10.17
Influence of the Recent Decadal Weakening of the East Asian Winter Monsoon on Large-Scale Air-Sea Interactions over the North Pacific
PAPER WITHDRAWN
Hisashi Nakamura, Univ. of Tokyo, Tokyo, Japan
The wintertime atmospheric circulation over the Far East and Northwestern (NW) Pacific is characterized by persistent southward transport of cold, dry air associated with a northwesterly monsoonal flow between the Siberian High and Aleutian Low. An abundant latent and sensible heat is supplied to this air flow from the warm ocean surface over the Kuroshio and its extension. This heat supply and strong meridional temperature gradients caused by the cold monsoonal flow both act to sustain the extremely strong near-surface baroclinicity that feeds baroclinic eddies to form a well-defined stormtrack downstream, marked as a zonally-oriented belt of local precipitation maxima in midlatitude. We found that the weakening of the winter monsoon, which occurred in the late 1980s, did substantially influence the heat and fresh water exchanges between the ocean and atmosphere over the entire North Pacific basin. The influence was through the associated decadal changes not only in the surface wind field but also in the stormtrack activity. The most profound signature of the latter was the disappearance of the distinct midwinter minimum in the seasonal march of the stormtrack activity over the NW Pacific, owing to the marked stormtrack activation in midwinter since the late 1980s. As captured in satellite measurements, this activation also increased precipitation in a midlatitude belt around southern Japan, while the local moisture supply from the ocean was reduced significantly in the presence of the weakened monsoon. Over the Northeastern (NE) Pacific, the enhanced diffluence of the stormtrack gave rise to more frequent penetration of storms into the subtropics than before, causing an increase in convective rainfall in the presence of enhanced evaporation from the nearby ocean. The enhanced stormtrack diffluence also caused less frequent passage of storms in the midlatitude NE Pacific and the associated reduction in fresh water supply into the ocean, whose signal was detected in sub-surface measurements near Hawaii. At the same time, the surface evaporation was enhanced over that region, as the advection of warm, moist air from the south weakened in association of the weakened Aleutian Low. The anomalous surplus and deficit in the net local moisture supply thus generated over the NE and NW Pacific, respectively, were balanced by anomalous westward transport of moisture in the midlatitude troposphere, as diagnosed in the NCEP/NCAR reanalyses.
Session 10, Continued
Thursday, 17 May 2001, 3:30 PM-4:44 PM
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