Precipitation in Nepal between 1987 and 1996

INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 27: 1753–1762 (2007) Published online 22 March 2007 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/joc.1492 Precipitation in Nepal between 1987 and 1996 Kimpei Ichiyanagi,a* Manabu D. Yamanaka,b Yoshitaka Murajic and Bijaya Kumar Vaidyad a Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka-city, Kanagawa 237-0061, Japan b Graduate School of Science and Technology, Kobe University, Nada-ku, Kobe 657-8501, Japan c EnergySharing Co. Ltd., 2-5-23 Toride, Toride 302-0004, Japan d Meteorological Forecasting Division, Department of Hydrology and Meteorology, Tribhuvan International Airport, Kathmandu, Nepal Abstract: Rain gauge station data from 1987 to 1996 were used to investigate spatial and temporal variability in monthly precipitation and annual and seasonal precipitation patterns over Nepal. Maximum annual precipitation increased with altitude for elevations below 2000 m but decreased for elevations of 2000–3500 m. The data revealed a negative relationship between annual precipitation and elevation only in western Nepal. Annual precipitation averaged on a 0.25° grid exceeded 3000 mm/yr in central Nepal but was less than 1000 mm/yr over Nepal’s northwestern mountains. Only winter precipitation over western Nepal was heavier than precipitation over central and eastern Nepal. A time series of standardized precipitation anomalies averaged over Nepal revealed no significant long-term trends. Further, almost no stations exhibited significant long-term trends by Kendall’s rank correlation analysis. A correlation analysis between summer monsoon precipitation and the All Indian Rainfall (AIR) index revealed positive and negative correlations in western and eastern Nepal, respectively. This analysis also revealed a positive correlation, but no negative correlation, between summer monsoon precipitation and the Southern Oscillation Index (SOI) in western and eastern Nepal. Composite differences in temperature, 850-hPa winds, outgoing longwave radiation (OLR), and precipitation rates between low and high AIR phases revealed that moist air from the Arabian Sea supported precipitation over western Nepal, whereas cold dry air from the Tibetan Plateau suppressed precipitation over eastern Nepal. However, composite differences in precipitation between low and high SOI phases revealed no anomalies for Nepal. Copyright  2007 Royal Meteorological Society KEY WORDS Asian summer monsoon; long-term trend; Nepal; orographic effect; precipitation Received 8 June 2005; Revised 8 June 2006; Accepted 4 December 2006 INTRODUCTION Located between the Indian plains and the high Himalayan mountains, Nepal is characterized by steep, complex topography that makes meteorological observations difficult. Numerous studies have investigated characteristics of the Indian monsoon (e.g., Hahn and Shukla, 1976; Parthasarathy et al., 1991; Meehl, 1994; Yang and Lau, 1998; Webster et al., 1998) and its connection to El Niño/Southern Oscillation (ENSO) (e.g., Webster and Yang, 1992; Ailikun and Yasunari, 2001; Wang and Fan, 1999; Lang and Barros, 2004). However, only few studies have examined the spatial and temporal variability of precipitation over Nepal, and most of the rain gauges in Nepal’s mountainous areas are located at valley bottoms (Lang and Barros, 2002). Consequently, precipitation variability over Nepal remains poorly understood. * Correspondence to: Kimpei Ichiyanagi, Institute of Observational Research for Global Change, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka-city, Kanagawa 2370061, Japan. E-mail: [email protected] Copyright  2007 Royal Meteorological Society Few researchers have investigated the local-scale precipitation patterns in central Nepal because of the difficulty of conducting studies in this mountainous area. Barros et al. (2000) examined rainfall in a mountainous area of Nepal, comparing monsoon rainfall measured using rain gauges to Tropical Rainfall Measuring Mission (TRMM) satellite-derived