Interannual Variability of the Asian Summer Monsoon and its Relationships with ENSO and Eurasian Snow Cover.

The onset of the Asian summer monsoon in relation to the land-sea thermal contrast, and its interannual variability and relationships with ENSO and Eurasian snow cover are studied using various data at NCAR data center. The onset of the Asian summer monsoon is marked by large temperature increases during summer over the Tibetan Plateau. Sensible heating of the Plateau surface in spring plays an important role in triggering the onset of the Asian summer monsoon. A monsoon index, based on the magnitude of the mean vertical shear of the zonal wind in the "South Asia" region, was used to classify the "strong" and "weak" broad -scale Asian summer monsoon seasons. Of the particular importance, the strong (weak) Asian summer monsoon years are associated with (a) strong (weak) Pacific trade winds; (b) positive (negative) air temperature anomalies in the midlatitudes, but negative (positive) air temperature anomalies in the tropics; and (c) negative (positive) SST anomalies in the equatorial eastern Pacific. Power spectral analyses show three significant peaks at periods near 4 ~ 6 years, 2 ~ 2.5 years and 15 months in the monsoon index, equatorial SST and Eurasian snow cover. Cross -spectral analyses show two coherent oscillations with periods near 3 ~ 6 years and 2 ~ 2.5 years in the monsoon index, equatorial SST and Eurasian snow cover. For the 3 ~ 6 year period range, phase relationships between the monsoon index, SST and Eurasian snow cover are complex and suggestive of two-way interactions. However, for the quasi-biennial period range, the Eurasian snow cover leads both SST and the monsoon index. High Eurasian winter and spring snow cover is followed by weak Asian summer monsoon and weak Pacific trade winds. The tropical east-west circulation provides a connection between the Asian monsoon and the atmosphere/ocean system in the tropical Pacific basin. Calculations of the sources and moisture sinks reveal that, over the Indian Ocean, the heat sources show distinct quasi-biennial oscillation. On the other hand, over the eastern Pacific a longer (4 ~ 6 years) periodicity is evident.