Satellite observations of sea level anomalies(SLA) from January 1993 to December 2012 are used to investigate the interannual to decadal changes of the boreal spring high SLA in the western South China Sea(SCS) using the Empirical Orthogonal Function(EOF) method. We find that the SLA variability has two dominant modes. The Sea Level Changing Mode(SLCM) occurs mainly during La Ni?a years, with high SLA extension from west of Luzon to the eastern coast of Vietnam along the central basin of the SCS, and is likely induced by the increment of the ocean heat content. The Anticyclonic Eddy Mode(AEM) occurs mainly during El Ni?o years and appears to be triggered by the negative wind curl anomalies within the central SCS. In addition, the spring high SLA in the western SCS experienced a quasi-decadal change during 1993–2012; in other words, the AEM predominated during 1993–1998 and 2002–2005, while the La Ni?a-related SLCM prevailed during 1999–2001 and 2006–2012. Moreover, we suggest that the accelerated sea level rise in the SCS during 2005–2012 makes the SLCM the leading mode over the past two decades.
This study analyzes monthly variability of thermocline and its mechanism in the South China Sea(SCS). The study is based on 51-year(1960–2010) monthly seawater temperature and surface wind stress data from Simple Ocean Data Assimilation(SODA), together with heat flux, precipitation and evaporation data from the National Centers for Environmental Prediction(NCEP), the National Oceanic and Atmospheric Administration(NOAA) and the Woods Hole Oceanographic Institution, respectively. The results reveal that the upper boundary depth(Z_(up)), lower boundary depth(Z _(low)), thickness(?Z) and intensity( T _z) of thermocline in the SCS show remarkable monthly variability. Being averaged for the deep basin of SCS, Z_(up) deepens gradually from May to the following January and then shoals from February to May, while Z low varies little throughout the whole year. Further diagnostics indicates that the monthly variability of Z_(up) is mainly caused by the buoyancy flux and wind stress curl. Using a linear method, the impacts of the buoyancy flux and wind stress curl on Z_(up) can be quantitatively distinguished. The results suggest that Z_(up) tends to deepen about 4.6 m when the buoyancy flux increases by 1×10^(-5) kg/(m?s ~3), while it shoals about 2.5 m when the wind stress curl strengthens by 1×10-^(7) N/m3.
The effects of typhoon intrusion on the Guangdong coastal upwelling system were investigated on the basis of in situ CTD (conductivity-temperature-depth) cruise observations and especially upward-looking ADCP (Acoustic Doppler Current Profil- ers) measurements obtained from a comprehensive survey of the Guangdong coastal region carried out by the Chinese Off-shore Investigation and Assessment Project in the summer of 2006. It was found that northeastward geostrophic advection driven by the summer monsoon has a significant near-seabed onshore component adjacent to Shantou, which in conjunction with upper-level offshore Ekman flow, constitutes the canonical Guangdong coastal upwelling system. Further analyses suggested that the Guangdong coastal upwelling system is sensitive to subtle changes in the typhoon intensity and migration pathway. On one hand, as a typhoon approaches from north of the upwelling system (e.g. Typhoon 0604 (Bilis) and Typhoon 0605 (Kaemi)) in the early phase of intrusion, the enhanced southwesterly leads to exceptional enhancement of the onshore flow; i.e., enhanced upwelling. Afterward, irrespective of the forced ocean responses resulting from the stronger local winds (Typhoon 0604) or the moderate typhoon-induced inertial oscillations (Typhoon 0605), the situation is not conducive to sustaining a stable, persistent upwelling system. On the other hand, when there is typhoon intrusion south of the upwelling system (e.g. Typhoon 0606 (Prapiroon)), the favorable southwesterly tends to be substituted by an anomalous northeasterly, which destroys the traditional coastal upwelling pattern. However, the canonical upwelling system tends to recover within 1-2 days of the typhoon passing.
PAN AiJunGUO XiaoGangXU JinDianHUANG JiangWAN XiaoFang
This paper investigates the response of the thermocline depth(TD) in the South China Sea(SCS) to the El Ni?o-Southern Oscillation(ENSO) events using 51-year(from 1960 to 2010) monthly seawater temperature and surface wind stress data acquired from the Simple Ocean Data Assimilation(SODA), together with heat flux data from the National Centers for Environmental Prediction(NCEP), precipitation data from the National Oceanic and Atmospheric Administration(NOAA) and evaporation data from the Woods Hole Oceanographic Institution(WHOI). It is indicated that the response of the SCS TD to the El Ni?o or La Ni?a events is in opposite phase. On one hand, the spatial-averaged TDs in the SCS(deeper than 200 m) appear as negative and positive anomalies during the mature phase of the El Ni?o and La Ni?a events, respectively. On the other hand, from June of the El Ni?o year to the subsequent April, the spatial patterns of TD in the north and south of 12°N appear as negative and positive anomalies, respectively, but present positive and negative anomalies for the La Ni?a case. However, positive and negative TD anomalies occur almost in the entire SCS in May of the subsequent year of the El Ni?o and La Ni?a events, respectively. It is suggested that the response of the TD in the SCS to the ENSO events is mainly caused by the sea surface buoyancy flux and the wind stress curl.
