INTERNATIONAL JOURNAL OF CLIMATOLOGY, VOL. 9,601-618 (1989) 551.577.32(548.7):551.513.7

RELATIONSHIPS BETWEEN THE SOUTHERN OSCILLATION A N D THE RAINFALL OF SRI LANKA

RAMASAMY SUPPIAH Institute of Geoscience. University of Tsukuba. Ibaraki 30.5, Japan

Received 26 July 1988 Revised 24 October 1988

ABSTRACT

The Southern Oscillation (SO) signals in the monthly and seasonal rainfall of Sri Lanka are studied by using statistical analyses. Correlation coefficients (CCs) between the rainfall of the first intermonsoon season (March-April) and the seasonal Southern Oscillation Index (SOI) of August-September-October (ASO) and November-December-January (NDJ) are positive and significant. In particular, March rainfall in region A is significantly correlated with the subsequent SO1 of A S 0 and NDJ. The CC between the rainfall of the southwest monsoon season (May-September) and the SO1 is also positive. Cumulative rainfall of the peak monsoon months (July and August) reveals a strong and positive correlation with the seasonal SOL

The CC between the rainfall of the second intermonsoon season (October-November) and the SO1 is negative and significant. The rainfall of this season is also significantly correlated with the SO1 (Tahiti-Darwin) of May-June-July (MJJ). The SO1 of MJJ serves as a particularly useful predictor for this seasonal rainfall. The CC between the rainfall of the northeast monsoon season (December-February) and the SO1 is also negative but weak.

Large signals appear in wind anomalies over Sri Lanka prior to El Nifio/Southern Oscillation (ENSO) events. Upper level easterlies (westerlies) and weak (strong) low-level easterlies are dominant in March before the La Nifia (El Nifio) years. These wind anomalies persist until the ENSO events reach maturity.

Based on the results of correlation analysis and wind anomalies, schematic models of the zonal circulation along the Equator between 60"E and 140"W are presented for La Niiia and El Niiio phases for northern hemispheric summer and winter seasons.

KEY WORDS Rainfall fluctuation regions Rainfall Srj Lanka Seasonal rainfall and Southern Oscillation

INTRODUCTION

The dominant large-scale atmospheric system in the tropics is the Southern Oscillation (SO), a phenomenon defined statistically by Walker and Bliss (1932). The SO is characterized by the variations of pressure difference between the Indonesian low and the South Pacific subtropical high. The SO signals have been observed in sea-surface temperature (SST) anomalies (Weare et al., 1976), rainfall (Berlage, 1957; Fleer, 198 I), surface wind anomalies (Rasmusson and Carpenter, 1982) and upper wind anomalies (Arkin, 1982). It is closely linked with the oceanic event called El Niiio, a striking phenomenon involving large-scale interannual variations of SSTs, sea level, currents, thermocline and wind and rainfall over the tropical Pacific Ocean, particularly in the east (Bjerknes, 1969; Wyrtki, 1975; Philander, 1983). El Niiio events are associated with warm SST anomalies, which are related to weak trade winds, and therefore to a weak pressure gradient between the Indonesian Low and the South Pacific subtropical high. Based on these conditions, Bjerknes (1 969) proposed a thermally driven east-west circulation pattern along the Equator for which the Indonesian region or 'maritime continent' acts as the main source region releasing a large amount of latent heat to the atmosphere (Ramage, 1968). The term La Niiia refers to a phase opposite to an El Niiio phase (Philander, 1985). El Niiio and the SO together comprise a complex system of climate fluctuations presently termed the El Niiio/Southern Oscillation (ENSO) phenomenon.

0899-841 8/89/060601-18$09.00 0 1989 by the Royal Meteorological Society

602 R. SUPPIAH

Recently, Rasmusson and Carpenter (1983) demonstrated below-normal rainfall during June-September over India, particularly in central India, and above-normal rainfall between October and December over Sri Lanka and south-east India associated with warm SST anomalies over the eastern equatorial Pacific. Such relationships have been confirmed by Behrend (1987) using the cross-correlation method. These studies have clearly shown the relationship between the SO and autumn or second intermonsoon season rainfall, but not for the other seasons in Sri Lanka where seasonality in rainfall and its relationships with atmospheric circulations of the lower latitudes are strong.

The main purpose of this study is to contribute to an understanding of the teleconnection patterns between the SO phenomenon and the monthly and seasonal rainfall over Sri Lanka, and their time lags. First, a brief description of seasonal rainfall distribution is given. Second, statistical relationships between the SO phenomenon and monthly and seasonal rainfall are given. Third, the relationship between wind variations over Sri Lanka and the ENS0 phenomenon is discussed. Finally, anomalous circulation features are given for northern hemispheric summer and winter seasons in relation to La Niiia and El Niiio years.

