Group seminar on 25. January, 14:15 CET
Shallow Convective Heating: A Key to Understanding Madden-Julian Oscillation
Prof. Dr. Zhaohua Wu
For the past five decades, Madden-Julian Oscillation (MJO) has been one of the most extensively studied phenomena in atmospheric and climatic sciences. While numerous features of MJO have been revealed through analyzing observational and/or modeling data, a relatively complete theory based on fundamental dynamics of atmosphere and ocean that can explain systematically most of the observed features of MJO appears still to be out of reach. Many fundamental questions regarding MJO still need to be answered, such as why variability on intraseasonal stands out and why MJO propagates eastward.
In this talk, the circulations driven by convective heating of different vertical structures are discussed through exploring the fundamental dynamical processes of tropical atmosphere. Special emphasis is placed on the low-level mass (moisture) convergence associated with the forced circulation and the maintenance of the shallow and deep heat sources. A new mechanism is inferred from the fundamental dynamics for the interaction between the large-scale convection and large-scale circulation. The new mechanism states that shallow heating drives a strong low-level moisture convergence so that the system of shallow heating and the forced large-scale circulation is unstable. When the unstable system reaches a certain amplitude, the stable cap layer immediately above the shallow heating erodes, and deep convection arises, which consumes most of the converged moisture at low levels without much feedback to the low-level convergence of moisture. The whole heating circulation system develops and dies; the estimated lifetime of such a system, based on the timescale of adjustment of tropical atmosphere to forcing, is on an intraseasonal timescale. Other observational features of MJO that can be explained by this theory are also discussed.
Our study infers that a key to understanding intraseasonal oscillation is the shallow convective heating, but not OLR or velocity potential of upper levels.