Linking Snow Cover and the Monsoon

To answer this question, Goés contacted John Fasullo, a scientist who suggested that the best description for what he does is “climate diagnostician,” someone who studies the underlying causes of climate phenomena. Fasullo specializes in the underlying causes of variability in the monsoon.

The Southwest Asian Monsoon is one facet of the larger Asian Monsoon. The basic cause of the Asian monsoon is that the Eurasian landmass and the Indian Ocean warm and cool at different rates through the seasons. In the summer, the land surface gets hotter than the ocean. Hot air above the land surface rises, and moist air from over the ocean flows in to replace it. In the winter, the opposite occurs. The air over land gets much colder and denser than the air over the ocean. Warmer air over the ocean rises, and cold air from the north blows in to replace it. Because the monsoon is driven by land-ocean temperature differences, anything that increases that difference could, in theory, strengthen the monsoon.

“Joaquim contacted me because of a paper I had published in 2004 in which I supported the existence of a link between the strength of the Asian monsoon and the amount of snow cover in Europe and Asia,” explained Fasullo. The basic idea is that before summer’s heat can kick the monsoon into high gear, it must first melt the winter snow cover and evaporate the resulting soil moisture.

“The idea that there is a link between snow cover and the monsoon has been around since the 1800s,” said Fasullo. “But the initial theory,” proposed in 1884 by a scientist named H.F. Blandford, “didn’t specifically include the Arabian Sea,” he added. “It was much more general; it basically said the less snow cover there was over the Himalaya, the sooner you would have warming in the spring and the warmer it would get over the summer. Therefore, snow cover would influence both the onset and intensity of the monsoon.”

“The trouble was the theory wasn’t supported by extensive observational studies of the modern satellite era. Scientists who looked for evidence of the relationship between snow cover and the strength of the monsoon couldn’t find it, or when they did, their methods were quite questionable,” Fasullo said. “In 2004, my colleagues and I published the results of an analysis we did of satellite-derived snow cover in Eurasia since 1967. We discovered that while El Niño-La Niña cycles had the biggest influence on the strength of the Asian Monsoon, Eurasian snow cover did play a major role in the monsoon intensity in non-El Niño/ La Niña years.”

Differences in temperature between land and ocean drive the Asian monsoon. During the winter in South Asia (upper), warm air over the Arabian Sea rises, drawing cold air from the land to the north. In summer (lower), after much of the snow melts, the land warms more than the ocean, and the wind direction reverses. (Maps by Robert Simmon, based on data provided by the Bigelow Laboratory for Ocean Sciences)

That paper established a general relationship between snow cover in Eurasia and the entire Asian monsoon from around 1970 to the present. Goés wanted something more specific. He wanted Fasullo to investigate whether snow cover changes in Eurasia since 1997 could be linked to the intensification of the Southwest Monsoon winds over the Arabian Sea. “We looked at changes in satellite observations of snow cover going back to 1967,” explained Fasullo, “and there is a very clear decline in snow cover in Eurasia since 1997—the rapid decline is certainly unique in the data record. We corroborated this decline by comparing the snow cover data with an independent data set—station observations of air temperature. The trend is verified from that data: air temperatures are going up considerably, and snow cover is going down.”

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Across Eurasia, snow cover (red line) has dropped significantly since the late 1990s. Warming temperatures are most likely to blame. Lack of snow allows the land surface to heat up more in the summer, and the widening temperature contrast between land and ocean in the summer strengthened the monsoon. (Graph by Robert Simmon, based on data provided by the Bigelow Laboratory for Ocean Sciences)