DATA AND METHOD

Rainfall data

The basic set of rainfall data used in the study consists of monthly mean rainfall values for 29 stations for 100 years (1881-1980). The data were obtained from a recent publication (Yoshino and Suppiah, 1982) and from manuscripts in the Department of Meteorology, Colombo, Sri Lanka. Figure 1 shows the location of the rainfall stations and the different rainfall regions of Sri Lanka used in this study. These regions were demarcated in an earlier study (Suppiah and Yoshino, 1984) using the space coefficients of an empirical orthogonal function analysis.

Southern Oscillation Index (Sol)

Walker and Bliss ( 1 932) incorporated pressure, temperature and rainfall in their Sol. However, pressure alone has been suggested for this index (Troup, 1965; Quinn and Burt, 1972; Chen, 1982), but other indices based on rainfall and temperature are still used. Among the weather stations of the Pacific and Indian Oceans, Tahiti (18"S,15ODW) and Darwin (12"S,13loE) show a strong negative relationship in their pressure anomalies (Trenberth, 1976). Pressure anomalies at these two stations were recommended by Chen (1982) for diagnostic analyses of interannual climate variation studies. In the present study, the index documented by Parker (1983) has been used to analyse the monthly and seasonal relationships. Because the Tahiti station only opened in 1935, another index, that of Wright (1975), was used to analyse the long-term relationship for the last 100 years. Wright's index was derived from a principal component analysis on pressure data from Cape Town, Bombay, Djakarta, Darwin, Adelaide, Apia, Honolulu, and Santiago. The seasonal SO1 is available for four different seasons, i.e. FMA (February-March-April), MJJ (May-June-July), AS0 (August-September- October) and NDJ (November-December-January).

Upper wind data

Upper air data for Colombo were obtained from the manuscripts and published reports of the Department of Meteorology, Colombo. Data at ten levels (850, 700, 600, 500, 400, 300, 200, 150, 100, and 70 mbar) were analysed for the period from 1961 to 1980. Wind data for Singapore and Darwin were taken from Monthly Climatic Data for the World.

Methods

To study the monthly and seasonal relationships between the SO1 and rainfall, mean monthly rainfall and seasonal rainfall for the rainfall regions and for Sri Lanka as a whole have been calculated. Normalized values

SOUTHERN OSCILLATION AND SRI LANKAN RAINFALL 603

Figure 1 . Locations of stations and different rainfall regions of Sri Lanka. A, B, C, D, and E represent the different rainfall regions explained in the text. After Suppiah and Yoshino (1984)

of rainfall were used in all calculations. Relationships between the SO1 and the rainfall of Sri Lanka were analysed by cross-correlation and lag-correlation methods.

SEASONAL RAINFALL DISTRIBUTION

Most Sri Lankan rainfall is received from weather phenomena associated with the Inter Tropical Convergence Zone (ITCZ) and from two monsoons, the southwest and the northeast. Therefore, the rainfall exhibits a marked seasonality in its spatial and temporal aspects. In particular, the rainfall of March and April is produced by the northward migration of the ITCZ and the rainfall of October and November is associated with the southward migration of the ITCZ. These two transitional seasons are known as the intermonsoon seasons. The months of March and April comprise the first intermonsoon season and the months of October and November the second intermonsoon season. May to September comprises the southwest monsoon season and December to February the northeast monsoon season. Together with the influences of the ITCZ

604 R. SUPPIAH

A l t i t u d e (m)

Figure 2. General topography of Sri Lanka

and the monsoons, topography (see Figure 2) also plays an important role in influencing the spatial pattern of rainfall. Therefore, the rainfall pattern of Sri Lanka has multiple origins. It is strongly influenced by global and synoptic circulations and also, from time to time, by meso-scale weather systems. In Sri Lanka, clear peaks in rainfall are observed in March-April and October-November, except for the eastern part. Even in the south- west monsoon months, the influence of the ITCZ is felt because its location is closely related to the summer monsoon trough over South Asia and the influence is well pronounced in the peak monsoon months, July and August (Krishnamurti, 1971). Moreover the seasonal rainfall pattern is mixed with the southwest monsoon rainfall in the western part and with the northeast monsoon rainfall in the eastern part of the island.

Detailed descriptions of the seasonal distribution of rainfall in Sri Lanka are given by Thambyahpillay (1954) and Domroes (1974). Therefore, only a brief description of the subject is given here, because this study mainly deals with the seasonal relationships. During the first intermonsoon season the southwest quadrant of Sri Lanka experiences a great amount of rainfall (more than 300 mm), while the remaining parts receive less, especially the north of the country (Figure 3a). In this season, rainfall is frequently associated with thunderstorms, depending on local thermal conditions due to the influence of the ITCZ.

