Decadal variability of wintertime North Atlantic and Pacific blockings: A possible cause

Decadal variability of the wintertime (DJFM) blocking occurring over the North Atlantic (NA) and North Pacific (NP) sectors is investigated by using the daily mean NCEP-NCAR reanalyses. It is found that the wintertime blocking activities over the NA and NP sectors exhibit a remarkable decadal variability, which is likely controlled by a decadal change in the basic-state (BS) baroclinicity associated with the large-scale atmosphere-ocean coupling in the midlatitudes. In each decadal cycle the high BS baroclinicity over the Northwest Atlantic corresponds to a higher blocking frequency over the Northeast Atlantic, but the low BS baroclinicity over the Northwest Pacific corresponds to a higher frequency of blockings over the Northeast Pacific. In climatological mean the blocking activity in the Northeast Atlantic and Pacific can exhibit a marked linear downward trend when the BS baroclinicity over the Northwestern Atlantic and Pacific sectors exhibits a linear upward trend. This may be because the excessively strong BS baroclinicity tends to suppress the onset of downstream blockings. It is thus concluded that a change in the atmospheric BS baroclinicity associated with the mid-latitude large-scale atmosphere-ocean coupling on decadal timescales is responsible for the decadal variability of the blocking activities over the NA and NP sectors.