cape G u a r d a f u i ) and a s t r o n g speed d i v e r g e n c e

occurs a l o n g the s t r e a m l i n e s . This a t a very g r e a t SW-monsoon cons t ancy

of 80-90% in the c e n t r e of an ama around 10°N. This i s p h y s i c a l l y caused

by the r ap id p r e s s u r e g r a d i e n t i n c r e a s e f r o m the e q u a t o r n o r t h w a r d i n t o the

r e g i o n o f t h e A r a b i a - I n d i a h e a t l o w . Th i s a c c e l e r a t i o n cannot b e b a l a n c e d

by the i nc r ea se w i t h I a t i t u d e o f the C o r i o l j / s P a r a m e t e r w h i c h , fo r a c c n s t a n t

p r e s s u r e g r a d i e n t , should p roduce a dec r ea se in the geos t rop / i c w i n d . / )

The r e l i a b i l i t y o f a v a i l a b l e mean pressure c h a r t s over A f r i c a i s a t

p r e s e n t u n f o r t u n a t e l y p o o r , because o f t he u n c e r t a i n r e d u c t i o n f r o m h i g h

p l a t eaux to sea leve l . Good new maps have been produced by L. WEICKMANN

j r . ( 2 9 ) ; n e v e r t h e l e s s , some d i s c r e p a n c i e s r e m a i n i n t he area o f i n t e r e s t

be tween the w e l l - d e f i n e d wind system and the c o r r e p p o n d i n g p r e s s u r e

d i s t r i b u t i o n . A second reason may r e s u l t f r o m the i m m e d i a t e n e i g h b o u r h o o dof the overheated land and the r e l a t i v e cool sea w h i c h , even at smal l d i s t a n c e

f r o m the e q u a t o r , produces a we 1 1 - b a l a n c e d wind sys tem approx ima te ly p a r a l l e lto the i so the rms .

The summer-dry area of the SW-monsoon in the r e g i o n of the speed

d i v e r g e n c e extends far out i n t o the A r a b i a n sea ( 2 4 ) r o u g h l y to 15° N, 65°E

(v iz . Table 1 . ) . The U .S . Navy A t l a s ( 2 8 ) g ives for a 3° square east of

Soco t r a (10-13° N , 57-60°E) under g e n e r a l l y low p r e c i p i t a t i o n f r e q u e n c i e sa c l e a r min imum in July/August w h i c h d i v i d e s the summe r r a ins äs in

S o m a l i a . A s i m i l a r s p l i t is shown by the m e r i d i o n a l c ross - sec t ion be tweenlong i tudes 62-68 E, p r e v i o u s l y p u b l i s h e d by the w r i t e r ( 1 2 ) , e s p e c i a l l y

pronounced in the w e l l documented a r e a s 8-12 N whose r a i n f r e q u e n c y

d e c r e a s e s f r o m 18% d u r i n g e a r l y June to 2-3% in A u g u s t . The c o r r e s p o n d i n g

wind component a n a l y s i s ( 12 ) shows t he r e s u l t a n t winds i n c r e a s i n g f r o m l . 9B e a u f o r t a t 0-2° N to 5 6 B e a u f o r t a t 14-16° N, w h i l e the r e s u l t a n t

d i r e c t i o n on ly changes f r o m 229 to 248 . A l t h o u g h the m e r i d i o n a l p r o f i l e

o f t h i s f l o w d i f f e r s c o n s i d e r a b l y f r o m t h e s t r e a m l i n e s o f t h e SW-monsoon,

i t shows the u n i f o r m i n c r e a s e in speed f r o m the equa to r to 15 N o r t h .

S i nee the SW-monsoon b l o w s l a r g e l y p a r a l l e l to the coas t , thed i f f e r e n c e i n s u r f a c e f r i c t i o n becomes a n o t h e r f a c t o r . T h e d e p a r t u r e o fthe s u r f a c e w i n d f r o m the g r a d i e n t wind towards low p r e s s u r e must over land

- where because o f the f l a t p o o r l y v e g e t a t e d s u r f a c e we shal l on ly assume30° - su re ly be g r e a t e r than over the sea ( 5 - 1 0 ° ) . Th is leads to t rue

f r i c t i o n a l d i v e r g e n c e a t the coas t in a l l c i r cums tances where the windblows p a r a l l e l to the coas t and h i g h e r p r e s s u r e is over the sea ( 1 4 ) . Thise f f e c t i s enhanced by f r i c t i o n a l d e f l e c t i o n o f w i n d - d r i v e n ocean s u r f a c e

c u r r e n t s : t h i s depa r t s Dy 35-40 towards the a n t i c y c l o n i c s ide , i . e .