SOUTHERN OSCILLATION AND S R I LANKAN RAINFALL 605

DECEMBER o : s w T O FEBRUARY

I

Figure 3. Seasonal rainfall (mm) distribution of Sri Lanka

Figure 3(b) shows the distribution of rainfall in the southwest monsoon season. It reveals a clear contrast in its spatial pattern which is strongly influenced by topography. The amount of rainfall received by places during the 5-month period ranges from under 100 mm to over 2000 mm. The region of maximum rainfall greater than 1000 mm covers the southwest quadrant of the island. A small rainy area is also found along the western slopes of the eastern hills. The Uva Basin and the southeast and northwest coastal belts of the island experience much less rainfall.

During the intermonsoon season the entire island receives a great amount of rainfall, with nearly everywhere experiencing more than 400 mm (Figure 3c). However, the southwest quadrant experiences most rainfall. The northwest and southeast parts of the island also receive considerable amounts of rainfall during this season.

There are two distinct maximum rainfall regions in the northeast monsoon season, as shown in Figure 3(d). One is located on the eastern slopes of the central highlands and the other is along the eastern coast from Batticaloa to Trincomalee. Two regions of minimum rainfall are also found, one on the west coast, the other

606 R. SUPPIAH

on the south coast. The spatial pattern of rainfall during this season does not reveal as much contrast from one area to another as does the pattern during the southwest monsoon.

SEASONAL RELATIONSHIPS BETWEEN RAINFALL AND THE SO1

First intermonsoon season

Weak positive correlation coefficients (CCs) appear between the rainfall of this season and the seasonal SOX when Sri Lanka is considered as a single unit. They vary considerably among the different rainfall regions, as shown in Table I. The rainfall of region A, where more rainfall is received from strong convective systems during this season, is positively correlated with the subsequent SO1 of A S 0 and NDJ. In particular, the March rainfall of this region reveals strong correlations with the SO1 of AS0 and NDJ, indicating r =0.26 and r = 0.22, significant at the 1 per cent and 5 per cent levels, respectively. Not only are the relationships weak for the other regions, but they are statistically insignificant.

Southwest monsoon seuson

The relationship between the SO1 and the rainfall of this season is also positive. During this season, CCs become stronger when the selected periods become shorter, i.e. May-September, r =Od6 ( n = loo), June -September, r = 0.26, July-September, r = 0.29 and July and August, r = 043. The pattern of relationships varies among the different rainfall regions of the island. As shown in Table 11, CCs are stronger for regions D

Table I. Correlation coefficients between the rainfall of the first intermonsoon season (March and April) and the seasonal SOI. First

letters of the months are given for the SO1 seasons at the top

Region FMA MJJ A S 0 NDJ

A 011 0.18 0.24 0.25 B 006 0.06 0.07 016 C 0.07 015 0.16 0.17 D 014 0.07 0.12 010 E 010 0.14 0.08 0.08

Sri Lanka 011 0.15 0.16 0.19

Significance levels for 1 per cent and 5 per cent are 0.25 and 0.20, respectively (n=IW).

Table 11. Correlation coefficients between the seasonal SO1 and the July and August rainfall of Sri Lanka. First letters of the months are

given for the SO1 seasons at the top

Region FMA MJJ A S 0 NDJ

A -0.12 0.22 0.21 0.14 B 000 0.30 0.33 0 1 5 C 0.0 1 0.28 0.35 0.16 D 0.25 0.46 0.55 039 E 0.13 0.39 0.44 0.33

Sri Lanka -0.01 0.37 0.43 0.25 ~ ~~~ ~ ~

Significance levels for 1 per cent and 5 per cent are 0.25 and 0.20, respectively (n = 100).

Tab

le 1

11.

Ran

ked

rain

fall

depa

rtur

es in

Jul

y an

d A

ugus

t dur

ing

the

sout

h-w

est m

onso

on se

ason

of

Sri L

anka

: E de

note

s th

e El

Niii

o ev

ents

as

take

n fr

om

Ras

mus

son

and

Car

pent

er (1983), L d

enot

es t

he L

a N

iiia

even

ts a

s ta

ken

from

Wri

ght (1975) a

nd P

arke

r (1983) an

d SD

is g

iven

for

norm

aliz

ed r

ainf

all

.-

Ran

k SD

Y

ear

Ran

k SD

Y

ear

Ran

k SD

Y

ear

Ran

k SD

Y

ear

Ran

k SD

1 2 3 4 5 6 7 8 9 10

11

12

13

14

15

16

17

18

19

20

-1.81

1943

-1.60

1888

-1.37

1929

-1.23

1952

-1.23

El899

-1.20

El911

-1.17

1940

-1.16

1908

-1.09

El905

-1.08

1920

-1.05

El930

-1.05

1890

-1.02

El918

-1.01

El914

-0.96

1894

-096

El925

-0.95

1979

-0.93

1980

-0.93

El972

-0.93

1922

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

-0.92

1977

-0.84

1945

-0-82

1978

-0.82

1913

-0.79

L1956

-0.79

1901

-0.78

1966

-0.76

1944

-0.76

El976

-0.76

El891

-0.75

1927

-0.74

El951

-0.68

1936

-0.68

El957

-0.67

El896

-0.65

1943

-0.62

1898

-0.61

1958

-0.6

0 1948

-0.5

8 1895

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

- 0.54

- 0.52

- 0.

