AGS-PRF: A Theoretical Framework to Investigate the Madden Julian Oscillation-Quasi Biennial Oscillation (MJO-QBO) Relationship--Upward Wave Radiation and Thin-Cirrus Feedbacks

Lin, Jonathan, Cambridge MA

Investigators

Abstract

The largest weather pattern in the world is the Madden-Julian Oscillation (MJO), an area of enhanced cloudiness and rainfall that forms over the equatorial Indian Ocean and propagates eastward over the course of a month or two. The MJO has worldwide impacts, modulating the number of hurricanes that form in the Gulf of Mexico, the rainy seasons of Asia, Australia, Africa, and the Americas, and the formation of the atmospheric rivers that cause flooding along the US West Coast. One issue that has gained prominence in recent years is how the MJO is influenced by the winds in the stratosphere above it. In the tropics the stratospheric wind blow from east to west or vice versa, changing direction over the course of about two years. The wind reversal starts at the top and descends in a progression known as the Quasi-Biennial Oscillation (QBO). Observations since 1980 show that the MJO is more robust in the easterly phase of the QBO (when the wind in the lower stratosphere is from the east) than in the westerly phase, a relationship which holds during the northern winter. The MJO is also more predictable in the easterly phase, thus the modulation of the MJO by the QBO has implications for subseasonal prediction. Work performed here seeks to understand why the MJO is sensitive to the phase of the QBO. Two possibilities are explored, the first being that the MJO is damped by the upward propagation of wave energy into the stratosphere. The work is an extension of earlier research by the Principal Investigator (PI) showing how the energy of convectively coupled equatorial waves can be lost through upward propagation. The second possibility is that the wind of the QBO influences the cirrus clouds that form at the tropopause level above convection in the MJO. Cirrus clouds are effective at trapping outgoing longwave radiation, and their radiative effect is thought to be important for creating the large areas of convective clouds found in the MJO. The QBO could affect the cirrus clouds either directly, as the QBO wind moves the clouds in one direction or the other, or indirectly through modulating the rising motions that favor cirrus clouds. The research is conducted through the development of an idealized theoretical model that captures the hypothesized effects and through simulations using a state-of-the art atmospheric model, the System for Atmospheric Modeling (SAM). The work has societal value due to the worldwide weather and climate effects of the MJO. The modulation of the MJO by the QBO has clear implications for subseasonal prediction, as noted above, but the reasons for the modulation are not known and the extent to which it can be harnessed for better predictions is difficult to assess. In addition, the Principal Investigator serves as a mentor to undergraduate and graduate students and participates in outreach activities such as the Lamont Open House. 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.