Seasonal evolution of the intraseasonal variability and its link with the upper ocean structure

The seasonal evolution of the 20–90-day intraseasonal variability (ISV) of the convection in the Indo-Pacific region is not characterized only by the well known eastward MJO perturbation during boreal winter and by the northeastward perturbation during boreal summer. Inspecting this seasonal cycle in more details reveals intriguing features related to monsoon dynamics such as: (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. These strong seasonal specificities are also objectively obtained by applying a hierarchical agglomerative classification (HAC) technique to the ensemble of intraseasonal events that can be extracted using a Local Mode Analysis (LMA) using the 1979-2006 OLR time series. The results of this HAC also show that the intraseasonal type varies with the SST field and then, but not only, in relation with the annual cycle and ENSO. Some types are mostly active over one single basin, confirming that the large-scale inter-basin organization of the convective perturbation is not a necessary condition for the existence of local intraseasonal perturbations.

The source of the ISV seasonal cycle is further studied using NOAA-OLR, NCEP re-analyses and a climatology of the seasonal cycle of the ocean mixed layer depth (MLD). For the northern Indian Ocean, sharp ISV maxima in May and June are related to an abrupt surface cooling associated with the setting of the monsoon low-level jet when the MLD is still thin (∼20 – 30 m). The ISV of the convection is small over these regions during July–September when the monsoon low-level wind forcing deepens the MLD (∼60 – 70 m). At this time, the low-level wind is weak over the west Pacific, the MLD is thin, 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. For these regions, a weak low-level jet, a thin MLD and a quite uniform ISV prevail during the whole monsoon season. These results support the hypothesis that a moderately shallow (∼20–30 m) ocean mixed layer is important to generate intraseasonal events, providing that other conditions (e.g., SST filed, large-scale circulation) are satisfied.