5 1

- 0.47

- 0.45

-0.33

- 028

-0.21

- 0.2 1

-0.16

-0.11

- 0.07

- 0.03

- 0-02

- 0.02

0.0 1

0.07

0.07

0.10

0.10

1912

1935

El884

1885

L1955

1973

L1893

E 1887

E 1969

L1968

1919

L1950

El941

1904

E 1902

1917

1970

1946

1967

1881

61

0.1 1

62

011

63

0.14

64

0.15

65

022

66

0.25

67

0.30

68

0.31

69

0.34

I0

0.42

71

0.45

72

047

73

0.55

74

062

75

069

76

0.78

77

0.78

78

080

79

0.86

80

092

E 1939

81

1954

82

L1938

83

L1962

84

1892

85

L1889

86

L1975

87

1942

88

1907

89

L1914

90

1926

91

1959

92

1937

93

L1971

94

L1903

95

L1921

96

El965

97

L1961

98

1910

99

1906

100

0.96

0.97

1 a4

1.13

1.13

1.21

1.3 1

1.36

1.36

1.40

1.51

1.57

1.57

1.68

2.02

2.03

2.18

2.25

2.50

2.52

Yea

r 5 5

1963

2: 0

1960

Ln fi

1928

r

El932

1915

4 s

L 1886

z L1924

1931

0 E

1883

L1933

r

El923

1900

1947

P

El953

L1916

L 1909

L1882

*1

L1897

?

P

P

Z v,

L1964

% E 3 XI

P r

1949

r

608 R. SUPPIAH

and E, where the southwest monsoonal winds usually blow as a warm fohn-like wind and thus cause high air temperature, and low rainfall and relative humidity. Regions D and E receive a great amount of convective rainfall during break-monsoon conditions. Regions A, B and C show weaker CCs than Regions D and E, though the values are still significant at the 1 per cent level. It is clearly seen from Table I11 that most of the El Niiio years had below-normal rainfall and most La Niiia years had above-normal rainfall.

However, CCs drop to 0.37 and -0.01 when the SO1 leads rainfall by one and two seasons, respectively. A similar pattern of relationship was reported between the Indian summer monsoon rainfall and the seasonal SO1 (Pant and Parthasarathy, 1981; Rasmusson and Carpenter, 1983; Parthasarathy and Pant, 1984). These findings suggest that the possibility of forecasting the mid-season southwest monsoon rainfall in Sri Lanka from the SO1 alone has a limited potential.

Second intermonsoon season

Unlike the relationships in the other seasons, the rainfall of the second intermonsoon season and the seasonal SOIs show a strong negative relationship, as shown in Table IV. The second intermonsoon season is related to the SO1 with from - 3 to 0 seasons' lag. The relationship is strong with the periods of MJJ and ASO. Ranked rainfall anomalies in Table V also indicate that the above- (below-) normal rainfall values are related to the El Niiio (La Niiia) years.

Since the SO1 provided by Wright (1 975) is not easily extendable, correlation coefficients between the SO1 of Tahiti minus Darwin pressure and the rainfall of the second intermonsoon months were calculated, because the pressure difference between these stations can be updated easily. Figure 4 illustrates the relationships between the monthly SO1 and the rainfall of October and November, which coincides with the beginning of the main crop season in Sri Lanka. Since CCs are stronger for May-June-July (MJJ) and the monthly pressure values show much noise (Wright, 1985), the mean value of the SO1 of MJJ is suggested as a predictor for rainfall anomalies of the second intermonsoon season. The relationship between the rainfall of the second intermonsoon season and the SO1 of MJJ is given in Figure 5. This figure reveals that the above- (below-) normal rainfall at the beginning of the main crop season over Sri Lanka could be predicted one season in advance using the SOI. Here the rainfall figure used in the calculation is the mean value for Sri Lanka, because the rainfall of this season reveals weak spatial variability over the island.

Figure 4 also suggests that the relationship between the monthly SO1 and October rainfall first appears in the month of March when signals in SST anomalies first appear off the South American coast (Rasmusson and Carpenter, 1982) and gradually build-up with minor fluctuations toward the concurrent month, which coincides with the mature stage of E N S 0 events. The gradual strengthening of the relationship between the rainfall of the second intermonsoon season and the SST anomalies of the Pacific and Indian Oceans has been demonstrated by Suppiah (1988). This pattern is closely related to the SO. However, Chiu and Newel1 (1983) reported that the equatorial Pacific SST changes lead those of the Indian Ocean by 2 4 months.

