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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade
smaller scale of India and South Asia. How is this connection manifested in the physical system?
Do irregularities in the ENSO/crop yield relationship indicate that intra-seasonal rainfall variability (e.g., the timing of the onset and first break of the monsoon in a particular summer, relative to plowing, planting, and harvesting) also influences total crop yield?
Do the irregularities in the relationship between SOI and AIRI suggest inherent limitations in their linkage? What are the factors involved in any such limitations?
How accurate must a seasonal forecast of monsoon rainfall be to be of use to the user community? How far in advance would a forecast have to be made?
In the preceding discussion, Indian crop yield is used as an example of the importance of discerning the ways that macroscale climate variability affects the local scale. The questions raised above are common to the monsoon regions of Australia, Africa, and the Americas.
In an effort to predict monsoons over a century ago, it was speculated that the varying extent and thickness of Himalayan snow exert some influence on the climatic conditions and weather over the plains of northwest India.8 Himalayan snow was therefore assessed via snowfall reports from various locations in the western Himalayan range as one of the predictors of Indian monsoon rainfall.9 Greater winter snowfall was found to be related to below-normal monsoon rainfall for the period 1880 to 1920. However, for the subsequent 30-year period, snowfall was highly variable and its relationship with the monsoon was reversed. Its use as a predictor was dropped.
Since the early 1970s, the Advanced Very High Resolution Radiometer (AVHRR) aboard National Oceanographic and Atmospheric Administration (NOAA) satellites has provided a snow cover dataset that is sufficiently accurate for continental-scale studies. Some pioneering work10 examined the snow-monsoon relationship using these satellite data. Several observational studies, some examining the role of Eurasian snow extent, others focusing on the Himalayan snow, suggested an inverse snow-monsoon relationship—that is, the less the snowfall, the greater the monsoon.
In the northern hemisphere snow cover ranges from 7 to over 40 percent of the total land area, making it the most rapidly changing natural surface. Snow cover and snow depth in a particular season can be related to atmospheric circulation of the next season through a series of feedback mechanisms.
The two main physical processes through which snow anomalies may affect climate on a seasonal timescale are the albedo effect and the hydrological ef-