Development of subsurface warm water in the East China Sea in fall

[1] The development of subsurface warm water (SSWW) in the East China Sea (ECS) in fall is analyzed using two observational data sets and the results of a one-dimensional numerical model. During October, the SSWW is developed in the ECS and remains till the end of November. The analysis shows that it is the Changjiang River (also known as Yangtze River) diluted water (CRDW) that maintains the vertical structure of water column with a subsurface maximum temperature in this region. In fall, the diluted water flows southward in the ECS, leading to a lower salinity layer in the upper ocean. The existence of this lower salinity layer limits the depth of the vertical mixing induced by the loss of heat on the sea surface in fall, and therefore, maintains the higher temperature of the subsurface water. In winter, when the density's vertical gradient of the low salinity layer cannot balance the negative surface buoyancy flux, the SSWW vanishes. However, this situation does not occur outside the region where the CRDW extends, because the stratification becomes unstable while the sea surface loses heat and the convective overturn will readjust the whole water column. Numerical results of a one-dimensional model reveal that the SSWW appears if the impact of the CRDW is included in the model; otherwise, the SSWW does not occur.