El Niño Southern Oscillation link to the Blue Nile River Basin hydrology

Abstract

Abstract:The objective of this study is to evaluate the relationships of El Niño Southern Oscillation (ENSO) indices and the Blue Nile River Basin hydrology using a new approach that tracks cumulative ENSO indices. The results of this study can be applied for water resources management decision making to mitigate drought or flood impacts with a lead time of at least few months. ENSO tracking and forecasting is relatively easier than predicting hydrology. ENSO teleconnections to the Blue Nile River Basin hydrology were evaluated using spatial average basin rainfall and Blue Nile flows at Bahir Dar, Ethiopia. The ENSO indices were sea surface temperature (SST) anomalies in region Niño 3Ð4 and the Southern Oscillation Index (SOI). The analysis indicates that the Upper Blue Nile Basin rainfall and flows are teleconnected to the ENSO indices. Based on event correspondence and correlation analysis, high rainfall and high flows are likely to occur during La Niña years and dry years are likely to occur during El Niño years at a confidence level of 90%. Extreme dry and wet years are very likely to correspond with ENSO events as given above. The great Ethiopian famine of 1888-1892 corresponds to one of the strongest El Niño years, 1888. The recent drought years in Ethiopia correspond to strong El Niño years and wet years correspond to La Niña years. In this paper, a new approach is proposed on how to classify the strength of ENSO events by tracking consecutive monthly events through a year. A cumulative SST index value of ½5 and cumulative SOI value of Ä 7 indicate strong El Niño. A cumulative SST index value of Ä 5 and cumulative SOI index of ½7 indicate strong La Niña.

Simulating the connections of ENSO and the rainfall regime of East Africa and the upper Blue Nile region using a climate model of the Tropics

“…The GPCP data show a strong signal of increased (decreased) rainfall during La Niña (El Niño) years over the Sahel region up to about 18 • N, including the upper catchment of the Nile River. This result agrees qualitatively with previous observational analyses, which suggested that La Niña years are associated with above normal rainfall and El Niño years with below normal rainfall in the upper catchment of the Blue Nile (Eltahir, 1996;Wang and Eltahir, 1999;Amarasekera et al, 1997;De Putter et al, 1998;Camberlin et al, 2001;Abtew et al, 2009). Figure 8 shows the corresponding fields averaged over the nine-member ensemble of Reg-TB simulations.…”

Droughts and floods over the upper catchment of the Blue Nile and their connections to the timing of El Niño and La Niña events

“…4, which shows the monthly discharge (JanuaryDecember) averaged over all El Niño (1965, 1968, 1969, 1972, 1982, 1983, 1986, 1987, 1991, 1992, 1995, 2002, and 2009), La Niña (1970, 1971, 1973, 1974, 1975, 1985, 1988, 1989, 2000, 2007, 2008, and 2010 and normal years (1966, 1967, 1976, 1977, 1978, 1979, 1980, 1981, 1984, 1990, 1993, 1994, 1996, 2001, 2003, 2005, 2006, 2011, and 2012). This confirms the results of previous studies that El Niño is mostly associated with below-average rainfall, and La Niña with above-average rainfall (Eltahir, 1996;Wang and Eltahir, 1999;Amarasekera et al, 1997;De Putter et al, 1998;Camberlin et al, 2001;Abtew et al, 2009). The JJAS discharge anomalies for the full analysis period are shown in Fig.…”

“…4. Monthly discharge at Eldiem station averaged during El Niño (1965, 1968, 1969, 1972, 1982, 1983, 1986, 1987, 1991, 1992, 1995, 2002and 2009), La Niña (1970, 1971, 1973, 1974, 1975, 1985, 1988, 1989, 2000, 2007, 2008and 2010 and normal years (1966, 1967, 1976, 1977, 1978, 1979, 1980, 1981, 1984, 1990, 1993, 1994, 1996, 2001, 2003, 2005, 2006, 2011 and 2012). , the red line represents the threshold for the extreme flood/drought, and the dashed red line represents the threshold for drought/flood.…”

“…De Putter et al (1998) presented a study of decadal periodicities of the Nile River historical discharge of the Roda Nilometer (Cairo, Egypt) and suggested that high-frequency peaks could be linked to ENSO. Abtew et al (2009) analyzed monthly rainfall observations from a 32-rain gauge monitoring network in the upper Blue Nile Basin and found that high rainfall is likely to occur during La Niña years and low rainfall conditions during El Niño years. He also found that extremely dry years are highly likely to occur during El Niño years and extremely wet years are highly likely to occur during La Niña years.…”

Explaining and forecasting interannual variability in the flow of the Nile River

“…Many studies investigated the teleconnections between the Ethiopian rainfall and the global sea surface temperature (SST) in order to find SST indices to use for Nile flow prediction (e.g., Eltahir, 1996;Abtew et al, 2009;Melesse et al, 2011). Eltahir (1996) showed that the SST anomalies over the tropical eastern Pacific Ocean explain 25 % of the interannual variability of Nile flow for the period 1872-1972.…”