The Sudanian savanna or Sudan region is a broad belt of tropical savanna that runs east and west across the African continent, from the Ethiopian Highlands in the east to the Atlantic Ocean in the west. It represents the central bioregion within the broader tropical savanna biome of the Afrotropical realm. The Sahel acacia savanna, a belt of drier grasslands, lies to the north, forming a transition zone between the Sudanian savanna and the Sahara Desert phytochorion. To the Sudan's south, the more humid forest-savanna mosaic forms a transition zone between the Sudanian savanna and the Guineo-Congolian forests that lie nearer the equator.