Precipitation Trends in Sri Lanka Since the 1870S and Relationships to El Nino–Southern˜ Oscillation

Home , Batticaloa, Hambantota, Kurunegala, Nuwara Eliya, Puttalam, Ratnapura

INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 23: 1235–1252 (2003) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/joc.921

PRECIPITATION TRENDS IN SRI LANKA SINCE THE 1870S AND RELATIONSHIPS TO EL NINO–SOUTHERN˜ OSCILLATION

BJORN¨ A. MALMGREN,a,* RANATUNGE HULUGALLA,b,c YOUSAY HAYASHIb and TAKEHIKO MIKAMIc a Department of Earth Sciences–Marine Geology, University of G¨oteborg, Box 460, SE-405 30 G¨oteborg, Sweden b Department of Earth Resources Sciences, National Institute of Agro-Environmental Sciences, Kannondai 3-1-3, Tsukuba-shi, Ibaraki-ken 305-8604, Japan c Department of Geography, Tokyo Metropolitan University, Minami Osawa 1-1, Hachioji-shi, Tokyo 192-0397, Japan Received 3 December 2002 Revised 26 March 2003 Accepted 26 March 2003

ABSTRACT The last 130 years of fluctuations in precipitation associated with the Southwest Monsoon (summer monsoon), Northeast Monsoon (winter monsoon), First Intermonsoon, and Second Intermonsoon seasons have been analysed at 15 climate stations in Sri Lanka. Analyses of trends in the interannual seasonal series indicated statistically significant temporal changes in Southwest-Monsoon-related precipitation at five of the stations, with three stations showing enhanced rainfall (total of 100 mm/month over the entire time interval) and two stations a decrease in rainfall with time (total of 150 mm/month). In addition, one station experienced a decrease of both First and Second Intermonsoon rainfall over time (total of 80 mm/month). The stations showing loss of rainfall are confined to higher elevation areas and those exhibiting enhanced rainfall are located in the lowlands in the southwestern sector of Sri Lanka. None of the stations show any significant change in Northeast Monsoon precipitation through time. Most of the stations are shown to have received significantly greater amounts of Second Intermonsoon precipitation during El Nino˜ years. The Second Intermonsoon rainfall is, on average, 70 mm/month greater during El Nino˜ years. However, a few stations show increased Southwest Monsoon precipitation (May to September) during La Nina˜ years. Two stations along the eastern and northern coastal areas receive, on average, between 14 and 20 mm/month more Southwest Monsoon rainfall during La Nina˜ years than during El Nino˜ years. Copyright  2003 Royal Meteorological Society.

KEY WORDS: Sri Lanka; trend analysis; ENSO relationship; precipitation; monsoon seasons

1. INTRODUCTION

Sri Lanka, formerly known as Ceylon, is a compact island, occupying a landmass of 65 610 km2 in the Indian Ocean off the southern tip of India. The island extends between 5°55 and 9°50 N, and between 79°41 and 81°53 E. Across the Central Highlands of Sri Lanka, with a highest elevation of 2524 m a.s.l. (Pidurutalagala peak), mean annual rainfall can be as high as 5500 mm at some places (e.g. Watawala, Kenilworth) on exposed southwest windward slopes at elevations between 1000 and 1300 m. On the other hand, in some areas in the southeast and northwest of the island, mean annual rainfall reaches amounts of between 800 and 1200 mm. The rainfall climate of Sri Lanka is predominantly governed by the seasonally varying monsoon system and the associated air masses that are part of the planetary wind regime over South Asia. Therefore, the climate of Sri Lanka can be characterized as a tropical monsoon climate. Owing to the annually alternating monsoon systems and their associated winds, two principal monsoon rainfall seasons, separated by two Intermonsoon rainfall seasons, can be identiﬁed in Sri Lanka (Domroes,

* Correspondence to: Bjorn¨ A. Malmgren, Department of Earth Sciences–Marine Geology, University of Goteborg,¨ Box 460, SE-405 30 Goteborg,¨ Sweden.

1974). The two principal monsoon seasons are the Southwest Monsoon (SWM; May through to September) and the Northeast Monsoon (NEM; December through to February). The two Intermonsoon rainfall seasons, the First Intermonsoon (FIM; March through to April) and Second Intermonsoon (SIM; October through to November) occur in association with the respective northward and southward migrations of the intertropical convergence zone (ITCZ) over Sri Lanka. During the Intermonsoon seasons, convectional type rainfall and tropical depressions (mainly during the SIM) originating in the Bay of Bengal are predominant, and heavy rainfall spells within a short period of time are frequent during these seasons. The nature of the seasonal cycle of monsoon rainfall, in association with regional and local topographic inﬂuences, leads to highly variable rainfall characteristics, both spatially and temporally. The Central Highlands, which control the prevailing moisture-laden monsoon winds, act as an important physiographical climatic barrier. Two major climatic zones can be distinguished to the west and east of the Central Highlands, the Wet Zone and Dry Zone respectively (Figure 1). The Wet Zone essentially comprises the southwest parts, which are directly exposed to the SWM winds, whereas the rest of Sri Lanka constitutes the Dry Zone, with less SWM rainfall. Apart from the SWM, the FIM also produces high rainfall in the Wet Zone. In contrast, the SIM, together with the NEM, produces substantial rainfall in the Dry Zone. In the Wet Zone, the greatest geographical and seasonal differences in the magnitude of rainfall can be attributed to the orographically inﬂuenced low-level wind circulation, which generates ‘fohn effect weather conditions’ among regions. Thambyahpillay (1958), Domroes (1971), Yoshino (1982), and Yoshino et al. (1983) have demonstrated that the Wet Zone and Dry Zone rainfall climatologies are related to local winds, and Thambyahpillay (1954) and Domroes (1974) discussed the annual and seasonal rainfall variability. Suppiah and Yoshino (1984a,b), Domroes and Ranatunge (1993a–c), Puvaneswaran and Smithson (1993), and Ranatunge (1994) discussed

10° N

Jaffna SRI LANKA

9° N Mannar Trincomalee

Anuradhapura

8° N Puttalam DRY ZONE

Kurunegala Batticaloa

Kandy Colombo Badulla 7° N Nuwara Eliya Diyatalawa Ratnapura WET ZONE Hambantota

6° N Galle

80° E81° E82° E

Figure 1. Locations of the 15 meteorological stations included in the analyses (further information about the stations is given in Table I)

