Abstract
The dominant large-scale interannual modes in the tropical Pacific and Indian Oceans—El Niño southern oscillation (ENSO) and the Indian Ocean Dipole (IOD)—dominate seasonal rainfall patterns in Ethiopia. However, there is a clear interaction between ENSO and the IOD, and it is unclear whether the IOD has an independent influence on seasonal monsoon patterns in Northern Ethiopia. We use monthly rainfall records from 15 stations from two drought–prone regions in Northern Ethiopia (Afar and Amhara) for the period 1966–2006 to explore relationships between rainfall and circulation patterns and sea surface temperature (SST) anomalies over the tropical Indo-Pacific region. Our analysis confirms that regional summer monsoon (Kiremt) rainfalls in these regions are predominantly modulated by ENSO. Warm and cold ENSO episodes (El Niño/La Nina) are associated with below and above average summer monsoon rainfall, respectively. Lagged relationship between the IOD and Kiremt rainfall shows that positive/negative phases of the IOD are generally conducive to Kiremt rainfall increases/decreases over large parts of Ethiopia. Regression models based on the large-scale circulation indices NINO3.4 and a Dipole Mode Index (DMI)NO-ENSO representing the “ENSO-free IOD” also highlight the role of ENSO. However, the relative-weights for the models with DMINO-ENSO, calculated using Akaike Information Criteria (AIC), were 1.5 and 1.1 times the weights for the ENSO only models for the Afar and Amhara regions, respectively. This suggests that the IOD has an independent regional influence. This is in line with the conception of the IOD as a unique coupled-mode in the tropics, and may have important implications in boosting seasonal forecasting skills in the regions. No statistically significant trends were found in the regional and modeled rainfall time-series.
Highlights
The East African monsoon system is predominantly linked to changes in sea surface temperatures (SSTs) in the Indian and Pacific Oceans [1,2,3,4,5,6,7] and, to a much lesser extent, the North Atlantic Ocean [8].Northern Ethiopia lies within the core moisture-sink region of the East African monsoon system, and rainfall changes over the region are associated with the El Niño southern oscillation (ENSO) and the Indian Ocean Dipole (IOD) [9,10]
It is unclear whether the IOD exerts influence on seasonal monsoon patterns in Northern Ethiopia, independent of ENSO
We develop an empirical forecast model for the monsoon rains in Northern Ethiopia, locally known as Kiremt (July–September) using regression with two sets of large-scale indices, after removing the direct and lagged ENSO signal from the IOD [21]
Summary
The East African monsoon system is predominantly linked to changes in sea surface temperatures (SSTs) in the Indian and Pacific Oceans [1,2,3,4,5,6,7] and, to a much lesser extent, the North Atlantic Ocean [8]. ENSO and the IOD are interrelated phenomena, and the extent to which the IOD is considered as a distinct oscillation of the climate system is still widely debated; for example, [12,13,14,15,16] It is unclear whether the IOD exerts influence on seasonal monsoon patterns in Northern Ethiopia, independent of ENSO. We build composite records for two drought-prone regions in Northern Ethiopia for the period 1966–2006 using monthly rainfall records from 15 stations, and we assess the contribution and impact of the “independent” component of the IOD on seasonal rainfall patterns To this end, we develop an empirical forecast model for the monsoon rains in Northern Ethiopia, locally known as Kiremt (July–September) using regression with two sets of large-scale indices, after removing the direct and lagged ENSO signal from the IOD [21]. Long-term trends in regional rainfall and station records are examined
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