Biweekly periodic variation of the Kuroshio axis northeast of Taiwan as revealed by ocean high-frequency radar

Abstract

An analysis of surface current data obtained from 2002 to 2005 using long-range high-frequency radar provides the first evidence for the presence of biweekly (11–14 day) periodic variations of the Kuroshio axis northeast of Taiwan. This analysis clarifies the spatiotemporal characteristics of these variations and reveals that cyclonic/anticyclonic eddies propagating along the shelf slope from the vicinity of the deep channel east of Taiwan induce these variations northeast of Taiwan. The behavior of the cyclonic/anticyclonic eddies on the shelf slope is well explained by 2nd-mode interior shelf waves advected by the Kuroshio's mean flow. Remote effects from the vicinity of the deep channel east of Taiwan, or from outside the East China Sea, are believed to play an important role in the generation of these biweekly periodic variations of the Kuroshio axis northeast of Taiwan. Moreover, on the shelf slope, these variations cause an onshore current across the shelf slope, suggesting topographically controlled upwelling. Therefore, the biweekly periodic variations of the Kuroshio axis northeast of Taiwan might contribute not only to the onshore transport of Kuroshio surface water but also to transport nutrient-rich Kuroshio subsurface water onto the shelf in the East China Sea.

Introduction

The Kuroshio is a western boundary current that is part of the subtropical gyre in the North Pacific. It enters the East China Sea (ECS) through a deep channel between Taiwan and Yonagunijima (hereafter, this channel is referred to as the East Taiwan Channel; ETC). In the ECS, the Kuroshio flows northeastward along the shelf slope to 30°N at isobaths of 200–500 m. Near 30°N, it veers clockwise; exiting the ECS through the Tokara Strait and then flowing eastward off the south coast of Japan (Fig. 1).

Short-term meandering variations of the Kuroshio axis in the ECS have been reported in many observational datasets, including those from short- and long-term mooring current meter records (Sugimoto et al., 1988; Feng et al., 2000; Nakamura et al., 2003), mooring inverted echo sounder data (James et al., 1999), satellite images of sea surface temperature (Qiu et al., 1990), and hydrographic data (Ichikawa and Beardsley, 1993). Prior studies identified consistent characteristics of the Kuroshio axis meanders in the ECS, such as their period (10–20 days), wavelength (100–375 km), and downstream phase speed (17–30 km day⁻¹). Baroclinic instability has been proposed as a dominant factor in driving the short-term Kuroshio axis meanders on the shelf slope across the PN line of the ECS (James et al., 1999). However, all of these studies are limited to the region from the middle of the Kuroshio in the ECS to downstream areas (from the PN line to the Tokara Strait; see Fig. 1 for geographic locations).

Zhang et al. (2001) reported strong meandering signals with a period of 10 days and 18 days from long-term mooring current meter records within the ETC, which is the entrance of the Kuroshio to the ECS. James et al. (1999) also indicated the possibility of downstream propagation of short-term Kuroshio axis meanders, with a period of 16 days, from the upstream area of the PN line. These previous studies imply the presence of short-term variations (or meanders) of the Kuroshio axis at the area between the ETC and the PN line and suggest that it propagates to the downstream area of the Kuroshio in the ECS. However, short-term Kuroshio axis variations over a time scale of 10–20 days and their downstream propagation as well as the linkage of such a signal to upstream and downstream areas have not been addressed. Additionally, if the downstream propagation of short-term variations along the Kuroshio from the ETC can be confirmed, the local effect of baroclinic instability on the shelf slope must not be the only cause for short-term variations of the Kuroshio axis on a time scale of 10–20 days from the PN line to the Tokara Strait.

The region northeast of Taiwan, located between the ETC and the PN line, is known to be an active upwelling area with nutrient supply from the Kuroshio subsurface layer (Liu et al., 1992; Hsueh et al., 1992; Hsueh, 1993, Hsueh, 2000; Sun and Su, 1994; Tang et al., 1999; Qiao et al., 2005). Short- and relatively long-term current variations in relation to the Kuroshio on time scales of a few days or a few months on the shelf slope have been reported from discontinuous hydrographic or short-term mooring current observations (Chuang and Wu, 1991; Tang and Yang, 1993; Chuang et al., 1993; Tang et al., 2000). In addition, upwelling of Kuroshio subsurface water on the shelf break is related to variations of the Kuroshio axis (Hsueh et al., 1992; Hsueh, 1993; Chuang and Liang, 1994; Liu et al., 1992). However, the presence of short-term variations of the Kuroshio axis on time scales of 10–20 days as well as the relationship between variations and upwelling of nutrient-rich Kuroshio subsurface water has not been addressed northeast of Taiwan.

Guo et al. (2006) estimated the total volume transport across the 200 m isobath along the shelf break in the ECS using a 1/18° nested ocean model and found that the total cross-shelf volume transport shows a clear short-term variation with a period of about 17 days, suggesting that the short-term variation of the Kuroshio axis can occur along the entire shelf break in the ECS and can thereby induce cross-shelf flows. This indicates a possible contribution of short-term Kuroshio axis variations with a time scale of 10–20 days to the cross-shelf transport of surface and nutrient-rich subsurface waters originating from the Kuroshio along the entire shelf break in the ECS, including the shelf break northeast of Taiwan. Therefore, it is very important from a biogeochemical point of view as well as from a physical point of view to investigate the temporal-spatial characteristics and dynamics of the short-term variations of the Kuroshio axis northeast of Taiwan.

In the present study, we present evidence for a short-term variation of the Kuroshio axis northeast of Taiwan with a time scale of 10–20 days by describing its spatiotemporal characteristics via an analysis of sea surface currents obtained using long-range high-frequency radar (HF radar). Finally, we also discuss the behavior and generation of this Kuroshio axis variation and its relationship with cross-shelf transport of Kuroshio surface and subsurface waters on the shelf slope northeast of Taiwan.