Copyright  2003 Royal Meteorological Society Int. J. Climatol. 23: 1235–1252 (2003) SRI LANKAN PRECIPITATION TRENDS 1237 the speciﬁc aspects of the spatial variation of rainfall in Sri Lanka, and Suppiah (1987) examined some distinct features of rainfall with regard to its inter-annual variability. Also, Fernando and Chandrapala (1992), Chandrapala (1996), Mikami et al. (1998), and Domroes and Schaefer (2000) noted the occurrence of temporal changes in the annual and seasonal rainfall. The present study focuses on the patterns of temporal and spatial ﬂuctuations in rainfall over Sri Lanka for the period mostly from 1869 or the early 1870s through to 2000. The series used are the longest available in Sri Lanka. The study uses precipitation patterns during the SWM, NEM, FIM, and SIM seasons as the basic units. The main objectives of the study are to investigate the potential existence of temporal trends and periodic oscillatory behaviour in the seasonal precipitation records using rigorous statistical testing procedures. Persistent changes in rainfall over long time intervals in historical precipitation records might result in adverse consequences for natural and man-made resources in Sri Lanka in the future. One further objective is to analyse the potential link between the monsoon seasonal series and El Nino–southern˜ oscillation (ENSO) signals.

2. DATA AND METHODS

The spatial pattern of the 15 meteorological stations used in this study is displayed in Figure 1, and Table I provides details of their specific locations and altitudes. These 15 stations are included in the study as they have the longest climate records available in Sri Lanka. Six stations are located in the Wet Zone, as defined by Domroes (1971), and the remaining nine stations belong to the Dry Zone (Figure 1). Of the 15 precipitation time series, eight encompass the time period from 1869 and four the time period from the early 1870s through to 2000 (Table I). The remaining three stations have slightly shorter annual series: Diyatalawa from 1901 to 2000; Jaffna from 1871 to 1994; and Kurunegala from 1885 to 2000. All of the time series have recorded monthly observations for all years, except for Jaffna, where the March 1992 observation is missing. This observation was replaced by the average of the March rainfall data for the three preceding and three succeeding years (the observation for March 1995 is available in spite of the series running only through to 1994). All series have been checked for data quality by the Meteorology Department of Sri Lanka. Seasonal time-series constituting the SWM, NEM, FIM, and SIM seasons were generated by computing the monthly means for each season. The mean is based on five months (May–September) for the SWM season,

Table I. Latitude, longitude, and altitude of the meteorological stations from Sri Lanka, and time span of the precipitation time series. N is the length of the time series

Latitude (N) Longitude (E) Altitude (m) Time span N (years)

Anuradhapura 08°21 80°23 93 1870–2000 131 Badulla 06°59 81°02 670 1869–2000 132 Batticaloa 07°43 81°42 3 1869–2000 132 Colombo 06°54 79°52 7 1869–2000 132 Diyatalawa 06°49 80°58 1248 1901–2000 100 Galle 06°02 80°13 13 1873–2000 128 Hambantota 06°07 81°07 16 1869–2000 132 Jaffna 09°39 80°01 4 1871–1994 124 Kandy 07°20 80°38 478 1870–2000 131 Kurunegala 07°28 80°22 116 1885–2000 116 Mannar 08°58 79°54 4 1870–2000 131 Nuwara Eliya 06°58 80°46 1895 1869–2000 132 Puttalam 08°02 79°50 2 1869–2000 132 Ratnapura 06°41 80°24 34 1869–2000 132 Trincomalee 08°35 81°15 3 1869–2000 132

Copyright  2003 Royal Meteorological Society Int. J. Climatol. 23: 1235–1252 (2003) 1238 B. A. MALMGREN ET AL. three months (December–March) for the NEM season, and two months for each of the FIM (March–April) and SIM (October–November) seasons. Trends in the seasonal time-series, in the sense of monotonically increasing or decreasing rainfall, were tested using standard linear regression analysis. The existence of randomness in the sequence of the residuals was tested using the Runs test of randomness. A second procedure, the Durbin–Watson test (Ryan, 1997: 46–48), was employed to analyse the independence among residuals. This method is designed to test for correlation between consecutive residuals in the case of cyclical patterns in time series, where the Runs test would not function well. The conformity of the residuals to a normal distribution was tested by means of the nonparametric test, introduced by Royston (1982a,b), which is an extension of the Shapiro–Wilk W test (Shapiro and Wilk, 1965) applied to sample sizes larger than 50. A useful feature of the Royston test is the conversion of the Royston W statistics to an approximate standard normal deviate. The strategy for testing the existence of trends follows standard regression analysis rules. Thus, if the residuals are random, then the slope of the regression line b is allowed to be used as a measure of the magnitude of the temporal change in precipitation. Furthermore, if the residuals are normally distributed, then the confidence interval can be used to assess whether the slope deviates significantly from zero or not; deviation from zero is indicative of a trend. If the residuals are not normally distributed then the Kendall test (Kendall and Ord, 1990) is used instead to test for trend. Von Storch (1999) showed that erratic results of the Kendall test may be obtained in the presence of pronounced serial correlations in time series (lag-1 autocorrelation r1 greater than 0.3). In the precipitation series investigated here r1 is always less than 0.25, which should not impair the results of the Kendall test. For tests of the potential influence of ENSO on the monsoon season precipitation patterns, El Nino,˜ neutral, and La Nina˜ years were defined in the way given in Table II. This definition follows that provided by the Center of Ocean–Atmospheric Prediction Studies (http://www.coaps.fsu.edu/∼legler/jma index1.shtml). The definition of the ENSO events is based on the southern oscillation index (SOI) data derived by the Japan Meteorological Agency (JMA). In this definition, an ENSO year extends between October of the ENSO year and September of the following year. Similar definitions of ENSO events have been provided by, for example, Quinn (1992) and Suppiah (1997). Since the precipitation values in all cases are univariate normal or depart only moderately from normality, and since the t-test procedure is known to be robust to departures from normality, the t-test was employed to test for differences in precipitation among El Nino,˜ La Nina,˜ and neutral events. Student’s t-test was used in cases where the variances of the precipitation values were homogeneous; the alternative Welch approximate t-test was applied when the variances were heterogeneous.

