From: Massachusetts Institute of Technology (MIT)
Published July 31, 2017 08:27 AM

Understanding tropical rainfall

The Intertropical Convergence Zone (ITCZ), also known as the doldrums, is one of the dramatic features of Earth’s climate system. Prominent enough to be seen from space, the ITCZ appears in satellite images as a band of bright clouds around the tropics. Here, moist warm air accumulates in this atmospheric region near the equator, where the ocean and atmosphere heavily interact. Intense solar radiation and calm, warm ocean waters produce an area of high humidity, ascending air, and rainfall, which is fed by converging trade winds from the Northern and Southern Hemispheres. The convected air forms clusters of thunderstorms characteristic of the ITCZ, releasing heat before moving away from the ITCZ — toward the poles — cooling and descending in the subtropics. This circulation completes the Hadley cells of the ITCZ, which play an important role in balancing Earth’s energy budget — transporting energy between the hemispheres and away from the equator.

However, the position of the ITCZ isn’t static. In order to transport this energy, the ITCZ and Hadley cells shift seasonally between the Northern and Southern Hemispheres, residing in the one that’s most strongly heated from the sun and radiant heat from the Earth’s surface, which on average yearly is the Northern Hemisphere. Accompanying these shifts can be prolonged periods of violent storms or severe drought, which significantly impacts human populations living in its path.

Scientists are therefore keen to understand the climate controls that drive the north-south movement of the ITCZ over the seasonal cycle, as well as on inter-annual to decadal timescales in Earth’s paleoclimatology up through today. Researchers have traditionally approached this issue from the perspective of the atmosphere’s behavior and understanding rainfall, but anecdotal evidence from models with a dynamic ocean has suggested that the ocean’s sensitivity to climate changes could affect the ITCZ’s response. Now, a study from MIT graduate student Brian Green and the Cecil and Ida Green Professor of Oceanography John Marshall from the Program in Atmospheres, Oceans and Climate in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS) published in the American Meteorological Society’s Journal of Climate, investigates the role that the ocean plays in modulating the ITCZ’s position and appreciates its sensitivity when the Northern Hemisphere is heated. In so doing, the work gives climate scientists a better understanding of what causes changes to tropical rainfall.

Continue reading at Massachusetts Institute of Technology (MIT)

Image: NOAA GOES Project Science Office and NASA

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