Anticorrelated intensity change of the quasi-biweekly and 30-50-day oscillations over the South China Sea

Over the South China Sea, the year-to-year variations in the intensity of the quasi-biweekly (QBW) and 30-50-day oscillations during June-July are anti-correlated. An explanation is offered for the out-of-phase relationship. We found during strong QBW years, the June-July mean convection and easterly vertical shear are enhanced over the equatorial westem-central Pacific, with corresponding low-level cyclonic meridional shear to the northwest. These conditions, concurring with the equatorial eastern Pacific warming, are favorable for emanation of moist Rossby waves from the equatorial western Pacific. On the other hand, the 30-50 day becomes active when the mean convection is enhanced over the eastern Indian Ocean and maritime continent where this mode stems from and develops, which often corresponds to cold western IO. One of these two large-scale settings often occurs in the absence of the other, so that when one mode is strong during June-July the other mode tends to be weak.