Epochal Changes in ENSO–Streamflow Relationships in Sri Lanka

Abstract

In an effort to use climate predictions for streamflow and malaria hazard prediction, the decadal variability of the El Niño-Southern Oscillation (ENSO) influence on streamflow and rainfall in the Kelani River in Sri Lanka was investigated based on records from 1925 to 1995. In the last half century, the warm ENSO phase was associated with decreased annual streamflow and the cold ENSO phase with increased streamflow. The annual streamflow had a negative correlation (warm ENSO associated with low streamflow) with the concurrent ENSO index of Niño-3.4 that was significant at the 5% level. This negative correlation with Niño-3.4 is enhanced to a 1% significance level if the aggregate streamflow from January to September alone is considered. There has been a transition in correlation between January-September streamflow and ENSO between the 1950s and 1970s from near or above zero to negative values that have 95% significance levels reminiscent of an epochal shift. This shift was evident when considering the period when the southwest monsoon dominates (April-September) or when correlations were undertaken between the seasonal streamflow and rainfall and the ENSO index in the month prior to each season. Since the relationship between ENSO and Sri Lankan streamflow has strengthened in recent decades the potential for ENSO-based prediction is retained. The epochal shift may also explain why malaria epidemics ceased to co-occur frequently with El Niño episodes after 1945.

Epochal changes in the association between malaria epidemics and El Niño in Sri Lanka

“…Note, that a more detailed analysis of the running windowed correlation between rainfall and ENSO indices for the Western region was previously reported [ 42 ] and this report also documents other physical evidence for decadal changes such as changes in atmospheric circulation patterns associated with ENSO and the reports of other researchers who have observed similar phenomenon across the South Asian and Indian Ocean region.…”

Annual flood sensitivities to El Niño–Southern Oscillation at the global scale

“…For example, in the Mekong Räsä-nen and Kummu (2013) found epochal behaviour in ENSOdischarge correlations, with strongly negative correlations in the pre-1940s and after the mid-1970s, but a rather weak relationship between these periods. Similar results were found by Zubair and Chandimala (2006), who investigated ENSOseasonal-stream-flow relationships in Sri Lanka, and found that the correlations changed from positive (pre-1950) to strongly negative (post-1970). These findings are supported by other studies, which found similar epochal behaviour in relationship between ENSO and Asian-Australian Monsoon (Wang et al, 2008), and ENSO and Indian Monsoon (Torrence and Webster, 1999).…”

Predictability of Sri Lankan rainfall based on ENSO

“…The reversal in ENSO influence from a largely drying tendency during El Niño events from April to September to a wetter tendency in October to December is related to the southeastward extension of the zone of subsidence that persists over the Indian subcontinent in summer (Zubair and Chandimala, 2006), leading to enhanced convection over Sri Lanka and southern India during the October to December season (Zubair and Ropelewski, 2006). The drying tendency in the following January to March is due to subsidence setting in over Sri Lankan region again during El Niño events (Rasmusson et al, 1999).…”