data. They produced a three-dimensional profile of radar-measured rain rates and discussed the interactions between mesoscale convective systems and steep terrain at elevations of 1–2 km. Barros and Lang (2003) compared precipitation data observed during the Monsoon Himalayan Precipitation Experiment to data extracted from a global atmospheric reanalysis data set and found that the reanalysis data severely underestimated precipitation. A few studies have investigated how the monsoon and ENSO affect large-scale precipitation patterns. Shrestha (2000) analyzed precipitation at 78 stations throughout Nepal and compared the temporal variability in precipitation to climatological parameters over Nepal. Their study revealed a significant relationship between precipitation and the Southern Oscillation Index (SOI), i.e., precipitation was lighter in Nepal when the SOI was low. Lang 1754 K. ICHIYANAGI ET AL. and Barros (2002) examined the onset of monsoon in central Nepal in 1999 and 2000 and found that the onset was associated with monsoon depressions in the Bay of Bengal. Kansakar et al. (2004) used data gathered from 222 stations over Nepal to derive climatological patterns of monthly precipitation, classifying regimes by the shape and magnitude of monthly precipitation. They found that precipitation patterns were controlled by the summer monsoon and by orographic effects induced by the mountain ranges. In this study we investigated spatial distributions of annual and seasonal precipitation, orographic effects, and long-term trends to improve understanding of how the Indian summer monsoon and ENSO affect precipitation over Nepal. meridional), and precipitation rate data at 2.5° × 2.5° resolution. In this study, these reanalysis data were used to calculate circulation anomalies and indices. We also computed several indices relevant to precipitation over Nepal and to monsoon circulation. The All Indian Rainfall (AIR) index measures monthly averaged precipitation over India. The Indian Institute of Tropical Meteorology provided homogeneous Indian monthly rainfall data sets, and the Climate Prediction Center provided the monthly averaged SOI, defined as the difference between pressures at sea level at Tahiti and Darwin. Precipitation and meteorological indices for Nepal were defined by deviations from monthly averages over the entire period (1987–1996). All data were normalized using their standard deviation. DATA SETS RESULTS The main data used in this study were amounts of monthly precipitation measured at 274 rain gauge stations throughout Nepal from 1987 to 1996. Precipitation amounts observed at these stations were extracted from data books published by His Majesty’s Government of Nepal, Department of Hydrology and Meteorology (DHM, 1992, 1997, 1999). We used the data for a specific station only if data were available for more than 80% of the entire period. The National Centers for Environmental Prediction/ National Center for Atmospheric Research (NCEP/ NCAR) global atmospheric reanalysis data set (Kalnay et al., 1996) includes surface temperature, outgoing longwave radiation (OLR), 850-hPa winds (zonal and Precipitation in relation to elevation Some previous studies of the Himalayas have considered orographic effects on precipitation (Singh et al., 1995; Singh and Kumar, 1997). Therefore, we examined the variability of precipitation with elevation. Figure 1 presents the locations of rain gauge stations in Nepal. Station elevations range from 70 to 4100 m, and many stations are concentrated in central Nepal. We calculated the average annual precipitation for individual rain gauge stations and also partitioned the station data into 500-m elevation bins. Figure 2 presents the average annual precipitation and the number of stations per specific 500-m elevation increments. More than 35 stations are located at elevations between 1500 and 2000 m, and fewer than Rain–gauge stations in Nepal 30.5N 30N 29.5N 29N 28.5N 28N 27.5N 27N 26.5N 26N 80E 81E 82E 83E 100 84E 1000 85E 3000 86E 87E 88E 89E 5000 Figure 1. Rain gauge stations of Nepal. Crosses and open circles indicate stations located below and above elevations of 3000 m, respectively. Copyright  2007 Royal Meteorological Society Int. J. Climatol. 27: 1753–1762 (2007) DOI: 10.1002/joc 1755 PRECIPITATION IN NEPAL BETWEEN 1987 AND 1996 6000 Prec. (mm/yr) 5000 4000 3000 2000 1000 0 0 500 1000 1500 2000 2500 3000 3500 4000 4500 0 500 1000 1500 2000 2500 3000 3500 4000 4500 num. stations 100 80 60 40 20 0 elevation (m) Figure 2. Annual precipitation and the number of stations in 500-m elevation bins over Nepal. Maximum, mean, and minimum annual precipitations are shown in the upper panel. Precipitation (mm/yr) (a) Western (80–82) (b) Central (83–85) (c) Eastern (86–88) 6000 6000 6000 5000 5000 5000 4000 4000 4000 3000 3000 3000 2000 2000 2000 1000 1000 1000 0 0 1000 2000 3000 4000 elevation (m) 0 0 1000 2000 3000 4000 elevation (m) 0 0 1000 2000 3000 4000 elevation (m) Figure 3. Relationships between annual precipitation and elevation divided into western (80 ° E–83 ° E), central (83 ° E–86 ° E), and eastern (86 ° E–89 ° E) Nepal. 10 stations are located between 2500 and 4000 m. Only one station is above 4000 m. Annual precipitation over 10 years (1987–1996) was averaged for each station. Maximum annual precipitation increased linearly with altitude from 3000 to 5500 mm/yr for elevations below 2000 m, and decreased to 1000 mm/yr for elevations above 2000 m. Mean annual precipitation was almost 2000 mm/yr below 3000 m. Singh and Kumar (1997) summarized the orographic effects found in previous studies. Some studies found maximum precipitation at 2000–2500 m and a subsequent decrease with elevation. Figure 3 presents the relationships between annual precipitation and elevation divided into three regions: western (80 ° E–82 ° E), central (83 ° E–85 ° E), and eastern (86 ° E–88 ° E) Nepal. A negative relationship was found only for western Nepal, where precipitation decreased by 100 mm for 500-m elevation increments. The correlation coefficient was approximately 0.4, indicating statistical significance. In western Nepal, maximum value was below 2500 mm/yr, whereas central and eastern Nepal had maximum values of approximately 5000 mm/yr. In central Nepal, maximum values increased and minimum Copyright  2007 Royal Meteorological Society values decreased below 2000 m. Precipitation ranged from 1000 to 3000 mm/yr for elevations below 3000 m in eastern Nepal. Spatial distribution Station data from 1987 to 1996 were averaged on a grid with a horizontal resolution of 0.25° in both longitude and latitude to examine the spatial distribution of precipitation. Figure 4 presents the spatial distribution of mean annual precipitation. Precipitation was relatively heavy (more than 2000 mm/yr) over central Nepal and eastern Nepal near Bhutan. Kansakar et al. (2004) described zones of heavy precipitation near Pokhara and northeast of the Kathmandu Valley. Annual precipitation was less than 1000 mm/yr in northwestern Nepal, where elevations exceed 3000 m. Most of Nepal had an average annual precipitation of 1000–2000 mm/yr. Figure 5 presents the spatial distribution of seasonal precipitation data composed of monthly precipitation averaged over three months. The four seasons include winter (December to February), pre-monsoon (March to May), monsoon (June to August), and post-monsoon Int. J. Climatol. 27: 1753–1762 (2007) DOI: 10.1002/joc 1756 K. ICHIYANAGI ET AL. Annual Precipitation (mm/year) 30.5N 5000 5000 30N 3000 5000 29.5N 5000 5000 5000 5000 29N 1000 5000 5000 5000 28.5N 5000 3000 1000 28N 500 27.5N 5000 100 3000 27N 1000 100 26.5N 26N 80E 81E 82E 83E 1000 84E 1500 85E 2000 86E 87E 88E 89E 3000 Figure 4. Annual precipitation in Nepal averaged from 1987 to 1996. This figure is available in colour online at www.interscience.wiley.com/ijoc (September to November). Figure 6 presents seasonal wind and precipitation features revealed by NCEP/NCAR reanalysis data. Horizontal and meridional winds and precipitation rates were averaged over the same period (from 1987 and 1996). In winter, total monthly precipitation exceeded 40 mm/month over western Nepal but was less than 20 mm/month over eastern Nepal. Figure 5(a) illustrates clearly that precipitation over western Nepal was heavier than precipitation over central and eastern Nepal only in winter. Figure 6(a) shows that a westerly wind dominated over the area north of latitude 25° N and that precipitation rates exceeded 50 mm/month in northwestern Nepal along the Himalayan range. In contrast, western Nepal had less precipitation than central and eastern Nepal during the pre-monsoon, monsoon, and post-monsoon seasons. During these seasons, a strong southwesterly wind from the Bay of Bengal supplied heavy precipitation to Nepal. Figure 5(b) shows that in the pre-monsoon season, the western region of Nepal had less than 50 mm/month of precipitation, while the central and eastern areas had more than 50 mm/month. As shown in Figure 6(b), a westerly wind dominated over the area of latitude 25° N–30° N, and the precipitation rate was more than 50 mm/month in northwestern Nepal in the pre-monsoon season. The precipitation rate was also greater than 100 mm/month in areas east of Nepal and Bangladesh. During the monsoon season, total precipitation exceeded 200 mm/month in all areas except northwest Nepal, as shown in Figure 5(c). Precipitation exceeded 800 mm/month at only two grids in central and eastern Nepal. As illustrated in Figure 6(c), a strong westerly wind dominated the area south of latitude 20° N during the summer monsoon season, from the Arabian Sea to the Bay of Bengal through the Copyright  2007 Royal Meteorological Society Indian subcontinent. Precipitation rates were more than 300 mm/month over the west coast of India and more than 400 mm/month over western Nepal and the west coast of Myanmar. During the post-monsoon season, precipitation was less than 100 mm/month over western Nepal and more than 100 mm/month over central and eastern Nepal, as shown in Figure 5(d). Even though Nepal has a zone of divergence, the precipitation rate was still over 50 mm/month in southern Nepal, as shown in Figure 6(d). Precipitation over the mountains of northwest Nepal was less than 200 mm/month during the monsoon season (Figure 5(c)) and less than 50 mm/yr during the post-monsoon season (Figure 5(d)). Kansakar et al. (2004) classified long-term mean precipitation data for specific stations by shape and magnitude. Their results indicated that July–August peaks were typical over western Nepal, that central Nepal had heavier precipitation, and that weather systems in the west supplied winter precipitation to the mountains of northwestern Nepal. The annual and seasonal precipitation patterns found in the current study resemble the results of previous studies (e.g., Chalise et al., 2003; Kansakar et al., 2004). However, this is the first study to reveal a local maximum precipitation over western Nepal in winter. DISCUSSION Long-term trends To examine long-term trends of precipitation in Nepal, standardized anomalies in monthly precipitation for all grid data were averaged for 1987–1996. Figure 7 presents the time series of precipitation anomalies Int. J. Climatol. 27: 1753–1762 (2007) DOI: 10.1002/joc PRECIPITATION IN NEPAL BETWEEN 1987 AND 1996 Precipitation (mm/month) 30.5N 30N DJF 29.5N 29N 28.5N 28N 27.5N 27N 26.5N 26N (a) 80E 81E 82E 83E 84E 85E 86E 87E 88E 89E 30.5N 30N MAM 29.5N 29N 28.5N 28N 27.5N 27N 26.5N 26N (b) 80E 81E 82E 83E 84E 85E 86E 87E 88E 89E 30.5N 30N JJA 29.5N 29N 28.5N 28N 27.5N 27N 26.5N 26N (c) 80E 81E 82E 83E 84E 85E 86E 87E 88E 89E 30.5N 30N SON 29.5N 29N 28.5N 28N 27.5N 27N 26.5N 26N 80E 81E 82E 83E 84E 85E 86E 87E 88E 89E (d) 80 60 40 20 200 150 100 50 800 600 400 200 200 150 100 50 Figure 5. Seasonal precipitation in Nepal for (a) December–February, (b) March–May, (c) June–August, and (d) September–November. This figure is available in colour online at www.interscience.wiley. com/ijoc over Nepal but does not reveal any significant long-term trends. Decreases occurred in 1990–1991 and 1993–1994, and increases occurred in 1992 and 