Tropical-wide teleconnection and oscillation. II: The ENSO-monsoon system

Two teleconnection indices, discussed in Part I, i.e. the Tropical-wide Osc illation Index (TOI) and the Walker circulation Index (WAI), are applied to the analysis of the ENSO-monsoon (El Nino Southern Oscillation-Asian monso ons) system. The first hypothesis presented in Part I was that the TOI for July-August-September (JAS) is closely related to the Indian summer monsoon index as well as the Southern Oscillation Index. As a result, the TOI repr esents the lead-lag characteristics of the tropical circulation variability over the eastern hemisphere (45 degrees E-170 degrees E) and simultaneousl y its interaction with the ENSO over the equatorial Pacific. The second hyp othesis was that there are two types of connection between the ENSO and Asi an monsoons: type I with distinct connection in space and time, and type II without connection. The WAI provides a measure for this connection. This i dea is supported by comparisons of observed and model teleconnection struct ures in Part I. Part II investigates these relations further. Time-lag correlations are cal culated between the key indices and atmospheric variables over the equatori al Indo-Pacific Oceans. If type II years, derived by the WAI, are removed f rom the 34-year time series, correlations between the TOI and these variabl es increase appreciably, now showing clearly the biennial character. The an alysis identifies a sequence of events involving biennial oscillation of th e ENSO-monsoon system from approximately JAS(-1) to JAS(0), followed by int ensification of the ENSO from JAS(0) to November-December-January(+1). The ENSO-monsoon oscillation system is not sinusoidal but skewed. To show the geographical patterns associated with the above sequence of eve nts, planar maps are presented of the lag correlation between the observed TOI(JAS) and (i) Vertical velocity at the 500 hPa level, (ii) precipitation , (iii) sea surface temperature (SST), and (iv) atmospheric sea level press ure. Distinct geographical distributions of the ENSO-monsoon oscillation em erge in both the observations and model data. One pattern is characterized by a horseshoe shape over the Pacific, which is generally symmetric around the equator, but with geographical differences depending on location in the lag sequence. The other pattern is a see-saw shape, primarily a standing o scillation located in the eastern South Pacific and the Indian Oceans, rese mbling the sea-level-pressure pattern found by Trenberth and Shea. Applying the lead-lag relationship, it is demonstrated that the SST over the centra l Pacific four months ahead can be projected, based on the TOI(JAS). Conver sely, the intensity of the Indian monsoon rainfall for non-type II years ca n be projected 15 months ahead by the SST over the eastern Pacific Ocean. T his indicates that the ENSO-monsoon oscillation system is quasi-periodic, a s opposed to irregular, with a two-year cycle; this is clearly revealed wit h the removal of type II years.