There is an increasing societal need for forecast at lead times between week three and month two. Probabilistic information about monsoon onsets and breaks, intensification/suppression of tropical cyclone activity etc. could benefit many sectors of the economy. The major phenomenon allowing for subseasonal forecasts is the Tropical Intraseasonal Oscillation. Additional sources of predictability at these lead times could be found in local atmosphere-ocean and atmosphere-land modes. Subseasonal forecasting is a new challenge and as such many questions are still open or even not yet well defined. Current state-of-the-art coupled numerical models are not representing adequately all aspects of the TIO in free simulations with different models showing different errors. How important in respect to TIO forecast is adequate initialization of these models and how far into the future can these models propagate the initial information in a useful sense? What are the sources for error growth? Can these sources be attributed to model deficiencies or to intrinsic physical reasons? Would forecast at higher horizontal resolution i.e., with improved scale interactions, be better?

Here we answer these questions using a version of the NCEP Climate Forecasting System (CFS) in a series of experiments conducted under the Climate Test Bed (CTB). After introducing a simplified measure of tropical intraseasonal oscillation activity we show that, for the CFS, forecast skill of the TIO is independent of horizontal resolution. The element that is crucial is atmospheric initial conditions; by ameliorating the atmospheric initial state from the Reanalysis-2 to the NCEP operational analysis we improved forecast of the TIO by 3-5 days. We also show that the forecast skill of the TIO in CFS depends on the phase of the oscillation with the fastest drop in pattern correlation occurring as the convectively active phase of the MJO approaches the Maritime Continent. We conclude by presenting some mechanisms responsible for this Maritime Continent Barrier.