Table IV. Correlation coefficients between the rainfall of the second intermonsoon season (October and November) and the seasonal SOL First letters of the months are given for the SO1 seasons at the top

Region FMA MJJ A S 0 NDJ

A B C D E

-0.16 -041 - 0.43 - 0.32 - 0.2 1 - 0.48 - 0.49 -0.35 - 0.30 - 0.57 - 0.60 - 0.42 - 0.22 - 0.49 - 0.50 -0.35 - 0.20 - 0.45 - 0.54 - 0.40

Sri Lanka - 0.24 - 0.54 -0.59 - 0.43

Significance levels for 1 per cent and 5 per cent are 0.25 and 0.20, respectively (n=lOo).

Tabl

e V.

Ran

ked

rain

fall

depa

rtur

es in

the

seco

nd in

term

onso

on se

ason

of S

ri L

anka

: (E)

den

otes

the

El N

iiio

even

ts as

take

n fr

om R

asm

usso

n an

d C

arpe

nter

(1

983)

, L d

enot

es th

e La

Niii

a ev

ents

as

take

n fr

om W

righ

t (1

975)

and

Park

er (

1983

) and

SD

is g

iven

for

nor

mal

ized

rai

nfal

l

Ran

k SD

Y

ear

Ran

k SD

Y

ear

Ran

k SD

Y

ear

Ran

k SD

Y

ear

Ran

k SD

3 4 5 6 7 8 9 10

11

12

13

14

15

16

17

18

19

20

- 2.

92

-2.6

1 - 1.

87

- 1.

64

- 1.

50

- 1.

46

- 1.

46

- 1.

41

- 1.

33

- 1.

26

- 1.

20

- 1.

14

-1.1

1 - 1.

10

- 1.

09

- 1.

06

- 1.

01

- 0.

92

- 0.

9 1

- 0.

89

L197

4 L1

938

L189

7 L1

971

1908

19

47

L191

6 L1

889

1917

19

42

L195

0 L

1924

L1

964

L190

3 19

27

L188

6 19

04

1952

19

49

L195

5

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

-0.8

7 -0

.86

-0.8

2 -0

.77

- 0.

70

- 0.

70

-059

-0

55

- 0.

48

- 0.

47

- 0.

45

- 0.

44

- 0.

44

- 0.

43

- 0.

42

- 0.

40

- 0.

34

- 0.

29

- 02

9 - 0.

25

1936

L1

909

1929

18

90

L196

8 19

19

1954

19

01

1915

19

73

1948

L1

975

1926

19

80

1892

L1

921

1937

19

58

1931

L1

962

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

-0.2

1 - 02

0 - 0.

20

-0.1

7 -0

.17

-0.1

6 -0

.1 1

- 0.

08

- 0.

07

- 0.

04

- 0.

02

- 0.

02

0.02

00

5 0.

06

0.09

01

0 01

1 0.

11

0.15

1960

L1

956

El95

1 El

918

El88

7 El

969

L193

3 19

59

1894

L1

893

1910

19

12

El97

6 E

1905

18

81

El95

3 19

66

1970

L1

882

1888

61

017

62

0.20

63

0.

23

64

0.31

65

0.

41

66

0.51

67

0.52

68

05

2 69

0.

62

70

0.63

71

0.

66

72

0.66

73

07

0 74

0.

72

75

0.77

76

Q

77

77

0.81

78

08

4 79

0.

86

80

088

1943

El

899

El92

3 19

34

L196

1 18

95

El93

9 19

00

1978

19

35

El9

11

El88

4 19

07

1883

19

46

1898

19

79

El93

2 19

13

El91

4

81

82

83

84

85

86

87

88

89

90

91

92

93

94

95

96

97

98

99

100

0.88

0.88

0.

92

1.01

1.

03

1.05

1.

06

1.18

1 -

20

1.30

1.

37

1.38

1.

41

1.43

1.

50

164

1.76

1.

80

1.99

2.

60

~

Yea

r Is s

1977

5

El96

5 z 2

1944

2

El89

6 r

1920

r 8

El92

5 18

85

5 19

45

1940

v)

1928

E

1922

2

U E

1967

2

El97

2 E

l930

1963

P

1906

r

El9

41

r

El8

91

El90

2

El95

7 L 5 >

610 R. SUPPIAH

01

0.0 Z Lu 2 -0.1 L L UJ

V O -0.2

!?

