Four-year Coupling between El Niño-Southern Oscillation and the Water Vapor Transport over East Asia

The summer moisture circulation over East Asia-western North Pacific (WNP) couples well with the El Niño-Southern Oscillation (ENSO) in four-year period. The moisture circulation is dominated by two well-separated modes. One mode (EOF1) exhibits an anomalous anticyclonic (cyclonic) moisture circulation over tropical-subtropical East Asia-WNP, with westward (eastward) transport anomaly over the tropical WNP and the Indian Ocean; the other mode (EOF2) displays a sandwich-like pattern with an abnormal anticyclonic (cyclonic) moisture circulation over subtropical WNP, layered between two cyclonic (anticyclonic) circulations. They couple well with the ENSO signal in four-year period. Within the four-year ENSO cycle, when a warm episode is developing continually, positive phase of EOF2 plays a key role, while in the transitional summer between a decaying warm episode and a developing cool episode, positive phase of EOF1 tends to take effect. In the summer of a developing cool episode, negative phase of EOF2 tends to play an important role, while negative phase of EOF1 tends to take effect in the transitional summer between a decaying cool episode and a developing warm episode.

Further study shows that the establishment of an anticyclone (cyclone) over the Philippine Sea region and its eastward extension during different phase of the ENSO episode plays an important role in the moisture circulation variation over East Asia-WNP. Conversely, the westward (eastward) wind anomaly to the south of the anticyclone (cyclone) is beneficial to the formation and the eastward propagation of the Kelvin wave, hence, to the development of the four-year periodic ENSO episode.