Interdecadal Change in South China Sea Tropical Cyclone Frequency in Association with Zonal Sea Surface Temperature Gradient

This study investigates the interdecadal changes in summertime tropical cyclone (TC) frequency over the South China Sea (SCS) during 1979–2010. Based on changepoint detection algorithms and spectral analysis, two inactive TC periods (period 1: 1979–93 and period 3: 2003–10) and one active TC period (period 2: 1994–2002) have been identified, with a dominant spectral peak of approximately 9–10 yr. Correlation analysis further reveals a significant negative relationship between TC frequency and the zonal sea surface temperature gradient (ZSG) between the northern Indian Ocean (NIO) and the western North Pacific (WNP) at both interannual and interdecadal time scales. That is, a positive ZSG between the NIO and the WNP tends to suppress cyclogenesis over the SCS, whereas a negative ZSG is generally favorable for SCS TC formation. The negative connection between cyclogenesis and ZSG may be explained by the influences of the ZSG on atmospheric circulations as well as Madden–Julian oscillation (MJO) activity over the SCS, which reveal prominent contrasts during the study periods. A positive ZSG between the tropical Pacific and the Indian Ocean induces an anomalous Walker-like circulation, which results in an anomalous subsidence and boundary layer divergence over the northern SCS. This also suppresses the moisture as well as MJO activity over the SCS, leading to a significant reduction in TC frequency during inactive periods 1 and 3. In contrast, a negative ZSG induces surface westerlies and favorable environmental conditions for TCs, thereby greatly enhancing SCS cyclogenesis during period 2.