5

Z 0 -0.3 - 2 UJ -0.4 a

V -0.5

- 0.6

\ \ \ \

1 1 1 1 1 1 1 1 1 1 1

J F M A M J J A S O N D

Sol- MONTH

Figure 4. Relationship between monthly SO1 (Tahiti-Darwin) and October and November rainfall anomalies of Sri Lanka

3 . 0 ~ 1 1 1 1 1 i l l I 1 1 1 1 I 1 1 1 1 1 I l I

- 1.5 -0.3 0.9 2.1 3.3

SOI ( MAY- JULY) Figure 5. Relationship between second intermonsoon season rainfall anomalies and the SO1 of MJJ showing a negative correlation coefficient ( r = -0.61). Regression equation for the line is y= - 0 . 5 6 4 ~ +0.636. Here y and x denote the rainfall anomalies and the SO1 for

MJJ. Mean rainfall of the season is 572 mm and the standard deviation is 127 mm

61 1 SOUTHERN OSCILLATION AND SRI LANKAN RAINFALL

Northeast monsoon season

Northeast monsoon rainfall is negatively correlated with the seasonal SOL The negative CC (r = - 0.22) between the northeast monsoon rainfall and the SO1 of AS0 is significant at the 5 per cent level, as shown in Table VI. Moreover, CCs are weak between the seasonal SO1 and the rainfall of this season when Sri Lanka is considered as a single unit, but they vary considerably among the different rainfall regions. In particular, CCs are weaker in regions D and E, where the northeast monsoon usually gives a great amount of rainfall due to orographic triggering, and stronger in regions A and B where the northeast monsoon gives little or no rain. Ranked rainfall departures for the northeast monsoon season in Table VII generally suggest that above- (below-) normal rainfall anomalies are associated with the weak (strong) SO1 though some El Niiio (La Niiia) years show below- (above-) normal rainfall. Above-normal rainfall anomalies in this season are also caused by tropical depressions and cyclones that originate in the southern part of the Bay of Bengal and strike Sri Lanka, particularly in December.

UPPER WINDS OVER SRI LANKA AND THE ENS0 PHENOMENON

For mean conditions, as illustrated in Figure 6, easterlies predominate at the lower and upper levels during the first intermonsoon season. In the southwest monsoon season, strong upper level easterlies and strong low- level westerlies are observed. During the second intermonsoon season, moderate westerlies are found at low- levels and easterlies at upper levels. In the northeast monsoon season, weak easterlies are observed at both upper and lower levels. The patterns of wind components described above vary considerably between the La Niaa and El Niiio years. Though variations in components are observed throughout the year, they are particularly striking at the beginning of the intermonsoon seasons in March and in October. Zonal wind components for March in Figure 7(a) reveal weak (strong) easterlies at the lower level and easterlies (westerlies) at the upper level in March. This pattern is strengthened in October, as shown in Figure 7(b).

Figures 8 and 9 show the composite patterns of wind components at Colombo for La Niiia and El Niiio years. Here the La Niiia years include 1964, 1968, 1971,1974, and 1975, and the El Niiio years include 1965, 1969,1972, and 1976. During the early stage of an El Niiio event, the ITCZ is usually weak and lies far south of its mean position and thus gives little or no rainfall to Sri Lanka. This condition is associated with strong lower easterlies and upper westerlies. By contrast, strong upper level easterlies and weak low-level easterlies were observed for La Niiia years. This situation is associated with an active ITCZ and therefore with more rainfall over Sri Lanka.

In the southwest monsoon season strong (weak) upper level easterlies are observed and strong (weak) low- level westerlies are found for La Niiia (El Niiio) years. Strong (weak) monsoon circulation and above- (below-) normal rainfall occur over India (Shukla, 1987) and Sri Lanka (Suppiah, 1987). Krishnamurti and Kanamitsu (1981) also reported weak upper easterlies and weak low-level westerlies for a drought monsoon condition in 1972.

Table VI. Correlation coefficients between the seasonal SO1 and north-east monsoon season (December to February) rainfall. First

letters of the months are given for the SO1 seasons at the top

Regions FMA MJJ A S 0 NDJ

A -0.22 - 025 - 026 - 020 B -0.26 - 0.2 1 - 0.24 -0.16 C -019 -0.10 -0.17 -0.13 D - 0.2 1 -0.11 -0.18 -018 E -0.10 -016 -0.17 -0.15

Sri Lanka -018 -017 -022 -0-18

Significance levels for 1 percent and 5 per cent are 0.25 and 0.20, respectively (n = loo).

Tabl

e V

II.

Ran

ked

rain

fall

depa

rtur

es in

the

nort

h-ea

st m

onso

on s

easo

n of

Sri

Lank

a: E

den

otes

the

El N

iiio

even

ts ta

ken

from

Ras

mus

son

and

Car

pent

er

(198

3), L

den

otes

the

La N

iiia

even

ts a

s ta

ken

from

Wrig

ht (

1975

) and

Par

ker

(198

3), a

nd S

D is

giv

en f

or n

orm

aliz

ed r

ainf

all

Ran

k SD

Y

ear

Ran

k SD

Y

ear

Ran

k SD

Y

ear

Ran

k SD

Y

ear

Ran

k SD

Y

ear

1 2 3 4 5 6 7 8 9 10

11

12

13

14

15

16

17

18

19

20

-1.7

9 19

80

-1.6

5 E

l905

-1

.64

1915

-1

.57

1979

-1

.57

1944

-1

.51

L195

5 -1

.38

1883

-1

.30

El9

39

-1-2

2 L1

893

-1.2

2 19

79

-1.2

2 E

l972

-1

.19

1892

-1

.15

1934

-1

.03

1928

-1

.03

El9

41

-1.0

1 L1

975

-1.0

1 19

45

-1.0

0 L1

889

-0.8

7 19

07

-0.8

5 19

58

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

- 0.

