oceans. The dry phase refers to the other half of the year, whenwinds bring cool, dry air from wintering continents. This distinctive variation of the annual cycle occurs over Asia, Australia, western Africa, and the Americas. In some locations (e.g., the Asia-Australia sector) the dry winter air flows across the equator, picking up moisture from the warm tropical oceans to become the wet monsoon of the summering continent. In this manner the “dry” of the winter monsoon is tied to the “wet” of the summer monsoon and vice versa. In contrast, regions closer to the equator have two rainy seasons. For example, in equatorial East Africa the two rainy seasons occur in March to May and September to December and fall between the two African monsoon circulations. These are referred to as the “long” and “short” rains, respectively.
Agrarian-based societies have developed in the monsoon regions because of the abundant solar radiation and precipitation, the two ingredients for successful agriculture. Agricultural practices have traditionally been tied strictly to the annual cycle. Whereas the regularity of the warm and moist and the cool and dry phases of the monsoon would seem to be ideal for agricultural societies, their regularity makes agriculture very susceptible to small changes in the annual cycle. Small variations in the timing and quantity of rainfall can have significant societal consequences. A weak monsoon year (i.e., with significantly less total rainfall than normal) generally corresponds to low crop yields. A strong monsoon usually produces abundant crops, although too much rainfall may produce devastating floods. In addition to the importance of the strength of the overall monsoon in a particular year, forecasting the onset of the subseasonal variability (e.g., the active periods and the lulls or breaks in between) is of particular importance. A late- or early-onset monsoon or an ill-timed lull in the monsoon rains may have very serious consequences for agriculture, even when mean annual rainfall is normal. As a result, forecasting monsoon variability on timescales ranging from weeks to years is an issue of considerable urgency.
An example of Indian rice yield susceptibility to monsoon variations is provided to illustrate these points. Figure 3.1a plots rice production in India between 1960 and 1996. Figure 3.1b plots the All-India Rainfall Index (AIRI).5 AIRI is a measure of total summer rainfall over India. The relationship between crop yield and AIRI was first noted in 1988.6 Figure 3.1a and Figure 3.1b provide an updated version of this relationship. In general, rice production has increased linearly during the past few decades. Superimposed on this trend are variations in crop production of about 15 to 20 percent. Some periods of production deficit are associated with El Niño years in the Pacific Ocean (shaded bars), while some abundant years are associated with La Niña, or “cold” events in the Pacific (diagonal bars). Figure 3.2a is a scatter plot of the AIRI and the crop yield as functions of their percent deviations from the mean. The correlation between the two time series is +0.61. All El Niño years (black triangles) fall in the negative quadrant, while all La Niña years (black squares) lie in the positive quadrant. Finally, the relationship between the preceding winter Southern Oscillation Index