A baffling signal in the tropics

Convection meets the Indian Ocean for supersize weather.

HANNAH Barnes remembers the day she saw it spawning. She was sitting in a shipping container, under a radar, on a tiny island in the Indian Ocean. “There was a lot of rain and low, gray clouds. But it wasn’t anything huge,” she said. “I’m from the Midwest, so it just reminded me of a fall storm.” Barnes, however, knew that the gray weather would grow into a cluster of storms more than 6,000 miles across. This armada of tall clouds would move eastward, bringing its fields of wind, humidity, and temperature. “The clouds and storms die out when they get to the dateline,” Barnes said. “But in the upper atmosphere, a signal remains and that signal propagates around the globe.”

Barnes was witnessing a massive atmospheric disturbance called the Madden-Julian Oscillation (MJO) brewing right above her and her colleagues. The MJO signal, a mélange of intense atmospheric convection, air pressure, and wind, is so strong that it can spike rainfall in the South Asian and Australian monsoons and increase the number of violent tornado outbreaks in the United States. It also interacts with other large atmospheric patterns, such as the El Niño Southern Oscillation and the North Atlantic Oscillation. Understanding it could improve long-range weather forecasts and enable scientists to further refine computer models of global climate. But scientists do not know much about the MJO and its signal, especially how and why it spawns every thirty to sixty days over the Indian Ocean. Researchers like Barnes think that important clues can be found in the massive expanse of convective clouds that help power the disturbance across the globe.

Originally published in Sensing Our Planet: NASA Earth Science Research Features. Read the full story here.