Interdecadal change of the connection between winter North Pacific Oscillation and summer precipitation in the Huaihe River valley

The relationship between the winter North Pacific Oscillation (NPO) and the following summer precipitation in the Huaihe River valley before and after the mid-1970s is investigated by using the Chinese 160-station precipitation data and NCEP/NCAR reanalysis data from 1951 to 2008. It is found that their linkage appears to have an apparent interdecadal variation. Before the mid-1970s, there was a prominent out-of-phase relationship between the winter NPO and the summer precipitation in the Huaihe River valley. However, such a relationship is significantly weakened afterwards. The change of atmospheric circulations related to the winter NPO before and after the mid-1970s is further addressed. Before the mid-1970s, a strong (weak) winter NPO was followed by the summer situations with anomalous low-level cyclonic (anticyclonic) circulation over the western Pacific and descending (ascending) over the Huaihe River valley. Meanwhile, the water vapor transporting to the Huaihe River valley was reduced (enhanced). These conditions are unfavorable (favorable) for the precipitation occurring in the Huaihe River valley, and thus the local precipitation was decreased (increased). After the mid-1970s, however, the impact of winter NPO on the summer atmospheric circulation system associated with the rainfall in the Huaihe River valley becomes diluted, thereby weakening its linkage to the summer precipitation in the Huaihe River valley.