towards ENE. The o f f - s h o r e winds p r o d u c e , l i k e anywhere e i s e , an u p w e l l i n g >of cold deep wa te r a long the coas t whose e f f e c t on t e m p e r a t u r e , v i s i b i l i t yand r a i n f a l l ( a r i d i t y ) i s w e l l known f r o m ma,ny papers (24 , 2 5 ) . A l t h o u g h

-14-

rea l fog i s not obse rved , m i s t and ~ h a z e are f r e q u e n t . But th is localcoas ta l e f f e c t a lone would never s u f f i c e to expla in the a r id i ty of the ^whole pen insu la up to 600-800 Km In l and . The anomalous lack of summerprecipi t.ation in tbe Somali peninsula and over the ad jacen t sea area out

A £ to 65 l must be exp l a ined by a combina t i on of a l l pos s ib l e d ive rgencee f f e c t s :(a) D i r e t j t i o n a l d ive rgence produced by ove rhea t ing of the E t h i o p i a n

highland», ^(b) Speed d ive rgence produced by no r thward incjeaae of the p res su re

gradient , ,( c ) F r i c t i o n a l d i v e r g e n c e in coas t -pa ra l l e l w inds ,(d) Def lec t ion of the wind-driven ocean surface currents and cold

u p w e l l i n g along the coas t .

The previous ly ment ioned a r id area on the nor th coast of SouthAmer ica owes its ex i s t ence , acco rd ing to LAHEY ( v i z . 22) and (11, 2 6 ) , toa s imi la r combina t ion o f mechanisms ( b ) , ( c ) , C d ) .

(September 1963).

«lThere exists in the northern part of the Somali peninsula, about

150 km from the northern coast, i.e. in the area of the plateau escarpmentand its southern slopes, a zone with relätively high rainfäll (84). It isalmost devoid of meteorological stations and has recently been used agric-ulturally äs the settlement of nomadic tribes progresses. From June andSeptember a convergence zone forms during the day in this area between theiarge SW-monsoon and the diurnal sea breeze circulation from the North.The latter, already described for Berbera (32), results from the Iargetemperature difference between the Gulf of Aden and the arid, summerlyhotinterior of Somalia and is very steady and intensive. It obviously reaches

TY from the north across the escajjment southward - in the same way äs theMaloja wind reaches across the asymmetric watershed from the SW into theSwiss Engadin -• and converges there with the SWrflow. This convergencehas been well documented by SAYER (83) through aerological and locustswarm observations; this, however, is not the ITC äs a compariaon withthe neighbouring Indian Ocean will cleafly show. Taking its definition inthe planetary sense, i.e. äs a connectiön betweeu the well defined lowpressure troughs, it extenda across Africa at approximately 18° K toPakistan - India at 25-29 N and acrosa southern Arabia and Baluchistanafter an orographically caused Interrupt!on in the Red Sea depression. Theregional convergence in the north of Somalia, at9.5-10 N, ia of anexclusively diurnal nature, it disappears during night and reforms daily inthe forenoon. More complicated is the Situation near the Eastern Hörn of Cape - V

-

-15-

G u a r d a f u i by the i n t e r f e r e n c e of a sea breeze c i r c u l a t i o n f r o m the IndianOcean. Here we have a day t ime copvergence be tween both sea b reezes wh ich

oppose the wes te rn component of the SW-monsoon.

Addendum (September 1965)

The d ive rgence of the l o w - t r o p o s p h e r i c f l o w is we l l evidei^ced by

some resu11ant upper w i n d s k i n d l y f u r n i s h e d by the East A f r i c a n

M e t e o r o l o g i c a l D e p a r t m e n t . The f o . l l o w i n g t ab le represents the d i r e c t i o n

of the average r e s u l t a n t f l o w d u r i q g July and Augus t , 09 and 15h.

3000 5000 7000 10000 ft .

ManderaLodwar

Gulu

3.93.1

2 .S

°N,°N,

>°N,

41.9°E35.6°E

32.3°E

207°80°

-

207°

(103°)( 48°)

220°

(111°)( 52°)

248°

(56°)161°

Values in bracke ts are weak r e s u l t a n t winds w h i c h may be s l i g h t l y

biased by a re la t ive ly large v a r i a b i l i t y . The occurrence of a d i r ec t i ona i

d i v e r g e n c e in the Kenya -E th iop i a bo rde r l and in th i s season is obvious lys t renger than ind ica t ed by s u r f a c e winds a l p n e .

Table 5 g ives some r e c e n t l y c o l l e c t e d p r e c i p i t a t i o n averages in the

Kenya-Eth iopia-Sudan border land in L a t . 3-6 N, showing at al l s ta t ions

east of Long. 35°E. t h e a b s e n c e of a p p r e c i a b l e summe r r a ins wh ich

g radua l l y d isappears west of Long. 35 E. In the same a r e a , the d ivergent

wind pa t t e rn south of the E th iop i an h igh lands s h i f t s into a convergent

pa t t e rn a t i t s sou theas te rn f l a n k s . In the mounta ins of southern E t h i o p i a

nor th of 6°N the summe r minimum d i sappears due to s t rong d iu rna lc i rcula t ions w i t h convec t ive shower a c t i v i t y .

