Relationships between North Pacific Wintertime Blocking, El Niño, and the PNA Pattern

The frequency of persistent high-latitude ridging events (commonly referred to as blocking) over the North Pacific–Alaskan region is investigated using a 44-winter record of daily 500-mb height fields. The winters are stratified in accordance with the phase of the El Niño–Southern Oscillation (ENSO) cycle and according to the sign of the seasonally averaged PNA index. It is found that the occurrence of blocking in the Bering Strait region is sensitive to the averaged polarity of the PNA pattern but is even more sensitive to the phase of the ENSO cycle. Sixty-nine percent more days of blocking are observed during winters occurring during the cool phase of ENSO, compared to those occurring during the warm phase. ENSO-related differences in blocking frequency are found to be associated with changes to both the mean and variance of the circulation over the North Pacific. The variance of geopotential heights on timescales corresponding to the lifetime of blocking events is found to be higher over the Bering Strait region in the cool phase of the ENSO cycle.