80

- 0.

79

- 0.

78

- 0.

76

- 0.

75

- 0.

75

- 0.

74

- 0.

70

- 0.

70

- 0.

63

-0.6

1 - 0.

50

- 0.

49

- 0.

48

-0.4

7 - 0.

45

- 0.

44

- 0.

4 1

- 0.

4 1

-0.4

1

L188

6 E

l976

19

06

L196

8 L1

964

1978

L1

938

L 19

09

1894

19

10

El9

30

El9

18

1952

18

88

1890

E

l899

L1

974

1927

19

60

1967

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

-0.4

0 L1

971

-0.3

9 L1

956

-0.3

5 19

47

-033

19

17

-0.3

2 19

73

-0.3

2 19

66

-024

19

70

-0.2

3 19

04

-0.2

1 19

00

-0.2

0 19

01

- 0.

1 1

5188

4 -0

.09

1908

--

0.06

19

20

-0.0

5 19

19

0.05

E

l925

0.

06

L192

1 0.

10

El9

02

0.11

19

26

0.14

19

22

0.16

L1

916

61

0.19

62

0.

22

63

0.26

64

0.28

65

0.

34

66

0.35

67

0.

39

68

0.39

69

0.

42

70

0.53

71

0.

57

72

0.58

73

0.

62

74

0.63

75

0.

68

76

0.71

77

0.

79

78

0.80

79

08

1 80

0.

82

1940

E

1965

L1

961

El9

53

1937

L

1924

19

49

El9

51

1929

E

l914

L1

933

1885

19

31

1913

19

63

1935

L

1903

19

42

1948

L1

950

81

083

82

0.84

83

08

5 84

08

9 85

0.

92

86

0.99

87

0.

99

88

1 .oo

89

1.09

90

1.

32

91

1.36

92

1.

45

93

1.58

94

1.

60

95

1.79

96

1.

94

97

2.10

98

2.

50

99

2.59

10

0 2.

96

El9

11

1898

E

l887

E

l923

L1

962

L 18

97

L188

2

1959

1943

19

46

1936

18

81

El8

96

1954

E

l969

E

l957

19

12

El8

91

P

1895

$ 5

El9

32

X

SOUTHERN OSCILLATION AND SRI LANKAN RAINFALL 613

70

100-

-

t-\,

200- I ’

150- I I ’

\ I L5’

300 -

400

500- -; 600

\ \

1

J F M A M J J A S O N C

Figure 6. Mean zonal wind (m s - ’ ) for Colombo, for the period 1961-1980 westerly winds are stippled

As a result of the continual process of zonal wind changes at the upper and lower levels and in the location and strength of the ITCZ through the first intermonsoon and southwest monsoon seasons, strong (weak) upper easterlies and strong (weak) low-level westerlies are observed for La Niiia (El Niiio) years and thus give below- (above-) normal rainfall in the second intermonsoon season. These findings in wind anomalies are consistent with the results of Gutzler and Harrison (1987). During a La Niiia event the monsoon trough over central India is very active (Krishnamurti, 1971) and is associated with strong Walker and weak Hadley Circulations and linked with the weak subtropical jet stream of the northern hemisphere. At this time, the location of the ITCZ remains north of its normal position resulting in below-normal rainfall over Sri Lanka in this season; but the conditions are reversed during El Niiio years. Similar processes have been reported by Kousky et al. (1984) and Harrison and Gutzler (1986) over the eastern Pacific region, which shows an in-phase relation with Sri Lankan rainfall.

This pattern persists for several months, overlapping the northeast monsoon season. During this season strong (weak) low-level northeasterlies and strong (weak) upper easterlies over Sri Lanka were observed for La Niiia (El Niiio) years. At the same time, the ITCZ and the northeast monsoon are more active during La Niiia years. Northeast monsoon winds usually bring much rainfall to the east coast and to the eastern flanks of the central highlands of Sri Lanka and give little or no rainfall to the southwestern lowlands. During the El Niiio years, the southwestern part of Sri Lanka receives much rainfall from convective systems because the ITCZ lies north of its normal position over Sri Lanka. The opposite processes are found for La Niiia years.

