Seasonality in the Trans-Atlantic Slave Trade

Seasonal rainfall in the Atlantic slaving world

In most regions in the Atlantic slaving world the growth of crops and sufficient pasture for livestock depended upon seasonal rainfall. Those people living in rainforests would have been familiar with intense summer downpours; those living towards deserts or in rain shadows would have known droughts lasting many months. Most Africans experienced greater precipitation extremes than those living in the Western Hemisphere. For people who survived their trans-Atlantic passage, ninety-five per cent would labor in tropical and sub-tropical regions in the Western Hemisphere marked by seasonal rainfall. Comparatively few enslaved Africans experienced the temperate year-round rains in Chesapeake and mid-Atlantic lands to the north or those temperate rains in the Rio de la Plata, the southernmost American slaving market.

In Atlantic Africa, shifting air masses produce July-October rains north of the equator and January-April rains south of the equator. In the principal slave-trading zone, from 15° North Latitude (above Senegal) to 15° South Latitude (below Benguela), coastal rains double the volume of precipitation just 20-30 miles inland. Deluges from Conakry (in modern Guinea) to Bassa (Liberia) match the rainfall in the Douala estuary of Cameroon—these are the rainiest pockets in the Atlantic world. Travelling south from the Senegal River, where twenty inches of rain fall annually, mostly in the summer, one reaches the northern extent of the rainforest above Sierra Leone (8-9° N), where ninety inches of rain fall in July and August. Heavy summer rains continue in dense rainforests stretching from Sierra Leone to the Windward Coast. Rainy season deluges commence along the eastern Bight of Benin and in the Bight of Biafra, precipitation amounts increasing during summer as one sails towards the equator. Once south of the Zaire River, one enters the driest coastline in Atlantic Africa, where farmers can expect to see less than fifteen inches of rain each year, mostly in February-April.

In contrast to precipitation in Atlantic Africa, there is less rainfall in most New World slaving markets and few droughts. The greatest downpours occur in the spring-summer in the tropical rainforests of northern South America from Surinam (5-6° N) to Pará (1-3° S). The 1,250-mile Brazilian coastline from Paraíba (6° S) to São Vicente (24° S) includes a tropical zone, with a hot and humid climate and no pronounced dry season; the hot and humid subtropics, with a two-month dry season; and a temperate region, with a warm climate and dry winters. In Bahia, there are two moderate rainy seasons, separated by heavy May rainfall. Farther south, in Rio de Janeiro, the year’s first substantial rain occurs in March. Most rain falls in the West Indies in the fall and winter, but the wettest low-lying areas only reach the precipitation levels of the driest rainy seasons in Atlantic Africa. In the Carolinas and the Chesapeake, June-August summers are the rainiest times, but there is also significant December-February precipitation, levels similar to those experienced during winters in maritime climates of Europe.