Collaborative Research: Cloud-Radiative Feedback as the Coupling Mechanism of the Madden-Julian Oscillation and Quasi-Biennial Oscillation

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A remarkable feature of weather in the tropics is the occasional appearance of a vast region of cloudy conditions moving slowly eastward along the equator from the Indian Ocean to the Pacific, causing an alternation of rain and clear skies over a period of a month or two. This alternation, called the Madden-Julian Oscillation (MJO), has a variety of long-range influences, for instance it affects the chances of hurricanes in the Gulf of Mexico and landfalling atmospheric rivers along the California coast. Given its slow propagation and worldwide effects the MJO is the dominant driver of predictable weather and climate variations on the subseasonal timescale. Naturally the MJO has been a topic of intensive research since its discovery in 1971 but we still lack a satisfactory understanding of it, and models used for weather predictions and climate simulations have difficulty representing it.One mystery of the MJO is its dependence on the direction of the stratospheric winds above it, which changes from easterly to westerly in a somewhat regular progression referred as the Quasi-Biennial Oscillation (QBO). The MJO is substantially more active in the easterly phase of the QBO, which features colder temperatures in the equatorial lower stratosphere along with the easterly winds. Some researchers have argued that the colder temperatures promote the MJO by reducing upper tropospheric stability and thus strengthening MJO-related convection. But the Principal Investigators (PIs) of this award hypothesize that colder temperatures matter because they promote cirrus clouds, which favor the MJO due to their infrared radiative feedback. The cirrus cloud hypothesis is motivated by several aspects of the QBO influence, among them that the influence does not appear prior to 1980. The 1980 onset makes sense because the stratosphere was warmer prior to 1980 and thus cirrus clouds would have been less susceptible to the cooling effect of the easterly QBO phase. The QBO-MJO relationship is explored through a variety of activities including analysis of data from several satellite missions (including COSMIC, CALYPSO, MLS) and reanalysis datasets, climate model simulations, and a simple linear model (based on the one used in AGS-2201441).The work is of societal as well as scientific value given the influence of the MJO on weather around the world and the possibility that the MJO could be predictable given better understanding of its behavior. The PIs conduct a webinar series to exchange research results with the MJO research community, and the lead PI is developing an online Spanish language workshop on the role of cloud processes in weather and climate. In addition, the project provide support and training to two graduate students.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.