Influence of the Recent Decadal Weakening of the East Asian Winter Monsoon on the North Pacific Climate
Hisashi Nakamura (hisashi@geoph.s.u-tokyo.ac.jp)
IGCR, Frontier Research System for Global Change also in Dept. of Earth and Planetary Physics, Univ. of Tokyo
The wintertime atmospheric circulation over the Far East and Northwestern (NW) Pacific is characterized by a low-level northwesterly monsoonal flow between the Siberian High and Aleutian Low and by an extremely intensified westerly jet aloft. The strong meridional temperature gradients sustained by the monsoonal flow and an abundant heat supply from the warm ocean surface both maintain the high baroclinicity over this region that feeds baroclinic eddies to form a well-defined stormtrack downstream. We found that the recent weakening of the winter monsoon did substantially influence the climate system over the entire North Pacific through the associated changes in the stormtrack activity. Its most marked signature is that the midwinter suppression of the stormtrack activity, which had been apparent in the early to mid-1980s, almost disappeared since then in association with substantial activation of the stormtrack activity in midwinter. As opposed to linear theory of baroclinic instability, this stormtrack activation occurred despite the decadal weakening of the Pacific jet. It is suggested that the weakening of the jet prolonged the residence time for growing migratory eddies within the baroclinic zone underneath, which seemingly overwhelmed the effect of the reduced mean-flow baroclinicity. It is also suggested that, as the excessively strong westerlies weakened, the eddies tended to change their structure so as sto be more efficient in converting the mean-flow available potential energy into their kinetic energy for growth. We also found that the observed weakening of the East Asian monsoon and Aleutian Low and the concomitant decadal modulation in the Pacific stormtrack activity significantly altered both the atmosphere-ocean heat and moisture exchange and hydrological cycle over the entire North Pacific basin.
