Interdecadal Decreases in Tropical Cyclone Activity Over the South China Sea During Fall and the Associated Interdecadal Index

ABSTRACT For the period 1979–2023, tropical cyclone (TC) activity in the South China Sea (SCS) during fall (September–November) exhibited an interdecadal decrease around 2000. The decrease occurred in the observed number of total TCs, migratory TCs moving from the western North Pacific (WNP) into the SCS, and local TCs forming within the SCS. Interdecadal decreases in TC activity from 1979–1999 to 2001–2023 are affected by interdecadal components of lower‐level anticyclonic anomalies extending from the Bay of Bengal across the SCS into the tropical WNP. These components effectively hinder TC genesis and subsequent passage frequency over the SCS and the tropical WNP, resulting in interdecadal decreases in TC activity over the SCS. These anomalous anticyclones owe their existence to significant interdecadal variations in sea surface temperatures (SSTs) over the southeastern Indian Ocean (74°–92° E, 16° S‐EQ; zone A), mid‐latitude North Pacific (170° E–160° W, 26°–40° N; zone B), and off‐equatorial eastern North Pacific (150°–126° W, 6°–24° N; zone C). Given these modulating processes, an interdecadal index based on areal‐mean values of interdecadal SST components over the above three zones is computed as C−(A + B)/2. This interdecadal index exhibits a relatively coherent phase with interdecadal variability in total TCs over the SCS. For these two variables, their interdecadal time series have a significant correlation coefficient of 0.89. Interdecadal SST variations over the above three zones reflect salient variability features of the Pacific Decadal Oscillation (PDO). On the other hand, the linear trend feature in the global warming time series is obviously different from the interdecadal oscillation feature in the total TC time series over the SCS. The PDO plays a more important role than global warming in modulating interdecadal variability of TC activity over the SCS during fall.