Climatological characteristics of the synoptic changes accompanying South China Sea summer monsoon withdrawal

Abstract

This manuscript analyses the climatological characteristics (especially the synoptic phenomena and changes) of the South China Sea (SCS) summer monsoon withdrawal (SCSSMW) based on monsoon retreat dates from the National Climate Center of China Meteorological Administration. The SCSSMW is mainly defined as the westerly to easterly shift in the zonal winds due to the westwards intrusion of the western North Pacific (WNP) subtropical high in the northern SCS region. At the low level (850 hPa), the weakening and retreat of the intertropical convergence zone (ITCZ) and rain belt in the SCS–WNP are pertinent, and the southwesterly winds over the north Indian Ocean and the SCS also weaken and retreat. The anomalous anticyclone centred over the northern SCS resembles a Rossby wave response to the reduced precipitation over the SCS and the Philippine Sea. Changes in the upper‐level (200 hPa) circulations include the deceleration of the tropical easterly stream (from the Indo‐China Peninsula to the Arabian Sea) and northerly cross‐equatorial flow (around the equatorial eastern Indian Ocean and Maritime Continent). At the mid‐level (500 hPa), the ascending centre moves equatorwards, shifting from the northern to the southern SCS. There appears an anomalous vertical circulation cell (descending/ascending in the northern/southern SCS), which links the anomalous horizontal circulation through the Sverdrup balance. In addition, the upper‐level divergent centre is shifted southeastwards, as is the generating centre of tropical cyclones. Moreover, an anomalous convergent centre over Taiwan Island is prominent in the upper level. The time series of several atmospheric variables (e.g., zonal wind, precipitation, convection) also exhibited abrupt changes during the SCSSMW. The reasons for not simultaneous retreat of monsoonal westerlies and rainy season in the SCS are also explored.

Statistical analysis of the impacts of intra‐seasonal oscillations on the South China Sea summer monsoon withdrawal

“…Most of the SCSSM withdrawal pentad determined by National Climate Center () and Zhang et al . () are the same, and their correlation is as high as 0.74 during 1951–2102, which is significant at the 99.9% confidence level according to the two‐tailed Student's t test (Hu et al ., ).…”

“…The enhanced convection and anomalous cyclone patterns seen in Figure b are not only statistically significant but also physically coherent due to the coupling of tropical circulation and rainfall, which is explained as follows: the anomalous cyclone pattern is located to the northwest of the enhanced convection, which can be regarded as a Rossby wave response to latent heating (Matsuno, ; Gill, ; Wang et al ., ; Hu et al ., ; ). This Rossby wave can accelerate the westerly wind south and southwest of the strongest convection.…”

“…Furthermore, there appears to be significant enhanced convection over the SCS–Philippine Sea (Figure b). Since the SCSSM is characterized by strong low‐level southwesterly winds and abundant rainfall (Wang et al ., ; Hu et al ., ), the above features of atmospheric circulation (i.e., anomalous westerly) and convection anomalies (i.e., enhanced convection) correspond well to the pattern of a delayed monsoon withdrawal.…”

“…In addition, Hu et al . () showed that SCSSM withdrawal tends to occur during the inactive phases of the MJO. However, the process by which this occurs remains unexplained and, thus, will be investigated in the following sections.…”

“…Hu et al . () found that the Madden–Julian oscillation (MJO; Madden and Julian, ) could modulate monsoon withdrawal over the SCS, which is more likely to occur during local inactive phases. The extremely late SCSSM withdrawal in 2010 is suggested to result from the combination of the 30–60‐day oscillation and the quasi‐biweekly oscillation (QBWO; Hu et al ., ).…”