1995. Intra-seasonal variation was low in 1991–1994 but high in 1987–1990 and 1995–1996. Shrestha et al. (2000) analyzed all the precipitation records for Nepal for 1948–1994; their results also revealed no significant long-term trends. Standardized anomalies in monthly precipitation at specific stations were also examined to test for longterm trends. Kendall’s rank correlation was used to test for significance in observed trends; Figure 8 presents stations with significant trends. Some stations in western Copyright  2007 Royal Meteorological Society 1757 and central Nepal showed an increase in precipitation; one station in central Nepal was the only one to show a decrease. Correlation coefficients at most stations ranged from −0.2 to 0.2, and long-term trends were not statistically significant. Correlations with climatological indices Many previous studies have related monsoon precipitation to ENSO (e.g., Webster and Yang, 1992; Ailikun and Yasunari, 2001). Kawamura (1998) discussed the indirect effects that both anomalous sea-surface temperature (SST) forcing and land–ocean thermal contrasts associated with ENSO had on the Asian summer monsoon. Shrestha et al. (2000) showed a strong relationship between precipitation over Nepal and the SOI in that less precipitation fell over Nepal during the ENSO warm phases. The relationship between the SOI and precipitation over Nepal was much stronger than the relationship between the SOI and AIR, especially after 1970. We examined the relationship between precipitation over Nepal during the summer monsoon season (from June to September) and climatological indices (the AIR and SOI). Correlation coefficients were calculated to relate a standardized anomaly of monthly precipitation for a specific station to the AIR and SOI. Figure 9 presents stations with significant correlations. Most stations having positive correlation with the AIR were in western Nepal, while a few stations in eastern Nepal were negatively correlated with the AIR, as shown in Figure 9(a). Figure 9(b) shows that some stations in western and eastern Nepal were positively correlated with the SOI, but none appeared to be negatively correlated. A composite analysis was used to elucidate the largescale impact of the AIR and SOI. High-phase years for the AIR were 1988, 1990, and 1994; low-phase years were 1987, 1993, and 1994. High-phase years for the SOI were 1988, 1989, and 1996; low-phase years were 1987, 1991, and 1992. Figures 10 and 11 show the differences in surface temperature, 850-hPa winds, OLR, and precipitation rates between low and high phases for the AIR and SOI, respectively. As shown in Figure 10(a), a negative temperature anomaly was observed over the Indian subcontinent. Figure 10(b) shows that cyclonic circulation over the Arabian Sea and the Indian subcontinent (centered at 25° N and 70 ° E) corresponded to a temperature anomaly. Figure 10(c) indicates a negative OLR anomaly observed over northern India, while Figure 10(d) shows the positive precipitation rate observed over India and western Nepal. The negative precipitation rate observed over eastern Nepal corresponded to the northerly wind anomaly (Figure 10(b)). These results suggest that moist air transported from the Arabian Sea supports precipitation over western Nepal, whereas cold dry air from the Tibetan Plateau suppresses precipitation over eastern Nepal. Therefore, monsoon precipitation over western and eastern Nepal is positively and negatively correlated with the AIR, respectively. Kripalani et al. (1996) also noted that monthly rainfall changes over Kathmandu corresponded to rainfall variations over northern India. Int. J. Climatol. 