A low-frequency oscillation over the tropical North Pacific——the Northern Oscillation

The Northern Oscillation, which is a zonal seesaw between sea level pressures in the eastern and western North Pacific, is a low-frequency atmospheric oscillation newly identified by Chen (1984). It is further studied in an attempt to examine its basic features of spatial and temporal variations including the seasonal and interannual fluctuations and to document its relationship with considerable fluctuations of the sea surface temperature, rainfall, trade wind and the Walker Circulation in the tropical Pacific.The empirical orthogonal function (EOF) analysis not only further verifies its existence, but also reveals its very large-scale nature and its most important place in the interannual variability of atmospheric circulation over the North Pacific.The long time series of sea level pressure at Ship N and Manila, which is extended from 1947 back to 1921, displays clearly that the Northern Oscillation is also evident for the period of 1921-1946. It is indicated that it is not a phenomenon peculiar to a