A Mechanism Relating the Indian Ocean SSTs, ENSO, and the Nile Flow

A significant fraction of the inter-annual variability in the Nile River flow is associated with ElNino Southern Oscillation (ENSO). Fluctuations of the Indian Ocean Sea Surface temperature (SST) are also associated with the Nile flow. Here, we investigate the intermediate role of Indian Ocean SSTs in the teleconnection between Nile flow and ENSO. Using the partial coherency analysis, we show that connection between the North and Middle of Indian Ocean SSTs and Nile flow is driven by ENSO. However, the South of Indian Ocean SSTs have an independent role from ENSO in shaping variability of Nile flow. This independence allows for indices of SSTs over the South of Indian Ocean and ENSO to explain about 60% of the variability of the Nile flow when they are used together. During ElNino events, the SSTs over the Indian Ocean increase following the warming of the Tropical Eastern Pacific (TEP), which forces a Matsuno-Gill circulation with enhanced westerlies winds over Indian Ocean that decreases the convergence of boundary layer air over the Upper Blue Nile (UBN) basin. On the other hand, the increased SSTs over South of Indian Ocean generate a cyclonic motion, which has a similar effect on the convergence of boundary layer air over the UBN. The proposed mechanism has an important implication in understanding the potential climate change impacts on the Nile flow induced by warming of over Indian Ocean and provide new indices for Nile flow forecast models.