27: 1753–1762 (2007) DOI: 10.1002/joc 1758 K. ICHIYANAGI ET AL. Prec. & UV850 (DJF) Prec. & UV850 (JJA) 45N 45N 50 40N 40N 100 35N 35N 200 100 50 100 30N 100 30N 50 200 300 300 400 25N 25N 20N 20N 400 200 50 300 300 100 15N 15N 50 10N 100 150 50 100 300 400 10N 200 300 50 200 100 100 5N 50E 55E 60E 65E 70E 75E 80E 85E 90E 95E 100E 105E 110E 5N 50E 55E 60E 65E 70E 75E 80E 85E 90E 95E 100E 105E 110E (a) (c) 10 20 Prec. & UV850 (MAM) Prec. & UV850 (SON) 45N 45N 50 40N 40N 50 35N 100 50 50 100 35N 150 30N 100 100 30N 50 100 150 50 150 25N 150 25N 100 20N 20N 200 200 250 15N 100 15N 250 250 200 200 10N 50 50 10N 300 300 150 50 100 250 5N 50E 55E 60E 65E 70E 75E 80E 85E 90E 95E 100E 105E 110E (b) 5N 50E 55E 60E 65E 70E 75E 80E 85E 90E 95E 100E 105E 110E (d) 8 6 Figure 6. Seasonal wind and precipitation rate in Nepal over the four periods (for periods see Figure 5). Anomaly / STD Nepal Precipitation (1987–1996) 2 1.5 1 0.5 0 −0.5 −1 −1.5 −2 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 Figure 7. Time series of standardized precipitation anomalies over Nepal. In contrast, Figure 11(a) indicates no temperature anomaly. Figure 11(b) shows that anomalies of both northwesterly and southerly winds combined over the Bay of Bengal into a strong westerly wind anomaly over Indochina. Figure 11(c) and (d) present the anomalies of negative OLR and positive precipitation rates observed over the Bay of Bengal and South China Sea. In Nepal, however, no precipitation anomalies were observed. Shrestha et al. (2000) found no good agreement Copyright  2007 Royal Meteorological Society between precipitation fluctuations over Nepal and India. When simultaneous correlations were performed in the present study, the results indicated that correlation coefficients were low throughout Nepal. Ichiyanagi and Yamanaka (2005) found that an anomaly in precipitation for Bangkok in May was a response to the Niño 3 SST anomaly in March. Lag–correlation analysis may yield a better understanding of the relationship between ENSO and the Asian monsoon. Int. J. Climatol. 27: 1753–1762 (2007) DOI: 10.1002/joc 1759 PRECIPITATION IN NEPAL BETWEEN 1987 AND 1996 Long–term trend (kendall's Rank Test) 30.5N 5000 30N 5000 4000 3000 5000 29.5N 5000 5000 5000 5000 29N 1000 5000 4000 5000 5000 28.5N 2000 1000 4000 3000 5000 28N 500 27.5N 5000 4000 3000 2000 2000 27N 1000 26.5N 26N 80E 81E 82E 83E 84E 85E 86E 87E 88E 89E Figure 8. Stations exhibiting increasing and decreasing trends by Kendall’s rank test. Open and closed circles indicate correlation coefficients of more than 0.2 and less than −0.2, respectively. (a) Correl. Monsoon Rainfall (JJAS) and AIR 30.5N 5000 30N 4000 3000 5000 29.5N 29N 5000 5000 5000 1000 5000 28.5N 5000 5000 4000 2000 3000 1000 28N 5000 4000 27.5N 2000 27N 5000 3000 1000 26.5N 26N 80E 81E 82E 83E 84E 85E 86E 87E 88E 89E (b) Correl. Monsoon Rainfall (JJAS) and SOI 30.5N 5000 30N 4000 3000 5000 29.5N 29N 5000 5000 5000 1000 5000 28.5N 4000 2000 3000 1000 28N 5000 5000 5000 4000 27.5N 2000 27N 5000 3000 1000 26.5N 26N 80E 81E 82E 83E 84E 85E 86E 87E 88E 89E Figure 9. Stations that correlated well with (a) All Indian Rainfall (AIR) and (b) Southern Oscillation Index (SOI). Solid (open) circles indicate a positive (negative) correlation. Copyright  2007 Royal Meteorological Society Int. J. Climatol. 27: 1753–1762 (2007) DOI: 10.1002/joc 1760 K. ICHIYANAGI ET AL. (a) Diff. Temp. (AIR) (c) Diff. OLR (AIR) 45N 45N 40N 40N 5 5 35N 35N 0 0 0 30N 0.5 −0.5 1 0 −1 −5 0 30N −10 −15 25N 0.5 0 0 25N −1 −20 20N 20N −5 −15 −5 0 −10 −10 −5 15N 15N −5 −0.5 0 0 10N 0 0 0 10N 0 5N 50E 55E 60E 65E 70E 75E 80E 85E 90E 95E 100E 105E 110E 5N 50E 55E 60E 65E 70E 75E 80E 85E 90E 95E 100E 105E 110E −5 (b) Diff. UV850 (AIR) (d) Diff. Prec. (AIR) 45N 45N 40N 40N 35N 35N 0 0 0 0 0 −30 30N 30N 25N 25N 20N 20N 60 30 0 0 0 30 0 0 30 15N 15N 10N 10N 0 0 5N 50E 55E 60E 65E 70E 75E 80E 85E 90E 95E 100E 105E 110E −30 0 −30 5N 50E 55E 60E 65E 70E 75E 80E 85E 90E 95E 100E 105E 110E 2 Figure 10. Composite differences between low and high All Indian Rainfall (AIR) phases in (a) surface temperature, (b) 850-hPa winds, (c) outgoing longwave radiation (OLR), and (d) precipitation rates during the monsoon season (June–September). CONCLUSIONS We statistically analyzed monthly precipitation data from 274 gauges in Nepal between 1987 and 1996 to determine spatial and temporal variability in precipitation. More than 35 stations were at elevations between 1500 