Sources of MJO and S2S Predictability: A Conundrum

Slowly-evolving phenomena such as the MJO, stratospheric conditions, upper-ocean heat content, soil moisture, and sea ice have been perceived as the sources of predictability on subseasonal to seasonal (S2S) timescales. However, as S2S prediction is advanced, it has become evident that these presumed sources of predictability also pose challenges to their own prediction and are the very phenomena targeted by S2S prediction. This raises a fundamental question: What are the ultimate sources of S2S predictability? Specifically, what are the sources of MJO predictability? Most successful MJO/S2S forecast systems are global coupled models without boundary forcing (except at the top of the atmosphere). Coupling between the Earth system components (e.g., the atmosphere, ocean and land) is important to S2S prediction. Global couple models can adequately represent such coupling only if their background states are correct, which can be achieved only through adequate initial conditions. Does this imply that the sources of S2S predictability reside solely in initial conditions? A thorough examination of the fundamental issue pertaining the sources of S2S predictability is needed to properly guide continued development of S2S prediction.