Showing papers by "Quanan Zheng published in 2019"

A case study of a Kuroshio main path cut-off event and impacts on the South China Sea in fall-winter 2013–2014

Abstract: This study examines a Kuroshio main path (KMP) cut-off event east of Taiwan Island occurred in fall-winter 2013–2014 and its impacts on the South China Sea (SCS) by analyzing satellite altimetry and mooring observations. Satellite altimeter sea level anomaly (SLA) images reveal a complete process that a huge cyclonic eddy (CE) from the Pacific collided with the Kuroshio and the western boundary from 15 October 2013 to 15 January 2014. Mooring observations evidenced that the Kuroshio upper ocean volume transport was cut off more than 82% from 17×106 m3/s in September to 3×106 m3/s in November 2013. The KMP cut-off event caused the Kuroshio branching and intruding into the SCS and strengthened the eddy kinetic energy in the northern SCS west of the Luzon Strait. Using the total momentum as a dynamic criterion to determine the role of eddy collision with the Kuroshio reasonably explains the KMP cut-off event.

Classification and 3-D distribution of upper layer water masses in the northern South China Sea

Abstract: Using the fuzzy cluster analysis and the temperature-salinity (T-S) similarity number analysis of cruise conductivity-temperature-depth (CTD) data in the upper layer (0–300 m) of the northern South China Sea (NSCS), we classify the upper layer water of the NSCS into six water masses: diluted water (D), surface water (SS), the SCS subsurface water mass (US), the Pacific Ocean subsurface water mass (UP), surface-subsurface mixed water (SU) and subsurface-intermediate mixed water (UI). A new stacked stereogram is used to illustrate the water mass distribution, and to examine the source and the distribution of UP, combining with the sea surface height data and geostrophic current field. The results show that water mass UP exists in all four seasons with the maximum range in spring and the minimum range in summer. In spring and winter, the UP intrudes into the Luzon Strait and the southwest of Taiwan Island via the northern Luzon Strait in the form of nonlinear Rossby eddies, and forms a high temperature and high salinity zone east of the Dongsha Islands. In summer, the UP is sporadically distributed in the study area. In autumn, the UP is located in the upper 200 m layer east of Hainan Island.

A Typhoon Wind-Field Retrieval Method for the Dual-Polarization SAR Imagery

Abstract: Synthetic aperture radar (SAR) is widely used to detect sea surface wind. It has been found that the signals of co-polarization SAR tend to become saturated under high wind-speed conditions. Also, low signal-to-noise ratios of cross-polarization SAR have been observed under low wind-speed conditions. In this letter, a typhoon wind-field retrieval method is developed by compositing the wind speeds retrieved from dual-polarized SAR images. This letter then applies the proposed method to eight typhoons (four from the Canadian RADARSAT-2 satellite and four from the Chinese Gaofen-3 satellite). The results show that the method is successful in regard to fine typhoon wind structure retrieval, with a root-mean-square error of wind direction of 12° and a wind speed of 1.7 m/s, against 12 oceanic buoy measurements in the South China Sea.

Unusual coastal ocean cooling in the northern South China Sea by a katabatic cold jet associated with Typhoon Mujigea (2015)

Abstract: This study deals with a unusual cooling event after Typhoon Mujigea passed over the northern South China Sea (SCS) in October 2015. We analyze the satellite sea surface temperature (SST) time series from October 3 to 18, 2015 and find that the cooling process in the coastal ocean had two different stages. The first stage occurred immediately after typhoon passage on October 3, and reached a maximum SST drop of −2°C on October 7 as the usual cold wake after typhoon. The second stage or the unusual extended cooling event occurred after 7 d of the typhoon passage, and lasted for 5 d from October 10 to 15. The maximum SST cooling was −4°C and occurred after 12 d of typhoon passage. The mechanism analysis results indicate that after landing and moving northwestward to the Yunnan-Guizhou Plateau (YGP), Typhoon Mujigea (2015) met the westerly wind front on October 5. The low-pressure and positive-vorticity disturbances to the front triggered meridional air flow and low-pressure trough, thus induced a katabatic cold jet downward from the Qinghai-Tibet Plateau (QTP) passing through the YGP to the northwestern SCS. The second cooling reached the maximum SST drop 4 d later after the maximum air temperature drop of −9°C on October 11. The simultaneous air temperature and SST observations at three coastal stations reveal that it is this katabatic cold jet intrusion to lead the unusual SST cooling event.

Seasonal and interannual variability of water mass sources of Indonesian throughflow in the Maluku Sea and the Halmahera Sea

Abstract: So far, large uncertainties of the Indonesian throughflow (ITF) reside in the eastern Indonesian seas, such as the Maluku Sea and the Halmahera Sea. In this study, the water sources of the Maluku Sea and the Halmahera Sea are diagnosed at seasonal and interannual timescales and at different vertical layers, using the state-of-the-art simulations of the Ocean General Circulation Model (OGCM) for Earth Simulator (OFES). Asian monsoon leaves clear seasonal footprints on the eastern Indonesian seas. Consequently, the subsurface waters (around 24.5σθ and at ~150 m) in both the Maluku Sea and the Halmahera Sea stem from the South Pacific (SP) during winter monsoon, but during summer monsoon the Maluku Sea is from the North Pacific (NP), and the Halmahera Sea is a mixture of waters originating from the NP and the SP. The monsoon impact decreases with depth, so that in the Maluku Sea, the intermediate water (around 26.8σθ and at ~480 m) is always from the northern Banda Sea and the Halmahera Sea water is mainly from the SP in winter and the Banda Sea in summer. The deep waters (around 27.2σθ and at ~1 040 m) in both seas are from the SP, with weak seasonal variability. At the interannual timescale, the subsurface water in the Maluku Sea originates from the NP/SP during El Nino/La Nina, while the subsurface water in the Halmahera Sea always originates from the SP. Similar to the seasonal variability, the intermediate water in Maluku Sea mainly comes from the Banda Sea and the Halmahera Sea always originates from the SP. The deep waters in both seas are from the SP. Our findings are helpful for drawing a comprehensive picture of the water properties in the Indonesian seas and will contribute to a better understanding of the ocean-atmosphere interaction over the maritime continent.