Progress of Studies on Circulation Dynamics in the East China Sea: The Kuroshio Exchanges With the Shelf Currents

Abstract

This paper reviews recent advances in the circulation dynamics of the Kuroshio and its interaction with shelf currents in the East China Sea (ECS). The annually averaged Kuroshio volume transport varies between 19 and 24 Sv, based on different observations, but there is no consensus on which season its volume transport peaks. The Kuroshio is intensified over the central slope of the ECS from that off the northeast of Taiwan. The total Kuroshio intrusion into the ECS shelf is estimated to be 1.3–1.4 Sv, deduced from the observed volume transport of exchange flow in the Taiwan and Tsushima Straits, based on the assumption of volume conversation over the shelf. However, the uncertainty regarding this estimation remains due to the absence of sufficient observations and understanding of the Kuroshio dynamics. The Kuroshio intrusions over the shelf off the northeast of Taiwan and southwest of Kyushu are stimulated by planetary or topographic β -effect associated with the alongshore variations in the ECS slope topography and altered by variations in the Kuroshio intensity, shear stress, and baroclinicity. Multilayered exchanges between the Kuroshio and shelf currents were found between 100- and 200-m isobaths along the central ECS slope. The spatial variations in these exchanges are governed by cross-isobath transport by geostrophy, whereas bottom Ekman transport may play a predominant role in altering the integrated exchange flow along the slope. Although the intrusion is greatly modulated along the path of the Kuroshio in the ECS by variable slope topography, there are few observations on the spatial variations of these exchange flows. The characteristics and variations in the circulation and hydrographic properties of waters between 100- and 200-m isobaths significantly determine the general ECS circulation, about which consensus has still not been attained.

Advances on Coastal and Estuarine Circulations Around the Changjiang Estuary in the Recent Decades (2000–2020)

“…The Kuroshio and the intrinsic characteristics of the ECS shelf current act as the major external factors that influence coastal and estuarine circulation. The annually average volume transport of the Kuroshio is ∼24.37 Sv in the central ECS (Liu et al, 2021), and a stronger/weaker Kuroshio is usually present in the warm/cold half of the year in the northern hemisphere (Yang et al, 2018). Observations based on the conservation of the total water volume over the shelf reveal that the integrated volume transport by the Kuroshio intrusion into the ECS is ∼1.3-1.4 Sv (Isobe, 2008).…”

Enhanced Diapycnal Mixing in the Deep Ocean Around the Island of Taiwan

“…In the distance range of ∼110–130 km close to the Ryukyu Arc, water layers are strongly uplifted over 100 m toward the steep southern flank. In the distance range of ∼50–110 km atop the Ryukyu Arc, the stratification also tilts southwardly, forming a front zone between the Kuroshio Water and the East China Sea Shelf Water (Liu et al., 2021; Sheu et al., 2009). Although reflections are strongly interrupted, their reflection strengths are significantly enhanced probably due to frontal strengthening of temperature/density contrasts at the periphery of the Kuroshio or due to an eddy carrying Kuroshio Water.…”

Long-Term Variability of the Kuroshio Shelf Intrusion and Its Relationship to Upper-Ocean Current and Temperature Variability in the East China Sea

“…With regards to current speed, the shelf region of the ECS shows vertically homogeneous changes according to KSI variability. The onshore advection of offshore water masses during strong KSI phases may accompany the transport of heat, salt, and nutrients, thereby influencing the regional ECS ecosystem (Sassa and Konishi, 2015;Chen et al, 2016;Lian et al, 2016;Liu et al, 2021). In the case of temperature, strengthening or weakening of the KSI induces different changes in the near-surface and deeper layers.…”

“…The KSI induces an exchange of water and materials between the deep western boundary current region and the shallow continental shelf region, and thereby changes the hydrographic properties in the ECS (Chern et al, 1990;Chen et al, 1995;Lian et al, 2016). The KSI is also responsible for variations in regional ocean circulation, climate, and ecosystem in the ECS (Chow et al, 2015;Sassa and Konishi, 2015;Chen et al, 2016;Chen and Guo, 2020;Liu et al, 2021).…”