MEKONG BASIN RESEARCH (SE-ASIA)
The Tibetan plateau has been identified as a tipping point for climate change impacts as accelerated glacier and snow melt causes a positive feedback accelerating the melting process by the enlarged dark rocky ground attracting more heat.
During the low flow season in April and May, 75-95% of the Mekong runoff in Vientiane originates in Tibet and during the peak flow months from July to September over 50%. Changes in snow melt driven hydrology in Tibet will hence significantly impact downstream hydrology and especially the dry season flow (Adamsonn, 2006). This strong vulnerability of the Mekong hydrology towards climate change impacts requires profound research on the climate-hydrology interactions under different scenarios.
The ENSO phenomenon and its impact on climate variability in the Mekong region
The El Niño–Southern Oscillation (ENSO) is a well-known phenomenon. In South East Asia it is leading to low rainfall rates during el Niño years and higher precipitation in la Niña years (AchutaRao and Sperber, 2002; Räsänen & Kummu, 2012). Räsänen and Kummu (2012) analysed the correlation of the ENSO phenomenon with recent climate variations in the Mekong Basin for rainfall data (149 precipitation stations, 1981-2005) and discharge (six Mekong gauging stations, 1910-2008) using spatial GIS analyses and statistical methods, such as linear correlations, spectral analysis and stochastic regression models. They detected that the hydrological dynamics of the Mekong River were significantly influenced by ENSO events, especially the hydrological processes in the southern and central part of the basin. The precipitation and discharge data correlated particularly in the decay years of ENSO events decreasing during El Niño years with shorter annual flood periods and increasing during La Niña years with longer flood periods. Furthermore, they found out that the correlation between ENSO and the hydrological processes of the Mekong changed a lot in the period 1910-2008. From 1910 to 1940 and from 1975 to 2008 a strong correlation in the data could be observed, whereas between 1940 and 1975 low correlations were recorded (Räsänen & Kummu, 2013).
The irregular (every 2-7 years) El Niño and La Niña years originate in the tropical Pacific and are accompanied by interactions between the ocean and the atmosphere of the tropical Indian and Pacific Oceans. They have clear signals in sea surface temperature (SST), atmospheric pressure patterns and a varying strength of the Pacific trade winds. While El Niño years account for higher sea surface temperatures and lower air pressure in the Eastern Pacific, La Niña conditions are generated by cold sea surface temperatures originating in the tropical Pacific (Mc Phaden et al., 2006; Tudhope et al., 2001). Higher El Niño SSTs lead to higher evaporation rates and hence to above-average rainfall in the eastern Pacific and South America during winter and late spring. At the same time, it causes droughts in eastern Australia, South East Asia and storms along the equator. Measuring sea surface temperature and atmospheric pressure is considered as a suitable tool to predict El Niño and la Niña events and hence precipitation more than several months ahead (Montecinos & Aceituno, 2003). For the Mekong Basin, it is suggested to have a high potential for prediction of ENSO induced hydro-meteorological extremes, ENSO index values from December-February explained approximately 50% of the 562 inter-inter-annual variation of the Mekong’s following year discharge (Räsänen & Kummu, 2013).
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