Development of the signal-to-noise paradox in subseasonal forecasting models: After how long? Where? Why?

Subseasonal forecast models are shown to suffer from the same  inconsistency found in climate models between the low strength of predictable signals and the relatively high level of agreement they exhibit with observed variability of the atmospheric circulation. That is, subseasonal forecast models show higher correlation with observed variability than with their own simulations, i.e., the signal-to-noise paradox. Also similar to climate models, this paradox is particularly evident in the North Atlantic sector.  The paradox is not evident in week 1 or week 2 forecasts, and hence is limited to subseasonal timescales. The paradox appears to be related to overly fast decay of Northern Annular  Mode regimes. Three possible causes of this overly fast decay and for the paradox in the Northern Hemisphere are identified:  a too-fast decay of polar stratospheric signals, overly weak downward coupling from the stratosphere to the surface (in some models),  and overly weak transient synoptic eddy feedbacks. While the paradox is clearly evident in the North Atlantic, things are qualitatively different in the Southern Hemisphere:  Southern Annular  Mode regimes persist realistically, the stratospheric signal is well maintained, and eddy feedback is, if anything, too strong and zonal.