Table II. Deﬁnition of El Nino,˜ neutral, and La Nina˜ years since 1869 following the Center for Ocean–Atmospheric Prediction Studies (http://www.coaps.fsu.edu/∼legler/jma index1.shtml)

El Nino˜ years 1877, 1880, 1888, 1896, 1899, 1902, 1904, 1905, 1911, 1913, 1918, 1925, 1929, 1930, 1940, 1951, 1957, 1963, 1965, 1969, 1972, 1976, 1982, 1986, 1987, 1991, 1997 Neutral years 1870, 1876, 1878, 1879, 1881, 1882, 1883, 1884, 1885, 1887, 1890, 1891, 1894, 1895, 1897, 1898, 1900, 1901, 1907, 1912, 1914, 1915, 1917, 1919, 1920, 1921, 1923, 1926, 1927, 1928, 1931, 1932, 1933, 1934, 1935, 1936, 1937, 1939, 1941, 1943, 1945, 1946, 1947, 1948, 1950, 1952, 1953, 1958, 1959, 1960, 1961, 1962, 1966, 1968, 1974, 1977, 1978, 1979, 1980, 1981, 1983, 1984, 1985, 1989, 1990, 1992, 1993, 1994, 1995, 1996, 1999 La Nina˜ years 1869, 1871, 1872, 1873, 1874, 1875, 1886, 1889, 1892, 1893, 1903, 1906, 1908, 1909, 1910, 1916, 1922, 1924, 1938, 1942, 1944, 1949, 1954, 1955, 1956, 1964, 1967, 1970, 1971, 1973, 1975, 1988, 1998, 2000

Copyright  2003 Royal Meteorological Society Int. J. Climatol. 23: 1235–1252 (2003) SRI LANKAN PRECIPITATION TRENDS 1239

3. RESULTS

3.1. Basic features of the seasonal precipitation Figures 2–5 respectively display the temporal patterns and trends in the SWM, NEM, FIM, and SIM precipitation series (monthly averages) for the 15 stations. For the NEM season, year 1984 is marked by exceptionally high precipitation at Anuradhapura, Jaffna, Mannar, Puttalam and Trincomalee from the Dry Zone and Ratnapura from the Wet Zone (Figure 3). No similar patterns of simultaneous precipitation peaks can be identified for any of the other monsoon seasons. Previously, Grove (1997) and Anderson (1999) have indicated that severe drought in 1877 caused extensive famine and human deaths in India and other parts of Asia and possibly Africa, but no such extreme event can be detected in the records from Sri Lanka (Figures 2–5). Figure 6 (left column) shows the monthly average precipitation for the four monsoon seasons at the 15 stations. All of the stations in the Wet Zone (Figure 1) display average monthly SWM rainfall of more than 125 mm over the time period analysed (Figure 6). The highest mean SWM rainfall of 386 mm/month is recorded at Ratnapura. The lowest mean SWM rainfall in the Dry Zone is 23 mm/month at Mannar and the highest is 85 mm/month at Diyatalawa. The coefficient of variation (V = 100 × mean/standard deviation) represents an index of the relative variability in rainfall amounts. The stations from the Wet Zone show relatively less variability in SWM-related precipitation, as displayed by V of less than about 33% (Figure 6, right column). On the other hand, a number of stations from the northern, northwestern (Jaffna, Mannar, Anuradhapura, and Puttalam), and southern parts (Hambantota) are marked by much wider fluctuations in SWM rainfall (V>50%). This may indicate a greater SWM rainfall variability at these stations from the Dry Zone. Of the 15 stations, Trincomalee and Batticaloa (mean > 262 mm/month) from the eastern coastal region and Ratnapura and Badulla (means of 164 mm/month and 196 mm/month respectively) from the south central parts show the high NEM-related precipitation (Figure 6), whereas the other stations show mean NEM precipitation of less than 137 mm. Inter-annual fluctuations in NEM precipitation are considerably more stable than the corresponding variations in SWM precipitation (Figure 6) as indicated by V values in the range of about 40 and 58%. The highest mean FIM rainfall of >140 mm/month is observed at Kurunegala, Kandy, Colombo, Ratnapura, Galle, Badulla and Diyatalawa; the first five stations are from the Wet Zone, and the latter two from the Central Highlands (Figure 6; Table I). Low amounts of FIM precipitation (mean < 77 mm/month) are found at Mannar, Jaffna, Trincomalee, Batticaloa, and Hambantota (Figure 1). These stations, together with Puttalam, however, also display the highest degree of relative variation in FIM precipitation (V>50%), indicating considerable instability in the FIM rainfall received. A distinguishing feature of the rainfall characteristics at all of the 15 stations is that the highest amounts of rainfall occur during the SIM season, with November being the rainiest month in Sri Lanka (Figure 6). With the exception of Nuwara Eliya from the highest altitude areas in the Central Highlands (Table I), stations in the Wet Zone receive the highest rainfall, with a mean of >280 mm/month. For example, the mean precipitation at Ratnapura is 416 mm/month. In addition, Jaffna and Trincomalee, from the north and northeast respectively, receive considerable SIM rainfall (mean 307 mm/month and 282 mm/month respectively). Hambantota in the south, on the other hand, records the smallest amount of rainfall in the SIM (mean 155 mm/month). The relative variation in SIM rainfall is comparatively stable, and no station exhibits a V>50%.

