Characteristic interdecadal change of quasi-biweekly and intraseasonal oscillations of summer convection over the South China Sea and the Western Pacific

Observational analyses indicate that there are significant interdecadal changes of quasi bi-weekly oscillation (QBWO) and lower frequency intra-seasonal oscillation (ISO) of convection in summer (May-October) over the South China Sea and the Western Pacific (SCS-WP) sector since the mid 1990s. For the QBWO modes, the northwestward propagation feature is clearly seen in both the periods, but its intensity is stronger in the later period (1998-2013). For the ISO modes, there is clear northeastward propagation of convection band in the tropical Indian Ocean in the later period and the convection band has a northwest-southeast tilted structure extending from the India Subcontinent to the Maritime Continent. Such feature is not seen in the earlier period. It is argued that the interdecadal change of the mean state is responsible for the QBWO and ISO intensity change. The increase of mean sea surface temperature in both the tropical Indian Ocean and the western Pacific in the later period and associated large-scale circulation changes such as enhanced rainfall and low-level moisture provide a favorable environmental condition for the growth of both the QBWO and ISO perturbations.