The ro le of d i u r n a l wind Systems for the annual and aeasona lp r e c i p i t a t i o n pa t t e rn in the area qf the Gu l f of Aden and the Red Sea has

been desc r ibed in more d e t a i l in a recen t p u b l i c a t i o n (H. Flohn, Con t r ibu-t ions to a Synoptic C l i m a t o l o g y of the Red Sea Trench and a d j a c e n t a r e a s ,

Bonner Meteorologische Abhandlungen No. 5, p. 2-35, 1965).

Appendix

The equation of continuity has been used which yields for an incomp-ressible medium, i e. assurning that the time variations in air density can

be neglected, the vertical component of the wind velocity w„ for a wind V

at height z above ground Z

-Wz = / divj£ V dz

•Tt

Kenya

VDH

Ethiopia

Sudan

TUTH-ftt,

3,a

TabLe 5. Average Precipüati on in Northern Kenya, Southern Ethiqpia and SE-Sudan .(in mm; data receivedfrorn Meteorologicat Services at Nairobi, Addis, Ababa and. Khartoum: n = number of years}.

Station Lat.N Long.E Beight

Archers Post C

Habbaswein 1

EL Hak S

Murn" 4

Todenyang 2) L

Lokochöggio l

Lokomor inyang l

Neghe l t i 1.) 5

.6° 37.7° 865m

.0 39.5 200

..8 41.0 366

.3 40.1 •• 1000

.5 35.9 400

.2 34.3 1040

„0 35.6 ?

.3 39.6 1444.

K a L L a f o . -5.6 44.2 ?

i Hagere Mari am i .8 38.2 1600

Bürge 5.7 37.8 1560

Adola 5

Kapoeta ' • *

.9 39.0 1750

.8 33.6 670

Loel l i 5,1 34.7 ?

JAN

5

5

0

0

5

6

15

9

3

16

48

24

6

9

FEB

6

5

0

1

7

18

12

5

3

17

39'

23

20

7

MÄR APR MAY

37 82 40

16 91. 8

37 82 20

28 96 48

48 35 37

82 83 37

70 . 93 39

1.6 159 95

46 59 35

96 1 98 1 88

73 156 1.20

88 134 156

57 70 1 03

43 73, . 93

JUN

2

0

0

0

13

23

18

7

0

66

40

45

80

65

JUL

6

1

2

0

8

38

19

8

1

28

66

35

132

71

ÄUG SEP

6 3

0 2

4 3

0 0

8 1

48 26

11 10

6 23

0 3

19 49

32 . 89

45 58

127 78

79 46

OGT NOV

26 84

24 42

51 96

54 80

7 ^38 66

42 1.54

126 21

23 23

123 29

177 28

152 51

62 35

48. 48

DEC

33

23

16

29

20

40

28

10

8

28

22

12

24

19

j_ YEAR

330

219 ,

310 ;

336

222

506

512

485

204 ,

857

888 }

823

794 !i

601 ,

1') 1952-61 2) Lake R u d o L p b >

£E£"6iW ^ W

Tftcto^c^v, 4Z* ^r':(jtoutfiff* #* •? r.'

f f . r f £ SULflfcsUvtAAiAviCiAn T 0 T,-

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*oo 9 g r? *G vrriro 4i ^T' 5"3 "7Xo ^ £?-̂ o "t3 5~ 3̂ 1 4^L ^K» -If 13 36 4> 2G0 _ « f l ^ / _ /•-% ^*rt J.

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ff

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6 S 0 <(?

•̂ 1 2 0 -4$j ^, ^*. ..̂ -/«*•

fr4-tn

^?39

n^5*

*t>~.

'ty-<&

i t?

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H>

37/7

U^

W

WVo-r^093t-/*)_0->

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16

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i

-17-

where the horizontal divergence of the. wind vector div V is obtained in an

orthogonal co-ordinate System from the wind components u and ;v along the

axes x and y by definition,

diVjj. V = Ou/0'x + dy/dyü

If we express the resultant wind? vectorla l ly, then i t is better Ip

express the divergence in a polar co-ordinate systemby the curvature radius

r and the turning angle Ct. If in such a, system s is taken äs a distance

ralong the atreamlines and V = wind velocity, then

div,-, v = dy/ds + v c/cc/öüi

Here , the f i r s t term on the r ighi side öf the equa t ion gives Uie

v e l o c i t y change a l o n g t.he s t r e a m l i n e , the second t e rm the f a n - l i k e d i f f l u e n c e

of the s t r e a m l i n e s dependent on the c u r v a t u r e r a d i u s , i . e . d i r e c t i o n ^ ld ive rgence . ' .

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