Tropical convective systems that grow larger than 100,000km2 sizes play a significant role in the water cycle and energy budget of the Earth system. Previously, we developed hybrid tropical cloud-precipitation regimes (TCPRs) derived from Moderate Resolution Imaging Spectroradiometer (MODIS) cloud observations and Integrated Multi-satellitE Retrievals for GPM (IMERG) precipitation data at a 1° scale, and demonstrated that TCPRs enabled a simple but effective identification of convective systems at the synoptic scale.

The Madden-Julian Oscillation (MJO) is the dominant mode of tropical intraseasonal variability, which is characterized as a planetary-scale envelop of convective clouds that propagates eastward over the Indo-Pacific warm pool. Recent studies showed a statistically robust correlation between the MJO and the quasi-biennial oscillation (QBO); MJO-related convective activities are enhanced and suppressed during an easterly and westerly phase of QBO, respectively. While the underlying mechanism of the MJO-QBO relationship has remained elusive, one of the most popular hypotheses is that the weakened stability in the upper troposphere and lower stratosphere during easterly QBO years provides a preferrable condition for deep convection to develop deeper and persist longer.

To test the stability hypothesis for the QBO control on the MJO, we examine properties of the convective aggregates of TCPRs in the southern Maritime Continent region, in which the contrast in MJO activities between easterly and westerly QBO years is most pronounced. By taking advantage of TCPRs, we composite the total size, fractions of stratiform clouds to core area, and top height of core for different phases of MJO and QBO, and the results are compared to find any systematic difference in the characteristics of convective aggregates. Our results show that, as consistent to previous studies, bigger convective aggregates tend to occur when the stability weakens. Further insight will be obtained by examining cloud radiative effects and atmospheric energy budget per convective aggregates.