3.2. Trend analysis of the seasonal precipitation series The results of the analyses for trend in seasonal precipitation are summarized in Table III. The Runs test of randomness indicates that the numbers of runs are greater than would be expected in all series, signifying that the residuals are random, which enables use of either regression analysis or the Kendall test for trend analysis. Three out of 15 stations (Colombo, Hambantota, and Puttalam) show statistically signiﬁcant increases in SWM-related precipitation through time (compare Figure 2, where the regression lines are displayed for

Copyright  2003 Royal Meteorological Society Int. J. Climatol. 23: 1235–1252 (2003) 1240 B. A. MALMGREN ET AL. sing precipitation as a function of Galle Kandy Nuwara Eliya 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 0 0 0 0 0 600 500 400 300 200 100 600 500 400 300 200 100 600 500 400 300 200 100 600 500 400 300 200 100 600 500 400 300 200 100 ght lines are regression lines, expres Year Jaffna Mannar Diyatalawa in Sri Lanka at the 15 stations. Strai 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 0 0 0 0 0 600 500 400 300 200 100 600 500 400 300 200 100 600 500 400 300 200 100 600 500 400 300 200 100 600 500 400 300 200 100 time, for the stations showing signiﬁcant temporal trends for this season (see also Table III) Puttalam Ratnapura Trincomalee Colombo Kurunegala Hambantota Anuradhapura Badulla Batticaloa 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 0 0 0 0 0

600 500 400 300 200 100 600 500 400 300 200 100 600 500 400 300 200 100 600 500 400 300 200 100 600 500 400 300 200 100 Monthly average precipitation (mm) (mm) precipitation average Monthly Figure 2. Fluctuations in SWM-related precipitation (monthly averages)

Copyright  2003 Royal Meteorological Society Int. J. Climatol. 23: 1235–1252 (2003) SRI LANKAN PRECIPITATION TRENDS 1241 1984 end for this season Galle Kandy Batticaloa Trincomalee Nuwara Eliya 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 0 0 0 0 0 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 1984 1984 1984 Jaffna Badulla Mannar Ratnapura Diyatalawa Year in Sri Lanka at the 15 stations. None of the stations is marked by a temporal tr 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 0 0 0 0 0 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 1984 1984 Puttalam Colombo Kurunegala Hambantota Anuradhapura 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 0 0 0 0 0

800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 Monthly average precipitation (mm) precipitation average Monthly Figure 3. Fluctuations in NEM-related precipitation (monthly averages)

Copyright  2003 Royal Meteorological Society Int. J. Climatol. 23: 1235–1252 (2003) 1242 B. A. MALMGREN ET AL. his season, marked by a Galle Kandy Trincomalee Nuwara Eliya Batticaloa 0 0 0 0 0 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 600 500 400 300 200 100 600 500 400 300 200 100 600 500 400 300 200 100 600 500 400 300 200 100 600 500 400 300 200 100 Year Jaffna e interval analysed (see also Table III) Mannar Ratnapura Diyatalawa Sri Lanka at the 15 stations. Only Nuwara Eliya shows a temporal trend for t 0 1860 1880 1900 1920 1940 1960 1980 2000 0 0 0 0 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 600 500 400 300 200 100 600 500 400 300 200 100 600 500 400 300 200 100 600 500 400 300 200 100 600 500 400 300 200 100 decrease in precipitation through the tim Colombo Puttalam Kurunegala Hambantota Anuradhapura Badulla 0 0 0 0 0 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000

600 500 400 300 200 100 600 500 400 300 200 100 600 500 400 300 200 100 600 500 400 300 200 100 600 500 400 300 200 100 Monthly average precipitation (mm) precipitation average Monthly Figure 4. Fluctuations in FIM-related precipitation (monthly averages) in

Copyright  2003 Royal Meteorological Society Int. J. Climatol. 23: 1235–1252 (2003) SRI LANKAN PRECIPITATION TRENDS 1243 tation displaying a temporal Galle Nuwara Eliya 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 0 0 0 0 0 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 Year Jaffna Kandy Mannar recipitation through time (see also Table III) Diyatalawa Sri Lanka at the 15 stations. For this season also Nuwara Eliya is the only s 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 0 0 0 0 0 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 trend, namely in the form of a decrease in p Colombo Puttalam Ratnapura Trincomalee Kurunegala Hambantota Anuradhapura Badulla Batticaloa 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 1860 1880 1900 1920 1940 1960 1980 2000 0 0 0 0 0

800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 800 700 600 500 400 300 200 100 Monthly average precipitation (mm) precipitation average Monthly Figure 5. Fluctuations in SIM-related precipitation (monthly averages) in

Mean 100 V 450 90 400 80 350 70 300 60 250 50 200 V SWM Mean 40 150 30 100 20 50 10 0 0 A Bd Bt C D G H J KaKu M N P R T A Bd Bt C D G H J KaKu M N P R T

100 450 90 400 80 350 70 300 60 250 50 200 V NEM Mean 40 150 30 100 20 50 10 0 0 A Bd Bt C D G H J KaKu M N P R T A Bd Bt C D G H J KaKu M N P R T

100 450 90 400 80 350 70 300 60 250 50 V

FIM 200

Mean 40 150 30 100 20 50 10 0 0 T A Bd Bt C D G H J KaKu M N P R T A Bd Bt C D G H J KaKu M N P R

100 450 90 400 80 350 70 300 60 250 50 V

SIM 200

Mean 40 150 30 100 20 50 10 0 0 T A Bd Bt C D G H J KaKu M N P R T A Bd Bt C D G H J KaKu M N P R

Figure 6. Mean values and coefﬁcients of variation V for the ﬂuctuations in SWM, NEM, FIM, and SIM precipitation over the time intervals studied

Copyright  2003 Royal Meteorological Society Int. J. Climatol. 23: 1235–1252 (2003) SRI LANKAN PRECIPITATION TRENDS 1245

Table III. Test of trends for SWM, NEM, FIM, and SIM precipitation time-series over the time intervals shown in Table I. For explanations of the procedures employed for test for trends, see text. ‘Obs.’ and ‘Exp.’ in the Runs test are respectively the observed and expected numbers of runs in a random sequence of events. Signiﬁcant trends are in bold

Station Runs test Durbin–Watson Normality Regression analysis Obs. (Exp.) test D;4− D test W R b Conf. Kendall interval of b test

