Contributions of Air-Sea Coupling to Northward-Propagating Intraseasonal Convection Over the South China Sea

Northward-propagating convection associated with intraseasonal oscillation (ISO) over the South China Sea (SCS) is closely linked to the evolution of the East China rainy season. In this study, we quantitatively examined the contribution of air-sea coupling to northward propagation of intraseasonal convection over the SCS based on observational diagnosis and model experiments. While oceanic feedback plays a limited role in northward propagation of Indian Ocean and western Pacific ISO, its positive effect on SCS ISO propagation is much larger and nearly comparable with that of atmospheric internal dynamics over the central SCS. Warm intraseasonal sea-surface temperature (SST) anomalies ahead of the convection induce boundary-layer convergence and surface turbulent fluxes, largely favoring growth of moist static energy to the north of the ISO convection. Conversely, weak negative SST anomalies induce divergence and reduced fluxes over the convective center. The underlying SST anomalies strongly promote northward propagation of SCS ISO, but destroy the maintenance of deep convection. We then verified these oceanic feedbacks using atmosphere-only (SPA) and atmosphere-ocean-mixed-layer coupled (SPK) configurations of the Super-Parameterized Community Atmospheric Model. The SPA simulations produced stronger intraseasonal convection but weaker propagation signals than the SPK simulations, confirming the positive contribution of intraseasonal SST to ISO propagation observed over the SCS. The onset of the South China Sea (SCS) summer monsoon is strongly linked with the beginning of both the East Asian summer monsoon and the rainy season in China. Notable intraseasonal signal exists in the SCS summer monsoon, which substantially affects the onset, outbreak, and rainfall amount of the SCS summer monsoon. As a response to the atmospheric activity, marked intraseasonal sea-surface temperature (SST) increases similar to 90 degrees before the active intraseasonal convection. The anomalous SST may also feedback to the atmosphere by inducing boundary-layer convergence and surface turbulent fluxes. However, the subject of oceanic feedback has received much less analysis and very limited qualitative discussion. Based on observational analysis and model experiments, our results show that warm SST ahead of the convection largely favors the northward propagation of SCS intraseasonal convection. This supporting role over the SCS is almost twice that of the Indian Ocean and the western Pacific. Therefore, accurate representation of the underlying ocean plays an important role in improving the numerical simulation and prediction skill of intraseasonal and monsoon activities over the SCS. Sea-surface temperature anomalies strongly promote northward propagation, but destroy the maintenance of intraseasonal convection over the South China Sea (SCS)Oceanic feedback shows greater contribution to propagation of intraseasonal convection over the SCS than over other tropical oceansPositive contribution of sea-surface temperature to intraseasonal convection is further confirmed by model simulations