The most important cycle in tropical weather is known as the Madden-Julian Oscillation (MJO). Similar to its better-known cousins El Niño and La Niña, it involves a complex coupling between atmosphere and ocean, which brings increased evaporation and rainfall. But unlike those cycles, which create a standing pattern of warmth on one side of the tropical Pacific, the MJO is nearly continuous, creating a migrating cluster of storms that spawns over the western Indian Ocean and moves eastward at 15–30 kilometers per hour. Every 30 to 60 days, the clouds respawn, and the cycle begins again.
Despite the MJO’s importance to weather across vast scales, scientists still don’t fully understand the basic physics of how it forms and propagates. Computer models that have tried to capture its physics often misrepresent it, which may degrade the forecast accuracy of hurricanes, monsoons, and other weather phenomena around the world.
In a new study intended to address those shortcomings, Ciesielski et al. examined data from the Dynamics of the MJO (DYNAMO) observation campaign, which collected information from a network of upper-air soundings, radar sites, and ships in the Indian Ocean that operated from October 2011 to March 2012.
One particularly thorny aspect of the MJO is determining how much heat is transferred between the ocean and throughout the atmosphere by convection and how much heat is absorbed or emitted in the form of radiation. The two are linked: Convection creates clouds that can block sunlight and trap radiation from Earth, heating and cooling the atmosphere, which in turn can enhance or suppress convection.
When the MJO was first discovered in the 1970s, attempts to model it focused on the effects of convection. But in the last 20 years, models have suggested that radiative effects play a key role. If they’re in phase with the convective effects and if they are strong enough, they might form a feedback loop that sustains the MJO. Models suggest that radiative heating must enhance convective heating by roughly 20% for this to kick in.
To study the effects of radiative heating, the authors focused on one site in particular, Gan Island in the central Indian Ocean. By looking at atmospheric profiles above Gan Island, they could see how clouds and moisture changed which altitudes were heated over the course of each cycle of the MJO. The observations showed that when the usual shallow cumulus clouds give way to the MJO’s towering cumulonimbus storms, radiant heat trapped by clouds and moisture gradually warms a deeper column of the lower atmosphere while the tops of the storms radiate heat into space, cooling the upper troposphere.
Their analysis shows that radiative heating is roughly 20% of convective heating, right around the predicted critical threshold for a convective-radiative feedback loop. The team suggests that as the effects of convective heating weaken, radiative heating takes over and keeps the MJO’s storms energized and moving eastward. (Journal of Geophysical Research: Atmospheres, https://doi.org/10.1002/2016JD025965, 2017)
—Mark Zastrow, Freelance Writer