SWM Anuradhapura 86 (66) 2.15; 1.85 0.96b −0.09 — −0.06d Badulla 89 (66) 2.10; 1.90 0.97d 0.05 −0.07–0.17 — Batticaloa 91 (66) 2.07; 1.93 0.94c 0.02 — 0.01d Colombo 88 (66) 1.71; 2.29 0.95c 0.50 — 0.24c Diyatalawa 71 (50) 2.29; 1.71 0.95c 0.04 — 0.04d Galle 76 (64) 1.73; 2.27 0.97d 0.16 −0.11–0.42 0.10d Hambantota 86 (66) 1.99; 2.01 0.91c 0.10 — 0.14a Jaffna 86 (62) 2.18; 1.82 0.92c −0.02 — −0.01d Kandy 78 (66) 2.05; 1.95 0.95b −0.42 — −0.22c Kurunegala 73 (58) 1.96; 2.04 0.98d 0.05 −0.18–0.28 — Mannar 95 (66) 2.18; 1.82 0.92c 0.01 — 0.02d Nuwara Eliya 75 (66) 1.80; 2.20 0.98d −0.75 −1.03–−0.48 — Puttalam 86 (66) 1.87; 2.13 0.94c 0.12 — 0.15a Ratnapura 80 (66) 1.88; 2.12 0.95c −0.12 — −0.04d Trincomalee 92 (66) 1.96; 2.04 0.98d −0.07 −0.19–0.06 — NEM Anuradhapura 81 (65) 1.99; 2.01 0.90c −0.07 — −0.07d Badulla 81 (66) 2.00; 2.00 0.90c −0.28 — −0.07d Batticaloa 83 (66) 1.73; 2.27 0.96b 0.12 — 0.03d Colombo 89 (66) 1.95; 2.05 0.96b 0.13 — 0.07d Diyatalawa 59 (50) 1.73; 2.27 0.94c −0.11 — −0.06d Galle 83 (64) 1.87; 2.13 0.91c −0.03 — −0.03d Hambantota 90 (66) 1.95; 2.05 0.96a −0.08 — −0.07d Jaffna 95 (61) 2.45; 1.55 0.93c 0.12 — 0.03d Kandy 87 (65) 1.98; 2.02 0.93c −0.17 — −0.09d Kurunegala 89 (58) 1.93; 2.07 0.91c −0.19 — −0.11d Mannar 89 (65) 2.06; 1.94 0.90c 0.08 — 0.03d Nuwara Eliya 80 (65) 1.95; 2.05 0.95c −0.15 — −0.05d Puttalam 81 (66) 2.06; 1.94 0.93c −0.07 — −0.09d Ratnapura 84 (66) 1.75; 2.25 0.97d 0.13 −0.15–0.40 — Trincomalee 78 (66) 1.80; 2.20 0.94c −0.04 — −0.02d FIM Anuradhapura 92 (66) 1.64; 2.36 0.96b −0.18 — −0.10d Badulla 86 (66) 1.87; 2.13 0.97d −0.22 −0.49–0.06 — Batticaloa 89 (66) 2.10; 1.90 0.92c 0.02 — 0.02d Colombo 86 (66) 1.85; 2.15 0.95c −0.02 — −0.01d Diyatalawa 64 (50) 1.86; 2.14 0.96a −0.07 — −0.02d Galle 77 (64) 1.73; 2.27 0.97d −0.28 −0.61–0.06 — Hambantota 86 (66) 1.79; 2.21 0.96b 0.10 — 0.06d Jaffna 87 (62) 2.03; 1.97 0.85c −0.03 — −0.02d Kandy 91 (66) 2.15; 1.85 0.96b −0.09 — −0.03d Kurunegala 77 (58) 1.92; 2.08 0.98d 0.12 −0.28–0.52 — Mannar 78 (66) 1.77; 2.23 0.97d 0.05 −0.12–0.21 — Nuwara Eliya 87 (66) 1.99; 2.01 0.98d −0.22 −0.42–−0.01 —

(continued overleaf )

Table III. (Continued)

Station Runs test Durbin–Watson Normality Regression analysis Obs. (Exp.) test D;4− D test W R b Conf. Kendall interval of b test

Puttalam 84 (66) 2.13; 1.87 0.90c 0.07 — 0.01d Ratnapura 92 (66) 2.36; 1.64 0.97d 0.08 −0.25–0.41 — Trincomalee 87 (66) 1.88; 2.12 0.91c 0.02 — 0.02d SIM Anuradhapura 90 (66) 2.03; 1.97 0.97d −0.09 −0.46–0.28 — Badulla 84 (66) 2.04; 1.96 0.88c −0.24 — −0.04d Batticaloa 91 (66) 2.11; 1.89 0.95b 0.24 — 0.04d Colombo 92 (66) 2.43; 1.57 0.97d 0.07 −0.87–1.15 — Diyatalawa 69 (50) 1.97; 2.03 0.95a −0.25 — −0.07d Galle 90 (64) 2.26; 1.74 0.96a −0.03 — −0.01d Hambantota 89 (66) 2.17; 1.83 0.96b 0.20 — 0.08d Jaffna 90 (62) 1.84; 2.16 0.96a 0.08 — 0.05d Kandy 97 (66) 2.14; 1.86 0.98d −0.03 −0.36–0.31 — Kurunegala 77 (58) 2.13; 1.87 0.96a −0.11 — −0.01d Mannar 88 (65) 1.71; 2.29 0.96a −0.21 — −0.02d Nuwara Eliya 84 (66) 1.94; 2.06 0.97d −0.37 −0.71–−0.04 — Puttalam 85 (66) 2.12; 1.88 0.98d 0.04 −0.35–0.42 — Ratnapura 88 (66) 2.32; 1.68 0.99d −0.06 −0.52–0.39 — Trincomalee 95 (66) 2.06; 1.94 0.98d −0.14 −0.61–0.33 — a p ≤ 0.05. b p ≤ 0.01. c p<0.001. d p>0.05. the stations marked by a significant trend). The increase ranges from about 0.1 mm/year, measured on a monthly basis at Hambantota and Puttalam, to 0.5 mm/year at Colombo. In contrast, rainfall has decreased pronouncedly since the late 1800s at two of the highest altitude stations from the Wet Zone, i.e. Kandy and Nuwara Eliya. This decrease is of the order of 0.4 mm/year at Kandy and 0.7–0.8 mm/year at Nuwara Eliya. No statistically significant change in NEM-related precipitation was found at any of the stations (Table III; Figure 3), which indicates no loss or gain in rainfall during the NEM season in Sri Lanka for the period under investigation. For the FIM and SIM seasons, station Nuwara Eliya is the only location that exhibits any statistically significant change in rainfall amounts (Table III; Figures 4 and 5). The tests indicate a decrease in precipitation for both seasons at this station: about 0.2 mm/year for the FIM season and about 0.4 mm/year for the SIM season. Hence, at this station, a considerable loss of precipitation (0.2 to 0.7–0.8 mm/year) is recorded for three of the rainy seasons (SWM, FIM, and SIM).

