Interaction of the Monsoon and Pacific Trade Wind System at Interannual Time Scales Part I: The Equatorial Zone

The time history of the Monsoon System over the Indian Ocean has been developed from ship observations and merged with the Wyrtki-Meyers Pacific Trade Wind field. The interaction of these two massive wind systems has been studied by a rather new empirical orthogonal function (EOF) analysis capable of detecting propagating features in the wind systems. The current study (Part I) was confined to variations within ±10° of the equator. Results show the two wind systems are strongly coupled at interannual time scales. The coupling is effected through cyclostationary pulsations and longitudinal shifts of the huge surface convergence over Indonesia. The interaction may also he thought of as the spatial expansion/contraction of the wind systems. These changes can be viewed as the transition of the Monsoon/Trade Winds between two preferred climate states. One sub-element of this apparent bimodality in the wind fields is the El Niño phenomenon. The zonal component of the combined wind fields seems to instigate the large-scale interaction noted above. Perturbations in the u-component are composed of two interesting elements. One is a traveling disturbance which moves from the Indian Ocean eastward into the Pacific, and has many features of an equatorially trapped Kelvin wave. The second is a standing wave pattern which has maxima in the Indian and western Pacific with a node over Indonesia (the Walker cell). Thew perturbations preceed El Niño events and are phase-locked to the seasonal cycle. Their time variation can be viewed as a frequency-modulated process. A quasi-biennial oscillation appears in both wind fields as a standing wave pattern with in-phase maxima off the cast coast of Africa and west cost of South America. Subsequent papers will describe the interaction of the Monsoon/Trade Winds system in the band ±30° of the equator (Part II) and, in Part III, build a mechanistic, physical picture for the results obtained in Parts I and II.