Joint Seminar: Is the Process of MJO Propagation into the Western Pacific Deterministic?
The Madden-Julian Oscillation (MJO) provides a source of weather and climate variabilities. In particular, it is of great importance to understand how and when the MJO propagates into the western Pacific (WP). Considering that synoptic-scale stochastic processes and/or large-scale SST forcing can support MJO propagation, MJO convective development over the WP may be more chaotic than a theory focusing on MJO-scale processes alone expect. To address this issue, we conducted a 5,000-member 14-km mesh global ensemble simulation for two MJO events realized in November and December 2018 (denoted as Nov-MJO and Dec-MJO) using the Nonhydrostatic Icosahedral Atmospheric Model (NICAM), and examined how much deterministically the timing of MJO propagation into the WP is predicted.
The 2,000-member control experiments revealed that while the timing of Nov-MJO propagation was almost uniquely determined, that of Dec-MJO propagation was bifurcated. Whether the timing of MJO propagation is unique or bifurcated depends on SSTs; additional 3,000-member experiments showed that SSTs in December lead to the bifurcation by increasing the degrees of freedom in supporting MJO propagation. The bifurcation seen in Dec-MJO propagation resulted from the difference in the moistening time scale over the WP. This was related to whether equatorial-wave disturbances intruding into the WP can develop and efficiently moisten the WP or not. The presence or absence of the development of the equatorial-wave disturbances was explained by the wave-convection coupling strength, which depended on the amplitudes of dynamical forcing from the mid-latitude Rossby wave trains refracting toward the tropics.
Our results suggest that MJO propagation into the WP can be inherently stochastic processes that are composed of the intraseasonal–synoptic and tropics–extratropics interactions, and that the seasonal components of SSTs can affect how much deterministic the MJO propagation is.
- Bundesstr. 53, room 022/023
- Seminar Room 022/023, Ground Floor, Bundesstrasse 53, 20146 Hamburg, Hamburg
- Daniel Klocke