and 2000 m, while fewer than 10 stations were above 2500 m. Maximum annual precipitation increased from 3000 to 5000 mm/yr as elevation increased to 2000 m, and then decreased to 1500 mm/yr at elevations from 2000 to 3500 m. A negative relationship was observed only in western Nepal. Precipitation decreased by 100 mm for specific 500-m elevations. Mean annual precipitation was less than 1000 mm/yr over the mountains in northwestern Nepal and more than 3000 mm/yr in central Nepal. Seasonal precipitation from December to February was greater over western Nepal than eastern Nepal. In contrast, western Nepal was drier from March to November. To investigate long-term trends in Nepal’s precipitation, we averaged the standardized anomalies of monthly precipitation for all grid data from 1987 to 1996 and examined the averages by applying Kendall’s rank correlation to specific stations. These analyses did not reveal any general trends, although some stations in central and Copyright  2007 Royal Meteorological Society eastern Nepal exhibited an increasing trend. One station in central Nepal showed a decreasing trend. We also investigated the relationships between precipitation during the summer monsoon and climatological indices for specific months and for the entire observational period. Precipitation in western Nepal was positively correlated with the AIR, while precipitation in eastern Nepal was negatively correlated. Monsoon precipitation in both western and eastern Nepal was positively correlated with the SOI. The NCEP/NCAR reanalysis data were used to analyze composite differences in temperature, 850-hPa winds, OLR, and precipitation rates between low and high phases of the AIR. The results indicated that moist air transported from the Arabian Sea supports precipitation over western Nepal, whereas cold dry air from the Tibetan Plateau suppresses precipitation over eastern Nepal. With regard to the SOI, anomalies of strong westerly winds, negative OLR, and positive precipitation rates were observed over the Bay of Bengal. However, no precipitation anomalies were observed over Nepal. Lag–correlation analysis is needed for a more comprehensive investigation of how precipitation variability in Nepal is related to ENSO. Int. J. Climatol. 27: 1753–1762 (2007) DOI: 10.1002/joc 1761 PRECIPITATION IN NEPAL BETWEEN 1987 AND 1996 (a) Diff. Temp. (SOI) (c) Diff. OLR (SOI) 35N 35N −0.5 0 30N 0 30N 25N −5 0 0 0 0.5 0 5 25N −5 −10 5 20N 20N −10 0 15N 15N −5 0 −5 −10 10N 0 0 10N 0 5 −5 10 5N 0 0 5N 5 10 EQ EQ 15 5 5 20 0 5S 60E 65E 70E 75E 80E 85E 90E 95E 100E 105E 110E 115E 120E 10 5S 60E 65E 70E 75E 80E 85E 90E 95E 100E 105E 110E 115E 120E −5 (b) Diff. UV850 (SOI) (d) Diff. Prec. (SOI) 35N 35N 0 0 30N 30N 0 0 25N 25N 20N 20N 0 0 0 −30 30 30 60 90 120 −30 30 60 60 60 15N 15N 10N 10N −60 0 30 60 30 −30 30 0 −30 30 0 −30 −30 5N 0 5N −30 −30 EQ EQ −30 0 5S 60E 65E 70E 75E 80E 85E 90E 95E 100E 105E 110E 115E 120E 0 30 −60 0 0 −30 5S 60E 65E 70E 75E 80E 85E 90E 95E 100E 105E 110E 115E 120E 3 Figure 11. Composite difference between low and high Southern Oscillation Index (SOI) phases in (a) surface temperature, (b) 850-hPa winds, (c) outgoing longwave radiation (OLR), and (d) precipitation rates during the monsoon season (June–September). ACKNOWLEDGEMENTS The authors thank the staff of Nepal’s Department of Hydrology and Meteorology (DHM) for their cooperation with the research and for providing the data books published by DHM. The Indian monthly rainfall data set and the Global land 1-km base elevation data set were provided by the Indian Institute of Tropical Meteorology and Climate Prediction Center and the National Geophysical Data Center, respectively. This research was supported by the Institute of Observational Research for Global Change (IORGC) in FY 2001-2005. REFERENCES Ailikun B, Yasunari T. 2001. ENSO and Asian summer monsoon: persistence and transitivity in the seasonal march. Journal of the Meteorological Society of Japan 79(1): 145–159. Barros AP, Lang TJ. 2003. 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