Monthly variability of Luzon Strait tropical cyclone intensification over the Northern South China Sea in recent decades

A number of tropical cyclones (TCs) in the western North Pacific (WNP) pass through Luzon Strait (LS) into the South China Sea (SCS) from June to November every year. The monthly variability of the ratio of TC intensity change, R _tc , shows that majority of the LSTCs achieve their lifetime maximum intensity (LMI) over the northern SCS (WNP) during August–September (June–July and October–November). Furthermore, compared to August, LSTCs in September are more easily intensified, suggesting that atmospheric and/or oceanic environments over the northern SCS in September are more favorable for TC development. The monthly-averaged oceanic and atmospheric environmental factors, including sea surface temperature, upper-ocean warm layer depth, vertical wind shear, relative humidity and large-scale low-level vorticity, are compared. The comparison between August and September is mainly studied because of the higher LSTCs frequency in these 2 months. The intensification tendency of LSTCs in September is primarily attributed to the relative thick upper-ocean warm layer and weak vertical wind shear. The transition of East Asian summer monsoon to winter monsoon tends to provide more favorable environmental conditions in September than in August for TC intensification in the northern SCS.