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.