3.3. Monsoon seasonal rainfall and ENSO Table IV summarizes the results of the t-test of the differences in precipitation for the 15 stations between El Nino˜ and La Nina,˜ El Nino˜ and neutral, and La Nina˜ and neutral events as defined in Table II. For the NEM and FIM seasons, no statistically significant differences in rainfall are found between any pair of these ENSO events. This indicates that the rainfall during the NEM and FIM seasons is not influenced by the ENSO phenomenon. Differences in SWM-season precipitation among the El Nino,˜ neutral, and La Nina˜ events can be found at two stations (Table IV). Batticaloa from the eastern part of Sri Lanka, and Jaffna, the most northerly station, show significantly larger precipitation during La Nina˜ years than during El Nino˜ and neutral years.

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Table IV. Analysis of the inﬂuence of ENSO on seasonal precipitation patterns in Sri Lanka: mean monthly SWM, NEM, FIM, and SIM precipitation for El Nino,˜ neutral, and La Nina˜ years, and t-tests of the difference in precipitation between El Nino˜ and La Nina,˜ El Nino˜ and neutral, and La Nina˜ and neutral events. Signiﬁcant differences are marked in bold. Asterisks for the precipitation values indicate that the series was detrended prior to the t-tests because of the existence of a trend in the series (compare Table III)

Station Precipitation (mm) t-test El Nino˜ Neutral La Nina˜ El Nino–La˜ Nina˜ El Nino–Neutral˜ La Nina–Neutral˜

SWM Anuradhapura 48.4 49.9 58.0 1.31 0.26 1.42 Badulla 77.3 77.8 83.8 0.94 0.09 1.08 Batticaloa 38.5 42.7 52.2 2.57a 1.02 2.57a Colombo∗ −7.8 6.7 −7.8 0.00 1.03 1.10 Diyatalawa 85.4 82.6 91.1 0.69 0.41 1.23 Galle 198.6 212.7 215.4 1.05 1.07 0.23 Hambantota∗ 4.95 −1.75 −0.3 0.59 0.79 0.28 Jaffna 28.4 36.1 48.8 3.75c 1.70 2.70b Kandy∗ −10.7 4.1 0.0 0.90 1.44 0.43 Kurunegala 140.8 139.8 136.0 0.41 0.10 0.41 Mannar 20.6 22.1 27.4 1.74 0.45 1.52 Nuwara Eliya∗ −9.8 4.5 −1.7 0.66 1.16 0.53 Puttalam∗ 6.2 −0.5 −3.8 1.57 1.16 0.69 Ratnapura 372.8 382.5 404.5 1.52 0.58 1.29 Trincomalee 64.6 68.1 75.4 1.45 0.55 1.32 NEM Anuradhapura 109.9 124.7 119.0 0.61 0.92 0.38 Badulla 262.8 268.1 247.4 0.54 0.21 0.91 Batticaloa 199.5 192.5 200.8 0.05 0.42 0.35 Colombo 94.9 106.5 91.1 0.32 1.07 1.43 Diyatalawa 137.9 137.3 133.4 0.26 0.04 0.30 Galle 110.2 122.3 116.2 0.44 1.12 0.53 Hambantota 83.7 79.0 95.4 0.93 0.50 1.91 Jaffna 122.9 131.9 114.4 0.57 0.61 1.24 Kandy 133.5 135.2 129.0 0.30 0.12 0.51 Kurunegala 92.7 104.2 107.9 1.04 0.91 0.31 Mannar 93.8 107.5 96.5 0.22 1.24 0.83 Nuwara Eliya 139.7 127.7 132.0 0.45 0.85 0.33 Puttalam 79.6 86.8 77.4 0.21 0.62 1.19 Ratnapura 149.8 162.2 163.1 0.96 1.40 0.47 Trincomalee 190.5 203.8 183.0 0.35 0.62 1.02 FIM Anuradhapura 125.6 122.9 124.9 0.05 0.26 0.15 Badulla 169.3 145.7 151.9 0.94 1.54 0.47 Batticaloa 63.9 63.9 73.8 0.75 0.00 1.02 Colombo 166.8 181.1 192.0 1.10 0.76 0.63 Diyatalawa 138.2 142.3 140.1 0.13 0.40 0.15 Galle 173.8 174.3 183.2 0.48 0.04 0.60 Hambantota 68.3 75.8 84.9 1.61 0.85 1.07 Jaffna 40.5 42.0 42.9 0.28 0.20 0.12 Kandy 144.7 137.1 139.8 0.33 0.67 0.25 Kurunegala 182.6 203.1 203.1 1.08 1.21 0.00 Mannar 54.3 61.4 61.7 0.80 0.90 0.04

(continued overleaf )

Table IV. (Continued)

Station Precipitation (mm) t-test El Nino˜ Neutral La Nina˜ El Nino–La˜ Nina˜ El Nino–Neutral˜ La Nina–Neutral˜

Nuwara Eliya∗ 8.6 −5.7 5.00 0.33 1.32 1.11 Puttalam 98.4 104.2 115.1 1.12 0.43 0.82 Ratnapura 261.9 271.7 286.2 1.21 0.63 0.95 Trincomalee 48.0 50.1 56.0 0.73 0.28 0.70 SIM Anuradhapura 289.6 248.1 214.2 3.81c 2.12a 2.31a Badulla 285.4 245.2 236.8 1.87 1.90 0.44 Batticaloa 301.9 248.8 231.5 3.15b 2.17a 0.98 Colombo 371.6 349.4 276.5 3.36b 0.76 3.47c Diyatalawa 296.9 244.4 223.4 2.71b 2.72b 1.08 Galle 351.5 311.7 298.8 2.04a 1.66 0.57 Hambantota 194.0 153.4 124.9 3.93c 2.46a 2.17a Jaffna 321.3 323.2 260.2 2.26a 0.07 2.35a Kandy 323.6 278.5 242.1 4.17c 3.01b 2.58a Kurunegala 390.0 335.6 278.6 3.97c 2.34a 2.68b Mannar 229.2 229.2 177.4 2.54a 0.00 2.66b Nuwara Eliya∗ 38.8 −7.1 −16.0 2.80b 3.07b 0.59 Puttalam 273.8 237.0 189.8 4.58c 1.92 2.81b Ratnapura 446.1 412.9 397.8 1.76 1.59 0.70 Trincomalee 321.9 281.7 251.8 2.57a 1.76 1.45 a p ≤ 0.05. b p ≤ 0.01. c p<0.001.

