Abstract
The Abdus Salam International Center for Theoretical Physics (ICTP) version 4.4 Regional Climate Model (RegCM4) is used to investigate the rainfall response to cooler/warmer sea surface temperature anomaly (SSTA) forcing in the Indian and Atlantic Oceans. The effect of SSTA forcing in a specific ocean basin is identified by ensemble, averaging 10 individual simulations in which a constant or linearly zonally varying SSTA is prescribed in individual basins while specifying the 1971–2000 monthly varying climatological sea surface temperature (SST) across the remaining model domain. The nonlinear rainfall response to SSTA amplitude also is investigated by separately specifying +1K, +2K, and +4K SSTA forcing in the Atlantic and Indian Oceans. The simulation results show that warm SSTs over the entire Indian Ocean produce drier conditions across the larger Blue Nile catchment, whereas warming ≥ +2K generates large positive rainfall anomalies exceeding 10 mm·day−1 over drought prone regions of Northeastern Ethiopia. However, the June–September rainy season tends to be wetter (drier) when the SST warming (cooling) is limited to either the Northern or Southern Indian Ocean. Wet rainy seasons generally are characterized by deepening of the monsoon trough, east of 40°E, intensification of the Mascarene high, strengthening of the Somali low level jet and the tropical easterly jet, enhanced zonal and meridional vertically integrated moisture fluxes, and steeply vertically decreasing moist static energy. The opposite conditions hold for dry monsoon seasons.
Highlights
Major features of the tropical atmospheric circulation, averaged over time-scales longer than a month or two, are largely determined by sea surface temperature (SST) variations
For the Equatorial Indian Ocean (EqIO) sensitivity experiments, linearly zonally varying SST anomaly forcing was prescribed across the EqIO (10°S–10°N, 35°–100°E)
The simulations were driven by the NOAA Optimum Interpolation (OI) SST V2 long-term climatological mean prescribed over the entire domain (Figure 2)
Summary
Major features of the tropical atmospheric circulation, averaged over time-scales longer than a month or two, are largely determined by sea surface temperature (SST) variations. Numerous observational and modeling studies have documented how SST variations affect the interannual and decadal fluctuations of seasonal rainfall over different parts of Africa (e.g., [1,2,3,4,5,6,7,8,9,10,11,12,13]). The role of the Atlantic and Indian Oceans in forcing climate anomalies is less well understood than the effects of ENSO-related forcing. Understanding the effects of the Indian and Atlantic Oceans is important because these oceans are the primary sources of moisture (e.g., [12]) and have direct bearing on the evolution of the large-scale circulation systems that affect rainfall in the
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