The Madden-Julian Oscillation (MJO) is a large-scale, eastward-moving atmospheric disturbance in the tropics that significantly influences global weather and climate patterns. With a cycle lasting approximately 30 to 80 days, it manifests as alternating periods of enhanced and suppressed convective rainfall. Originating over the Indian Ocean, the MJO propagates eastward across the Maritime Continent and into the Pacific Ocean, modulating phenomena such as monsoon systems, tropical cyclone development, and extratropical weather extremes. Unlike stationary climate modes (e.g., ENSO), the MJO is characterized by its planetary-scale circulation anomalies and dynamic progression, making it a critical driver of subseasonal variability.
Real-time Multivariate MJO (RMM) index, developed by Wheeler and Hendon (2004), is used to monitor and quantify the MJO’s behavior. This index employs empirical orthogonal function (EOF) analysis to extract the MJO’s dominant spatial and temporal patterns from variables like outgoing longwave radiation (OLR) and zonal winds at 200 and 850 hPa. The RMM index is represented by two principal components, RMM1 and RMM2, which together define the MJO’s phase and amplitude in a two-dimensional phase space diagram. Here, we utilize OLR fields from the FY-3D satellite and zonal winds at 200 hPa and 850 hPa provided by the CRA-40 reanalysis to achieve real-time monitoring of the RMM index.
Interpreting the RMM Phase Space Diagram:
1. Phases (1–8): The circular diagram divides the MJO’s lifecycle into eight phases, corresponding to the geographic location of its convective core. For example:
o Phases 2–3: Enhanced convection over the Indian Ocean.
o Phases 4–5: Active convection shifts to the Maritime Continent/Western Pacific.
o Phases 6–8: Suppressed rainfall returns to the Indian Ocean as the MJO moves eastward.
2. Amplitude: The distance from the origin (center) reflects the MJO’s strength. A radius >1 indicates a strong MJO event, while values<1 suggest weak or inactive conditions.
3. Trajectory: The plotted path (time series of RMM1/RMM2) shows the MJO’s propagation speed and direction. Clockwise movement signifies eastward progression, with tighter loops implying faster cycles. Stalling near certain phases may signal delayed impacts on regional weather.
By analyzing this diagram, forecasters can predict shifts in rainfall, temperature, and extreme events weeks in advance, leveraging the MJO’s role as a "bridge" between weather and climate.