By far the greatest differences in precipitation between the ENSO events are found for the SIM season (Table IV). Here, all of the 15 stations under study, except Badulla and Ratnapura, show pronounced differences between one or several of the ENSO events. All of the remaining stations show statistically significant greater precipitation amounts during El Nino˜ than during La Nina˜ years. For example, at some of these stations (Batticaloa, Diyatalawa, and Nuwara Eliya) the El Nino˜ years stand out above both the neutral and La Nina˜ years, as shown by significant differences between El Nino–La˜ Nina˜ and El Nino–neutral˜ years but no significant difference between La Nina˜ and neutral years. Some of the other stations (Colombo, Jaffna, Mannar, and Puttalam) are instead characterized by the neutral years showing precipitation indifferent from the El Nino˜ years. For another group of stations (Galle and Trincomalee), the El Nino˜ and neutral, and La Nina˜ and neutral years cannot be distinguished despite the recorded difference between the El Nino˜ and La Nina˜ years. In contrast, four of the stations (Anuradhapura, Hambantota, Kandy, and Kurunegala) revealed a statistically significant difference in precipitation between all three ENSO phases. At these stations, precipitation during the neutral years is found to be intermediate between those of El Nino˜ and La Nina˜ years.

4. DISCUSSION

Analyses of long-term series of precipitation fluctuations at 15 meteorological stations in Sri Lanka have demonstrated the existence of statistically verified temporal increases in SWM-related precipitation at three stations, (i.e. Colombo, Hambantota, and Puttalam; (z) Figure 1; Table III), and decreases in precipitation during the same season at two stations (Kandy and Nuwara Eliya) since the later part of the 19th century. As stations of the former group are located along the southwestern, western, and southeastern parts of the coastal belt, and the stations in the other group are situated in the Central Highlands, the strong influence of the SWM

Copyright  2003 Royal Meteorological Society Int. J. Climatol. 23: 1235–1252 (2003) SRI LANKAN PRECIPITATION TRENDS 1249 can be identified. Noticeably, precipitation in both groups is under the influence of the SWM in contrast to the FIM, SIM and NEM seasons, indicating a difference in gain and loss of SWM precipitation between the low lands and high mountains. However, although three coastal stations exhibit increasing precipitation and two highland stations exhibit decreasing SWM precipitation, the patterns of decreasing and increasing rainfall are not spatially coherent across distinct coastal and highland regions. The results, therefore, do not indicate any broad precipitation trends in these regions of Sri Lanka. Furthermore, Nuwara Eliya (1895 m a.s.l.) has experienced decreases in both FIM- and SIM-related rainfall. The increases in SWM precipitation over this time period are of the order of 73 mm/month at Colombo, 11 mm/month at Hambantota, and 15 mm/month at Puttalam. Thus, the total average gain in SWM rainfall at these stations is about 100 mm/month. This gain is, however, less than the loss of precipitation experienced at stations such as Kandy, average loss of 53 mm/month, and particularly Nuwara Eliya, average decrease of 100 mm/month, over the time interval of study, yielding a total net loss of more than 50 mm/month in SWM rainfall since the latter part of the 19th century. Likewise, the decreases in FIM and SIM precipitation at Nuwara Eliya correspond to average losses of 30 mm/month and 50 mm/month of rain respectively, and thus the combined loss of rainfall at this station is equivalent to 180 mm/month for the SWM, FIM, and SIM seasons. Relatively few previous studies have been devoted to investigations of longer-term changes in precipitation in Sri Lanka. Ranatunge (1988) noted a general trend toward loss of annual rainfall at some stations in the higher elevation areas, including Nuwara Eliya. Madduma Bandara and Kuruppuarachchi (1988) also observed a similar temporal trend at Nuwara Eliya. Chandrapala (1996) indicated a similar pattern of loss of total rainfall at Nuwara Eliya, where Domroes (1996) also reached a similar conclusion on the basis of analyses of annual rainfall data. Kayane et al. (1998) reported a decreasing pattern of SWM rainfall over time at Nuwara Eliya and an increasing pattern at Colombo, which is consistent with the results of the present study. Furthermore, Domroes and Schaefer (2000) analysed annual precipitation records encompassing two time periods, 1895–1996 and 1960–96, at 13 of the stations considered in this study (all except Jaffna and Diyatalawa) in order to assess the existence of temporal trends in the records. They concluded, using linear regression analysis and the Mann trend test (similar to the Kendall test used here), that Sri Lanka has experienced a considerable decrease of precipitation on an annual basis during this period of time. However, in their study, only three stations, i.e. Anuradhapura, Kandy, and Nuwara Eliya, actually showed statistically significant decreases over the longer 1895–1996 period, whereas only one station, Batticaloa, is marked by any significant loss of precipitation over the shorter 1960–96 period. Thus, the results obtained point to a significant decrease of SWM rainfall at two of the stations, Kandy and Nuwara Eliya, where a loss of annual precipitation has been reported in previous studies, including a recent study by Domroes and Schaefer (2000). In addition, we also found a similar loss of rainfall during the FIM and SIM seasons at Nuwara Eliya. Domroes and Schaefer (2000) estimated the total annual loss of precipitation at 620 mm at Nuwara Eliya over the 1895–1996 period. We arrived at an estimate of 180 mm for the SWM, FIM, and SIM seasons using a longer time period (an additional 34 years) than that encompassed in the analysis by Domroes and Schaefer (2000). For a comparison of the results obtained by Domroes and Schaefer (2000) for the shorter 1960–96 period, we carried out a renewed analysis for trend over the last 40 year period (1961–2000) for the different monsoon seasons. The results revealed that none of the stations has experienced any statistically significant changes in rainfall for any monsoon season over this period. Several previous studies have explored the relationship between ENSO and rainfall over the Indian subcontinent, including Sri Lanka. The studies by Rasmusson and Carpenter (1983), Shukla and Paolino (1983), and Ropelewski and Halpert (1987), dealing with large-scale precipitation patterns, suggested an El Nino-related˜ deficiency in SWM precipitation in northern, central, and peninsular India. Decreased SWM rainfall over the Indian subcontinent, and in turn in Indian rice production, during El Nino˜ events is supported by a comprehensive study by Webster et al. (1998). A number of other studies specifically dealing with Sri Lanka have also demonstrated a clear link between ENSO and seasonal rainfall over Sri Lanka. Suppiah (1989) analysed the relationship between seasonal varieties of the SOI and seasonal rainfall for different rainfall regions of Sri Lanka and the entire island. For Sri Lanka as a whole he observed positive correlations between the July–August precipitation and the

