To answer this question, we unite three methodologies: investigation and reconstruction of previous climates; analysis of the impacts of historical weather and climate change on societies; and the analysis of discourses and the social representation of climate. Norrgård (2013), for example, applied these methodologies in a study of the climatic periodization of the Gold and Guinea Coasts in West Africa. His study, which is based on “documents with descriptive information and without measured data,” spanned 48 years (1750-1798). He found that some West African droughts, but not all, affected the TAST. Like Norrgård's study, we examine the effects of droughts on the TAST, but in sharp contrast to Norrgård, we consider a much broader range of space and time scales: from the African coast across the Atlantic and from the mid-18th through the mid-19th centuries. Because direct instrumental measurements of wind speed and direction, temperature, cloud cover etc. were often not available for the region and time scales to be examined, a variety of proxy data is used. Such data includes logs kept by the captains of the slave ships. The captains were required to record in their logs daily meteorological conditions—rain, fog, squall lines, wind direction etc.—as well as ship departure and arrival dates, embarking and disembarking ports, and number of slaves carried on each voyage. The data from these logs is compiled in the Global Ship Observations, The Maury Collection, 1792–1910 and the Slave Voyages Data Set. The ship data, however, poses a challenge: it is asynoptic, meaning the observations are recorded at different locations at different times. To construct weather maps based on the ship observations, we use standard algorithms for converting asynoptic data to synoptic data. In light of the weather and climate patterns constructed from the data, we discuss the effects of droughts on the TAST.
Norrgård, Stefan, A New Climatic Periodisation of the Gold and Guinea Coasts in West Africa, 1750 – 1798. Abo, Finland: Abo Akademi University Press, 2013.
The Maury Collection: Global Ship Observations 1792-1910. CD-ROM, version 1.0. U.S. Department of Commerce, National Oceanic Graphic and Atmospheric Administration, National Climatic Data Center, 1998.
Voyages Database (Voyages: The Trans-Atlantic Slave Trade Database), http://www.slavevoyages.org (accessed August, 2015).