DISCUSSION: A monsoon is defined by a seasonal reversal in wind direction. A consequence of this wind direction reversal is often a large seasonal change in rainfall. For example, over India and southeast Asia during the summer, the land heats up more/faster than the surrounding water, leading to the development of relatively low pressure over the land. This low pressure leads to onshore flow of warm, moist winds which slow down over land, converge, rise, and lead to heavy precipitation. During winter, the opposite occurs. The land is cooler than the surrounding water, leading to high pressure, offshore winds, and dry conditions.
These monsoon rains occur every year, but the length, intensity, and total amount of rainfall vary from year to year and over the course of a season. For example, relatively little rain fell in the early part of this year’s rainy season in India. But, beginning on 7 August, much higher-than-normal rain fell along India’s southwest coast. Specifically, 453.4 mm (~18 inches) fell in the state of Kerala over six days (almost 5 times the amount that normally falls over that same period in that region). Drier than normal conditions can pose serious problems for agricultural productivity in India, while wetter conditions can destroy crops and infrastructure via flooding. The picture above shows people fleeing rising flood waters from the recent heavy rains in southwest India. Since May, millions of people in India have been forced to flee rising water and greater than 1,000 have been killed. Similar flooding issues occurred in India during the rainy season last year.
In addition to flooding, heavy rains can bring an increased risk of landslides. Increased water in the soil increases pressure between soil grains, increasing the risk of slide. In the past two years, there have been at least 3,000 landslides in India. Many of these are in areas of new construction. Adding material to the upper portions of a slope can also increase the risk of a slide. Thus, limiting development on slopes may be a worthwhile policy strategy to help reduce damages and fatalities from landslides in the future in India and elsewhere.
The temporal and spatial scales of global climate and the Asian monsoon are very different, making it difficult to directly correlate the two. Nevertheless, a warming background climate increases the probability of intense rainfall events (warmer air can “hold” more water vapor) and may increase the day-to-day variability of precipitation. Thus, the heavy rains and associated flooding/landslides of the last two rainy seasons may become a more common occurrence, requiring the implementation of long-term adaptation strategies.
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© 2019 Meteorologist Dr. Ken Leppert II