May–July, August–October, and November–January SOI, indicating more summer rainfall when the SOI is high for these seasons. He also noted negative correlations between the SOI and SIM rainfall for all ‘ENSO seasons’ (also for February–April in addition to those mentioned above), pointing to higher amounts of SIM rainfall over Sri Lanka for lower SOI. In addition, he did not detect any clear relationships between the FIM season rainfall and SOI. However, he found an inverse relationship between NEM season rainfall and the August–October SOI. A subsequent study by Suppiah (1997) adopted a different approach to the study of ENSO–Sri Lankan precipitation relationships than that in his previous study (Suppiah 1989). Similar to the present study, Suppiah’s (1997) study was based on seasonal rainfall differences between defined El Nino˜ and La Nina˜ years. Overall, Sri Lankan time series were generated using a weighted average based on a principal component analysis of rainfall fluctuations over the time period from 1881 to 1990 at 29 climate stations. Suppiah’s (1997) study indicated that La Nina˜ events result in above-normal rainfall over Sri Lanka during the SWM season. Similarly, Kane (1998) found that Sri Lanka experienced ‘drought’ during the SWM season in the years marked by a pronounced El Nino˜ (‘unambiguous’ ENSO in his terminology). Our study showed an increase in SWM precipitation during La Nina˜ years only at two stations (Batticaloa and Jaffna) along the eastern and northern coastal areas, suggesting that the whole of Sri Lanka did not respond to these events in the same manner. These stations receive, on average, between 14 (Batticaloa) and 20 mm/month (Jaffna) more SWM rainfall during La Nina˜ years than during El Nino˜ years. Similar to the present investigation, no clear coupling was found between the ENSO events and NEM and FIM season precipitation in Sri Lanka in the study by Suppiah (1997). However, this is in contrast with the studies by Rasmusson and Carpenter (1983), Shukla and Paolino (1983), and Ropelewski and Halpert (1987), which pointed to enhanced El Nino˜ rainfall in extreme southern India and Sri Lanka during the NEM season. As for the SIM season, for which our study demonstrated significantly greater rainfall during El Nino˜ years than during La Nina˜ years at 13 out of the 15 stations, Suppiah (1997) and Kane (1998) also found positive rainfall anomalies for El Nino˜ years and negative anomalies for La Nina˜ years that were in close agreement with the present study. The 13 stations displaying a significant difference with regard to SIM precipitation receive between 52 (Mannar) and 111 mm (Kurunegala) more SIM rainfall per month during El Nino˜ years than during La Nina˜ years. On average, rainfall is 70 mm/month greater during El Nino˜ years. Viewed over all the 13 stations, the total rainfall received during El Nino˜ events exceeds that for the La Nina˜ years by about 900 mm/month, corresponding to more than 1800 mm for the 2-month period comprising the SIM season. A similar comparison of the El Nino˜ and neutral years indicates that an average of about 50 mm/month (total of about 340 mm/month) more SIM rainfall has been received over the seven stations for which a significant discrepancy between these ENSO events was found during El Nino˜ years than during neutral years (Table IV). In view of the emerging 2002–2003 El Nino˜ event, about 70 mm/month more SIM-related rain may thus be anticipated over most of the climate stations during the year 2003 than during an average La Nina˜ year. The expected gain in SIM rainfall in 2003 at the stations where there is a significant difference between El Nino˜ and neutral ENSO years is about 50 mm/month compared with an average neutral year.

5. SUMMARY

We have analysed monsoon seasonal precipitation records, comprising the SWM, NEM, FIM, and SIM, from 15 climate stations in Sri Lanka from the latter part of the 19th century through to 2000 using a number of statistical methods. The objectives of the study were to analyse the time series from individual stations for potential trends, and then to examine seasonal precipitation differences between ENSO events (i.e. El Nino,˜ neutral, and La Nina˜ years). The results of the study can be summarized as follows.

1. Five of the 15 stations under study display statistically signiﬁcant changes in SWM-related precipitation with time. An increasing trend is recorded at three stations (Colombo, Hambantota, and Puttalam),

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resulting in a gain of SWM rainfall of between 11 and 73 mm/month over the period of study. The total increase in SWM rainfall at these stations is nearly 100 mm/month. This increase is, however, counterbalanced by decreasing rainfall amounts at two stations (Kandy and Nuwara Eliya) equalling in total about 150 mm/month since the late 19th century (50–100 mm/month at the individual stations). The trends of increasing and decreasing rainfall are, however, not consistent across wider locales or distinct regions of Sri Lanka, indicating a lack of spatially coherent patterns. No change in NEM precipitation was observed. One station (Nuwara Eliya) showed decreasing trends in both FIM and SIM rainfall equivalent to 30 mm/month and 50 mm/month respectively. 2. The SIM season rainfall at most stations (13 altogether) is, on average, 70 mm/month greater during El Nino˜ than during La Nina˜ years, and nearly 50 mm/month greater during El Nino˜ than during neutral ENSO years. On the other hand, two stations (Batticaloa and Jaffna) have together received nearly 35 mm/month more SWM rain during La Nina˜ than during El Nino˜ years. No similar inﬂuence from ENSO is seen for the NEM and FIM seasons.

ACKNOWLEDGEMENTS The rainfall data used in this study were obtained from the Meteorology Department of Sri Lanka.

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