Alongshore and cross-shore circulations and their response to winter monsoon in the western East China Sea

Abstract

a b s t r a c tAn array of four bottom-mounted acoustic Doppler current profilers (ADCPs) were deployed during the winter of 2008 (28 December 2008 to 12 March 2009) along a cross-shelf section in the western East China Sea to investigate the winter circulation and its response to wind. During the observation period, the observed subtidal currents exhibit coherent spatial structure and temporal variation in terms of their mean (seasonal), trend (intra-seasonal), and synoptic variability. The subtidal currents are polarized roughly in the alongshore direction parallel to local isobaths, and the weak cross-shore current is closely linked to the alongshore component. The temporal variation of the currents follows the rhythm of wind stress, sea level, and sea level difference at the synoptic scale.The mean currents are basically composed of two anti-parallel currents in the alongshore direction: the East China Sea coastal current (ECSCC) flows southwestward along the inner shelf and the Taiwan warm current (TWC) flows in the opposite direction along the outer-shelf. The strongest current occurs over the mid-shelf as a coastal jet. The intra-seasonal currents exhibit an expansion and intensification of the ECSCC along with shrinking and weakening the alongshore component of the TWC. There is a significant increase in onshore current particularly over the mid-shelf.The fluctuations of synoptic currents show a significant positive correlation with wind stress, and the fluctuations are negatively correlated with sea level and sea level difference. The coherent spatial structure of the currents indicates that the depth-independent column motion is related to the sea level difference through a barotropic pressure gradient. The vertical shear of currents is related to the densityrelated baroclinic pressure gradient in the whole water column and to the friction within the surface and bottom boundary layers.

Synoptic fluctuation of the Taiwan Warm Current in winter on the East China Sea shelf

“…This episodic occurrence of the TWC agrees with the results from a previous study based on four mooring surveys off the Zhe-Min coast (Zeng et al, 2012). The mechanism of the episodic occurrence of the TWC was mainly associated with the winter monsoon, which agrees with the analysis of observational data by Huang et al (2016). However, the overall magnitude of the TWC fluctuation, and its role in the cross-shore flux, is still not fully understood due to the short-term nature of the observational data.…”

“…Previous studies on the wintertime TWC were based on cruise surveys (Su and Pan, 1987;Chen et al, 1994;Chen and Wang, 1999), anchored mooring observations (Zhu et al, 2004;Zeng et al, 2012;Huang et al, 2016) and numerical simulations (Guo et al, 2003(Guo et al, , 2006Yang et al, 2011Yang et al, , 2012Xuan et al, 2012aXuan et al, , 2016. The observation data are limited in terms of temporal and spatial coverage; hence, they cannot fully reveal the synoptic fluctuations of the TWC and their regional differences.…”

“…The observed and simulated currents were both averaged for the observational period, which was from 1 January to 28 February 2009. Using the same method as in Huang et al (2016), we defined the positive alongshore current direction as from the southwest (218 • ) to the northeast (38 • ), which is the mean tangential direction of the isobaths on the southwestern shelf of the ECS. The positive cross-shore direction is from the north- west (308 • ) to the southeast (128 • ), normal to the isobaths.…”

“…The wintertime TWC on the ECS shelf shows synoptic fluctuations (Cui et al, 2004;Zhu et al, 2004;Zeng et al, 2012;Huang et al, 2016). These synoptic fluctuations show some features common to those over other continental shelves; i.e., they have periods between 3 and 15 days and are associated with coastal sea level changes, which can be explained by local winds or by coastal trapped waves (Huyer, 1990;Brink, 1991;Huthnance et al, 1986).…”

“…These synoptic fluctuations show some features common to those over other continental shelves; i.e., they have periods between 3 and 15 days and are associated with coastal sea level changes, which can be explained by local winds or by coastal trapped waves (Huyer, 1990;Brink, 1991;Huthnance et al, 1986). Huang et al (2016) showed that the wind was a main physical factor, which caused the temporal variation of the wintertime currents at the synoptic scale in the coastal area of the ECS. However, the dominant physical factors of the TWC fluctu- ations still lack study; the fluctuations on the whole shelf of the ECS may be complicated due to the complex bottom topography, alternating wind forcing and conjunction of several current systems such as the Kuroshio Current, the Taiwan Strait Current (TSC) and the Zhe-Min Coastal Current (ZMCC).…”