Transac. Oceanol. Limnol., 2, 26-40
Bin WANG and Yan WANG
Department of Meteorology, School of Ocean and Earth Science and Technology
University of Hawaii, Honolulu, Hawaii ABSTRACT
The El Niņo-Southern Oscillation (ENSO) cycle is demonstrated to be highly variable. The dominant frequency changes with time, resultuing in a broad band of period ranging from 2 to 7 years. In the mid-1960s the dominant period changed suddenly from 6 years to about 2 years. From the mid-1960s to mid-1970s the period increased gradually from 2 years to 4 years. After the mid-1970s the ENSO cycle has been a quasi 5 year oscillation with a significant quasi-biennial component, and the ENSO events bear considerable similarities in their transition from a cold to a warm state.
Multi-variate empirical orthogonal function (MV-EOF) analysis of the post-1970 data indicates that the most significant mode describes the mature phase of ENSO, and the second most significant mode describes an early stage in the ENSO warming. The development of a warm episode is characterized by (a) a planetary-scale low pressure anomaly in the tropical North Pacific, (b) an accompanying quasi-geostrophic cyclonic gyre with relatively strong westerly anomalies in the western Pacific and enhanced trades in the eastern equatorial Pacific, (c ) a convection anomaly everlapping with the westerly anomaly over the western Pacific due primarily to the meridional wind convergence. This mode displays a significant quasi-biennial component, in contrast to the frist EOF mode which depicts the mature phase of El Nino and exhibits a dominant 5-year period.
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