Weakening of the Kuroshio Intrusion into the South China Sea over the Past Two Decades

Abstract

Inferred from the satellite and in situ hydrographic data from the 1990s and 2000s, the Kuroshio intrusion into the South China Sea (SCS) had a weakening trend over the past two decades. Associated with the weakened Kuroshio intrusion, the Kuroshio loop and eddy activity southwest of Taiwan became weaker, whereas the water above the salinity minimum became less saline in the northern SCS. The sea surface height southwest of Taiwan increased at a slower rate compared to other regions of the SCS because of the weakened Kuroshio intrusion. Simulations using the Regional Ocean Modeling System (ROMS) Pacific model show that the strength of the Kuroshio intrusion into the SCS decreased from 1993 to 2010 with a negative trend, −0.24 sverdrups (Sv) yr−1 (1 Sv ≡ 106 m3 s−1), in the total Luzon Strait transport (LST). Although wind-induced Ekman transport through the Luzon Strait became weaker, the magnitude at 0.001 Sv yr−1 was too small to compensate for the negative trend of the LST. On the other hand, the piling up of the water induced by monsoon winds was an important mechanism for changing the pressure gradient across the Luzon Strait and eventually affecting the LST. The sea level gradient between the western Pacific and the SCS had a negative trend, −0.10 cm yr−1, corresponding to a negative trend in the geostrophic transport at −0.20 Sv yr−1. The Kuroshio transport east of Luzon Island also had a negative trend, which might also be linked to the weakening Kuroshio intrusion.

Comparison and validation of global and regional ocean forecasting systems for the South China Sea

“…The Kuroshio intrudes into the SCS through the LUS, carrying the warm and salty water from the NWP, significantly affecting the circulation pattern and the budgets of heat and salt in the NSCS (Farris and Wimbush, 1996;Wu and Chiang, 2007;Liang et al, 2008;Nan et al, 2013). However, this is still not in accordance with how the Kuroshio intrudes into the NSCS.…”

Toward a Mesoscale Hydrological and Marine Meteorological Observation Network in the South China Sea

“…The minimum distance between cruise stations is approximately 20 km, which is within the Rossby radius of deformation of both the ocean (~20-120 km; Cai et al 2008) and atmosphere (10-200 km, with the time scale from hours to days for mesoscale processes; Fujita 1981). Subsequently, the observations from these cruises and fixed stations have been capable of resolving mesoscale processes, such as eddies (Nan et al 2011;He et al 2013), fronts , and coastal upwelling (Wang et al 2013a) in the ocean, as well as synoptic weather processes such as cold surges and tropical cyclones in the atmosphere. For example, Li et al (2006) investigated the impact of cold surge on SST and air-sea heat fluxes.…”

“…Then, from 1989 to 1999, several cruises took place and yielded larger datasets for the southern SCS (SSCS) (Fang et al 2002). At that time, there were only a few cruise stations in the central SCS (between 12° and 18°N) and around the dynamically energetic regions (e.g., west of the Luzon Strait and east of the Vietnam coast), where mesoscale eddies are active (G. Chen et al 2010;Zhuang et al 2010;Nan et al 2011). Most of these earlier observations did not include meteorological observations, consisting only of conductivity-temperature-depth (CTD) measurements in the ocean.…”

“…For example, the variation of the MABL height in the SCS before and after SCS monsoon onset has been well depicted by sounding data (Wang et al 2010b). The vertical structure of mesoscale eddies can only be captured by in situ measurements (e.g., Nan et al 2011). Furthermore, in situ observations can also promote the study of the formation of the SCS warm current in the intermediate ocean layer of the northern SCS, which is mainly controlled by baroclinicity combined with topography and is normally referred to as the joint effect of baroclinicity and relief (JEBAR).…”

Regime shift of the South China Sea SST in the late 1990s

“…Inter-basin pressure gradient between the northwest Pacific Ocean and the SCS is suggested be an important mechanism influencing the LST and hence the SCSTF. Nan et al (2013) showed that the pressure gradient between these two basins has a negative trend in last two decades. The basin-mean rate of sea level rise in the SCS during 1993-2012 is 5 mm year −1 and about 72 % (3.6 mm year −1 ) of the total sea level rise is contributed by the PDO (Cheng et al 2015).…”