Results of the correlation analysis, wind and rainfall anomalies for Sri Lanka and selected other stations from elsewhere in the tropics suggest different patterns of Walker Circulation. Since the relationships between the rainfall of Sri Lanka and the SO1 are different in summer and winter, different schematic models-are presented to depict them. Figures 10 and 11 present schematic models for the La Niiia and El Niiio years of the northern hemisphere summer and northern hemisphere winter months, respectively. The models show

614

100-

150-

200

300

400-

500

600- 700-

850

R. SUPPIAH

-

-

-

-

mb 70D

L

MARCH

\

I I I /

\ I

/ / /

1 -

4 2 0 - 2 - 4 - 6 - 8

70

100-

-

150-

200 -

300-

400-

500-

600- 700-

850-

OCTOBER

1 1 1 1 I I I I I I I I I I

8 4 0 -4 -8 -12 -16 -18 ZONAL WIND ( m r s ) ZONAL WIND ( m l s )

Figure 7. Zonal winds for March (a) and October (b). 1 , mean condition for 20 years; 2, La Nifia condition; 3, El Nifio condition

that a La Niiia year is associated with above-normal rainfall over Sri Lanka in the summer half of the year and that the reverse is the case in the winter half of the year. They also show that an El Niiio year is associated with below-normal rainfall over Sri Lanka in summer, and again that the reverse circulation pattern is true in winter.

CONCLUSIONS

The different rainfall regions of Sri Lanka exhibit either a winter maximum or winter and summer maxima. That pattern is closely related to the migrations of the ITCZ and to the southwest and northeast monsoons. Therefore, interannual variations in rainfall indicate distinct seasonal relationships with the variations of atmospheric circulations in the lower latitudes. Notably, the rainfall of the summer and winter halves of the year reveal significant positive and negative CCs with the SO phenomenon, which is stronger in the winter months of the northern hemisphere (van Loon and Madden, 1981).

Relationships are not clear in the first intermonsoon and northeast monsoon seasons if Sri Lanka is considered as a single unit. Yet the relationships are clear between the rainfall of the different regions and the seasonal SOI. In particular, the first intermonsoon rainfall of region A is positively correlated with the SO1 of

200-

300-

400-

50

0-

600-

, 7O

Or

850

-

1 I 1 / / I I I I I I I Il

ll

ll

ll

ll

Il

J

J F

MA

MJ

JA

SO

ND

Fi

gure

8.

Com

posi

te z

onal

win

d (m

s-')

con

ditio

n fo

r L

a N

iiia

even

ts.

Wes

terl

y w

inds

are

stip

pled

Fi

gure

9.

Com

posi

te z

onal

win

d (m

s-')

con

ditio

n fo

r El

Niii

o ev

ents

. W

este

rly

win

ds a

re s

tippl

ed

616

15-

';i 10- x -

c - I -

u -

!? w - x -

R. SUPPIAH

-

5 -

- -

0-

15

U

I 0 - 5 I W

60"E 80"

E x

!- U

I 5 t3 W

OmO( 100" 120" 140" 160" 180" 160" A

140" W Srl Lanka Singapore Darwin Canton

Island

Figure 10. Schematic models of the Walker Circulation along the Equator as determined from wind, SST and rainfall anomalies for the La Nifia (a) and El Niiio (b) phases of the northern summer. Warm and cold denote the warm and cold SST anomalies, respectively

Srl Lanka Slngaporo Darwln Canton Ieland

Figure 11. As in Figure 10, but for the northern winter

SOUTHERN OSCILLATION A N D SRI LANKAN RAINFALL 617

AS0 and NDJ, giving a signal in rainfall for the subsequent ENSO events. During the northeast monsoon season, the rainfall of region A reveals a significant negative relationship with a - 1 season lag.

During the southwest monsoon season the relationship is especially strong between the SO1 and the cumulative rainfall of July and August in the concurrent period and one season later. During these months, the rainfall of regions D and E shows a strong relationship with the SO1 because the regions have little rainfall with strong interannual variabilities.

The rainfall of the second intermonsoon season is negatively correlated with the SO1 at least two seasons earlier. This negative relationship first appears in the month of March and gradually develops to be stronger toward the October and November period, though the southwest monsoon masks the relationship between May and September. In particular, the SO1 of MJJ serves as a useful predictor for rainfall anomalies of the second intermonsoon season.

Signals in wind anomalies are observed over Sri Lanka prior to moderate and strong ENSO events. Upper easterlies (westerlies) and weak (strong) low-level easterlies are noticed for La Niiia (El Niiio) years. These wind anomalies persist until the maturity of ENSO events, and thus cause different patterns of the Walker Circulation in the summer and winter halves of the northern hemisphere.

ACKNOWLEDGEMENTS

I am grateful to Professor Masatoshi Yoshino and Dr Tetsuzo Yasunari of the University of Tsukuba for their helpful suggestions and encouragements. Thanks are due to anonymous referees who made useful comments on the manuscript. I would like to thank the Director, Department of Meteorology, Colombo, Sri Lanka for providing rainfall data. Most of the computations were made in the Sclence Information and Processing Center, University of Tsukuba.

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