Showing papers by "Quanan Zheng published in 2017"

Statistical features of eddies approaching the Kuroshio east of Taiwan Island and Luzon Island

Abstract: This study examined the statistical features of eddies approaching the Kuroshio east of Taiwan Island and Luzon Island. In total, 315 eddies (138 anticyclonic and 177 cyclonic eddies) were detected from 19.5 years of satellite altimeter sea-level data, with more than 95% of these eddies being generated in the ocean west of the Mariana Islands. Eddy trajectory statistics indicated that eddies frequently intrude into the Kuroshio regime at two latitude bands, namely 18°N–19°N and 22°N–23°N, with periods of 146 ± 62 and 165 ± 46 days, respectively. The interaction time is longer within the two active bands (33 ± 10 days at 18°N–19°N and 45 ± 17 days at 22°N–23°N) than at other latitudes. These two eddy-intrusion bands are associated with the northern and southern Subtropical Countercurrents (STCCs). These STCCs have a vertically reversed sign of the meridional potential vorticity gradient, thus providing a key energy source for eddy generation. In addition, when westward-propagating eddies approach the Ryukyu Islands, the southwestward recirculation flow east of the island chain as well as topographic effects cause some eddies to head southwestward to the east of Taiwan and intrude into the Kuroshio at 22°N–23°N, rather than to dissipate directly. Therefore, we suggest that the STCCs play a key role in inducing the eddies to frequently intrude into the Kuroshio at 18°N–19°N and 22°N–23°N. In addition, the Ryukyu Islands are responsible for concentrating the eddies within 22°N–23°N.

Diagnosis of 3D Vertical Circulation in the Upwelling and Frontal Zones East of Hainan Island, China

Abstract: Using the generalized omega equation and cruise observations in July 2012, this study analyzes the 3D vertical circulation in the upwelling region and frontal zone east of Hainan Island, China. The results show that there is a strong frontal zone in subsurface layer along the 100-m isobath, which is characterized by density gradient of O(10−4) kg m−4 and vertical eddy diffusivity of O(10−5–10−4) m2 s−1. The kinematic deformation term SDEF, ageostrophic advection term SADV, and vertical mixing forcing term SMIX are calculated from the observations. Their distribution patterns are featured by banded structure, that is, alternating positive–negative alongshore bands distributed in the cross-shelf direction. Correspondingly, alternating upwelling and downwelling bands appear from the coast to the deep waters. The maximum downward velocity reaches −5 × 10−5 m s−1 within the frontal zone, accompanied by the maximum upward velocity of 7 × 10−5 m s−1 on two sides. The dynamic diagnosis indicates that SADV...

Mechanisms of SAR Imaging of Shallow Water Topography of the Subei Bank

Abstract: In this study, the C-band radar backscatter features of the shallow water topography of Subei Bank in the Southern Yellow Sea are statistically investigated using 25 ENVISAT (Environmental Satellite) ASAR (advanced synthetic aperture radar) and ERS-2 (European Remote-Sensing Satellite-2) SAR images acquired between 2006 and 2010. Different bathymetric features are found on SAR imagery under different sea states. Under low to moderate wind speeds (3.1~6.3 m/s), the wide bright patterns with an average width of 6 km are shown and correspond to sea surface imprints of tidal channels formed by two adjacent sand ridges, while the sand ridges appear as narrower (only 1 km wide), fingerlike, quasi-linear features on SAR imagery in high winds (5.4~13.9 m/s). Two possible SAR imaging mechanisms of coastal bathymetry are proposed in the case where the flow is parallel to the major axes of tidal channels or sand ridges. When the surface Ekman current is opposite to the mean tidal flow, two vortexes will converge at the central line of the tidal channel in the upper layer and form a convergent zone over the sea surface. Thus, the tidal channels are shown as wide and bright stripes on SAR imagery. For the SAR imaging of sand ridges, all the SAR images were acquired at low tidal levels. In this case, the ocean surface waves are possibly broken up under strong winds when propagating from deep water to the shallower water, which leads to an increase of surface roughness over the sand ridges.

New insight into the South China Sea: Rossby normal modes

Abstract: The South China Sea (SCS), the largest marginal sea of the Northwest Pacific Ocean, is characterized by frequent occurrence of energetic mesoscale eddies. The eddy diameters range from 100 to 300 km. The eddy lifespan varies from several days to several months with the longest time of seven months (Zheng et al., 2017). The eddy disturbance reaches down to the ocean bottom layer. Before 2011, eddies in the SCS were treated as a single process. Thus, their generation was individually attributed to various mechanisms, such as the Kuroshio intrusion, local winds and disturbances from the Pacific (Wang and Chern, 1987; Li et al., 1998; Wang et al., 2003; Hu and Kawamura, 2004; Xie et al., 2011; Zheng et al., 2011). Since 2011, some investigators have found that eddies in the SCS behaved as grouped phenomena. Nan et al. (2011) observed three long-lived anticyclonic eddies appearing as an eddy train along 18°N in the northern SCS in summer 2007, and suggested the frontal instability of the local current as their generation mechanism. Zheng et al. (2014) found that there is internal coherence among alternatingly distributed anticyclonic and cyclonic eddies in the SCS, and suggested existence of 2-D standing wave modes in the SCS deep basin. Recently, Xie et al. 1 adopted theories of the Rossby normal modes in the enclosed ocean basin to analyze 2-D distribution patterns of the sea level anomaly (SLA) in the SCS deep basin. Their results give a new insight into the SCS eddy dynamics. The Rossby normal mode solutions to the linear potential vorticity equation on the β-plane for a rectangular ocean basin with a zonal length L (0≤x≤L), a meridional width l (0≤y≤l), and a uniform depth are

A Model of Radar Backscatter of Rain-Generated Stalks on the Ocean Surface

Abstract: In this paper, a model of radar backscattering from rain-generated stalks on the ocean surface in a rain field is proposed. In the model, stalks in the rain field form an array and are considered as finite water cylinders standing out of an infinite water surface. The radar backscattering coefficient from these stalks is derived. Both incoherent and coherent backscattering mechanisms from the stalks are considered. The model shows that the radar backscattering intensity is a function of the average distance between stalks on the water surface, the radar wave frequency, and the incident angle of radar waves. For light/moderate rain (at low rain rates), the radar backscattering intensity increases with increasing rain rate. For heavy rain (at high rain rates), the radar backscattering intensity decreases with increasing rain rate. The maximum radar backscattering intensity occurs at a rain rate that depends on the radar wave frequency and the incident angle of radar waves. The present model is used to explain the radar signatures of a rainfall event simultaneously observed by C-band ENVISAT (European satellite) Advanced Synthetic Aperture Radar (ASAR) and ground-based weather radar in the Northwest Pacific. The relationship between the radar return intensity extracted from the C-band ASAR image and the rain rate obtained from ground-based weather radar is in agreement with the model’s calculation. Also, the air–sea interface in rain fields and its effects on the attenuation of radar backscattering are experimentally studied in the laboratory.

The impact of oceanographic conditions on fishing ground distribution of flying squid (Ommastrephes bartrami) in the Western North Pacific using remotely sensed satellite data

Abstract: The spatiotemporal fluctuation of flying squid resources from July to November in 2009 to 2014 and its relationship with ocean environmental variables are analyzed with the GAM model in this study. The most suitable SST, SLA and Chla are 16–18°C, 0.5–4 cm and 0.25–0.3 mg/m3, respectively. The model results showed that the Month and the Latitude have greatest effect on the CPUE. The influence of environmental factor followed by G 0–100 , G 200–300 , SST, T 300 , T 100 , SSTG, Chla, SLA and EKE.