Monsoons: Wet, Dry, Repeat...

Narration: George Huffman, Gail Skofronick-Jackson


Huffman: The monsoon is a seasonal wind and rain pattern that was first described over south Asia. You see the clouds blossoming here during the summer part of the monsoon. For centuries people have known about it, but only recently have we received enough data from satellites to really describe what's going on.

Skofronick-Jackson: What you can see here is moist air that has evaporated from the ocean, coming across India and providing rainfall, driving the monsoon season. The great thing about GPM is that it allows us to see precipitating systems as a whole, over land and oceans, and then as they transition from one boundary to the next.

Huffman: All this rainfall drives soil moisture over land. It's beneficial because it promotes the economic activity that people depend on, for example, agriculture. As well it fills the rivers, which provides water for human activity and the natural environment, as well as transportation. If the rivers get too full, of course, it becomes flooding. At first the floods you see here are fairly minor and broad-scale but then they concentrate in the few wiggly lines which are the river basins, for example, in central eastern China.

Skofronick-Jackson: In mountainous regions when the ground becomes saturated due to heavy rains it can lead to landslides. Landslides kill thousands of people every year and are primarily triggered by rainfall. They are especially common in the Himalayan region each monsoon season.

Huffman: One really cool way to look at the monsoon is to do a split-screen and look at the summer and the winter at the same time. In the summer the wind is blowing onshore, bringing the moist rain-laden air into the continent. In the winter time, it blows off the continent.

Skofronick-Jackson: Now those winds are basically driven by temperature differences between the ocean and the land. And where the land is nice and warm, the air expands and it draws in the moist precipitation from the ocean waters, but in the winter time, it's very cold and you can see that the moisture then it goes from the continent back into the oceans. Over the past fifty years or so, satellites have been used to measure precipitation all around our Earth. With that dataset we're able to understand that monsoons occur not only in south Asia and India, but in other parts of the world as well.

Huffman: For example, Africa, where the temperature gradient is between the Atlantic Ocean and the Sahara Desert, the wind blows from the moist Atlantic Ocean onto west Africa providing the moisture for the precipitation. Some of these westward moving storms provoke hurricanes over the Atlantic that occasionally make it to the U.S. Southwestern North America also has a summertime monsoon. You see high soil moisture in regions where there's a lot of precipitation in western Mexico, and later in the season this extends up into the southwestern U.S. And the Southern Hemisphere has a monsoon as well. This occurs in the Northern Hemisphere winter, which is the Southern Hemisphere summer when Australia is warmer than the ocean to the north.

Skofronick-Jackson: Having a better understanding of the global water cycle and monitoring changes over time is important for society, for our everyday lives and our long-term future.