Weakening Trend of Luzon Strait Overflow Transport in the Past Two Decades

Abstract

The Luzon Strait overflow is a deep current of dense water flowing persistently into the South China Sea (SCS) from the Pacific. It is the primary driving force of the deep SCS circulation and potentially influenced by ocean mass changes. Limited by the lengths of in situ observations, long‐term changes in the Luzon Strait overflow transport (LSOT) have not been ascertained. Based on 14 years of satellite‐based ocean bottom pressure (OBP) time series, here we provide an evidence of a weakening of the LSOT by −0.43 Sv decade−1 between 2003 and 2016. Moreover, both OBP data and mooring time series reveal a large annual cycle in the LSOT. The weakening trend in the LSOT may profoundly influence the deep SCS circulation, as well as heat and mass fluxes between the SCS and surrounding oceans.

Decadal Weakening of Abyssal South China Sea Circulation

“…When the LPVF weakens, the domain‐averaged PV will decrease accordingly, accompanied by an anticyclonic boundary circulation anomaly to reduce the cyclonic circulation. The Luzon Strait overflow is the sole LPVF source flowing into the abyssal SCS basin and recently ascertained a weakening trend of $$-$$0.43 $$\mathrm{S}\mathrm{v}\,{\text{decade}}^{-1}$$ during 2003–2016 (Zhu et al., 2022). Thus, the associated LPVF change is, $$${\Delta}\mathrm{L}\mathrm{P}\mathrm{V}\mathrm{F}={\Delta}Qf/H,$$$ where $${\Delta}Q$$ is the Luzon Strait overflow change rate, $$f$$ is the typical Coriolis parameter at 20°N, and $$H$$ is the mean thickness of the Luzon Strait overflow (Zhu et al., 2017).…”

“…If taken $$H$$ = 850 m, the decadal change rate in the Luzon Strait overflow transport, $${\Delta}Q=-$$0.43 $$\mathrm{S}\mathrm{v}\,{\text{decade}}^{-1}$$ from Zhu et al. (2022), yields a decreasing $${\Delta}$$LPVF of $$-$$0.025 $${\mathrm{m}}^{2}{\mathrm{s}}^{-2}\mathrm{d}\mathrm{e}\mathrm{c}\mathrm{a}\mathrm{d}{\mathrm{e}}^{-1}$$. Meanwhile, the quantitative analysis of Gan et al.…”

“…The ECCO OBP data was recently validated by mooring data and proven to be a useful data set to effectively reflect circulation changes relative to the mean (Zhu et al., 2022). The annual cycle and interannual variation in the Luzon Strait transport changes derived from the ECCO OBP reached a substantial agreement with those from the mooring velocity observed during October 2009–April 2013 in the Luzon Trough by Zhou et al.…”

“…In the SCS, how does the abyssal circulation respond to the ocean mass changes is an important and interesting issue to be examined. Furthermore, the weakening of the Luzon Strait overflow (Zhu et al., 2022) may be also directly linked to the abyssal SCS circulation variation. Therefore, we are motivated to analyze the OBP changes in the SCS and explore the abyssal SCS circulation variation.…”

“…The Luzon Strait overflow is the sole LPVF source flowing into the abyssal SCS basin and recently ascertained a weakening trend of 𝐴𝐴 − 0.43𝐴𝐴 Sv decade −1 during 2003-2016(Zhu et al, 2022). Thus, the associated LPVF change is,…”