Relationships between Rwandan seasonal rainfall anomalies and ENSO events

Abstract

This study aims primarily at investigating the relationships between Rwandan seasonal rainfall anomalies and El Niño-South Oscillation phenomenon (ENSO) events. The study is useful for early warning of negative effects associated with extreme rainfall anomalies across the country. It covers the period 1935-1992, using long and short rains data from 28 weather stations in Rwanda and ENSO events resourced from Glantz (2001). The mean standardized anomaly indices were calculated to investigate their associations with ENSO events. One-way analysis of variance was applied on the mean standardized anomaly index values per ENSO event to explore the spatial correlation of rainfall anomalies per ENSO event. A geographical information system was used to present spatially the variations in mean standardized anomaly indices per ENSO event. The results showed approximately three climatic periods, namely, dry period , semi-humid period (1961)(1962)(1963)(1964)(1965)(1966)(1967)(1968)(1969)(1970)(1971)(1972)(1973)(1974)(1975)(1976) and wet period (1977)(1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992). Though positive and negative correlations were detected between extreme short rains anomalies and El Niño events, La Niña events were mostly linked to negative rainfall anomalies while El Niño events were associated with positive rainfall anomalies. The occurrence of El Niño and La Niña in the same year does not show any clear association with rainfall anomalies. However, the phenomenon was more linked with positive long rains anomalies and negative short rains anomalies. The normal years were largely linked with negative long rains anomalies and positive short rains anomalies, which is a pointer to the influence of other factors other than ENSO events. This makes projection of seasonal rainfall anomalies in the country by merely predicting ENSO events difficult.

Evaluation of ENACTS‐Rwanda: A new multi‐decade, high‐resolution rainfall and temperature data set—Climatology

“…For Rwanda specifically, one study of historical station rain gauge data (up to the early 1990s) found that a little over 40% of the annual rainfall occurred in the March-May season, between 30 and 40% occurred in the September-December season, and about 15-20% occurred in January and February with June-August typically having only about 5% of the annual rain (Muhire et al, 2015). One study found that the pluviometric coefficient (ratio of monthly rainfall to annual rainfall) is more spatially heterogeneous across the country in February and October than in other months, potentially suggesting a role of additional sources of moisture (e.g., advection from the Congo and/or Atlantic).…”

“…There has been regional hydro-climatic variability on multi-decadal timescales (Ilunga and Muhire, 2010;Mbungu et al, 2012;Muhire et al, 2015;Muhire and Ahmed, 2015a). On shorter, inter-annual timescales, the El Nino-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) play a significant role in the rainfall patterns of Rwanda in particular and East Africa more generally (Ropelewski and Halpert, 1987;Saji and Yamagata, 2003;Ilunga and Muhire, 2010;Muhire et al, 2015;Ndomeni et al, 2018).…”

“…Several prior studies of Rwandan climate subdivide the nation into four physiographic regions: the area near Lake Kivu (entirely in the western province), the Nile-Congo divide (mostly in the western and northern provinces), the plateau region (including much of Kigali, much of the southern province and parts of the northern province) and the eastern lowlands (including most of the eastern province and parts of Kigali) (Ilunga et al, 2004;Ilunga and Muhire, 2010;Muhire et al, 2015). The eastern lowland region is the driest and warmest (and topographically lowest) region of the country.…”

“…The Mascarene high influences the advection of moisture and path of storm systems into East Africa from the Indian Ocean while the St. Helena high influences the advection of moisture and path of storm systems into the southwest and central Africa from the Atlantic. The Azores and Arabian high-pressure systems have somewhat more remote influences on the positioning and persistence of high-pressure systems which can influence regional droughts (Muhire et al, 2015). During much of the year, the majority of rainfall in Rwanda comes primarily from the Intertropical Convergence Zone (ITCZ), Indian Ocean/Lake Victoria and storms influenced by the Mascarene high (Ilunga et al, 2004).…”

“…The weak wind climatology over Rwanda in most seasons may be partly product of a disruptive influence of the topography on the mean wind field while also reflecting the equatorial doldrums and larger‐scale patterns of atmospheric convergence. The MAM and SOND rains are strongly influenced by the near‐equatorial positioning of the ITCZ and while the majority of moisture flux into Rwanda comes from the Indian Ocean from the (south) east, there is some moisture advection from the DRC between January and May (Muhire et al ., ). While this moisture advection from the DRC is not explicitly visible in the mean wind fields shown in Figure a,b, episodic westerly wind anomalies and storms that convey moisture from the Congo air mass into Rwanda are counterbalanced by weak prevailing easterlies.…”