Diverse relationship between the tropical night in South China and the water vapor transport over the South China Sea and the plausible causes

Abstract

South China (SC) frequently suffers from tropical night (TN), which features extreme high nighttime temperature with abnormal high humidity usually. This study explores the relationship between the TN in SC and the water vapor transport (WVT) over the South China Sea (SCS), an important water vapor source. 64% and 36% of the TN occurs under abnormally increased and decreased SCS WVT (named positive and negative SCS-WVT cases), manifesting their diverse relationship. The positive SCS-WVT cases feature above-normal humidity and subsidence over SC, favoring TN via adiabatic heating, longwave radiation and sensible heat persistently from day to night. The negative SCS-WVT cases feature strong subsiding anomaly over SC, which not only induces adiabatic heating, but also reduces the cloud cover, increases the daytime solar insolation and provides pre heating for TN.The diverse relationship between the TN in SC and SCS WVT is modulated by an anomalous anticyclone over the western North Pacific. For positive SCS-WVT cases, the anticyclone is related to two suppressed convection originating from the southwest and south of the subtropical high and moves westward. The anticyclone is centered east of the SCS on TN onset, inducing anomalous southerly SCS WVT. For negative SCS-WVT cases, the anticyclone is related to a suppressed convection originating from the southwest of the subtropical high and moves northwestward. The anticyclone is centered northwest of the SCS on TN onset, inducing anomalous northerly SCS WVT. SC is dominated by the anomalous subsidence in the northwest (east) of the anticyclone for positive (negative) SCS-WVT cases, and the subsidence is stronger for negative cases due to the vertically upward increase of negative vorticity advection associated with a northward-tilting structure. It is highlighted that different structures of anticyclonic anomaly could result in distinct features of extreme high temperature.