In the 1920s, Sir Gilbert Walker made the seminal connection between barometer readings of air pressure at sea level at stations on the eastern and western sides of the Pacific Ocean (Tahiti and Darwin, Australia). He observed that when pressure rises in the east, it usually falls in the west, and vice versa. Walker and his team sorted through data on weather records till they found some patterns of rainfall in Latin America which could be associated with changes in the temperatures of ocean waters (See diagram: The Walker effect).

What robbed Walker of the credit for establishing the vital link between patterns of rainfall in South America and the ocean temperatures was his theory of a connection between monsoons (or the want of it) in India and unusually mild winters in Canada. Walker was criticised for his thesis: How could climatic conditions over such widely separated region be linked?