Just prior to these two dates, a hybrid combination of an Equatorial Rosby wave and Equatorial Kevin wave, known as a Madden Julian Oscillation (MJO), past just north of Papua New Guinea.
Associated with the Rosby wave component of the MJO were a pair of tropical low-pressure cells located on either side of the Equator that produced a burst of strong westerly winds that dramatically reduced the strength of the West Pacific trade winds a few days later.
The figure below displays the time-longitude plots for 850 hPa westerly wind anomalies covering this period of time. It clearly shows the Aug-Sep and Oct-Nov MJO events that "kill" the West Pacific trade winds around September 17th and November 10th.
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