Yiquan Qi

A Review on the Currents in the South China Sea: Seasonal Circulation, South China Sea Warm Current and Kuroshio Intrusion

Researches on the currents in the South China Sea (SCS) and the interaction between the SCS and its adjacent seas are reviewed. Overall seasonal circulation in the SCS is cyclonic in winter and anticyclonic in summer with a few stable eddies. The seasonal circulation is mostly driven by monsoon winds, and is related to water exchange between the SCS and the East China Sea through the Taiwan Strait, and between the SCS and the Kuroshio through the Luzon Strait. Seasonal characteristics of the South China Sea Warm Current in the northern SCS and the Kuroshio intrusion to the SCS are summarized in terms of the interaction between the SCS and its adjacent seas.

Seasonal variability of thermal fronts in the northern South China Sea from satellite data

The 8-year (1991-1998) Pathfinder sea surface temperature data have been applied here to produce the objectively derived seasonality of the oceanic thermal fronts in the northern South China Sea from 17 degreesN to 25 degreesN. Several fronts have been clearly distinguished, namely, Fujian and Guangdong Coastal Water, Pear River Estuary Coastal Taiwan Bank, Kuroshio Intrusion, Hainan Island East Coast and Tonkin Gulf Coastal fronts. The frontal patterns in winter, spring and summer are quite similar, whereas individual fronts display different modes of seasonal variability due to different mechanisms favoring those fronts.

An exceptional anticyclonic eddy in the South China Sea in 2010

The highest sea level near the Xisha Islands in recent 20 years occurred during August 2010. Satellite altimeter data indicated that the extreme event was largely due to an anticyclonic eddy, whose amplitude exceeded 20 cm and size exceeded 400 km on 11 August 2010. Cruise observations showed the eddy raised the center temperature by 7.7 degrees C at 75 m and vertically extended to 500 m. Eddy tracking showed it had a life span of more than 8 months and propagated far from the south of Xisha Islands. Such strong and long-lasting eddy that moved northward for such a long distance was observed for the first time in the South China Sea (SCS). Observational data from CTD/XBT and the reconstructed three-dimensional temperature and salinity were used to explore the eddys features and vertical structure. Our analyses show the 2010 summer monsoon and current in the western boundary of the SCS were largely altered after the 09/10 El Nino event. From May onward, the wind blew northward and strengthened over the northwestern SCS. Such wind drove a strong northward current along the western boundary, which carried the eddy northward by advection from May to July. Energy budget showed, during the eddy northward propagation, the boundary current passed energy to the eddy, which led to the continuing growth of the eddy in both strength and size.

South China Sea Wind-Wave Characteristics. Part I: Validation of Wavewatch-III Using TOPEX/Poseidon Data

A full-spectral third-generation ocean wind-wave model, Wavewatch-III, has been implemented in the South China Sea (SCS) for investigating wind-wave characteristics. This model was developed at the Ocean Modeling Branch of the National Centers for Environmental Prediction (NCEP). The NASA QuickSCAT data (0.25degrees resolution) 2 times daily were used to simulate the wind waves for the entire year of 2000. The significant wave heights from Wavewatch-III are compared to the TOPEX/Poseidon (T/P) significant wave height data over the satellite crossover points in SCS. The model errors of significant wave height have Gaussian-type distribution with a small mean value of 0.02 m (almost no bias). The model errors are comparable to the T/P altimeter accuracy (0.5 m) in the central SCS and are smaller than the T/P altimeter accuracy in the northern and southern SCS, which indicates the capability of Wavewatch-III for SCS wave simulation.

Summer upwelling and thermal fronts in the northwestern South China Sea: Observational analysis of two mesoscale mapping surveys

Persistent coastal upwelling and upwelling-induced thermal fronts in the northwestern South China Sea are investigated using satellite measurements, two intensive mesoscale mapping surveys and three bottom-mounted ADCPs. The results indicate that pronounced surface cooling and upwelling-related fronts with a width of 20-50 km occur around Hainan Island and persist through the summer upwelling season. Driven by the prevailing southwesterly monsoon, the subsurface cooling band is approximate to 6 degrees C colder than the water offshore of the East Coast, where the thermal gradients are generally more than 0.1 degrees C/km. The cold and nutrient-rich coastal water is identified to be derived primarily from the deep water of the outer shelf. At the same time, the spatial structure of the upwelling and thermal front, as well as the upwelling-related coastal currents, is significantly regulated by wind forcing. A prominent lagged correlation between the moored temperature records and alongshore wind stress is detected in the East Coast. The correlation coefficient is -0.8 with the temperature lagging behind wind stress by 2.2 days, indicating that the cooling band off the East Coast is dominated mostly by the alongshore southwesterly monsoon during the upwelling season.

Observed deep energetic eddies by seamount wake.

Despite numerous surface eddies are observed in the ocean, deep eddies (a type of eddies which have no footprints at the sea surface) are much less reported in the literature due to the scarcity of their observation. In this letter, from recently collected current and temperature data by mooring arrays, a deep energetic and baroclinic eddy is detected in the northwestern South China Sea (SCS) with its intensity, size, polarity and structure being characterized. It remarkably deepens isotherm at deep layers by the amplitude of ~120 m and induces a maximal velocity amplitude about 0.18 m/s, which is far larger than the median velocity (0.02 m/s). The deep eddy is generated in a wake when a steering flow in the upper layer passes a seamount, induced by a surface cyclonic eddy. More observations suggest that the deep eddy should not be an episode in the area. Deep eddies significantly increase the velocity intensity and enhance the mixing in the deep ocean, also have potential implication for deep-sea sediments transport.

