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Tropical‐wide teleconnection and oscillation. II: The enso‐monsoon system
Author(s) -
Miykoda K.,
Navarra A,
Ward M. N.
Publication year - 1999
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49712556008
Subject(s) - teleconnection , climatology , walker circulation , monsoon , el niño southern oscillation , multivariate enso index , southern oscillation , oscillation (cell signaling) , la niña , atmospheric circulation , southern hemisphere , pacific decadal oscillation , quasi biennial oscillation , tropics , environmental science , atmospheric sciences , geology , troposphere , biology , genetics , fishery
Two teleconnection indices, discussed in Part I, i.e. the Tropical‐wide Oscillation Index (TOI) and the Walker circulation Index (WAI), are applied to the analysis of the ENSO‐monsoon (El Niño Southern Oscillation‐Asian monsoons) 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 represents the lead‐lag characteristics of the tropical circulation variability over the eastern hemisphere (45°E‐170°E) and simultaneously its interaction with the ENSO over the equatorial Pacific. the second hypothesis was that there are two types of connection between the ENSO and Asian monsoons: type I with distinct connection in space and time, and type II without connection. the WAI provides a measure for this connection. This idea is supported by comparisons of observed and model teleconnection structures in Part I. Part II investigates these relations further. Time‐lag correlations are calculated between the key indices and atmospheric variables over the equatorial Indo‐Pacific Oceans. If type II years, derived by the WAI, are removed from the 34‐year time series, correlations between the TOI and these variables increase appreciably, now showing clearly the biennial character. the analysis identifies a sequence of events involving biennial oscillation of the ENSO‐monsoon system from approximately JAS(‐1) to JAS(O), followed by intensification of the ENSO from JAS(O) 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 events, planar maps are presented of the lag correlation between the observed TOI(JAS) and (i) vertical velocity at the 500hPa level, (ii) precipitation, (iii) sea surface temperature (SST), and (iv) atmospheric sea level pressure. Distinct geographical distributions of the ENSO‐monsoon oscillation emerge 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 oscillation located in the eastern South Pacific and the Indian Oceans, resembling the sea‐level‐pressure pattern found by Trenberth and Shea. Applying the lead‐lag relationship, it is demonstrated that the SST over the central Pacific four months ahead can be projected, based on the TOI(JAS). Conversely, the intensity of the Indian monsoon rainfall for non‐type II years can be projected 15 months ahead by the SST over the eastern Pacific Ocean. This indicates that the ENSO‐monsoon oscillation system is quasi‐periodic, as opposed to irregular, with a two‐year cycle; this is clearly revealed with the removal of type II years.