E. Linacre and B. Geerts
Yields of the staple crop of maize (or corn) in the relatively arid region of northeast Brazil (the Nordeste) depend critically on the rainfalls. The nine states that constitute the Nordeste contain the driest areas of Brazil, and the interannual variation of their annual rainfall is higher than anywhere else in Brazil. Rain falls mainly in autumn (around May) within about 400 km of Brazil’s coast, and in late summer (around March) further to the west, and also north of 6° S. The region has experienced devastating droughts (as in 1915, 1958, 1972, 1983, and 1993) and floods (as in 1984-86) (Fig 1).
Fig 1: Annual precipitation anomalies in Northeastern Brazil. Data compiled by the Joint Institute for the Study of the Atmosphere and the Oceans (JISAO)(2).
Corn yields in the Nordeste averaged 227 kg/ha in 1983 (a drought year) and 593 in 1984-85 (wet years). (These figures are the means of the nine state averages in each year.) The values are about 283 kg/ha in the three El Niño years, 1983, 1987 & 1990, but over twice as much in the two La Niña years, 1984 & 1985.
The annual corn yield in six of the states correlate significantly (confidence level over 99%) with the Southern Oscillation Index averaged from February to April, which generally is the corn planting time. Thus the SOI offers some indication of subsequent harvests in those states. The other three states (Alagoas, Sergipe and Bahia) cover the coastal region between 9-17° S. Much of the rain in this area results from shallow convection in southerly postfrontal air, whereas the rain in the other 6 states (located inland and/or north) is largely due to the southeastward migration of the Inter-Tropical Convergence Zone (ITCZ) during late summer.
The movement and intensity of the ITCZ over South America is strongly related to the Southern Oscillation. Under El Niño conditions high pressure over the tropical Atlantic holds up the southward migration of the ITCZ towards the Nordeste.