Interannual variability in the lower-tropospheric poleward sensible heat fluxes over the Northwestern Pacific associated with migratory baroclinic waves and with standing waves in bimonthly mean fields is investigated for the winter based on the NCEP/NCAR reanalyses for 1979-1995. The former is defined as the covariance (VhTh) of 8-day highpass-filtered temperature and meridional wind, and the latter (VsTs) can be regarded as heat transport by the northwesterly East Asian winter monsoon. Although the climatological-mean VhTh is suppressed in midwinter, its interannual variability is the largest. The first EOF of January-February VhTh, which accounts for nearly 50% of the total interannual variance over the Northwestern Pacific, represents anomalous VhTh along the climatological-mean stormtrack. The linear regression map of VsTs with the first principal component (PC1) timeseries of VhTh shows that the enhanced VsTs tends to be accompanied by the suppressed VhTh along the stormtrack and vise versa. At 40-45N nearly 2/3 of anomalous VsTs tends to be compensated by anomalous VhTh in the opposite sense, which leaves fairly small changes in the total atmospheric sensible heat transport from one winter to another.
The seasonal-mean circulation anomalies associated with the first mode of VhTh are identified in the linear regression maps with PC1. The enhanced VhTh is associated with anticyclonic SLP (sea-level pressure) anomalies that cover almost entire midlatitude North Pacific and weaker cyclonic anomalies over Siberia; in other words, the weakening of the Aleutian Low and Siberian High. These surface anomalies are accompanied by a PNA-like pattern over the North Pacific in the upper troposphere but its anticyclonic "tail"extends into northern China that is in seesaw with cyclonic anomalies over Siberia. This baroclinic structure is consistent with lower-tropospheric anomalous VsTs over the Far East.
Nakamura and Yamagata (1998) pointed out that two of the three dominant modes of the decadal wintertime SST variability over the Northwestern Pacific changed drastically in the second half of the 1980s. One is the midlatitude mode characterized by SST anomalies along the subarctic front with the anomalous Aleutian Low and PNA aloft, and the other represents decadal modulation of the East Asian winter monsoon. Decadal variations associated with these two modes are evident in the unsmoothed timeseries of PC1. In the first half of the 1980s, the stormtrack activity was strongly suppressed in midwinter, manifested as the marked minimum in the seasonal cycle of VhTh, in association with the enhanced monsoonal cold air advection in the lower-troposphere and the enhanced westerly jet aloft. Since the late 1980s, the monsoon and westerly jet both became weaker whereas baroclinic waves became active in midwinter, leaving almost constant VhTh throughout the winter half of the year.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics