Abstract
In the present study, the northward-propagating intraseasonal oscillations (ISOs) over the South China Sea (SCS) during the summer monsoon season are divided into two types. One is the northward propagation of ISOs confined in the SCS (defined as P1-ISO), another for ISOs moving from the SCS to Southern China (SC) defined as P2-ISO. The spatial and temporal structures of the two types of ISOs are compared based on the analysis of the NCEP–NCAR reanalysis datasets. It is found that a significant difference appears in the heat flux anomalies over SC that is attributed to the cloud-radiation effect. A mechanism associated with the changes in the convective instability is proposed to understand the cause of the northward propagation of the ISO from the SCS to SC. For the P1-ISO, a more precipitation over SC leads to a less surface solar radiation and then a cold land surface temperature (LST), which induces an increase of static stability via surface longwave radiation and sensible heat flux. Whereas the total contribution to the convective instability changes does create an unfavorable for ISOs propagating northward, the moisture convergence, mainly controlled by the baroclinic vorticity advection effect, may contribute to an increase of convective instability that favors the ISOs moving northward. For the P2-ISO, a less precipitation over SC results in a more shortwave radiation reaching the ground that induces a warm LST, and thus leading to a warm low-level air temperature and destabilizing the atmosphere ahead of the convection. Moreover, the moisture convergence, dominated by the warm low-level air temperature, also contributes to the increase of convective instability. Thus the convective instability increment should favor the ISOs moving northward. It is worth noting that the baroclinic vorticity advection and moisture advection are not essential for ISOs propagating from the SCS to SC.
Highlights
The South China Sea (SCS) summer monsoon (SCSSM) is a subsystem of the Asian monsoon, with unique characteristics that different from those of other Asian monsoon subsystems
To understand key processes of the northward-propagating intraseasonal oscillations (ISOs) from the SCS to Southern China (SC), we divide the ISOs into P1-ISO with the northward propagation confined in the SCS (Fig. 3a) and P2-ISO that can move from the SCS to SC (Fig. 3b)
The change in convective instability over SC, mainly induced by the q′ and T′, is a key factor for the ISOs propagating from the SCS to SC
Summary
The South China Sea (SCS) summer monsoon (SCSSM) is a subsystem of the Asian monsoon, with unique characteristics that different from those of other Asian monsoon subsystems. The SCS intraseasonal oscillations (ISOs) usually include two periods of 10–20day and 30–60-day (Mao and Chan 2005; Li et al 2018; Zheng and Huang 2019), and impact the precipitation over Southern China (SC) with a northward or northwestward movement (Chen et al 2015; Li et al 2015; Gao et al 2016; Zheng and Huang 2018). The northwardpropagating BSISOs appear in South Asian summer monsoon (Yasunari 1979, 1980; Krishnamurti and Subrahmanyam 1982), the WNP and SCS summer monsoon regions (Murakami and He 1984; Chang et al 1996; Wang and Xie 1997; Zheng et al 2011; Zheng and Huang 2018), with a variance equivalent to the sum of the eastward- and westward-propagating modes (Lin et al 2010).
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