An extraordinarily persistent and heavy rainfall event in Hainan Island, from 1 October to 9 October 2010, was the heaviest event ever recorded in the region. The accumulated rainfall amounts during these 9 days exceeded 600 mm on southeastern Hainan Island, with extreme rainfall amounts of greater than 1000 mm in coastal areas. The heavy rainfall event (abbreviated as OCT10) was in association with one extended tropical depression (TD). Based on rain-gauge precipitation, satellite altimetry, in situ Argo profile, air–sea enthalpy flux, and reanalysis data, the warm oceanic anomaly from surface to subsurface layer in the central SCS, the strongest and longest lasting one in October over 29 years (1993–2021), was illustrated as significant element in the development of the prolonged TD. The objective of this study was to investigate the thermodynamic impact of such warm ocean anomaly on the genesis and maintenance of the extended TD, and its resultant reinforcing effect on TD-related persistent rainfall in Hainan Island during the OCT10 event.

The pre-existing warm sea surface temperature (SST) anomalies over the SCS aroused the formation of anomalous surface southerlies which supported the growth and maintenance of the the TD. And an extraordinarily thicker warm subsurface layer under the positive SST anomalies, with the tropical cyclone heat potential (TCHP) was as high as 97–108 KJ cm⁻², more than twice of the climate state (33.5–47.5 KJ cm⁻²). Such a thickened warm subsurface layer obviously contained sufficient TCHP to counteract the TD-induced cooling effects from both the thermocline and the sea surface. Indeed, daily SST cooling was only around 0.125 °C throughout the TD developing stage. Thus the available enthalpy fluxes were ample to sustain the extended TD.

The composite analyses were conducted to confirm that the warm ocean anomalies in the central SCS reinforced the heavy rainfall related to TCs in Hainan Island in October. From the 29-year data (1993–2021) in October, 12 cases where southward cold air accompanied by the southerly wind anomalies associated with TCs centered in the northern SCS were identified. Four of the incidents involve meso-scale warm eddies or warm anomalies that are stronger than the climatology in the central SCS, referred to as “Warm eddy” cases. The remaining eight individual cases exhibit “Normal” upper-ocean layers. In “Warm eddy” cases, the composite precipitation on the eastern coast of Hainan Island is five times more than in the “Normal” cases.