Intraseasonal convective perturbations related to the seasonal march of the Indo-Pacific monsoons

The seasonal evolution of the 20 - 90- day intraseasonal variability ( ISV) of the convection in the Indo-Pacific region presents intriguing features related to monsoon dynamics: ( i) a sharp ISV maximum in May for the southern Bay of Bengal and in June for the eastern Arabian Sea, ( ii) a maximum ISV over the west Pacific in July - September when the ISV over the northern Indian Ocean is weaker, ( iii) a persistent ISV north of Australia from December to March, and ( iv) a weak ISV over continental regions. The source of these behaviors is investigated from time series of satellite observations, meteorological reanalysis, and from an ocean mixed layer depth ( MLD) climatology. For the northern Indian Ocean, sharp ISV maxima in May and June, when the MLD is still small ( similar to 20 - 30 m), are related to an abrupt surface cooling associated with the setting of the monsoon low- level jet. The ISV of the convection is smaller over these regions during July - September when the MLD is deepened ( similar to 60 - 70 m) by the monsoon low- level wind forcing. At this time, the low- level wind is weak over the west Pacific, the MLD is small, and the amplitude of the ISV remains large. North of Australia, and also over the south equatorial Indian Ocean, there is no particular reinforcement of the ISV near the onset. A weak low- level jet and a small MLD prevail during the whole monsoon season in good agreement with a more uniform ISV. These results support the hypothesis that a moderately shallow ( similar to 20 - 30 m) ocean mixed layer is important for maintaining locally the ISV of the convection, when the conditions ( e. g., SST, large- scale circulation) for such an ISV to exist are satisfied. The seasonal variability of the MLD, related in part to the monsoon low- level jet variability over the tropical oceans, thus gives a link between the seasonal march of monsoon circulations and the seasonal evolution of the ISV of the convection. This has implications for a better understanding of the origin of the ISV and for the predictability of its amplitude, especially near the monsoon onset date.