Moisture Mode Theory for the MJO and the BSISO
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Prof. Adam Sobel
Monday April 18, 2022, 2 PM ET
The Madden Julian Oscillation (MJO) and Boreal Summer Intraseasonal Oscillation (BSISO) are fundamental modes of variability in the tropical atmosphere on the intraseasonal time scale, and important sources of predictability on that time scale. I will review the evidence for, and development of, what has been called the “moisture mode” theory as an explanation of these modes’ fundamental dynamics. This view posits that these modes would not exist in a dry atmosphere, that a prognostic equation for moisture is essential in any mathematical model of them, and that the moist static energy (or moist entropy) budget is the most useful framework within which to understand them. Then I will present some new work, by Shuguang Wang and myself, that unifies the two modes by presenting them as leading unstable modes of a single idealized model. The essential ingredients of the model physics are similar to those in many earlier theoretical studies. The key new ingredients are inclusion of both zonal and meridional gradients of moisture in the basic state (both of them based on the seasonal climatology over the Indian ocean), some mathematical steps which make the solutions easier to interpret, and consideration of a hierarchy of modes that emerge as solutions under different simplifying assumptions. In this model, the structure of the modes is controlled by the strength of the meridional moisture gradient, which varies with the seasons. In northern winter, the meridional gradient of moisture is strong, and the model produces an eastward-propagating MJO-like mode. In northern summer, the meridional gradient of moisture is weak (in the northern hemisphere) and the model produces a mode with apparent northward as well as eastward propagation and a BSISO-like structure.
Adam Sobel is a professor at Columbia University’s Lamont-Doherty Earth Observatory and Engineering School. He studies the dynamics of climate and weather, particularly in the tropics. In recent years he has become particularly focused on understanding the risks to society from extreme weather events and climate change. Sobel has received awards from the American Meteorological Society, the AXA Research Fund, and the American Geophysical Union. He is author or co-author of over 200 peer-reviewed scientific articles; a book, Storm Surge (Harper-Collins), about Hurricane Sandy and climate change; and many op-eds and articles in the mainstream media, including the New York Times, CNN, Los Angeles Times, and many others. He currently hosts a podcast, Deep Convection, featuring wide-ranging conversations with other climate scientists.
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