Precipitation Trends in Sri Lanka Since the 1870S and Relationships to El Nino–Southern˜ Oscillation
Total Page：16
File Type：pdf, Size：1020Kb
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. Copyright 2003 Royal Meteorological Society 1236 B. A. MALMGREN ET AL. 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.