Investigation into Winter Blocking Regimes: Mechanisms for Onset and Predictability

The physical mechanisms and predictability of blocking onset are examined for four main blocking locations during boreal winter: the Atlantic, Europe, Asia, and the North Pacific. The four sectors are selected based on the EOF analysis, and different evolution scenarios are revealed for the different locations, implying different physical mechanisms. The Atlantic blocking events resemble the negative phase of the NAO, and spectral analysis shows a persistent contribution from the low-frequency (> 30 days) flow. Contribution from Rossby wave breaking (RWB) and diabatic impacts are also evident. The Europe blocking events are preceded by slowly propagating Rossby wave trains, and blocking onset is manifested as a large-scale Rossby wave break with the abrupt development of a blocking high over a very short period (< 2 days). Asian blocks are related to a stationary wave train originating from the Atlantic, and blocking onset is associated with strong convergence of wave activity flux (WAF). The blocking high has an equivalent barotropic structure, with a relatively weak contribution by diabatic heating. The Pacific blocks show a contribution from the low, intermediate, and high frequency bands; high-frequency eddy activity increases before onset, and a strong contribution from RWB is evident.

The predictability of blocking frequency and onset for each sector is investigated using the subseasonal reforecast produced from the ECMWF model. Results suggest higher predictability over certain regions, depending on the physical mechanisms involved in blocking onset.