Summer monsoon–induced upwelling dominated coastal sea surface temperature variations in the northern South China Sea over the last two millennia

Abstract

The South China Sea (SCS), situated to the north of the Indo-Pacific Warm Pool (IPWP), is under the strong influence of the Asian monsoon system. However, coastal sea surface temperature (SST) records from the SCS, which are of vital importance to exploring ocean-atmosphere-land interactions behind the Asian monsoon system, remain scarce. Here, we use a sediment core collected at the coast of northern SCS to investigate alkenone-SST variations over the past two millennia. On multi-centennial timescale, SST changes in our record exhibit an opposite pattern to that of Northern Hemisphere temperature and solar irradiance, for example, relatively cool SST during the Medieval Warm Period (MWP) and warm conditions during the Little Ice Age (LIA). Together with alkenone content and existing records, we suggest that the regional SST changes result from a strengthening (weakening) of wind-driven coastal upwelling, associated with variability of the Asian summer monsoon intensity during the MWP (LIA).

Asian Winter Monsoon Imprint on the Water Column Structure at the Northern South China Sea Coast

“…In analogy with this scenario, it is possible that a stronger winter AMC during the LIA would have promoted vertical mixing of the onsite water column which; as a result, it would have brought more cold waters and production of (halophilic) Euryarchaeota/ Archaea community (archaeol, the major driver of ACE values) at the subsurface layers toward upward, thereby decreasing SSTs while increasing its salinity. Notably, during the LIA cold interval, a less input of riverine discharge like the Pearl River drainage, due to the concomitant reduction of summer AMC intensity, as effectively corroborated by a growing body of compelling and independent evidence (e.g., Dykoski et al, 2005;Wang et al, 2005;Zhang et al, 2008;Wang et al, 2012;Lee et al, 2019), may have also somewhat contributed to the inferred salinity increase here. Because these two processes are naturally coupled together from a climatological perspective, it is still difficult to assuredly claim which should play a major role in driving the higher salinity during the LIA.…”

Asian Winter Monsoon Imprint on Holocene SST Changes at the Northern Coast of the South China Sea

“…The U K ′ 37 -SST record shows warm conditions during the HCO and MCA and remarkably cool temperatures during the LIA. In addition, hopane compounds exhibit (Lee et al, 2019). The color bars and green arrow line outline the same intervals and SST gradient, respectively, as in Figure 3, and the dashed gray lines hint at the correspondence of three independent records within their chronological uncertainties.…”

“…The reason behind this is that the prevailing southwesterly winds, activated along with the ASM, are apparently reversed in direction to deliver land‐sourced biomass toward the northern coast of the SCS (Figure b). In addition, the postulated ASM enhancement over these cool intervals also contradicts a growing body of compelling evidence in line with a weakened ASM for the same time slices (Hu et al, ; Lee et al, ; Wang et al, ). Instead, variations in AWM strength could feasibly explain our multiple proxy records for both cold and warm intervals from multicentennial to millennial scales.…”

“…The last millennium, in which both the LIA and MCA emerged as climate anomalies (Mann et al, 2008), is an excellent time window to disentangle the spatial structures of how northern SCS coastal conditions interact with the Asian monsoon system (Figure 4). Our recent work on another sediment core HKGS-A, in the southeast waters offshore of Hong Kong (Figure 1b) actually revealed the impact of summer monsoon-driven upwelling on coastal SST changes in the northern SCS (Lee et al, 2019). It is evident that the ASM cooling effect outweighed the original signal of global temperature change, resulting in relatively warm SST during the LIA and cool conditions during the MCA (Figure 4d).…”

Comparison of the U<sub>37</sub><sup>K<sup>′</sup></sup>, LDI, TEX<sub>86</sub><sup>H</sup>, and RI-OH temperature proxies in sediments from the northern shelf of the South China Sea

“…Wang et al, 2015. However, it is still a matter of debate whether MG-II Euryarchaeota can produce iGDGTs or not (e.g., Lincoln et al, 2014;Schouten et al, 2014;Besseling et al, 2020;Ma et al, 2020) due mainly to a current lack of cultured representatives of MG-II Euryarchaeota.…”