Seasonal eddy kinetic energy modulations along the North Equatorial Countercurrent in the western Pacific

Seasonal eddy kinetic energy (EKE) variability and its associated eddy energy conversion processes in the western tropical Pacific are investigated using satellite altimeter observations and a global, eddy-resolving, ocean general circulation model (OGCM). Both the altimeter-observed sea surface height anomalies and the OGCM simulation show an area with enhanced EKE east of the Mindanao Island centered around 133°E and 5°N. This enhanced EKE area corresponds to the location of the quasi-stationary meander of the North Equatorial Countercurrent (NECC) and is bordered to the south by the Halmahera Eddy. The mesoscale EKE in this area exhibits a clear seasonality, strong in summer (July–August) and weak in winter (November–January), and much of this seasonality is confined to the upper 200 m layer. An investigation into the upper ocean eddy energetics based on the OGCM simulation reveals that the areal barotropic eddy energy conversion rate has an annual cycle similar to the EKE variations, while the areal baroclinic eddy energy conversion is found to be much smaller that the barotropic conversion rate and exhibits no clear seasonal changes. This indicates that the EKE variations are largely controlled by barotropic conversion of the seasonally varying regional circulation. By examining the seasonal background circulation changes, we find that the amplification of the barotropic eddy energy conversion rate in July–August is related to the seasonal evolution of the Mindanao Current and the New Guinea Coastal Current that amplifies the curvature and amplitude of the quasi-stationary meander of the NECC and results in an elevated EKE level through increased regional barotropic conversion.

Intraseasonal‐to‐semiannual variability of sea‐surface height in the astern, equatorial Indian Ocean and southern Bay of Bengal

Intraseasonal-to-semiannual variability of sea-surface height (SSH) in the eastern, equatorial Indian Ocean (EEIO) and southern Bay of Bengal (BoB) is investigated using altimetric data, and solutions to 1½−layer (first baroclinic mode) and linear, continuously stratified (LCS; multi-baroclinic-mode) models. The amplitude and dominant period of SSH variability differ regionally. Large-amplitude variability is found along the west coast of Sumatra, in a zonal band across the BoB centered along 5°N, east of Sri Lanka, and in the northwestern BoB, respectively. Along the Sumatran west coast, SSH variability peaks at 30 − 60 days, 90 days, and 180 days. Along 5°N and east of Sri Lanka, the 30 − 60-day variability is dominant. Sensitivity experiments using a nonlinear version of the 1½−layer model forced by realistic winds reproduce the observed patterns of intraseasonal variability in the southern BoB. At 30 − 60 days, the solutions show that eddies (nonlinear Rossby waves) propagating from the east, rather than local wind forcing, account for most of the variance east of Sri Lanka; furthermore, they demonstrate that the variance is significantly enhanced by the nonlinear transfer of 90 − 120-day energy into the intraseasonal band of 30 − 60 days. The LCS solutions show that the first two baroclinic modes explain most of the SSH variance at 90 − 180 days. The second baroclinic mode dominates the SSH variance at 180 days, a consequence of basin resonance and strong wind forcing.

Interannual and interdecadal variability of the North Equatorial Countercurrent in the Western Pacific

Interannual and longer timescale variations of the North Equatorial Countercurrent (NECC) in the western Pacific are investigated using the multi-decade (1960-2014) hindcast by the Ocean general circulation model for the Earth Simulator (OFES). The OFES-simulated sea level and upper ocean circulation changes show favorable comparisons with available tide gauge data and repeat hydrographic surveys along the 137oE meridian. An empirical orthogonal function (EOF) analysis reveals that the low-frequency NECC variability is dominated by two distinct modes. The first mode fluctuates interannually and shows strengthening and southward migration of the NECC concurrent with the development of El Nino events. Unlike the extra-tropical western Pacific Ocean circulation variability controlled by wind forcing west of the dateline, the interannual NECC variations are forced by equatorial wind forcing cumulative across the entire Pacific basin. The second mode of the NECC variability has a inter-decadal timescale and is characterized by NECCs progressive weakening in strength, migrating poleward, and broadening in width over the past 50 years. These long-term changes in NECC are caused by the corresponding changes in the trade wind system that weakened and expanded poleward in the past half a century across the Pacific basin. This article is protected by copyright. All rights reserved.

Entropy budget of the ocean system

An entropy balance equation including the transfers of heat and mass, work of external force of the ocean is presented and discussed. Entropy flux through the sea surface are calculated, and the results show that entropy flux due to heat transfer is about - 555.6 mWm(-2) K-1, entropy flux connected with the work of wind stress is relatively smaller at about - 0.09 mWm(-2) K-1, entropy flux due to mass transfer is - 0.02 mWm(-2) K-1, the sum of all the entropy flux is - 555.7 mWm(-2) K-1, the mean rate of generation of entropy inside the ocean must be 555.7 mWm(-2)K(-1) when the ocean is in a climatic steady condition.