tropical cyclones. Humid subtropical climates lie on the east side continents, roughly between latitudes 20° and 40° degrees away from the equator. An oceanic (or maritime) climate is typically found along the west coasts at the middle latitudes of all the world's continents, bordering cool oceans, as well as southeastern Australia, and is accompanied by plentiful precipitation year-round. The Mediterranean climate regime resembles the climate of the lands in the Mediterranean Basin, parts of western North America, parts of Western and South Australia, in southwestern South Africa and in parts of central Chile. The climate is characterized by hot, dry summers and cool, wet winters. A steppe is a dry grassland. Subarctic climates are cold with continuous permafrost and little precipitation. The tropical zones have the highest number of storm events followed by the temperate climate. In a recent study, researchers from 63 countries combined 30-minutes rainfall data in order to estimate the global rainfall erosivity (an index combining the amount, frequency and intensity of rainfall). The arid and cold climate zones have very low number of erosive events. Africa The northern half of the continent is primarily desert, containing the vast Sahara Desert, while its southern areas contain both savanna and plains, and its central portion contains very dense jungle (rainforest) regions. The equatorial region near the Intertropical Convergence Zone is the wettest portion of the continent. Annually, the rain belt across the continent marches northward into Sub-Saharan Africa by August, then moves back southward into south-central Africa by March. Mesoscale convective systems which form in tandem with tropical waves that move along the Intertropical Convergence Zone during the summer months become the seedlings for tropical cyclones which form in the northern Atlantic and northeast Pacific oceans. Areas with a savannah climate in Sub-Saharan Africa, such as Ghana, Burkina Faso, Darfur, Eritrea, Ethiopia, and Botswana have a distinct rainy season. Within of Madagascar, trade winds bring moisture up the eastern slopes of the island, which is deposited as rainfall, and brings drier downsloped winds to areas south and west leaving the western sections of the island in a rain shadow. This leads to significantly more rainfall over northeast sections of the island than the southwestern portions of Madagascar. Southern Africa receives most of its rainfall from summer convective storms and with extratropical cyclones moving through the Westerlies. Once a decade, tropical cyclones lead to excessive rainfall across the region.