Issues related to S2S and MJO predictability

One main justification for subseasonal-to-seasonal (S2S) prediction is its identified sources of predictability. These sources include slowly varying phenomena, such as the MJO, stratospheric conditions, upper-ocean heat content, soil moisture, and sea ice. In practice, however, these presumed sources of S2S predictability have become the main targets of S2S predictability. For example, predicting the MJO has faced challenges, its propagation over the Indo-Pacific Maritime Continent being a case in point. This raises fundamental questions: What are the sources of predictability of the MJO? Can dominant physical processes of the MJO (e.g., moisture, air-sea interaction) be viewed as its sources of predictability? If yes, then what are the sources of predictability of these processes? If not, then is the MJO predictability solely from the initial conditions when its predictions are made by global coupled models? These and other fundamental issues about the sources of S2S and MJO predictability need to be addressed to properly guide the development of S2S and MJO predictions.