Evidence of an extratropical atmospheric influence during the onset of the 1997-98 El Nino

The major 1997-98 El Nino episode was initiated by a series of very energetic westerly wind bursts in late 1996 and early 1997. Downwelling oceanic Kelvin waves were subsequently generated and propagated rapidly eastward across the equatorial Pacific Ocean and induced significant warming in the eastern basin. By analyzing newly available wind products derived from SSM/I satellite observations, we found indications that these westerly wind bursts were embedded within the active phase of the tropical intraseasonal oscillation (the Madden-Julian Oscillation) but their amplitudes were greatly enhanced in the western Pacific sector. The local enhancement involved the development of equatorial twin cyclones which themselves were induced by northerly cold surges from East Asia/Western North Pacific into the tropical Pacific, demonstrating an extratropical atmospheric influence on tropical processes.