The Role of Radiative Heating in the Madden-Julian Oscillation
Large international field campaign demonstrates importance of radiative heating to a key large-scale tropical weather phenomenon that is poorly represented in current Earth system models.
The Madden-Julian Oscillation (MJO) is a large-scale weather phenomenon that manifests as an eastward-moving system of anomalous cloudiness and rainfall near the equator. It is an important coupling between the atmosphere, ocean, and convection and dominates tropical variability on intra-seasonal (weekly to monthly) timescales. Despite its broad impacts on tropical and global weather and climate, the MJO remains poorly represented in many weather and Earth system model simulations because of its complex nature. Key elements of the MJO that need to be better understood are its initiation and how it maintains its organization as it moves eastward. This study uses unprecedented observational datasets of the MJO collected during an international field campaign (DYNAMO: Dynamics of the MJO) to focus on how relationships between radiation, clouds, and convection impact the MJO.
Early MJO studies tended to emphasize the crucial role of heating connected with convective processes. More recent studies have raised the question of whether radiative heating (heating due to absorption of sunlight and absorption and emission of thermal radiation) may play a key role in maintaining the organization of the MJO. While radiative-convective instabilities can result in MJO-like modes, these instabilities only occur when radiative heating reaches a critical fraction (~20%) of the overall convective heating. Previous multi-model results and western Pacific field data analysis did not conclusively resolve the question and debate on the impact of radiative heating on MJO continued. This DYNAMO study confirms that radiative heating can reach and even exceed 20% of convective heating, implying that convective-radiative instability is a key factor in the MJO. The study suggests that as the MJO convective envelope weakens over the central Indian Ocean, cloud-radiative feedbacks help maintain the mature MJO organization as it moves eastward. These results may lead to improved model representation of the MJO and improved prediction of tropical weather and climate in global models.
This study focuses on three MJO events during 3-month period (October to December 2011) over Gan Island in the Indian Ocean during the DYNAMO campaign. Observations and advanced radiative flux and heating rate data products from the Atmospheric Radiation Measurement (ARM) Mobile Facility deployment at Gan Island were key to the study. Based on column-net, as well as vertical profiles of radiative heating rates, this study shows that MJO modulates radiative fields with near-zero net-tropospheric cooling during its active phase due to longwave absorption and re-emission. MJO-composited radiative heating rates exhibit tilted structures with height reflecting changes in the cloud population and associated water vapor fields. Over the MJO lifecycle net-tropospheric radiation varies ~0.5 K/d enhancing the amplitude of the convective heating signal by ~20% in the mean with a minimum in this enhancement ~10 days prior to peak MJO rainfall and maximum ~7 days after. This suggests that as MJO convective envelope weakens over the central Indian Ocean, cloud-radiative feedbacks help maintain the mature MJO as it moves eastward.
Colorado State University
This research was supported by the National Oceanic and Atmospheric Administration under grant NA15OAR4310177, by Department of Energy, Office of Biological and Environmental Research, Atmospheric System Research Grant DE-SC0008582, by the National Aeronautics and Space Administration under grant NNX13AF74G, and by the National Science Foundation (NSF) under grant AGS-1360237. Work at Lawrence Livermore National Laboratory (LLNL) was supported by the DOE Atmospheric Radiation Measurement (ARM) Facility and performed under the auspices of the U.S. Department of Energy by LLNL under contract DE-AC52-07NA27344.
Ciesielski, P. E., R. H. Johnson, X. Jiang, Y. Zhang, and S. Xie. “Relationships between radiation, clouds, and convection during DYNAMO.” J. Geophys. Res. Atmos., 122, 2529-2548 (2017) [DOI:10.1002/2016JD025965]