Recent Trends in the Onset and Withdrawal of Summer Monsoon over Nepal

Authors
D. Gautam 1
S. Regmi 2
1 Regional Integrated Multi-Hazard Early Warning System for Africa and Asia, Asian Institute of Technology campus, Klong Luang, Pathumthani 12120, Thailand.
2 Department of Hydrology and Meteorology, Tribhuvan International Airport, Kathmandu, Nepal.
Abstract
This paper presents the analysis of the onset and withdrawal dates and the duration of summer monsoon over Nepal for last 63 years (1951-2013). The trend analysis revealed that both the onset and withdrawal of summer monsoon are delayed in recent years. The statistical analysis using cumulative deviation and Worsley likelihood tests showed a step jump in mean withdrawal date after the year 1997. The mean withdrawal date after 1997 is later than the normal withdrawal date by 10 days. Although, the trend for onset is statistically insignificant, the trend for withdrawal is significant at 5% level. Stationarity tests for the period of 1951-1996 and 1997-2013 also revealed that both the onset and withdrawal series are non-stationary. A step change in withdrawal date with delayed trend on both onset and withdrawal coupled with longer duration suggests a temporal shift in the monsoon pattern over Nepal after 1997. The temporal shift in the monsoon is consistent with published findings on the effect of global climate change on the dynamics of the South Asian summer monsoon precipitation. The shifting pattern of the summer monsoon exerts a strong stress on agriculture, water resources, ecosystems, human health and the overall economy of the country.
Keywords
Nepal
Monsoon
Onset
Trend
Withdrawal
Ananthakrishnan, R., Acharya, U.R. and Ramakrishnan, A.R. On the criteria for declaring the onset of the south-west monsoon over Kerala; Forecasting Manual. FMU Report No. IV-18.1: 52, 1967, India Meteorological Department, Pune, India.
Ananthakrishnan, R. and Rajagopalachari, C.J. Patterns of Monsoon Rainfall Distribution over India and Neighbourhood. Proc of Symposium on Trop. Meteor., Wellington, New Zealand Met. Service, 1964; 192-200.
Ashfaq, M., Shi Y., Tung, W.-w., Trapp, R.J., Gao, X., Pal, J.S. and Diffenbaugh, N.S. Suppression of south Asian summer monsoon precipitation in the 21st century, Geophys. Res. Lett, 2009; 36: L01704.
Baidya, S.K., Shrestha, M.L. and Sheikh M.M. Trends in Daily Climate Extremes of Temperature and Precipitation in Nepal, J. Hydrol. Meteorol., 2008; 5(1): 38-51.
Cook B.I. and Buckley B.M. Objective determination of monsoon season onset, withdrawal, and length, J. Geophys. Res., 2009; 114: D23109.
CRCCH. Cooperative Research Centre for Catchment Hydrology, Australia, Trend Software, 2005; Available online: http://www.toolkit.net.au/Tools/TREND (accessed on 4/11/2013).
Dash S.K., Jenamani R.K. and Shekhar M.S. On the decreasing frequency of monsoon depressions over the Indian region, Curr. Sci., 2004; 86(10): 1404-1411.
Devkota L.P. Onset of Summer Monsoon in Nepal. The Himalayan Review, 1983; XV, Nepal Geographical Society 1983; 84: 11-20.
DHM. Preliminary Weather Report of Monsoon Season, 2010 across Nepal. Climate Section, Department of Hydrology and Meteorology, J. Hydrol. Meteorol., 2010; 7(1): 92-93.
Dickey, D.A. and Fuller, W.A. Distribution of the Estimators for Autoregressive Time Series with a Unit Root, J. Am. Stat. Association, 1979; 74: 427-431.
Duan, K.L.G., Thompson, T., Yao, M.E. and Davis, E. Mosley-Thompson. A 1000 year history of atmospheric sulfate concentrations in southern Asia as recorded by a Himalayan ice core, Geophys. Res. Lett., 2007, 34, L01810.
Francis, C. Lionel S. TREND - User Guide, 2005, CRC for Catchment Hydrology, Australia.
GON. Crop Situation Update, Issue 10, February 2010, National Food Security and Monitoring System, Government of Nepal.
Goswami, B.N., Kulkarni, J.R., Mujumdar, V.R. and Chattopadhyay R. Short communication on factors responsible for recent secular trend in the onset phase of monsoon intraseasonal oscillations. Int. J. Climatol., 2010; 30: 2240-2246.
Grayson, R.B., Argent, R.M.; Nathan, R.J. and McMahon, T.A. Mein R. Hydrological Recipes: Estimation Techniques in Australian Hydrology. Cooperative Research Centre for Catchment Hydrology, Australia, 1996; 125P.
IPCC. Climate Change 2007: the physical science basis. In: Solomon S. et al. (eds) Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007, Cambridge, UK.
Kundzewicz, Z.W. and Robson A. (Editors), Detecting Trend and Other Changes in Hydrological Data. World Climate Program – Water, WMO/UNESCO, WCDMP-45, WMO/TD 1013, Geneva, 2000; 157P.
Kwiatkowski, D., P. C. B. Phillips, P. Schmidt and Y. Shin, Testing the Null Hypothesis of Stationarity against the Alternative of a Unit Root, J. Econometrics,  1992; 54: 159-178.
Lang, T.J. and Barros A.P. An Investigation of the Onsets of the 1999 and 2000 Monsoons in Central Nepal, Monthly Weather Review, 2002; 130: 1299-1316.
Malla G. Climate Change and Its Impact on Nepalese Agriculture. J. Agr. Environ., 2008; 9: 62-71.
MOAC. Nepal Agriculture and Food Security Country Investment Plan, Government of Nepal, Ministry of Agriculture and Cooperatives. 2010.
Nayava J.L. Rainfall in Nepal. The Himalayan eview, 1980; 12: 1980.
Nayava J.L. Variations of rice yield with rainfall in Nepal during 1971-2000. J. Hydrol. Meteorol., 2008; 5(1): 93-102.
Prasai B.K. National Issue Paper on the Agriculture Sector (Adaptation), 2010, UNDP, Kathmandu.
Practical Action. Temporal and Spatial Variability of Climate Change over Nepal (1976-2005), Practical Action Nepal Office, Kathmandu, 2009, available online: http://practicalaction. org/file/region_nepal/Climate Change 1976-2005 .pdf (accessed on 04/11/2013).
Sabeerali C.T., Rao S.A., Ajayamohan R.S. and Murtugudde R. On the relationship between Indian summer monsoon withdrawal and Indo-Pacific SST anomalies before and after 1976/1977 climate shift, Climate Dynamics, 2011, doi: 10.1007/s00382-011-1269-9.
Sapkota C. Failure to yield, Nepali Times, 3-9 August, 2012.
Sigdel M. Ikeda M. Summer Monsoon Rainfall over Nepal Related with Large-Scale Atmospheric Circulations. Journal of Earth Science & Climate Change, 2012; 3(2): 112, doi: 10. 4172/2157-7617. 1000112.
Shrestha M.L. Interannual variation of summer monsoon rainfall over Nepal and its relation to southern oscillation Index, Meteorol. Atmos. Phys., 2000; 75: 21-28.
WFP. Summer Crop Nepal 2009/2010, Crop and Food Security Update, 2010, Joint Report by Ministry of Agriculture and Cooperatives and World Food Programme.
ECOPERSIA
Volume 1, Issue 4 - Serial Number 6
December 2013
Pages 353-367