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Chu X.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology | Dong C.,University of California at Los Angeles | Qi Y.,Chinese Academy of SciencesGuangzhou China
Journal of Geophysical Research: Oceans | Year: 2016

An eddy pair off the Vietnam coast is one of the most important features of the summertime South China Sea circulation. Its variability is of interest due to its profound impact on regional climate, ecosystems, biological processes, and fisheries. This study examines the influence of the El Niño-Southern Oscillation (ENSO), a basin-scale climatic mode, on the interannual variability of this regional eddy pair using satellite observational data and historical hydrographic measurements. Over the last three decades, the eddy pair strengthened in 1994 and 2002, and weakened in 2006, 2007, and 2008. It was absent in 1988, 1995, 1998, and 2010, coinciding with strong El Nino-to-La Nina transitions. Composite analyses showed that the strong transition events of ENSO led to radical changes in the summer monsoon, through the forcing of a unique sea surface temperature anomaly structure over the tropical Indo-Pacific basin. With weaker zonal wind, a more northward wind direction, and the disappearance of a pair of positive and negative wind stress curls, the eastward current jet turns northward along the Vietnam coast and the eddy pair disappears. © 2016. American Geophysical Union. All Rights Reserved.


Li H.,State Key Laboratory of Satellite Ocean Environment DynamicsSecond Institute of Oceanography | Zhou W.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology | Wang D.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology | Liu Z.,State Key Laboratory of Satellite Ocean Environment DynamicsSecond Institute of Oceanography
Journal of Geophysical Research: Oceans | Year: 2017

A new 11 year (2004-2014) monthly 1° gridded Argo temperature and salinity data set with 49 vertical levels from the surface to 1950 m depth (named BOA-Argo) is generated for use in ocean research and modeling studies. The data set is produced based on refined Barnes successive corrections by adopting flexible response functions based on a series of error analyses to minimize errors induced by nonuniform spatial distribution of Argo observations. These response functions allow BOA-Argo to capture a greater portion of mesoscale and large-scale signals while compressing small-sale and high-frequency noise relative to the most recent version of the World Ocean Atlas (WOA). BOA-Argo data set is evaluated against other gridded data sets, such as WOA13, Roemmich-Argo, Jamestec-Argo, EN4-Argo, and IPRC-Argo in terms of climatology, independent observations, mixed-layer depth, and so on. Generally, BOA-Argo compares well with other Argo gridded data sets. The RMSEs and correlation coefficients of compared variables from BOA-Argo agree most with those from the Roemmich-Argo. In particular, more mesoscale features are retained in BOA-Argo than others as compared to satellite sea surface heights. These results indicate that the BOA-Argo data set is a useful and promising adding to the current Argo data sets. The proposed refined Barnes method is computationally simple and efficient, so that the BOA-Argo data set can be easily updated to keep pace with tremendous daily increases in the volume of Argo temperature and salinity data. © 2017. American Geophysical Union. All Rights Reserved.


Jing Z.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology | Qi Y.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology | Du Y.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology
Journal of Geophysical Research C: Oceans | Year: 2015

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 ∼6°C colder than the water offshore of the East Coast, where the thermal gradients are generally more than 0.1°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. © 2015. American Geophysical Union.


Jing Z.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology | Qi Y.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology | Fox-Kemper B.,Brown UniversityProvidence | Du Y.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology | Lian S.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology
Journal of Geophysical Research: Oceans | Year: 2016

Seasonal thermal fronts associated with wind-driven coastal downwelling/upwelling in the northern South China Sea are investigated using satellite measurements and three repeated fine-resolution mapping surveys in winter, spring, and summer. The results show that vigorous thermal fronts develop over the broad shelf with variable widths and intensities in different seasons, which tend to be approximately aligned with the 20-100 m isobaths. Driven by the prevailing winter/summer monsoon, the band-shaped fronts were observed with a magnitude exceeding 0.1°C/km in the subsurface, and accompanied by energetic coastal downwelling/upwelling due to shoreward/offshore Ekman transport. The downward/upward tilting of seasonal thermoclines across the shelf exceeds 20 m, significantly contributing to the development of thermal fronts over the shelf. In addition, the diagnostic analysis of Potential Vorticity (PV) suggests that the summer frontal activities induced by the coastal upwelling are more stable to convection and symmetric instabilities in comparison to the winter fronts associated with downwelling-favorable monsoon forcing. This is primarily due to their essential differences in the upper ocean stratification and horizontal buoyancy gradients arising from wind forcing. At the same time, the coastal currents are substantially regulated by the seasonal winds. An expected lag correlation between the velocity from mooring measurements and alongshore wind stress is detected near the frontal region. These results indicate that seasonal wind forcing plays an important role in the frontal activities and coastal water transport over the shelf. © 2016. American Geophysical Union. All Rights Reserved.


Xie X.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology | Liu Q.,University of the Sea | Shang X.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology | Chen G.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology | Wang D.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology
Journal of Geophysical Research: Oceans | Year: 2016

This study presents two sets of current records obtained from the South China Sea and satellite altimeter data, and it suggests that near-inertial waves induced by parametric subharmonic instability (PSI) associated with internal tides can be transported poleward beyond their critical latitude ϕc by background geostrophic flow (BGF). The two mooring locations were poleward of ϕc (≈14°N) for diurnal subharmonics (0.5D1; half diurnal frequency D1); however, both of the current records revealed clear signals at 0.5D1. The enhanced subinertial motion at 0.5D1 exhibited a fortnightly spring-neap cycle but did not agree with that of D1, indicating that it may not be generated via PSI associated with the local D1. Observations from the altimeter data and a ray-tracing simulation suggested that these nonlocally generated 0.5D1 waves may be excited near their ϕc, after which they propagated poleward under the role of the BGF to the observation site with a latitude higher than ϕc. The poleward propagation of near-inertial waves can produce elevated vertical shears; thus, it may play an important role in enhancing the local turbulent mixing. © 2016. American Geophysical Union. All Rights Reserved.


Guo S.-X.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology | Zhou S.-Q.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology | Qu L.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology | Lu Y.-Z.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology
Journal of Geophysical Research: Oceans | Year: 2016

We studied the thickness of diffusive convective layers that form when a linearly stratified fluid is subjected to heating from below in the laboratory. The thickness of the bottom convecting layer is much larger than subsequent layers. These thicknesses are systematically identified and used to examine the available convecting layer thickness parameterizations, which are consisted of the measured heat flux F (or thermal buoyancy flux qT), initial stratification N, density ratio Rρ, thermal diffusivity κT, etc. Parameterization with an intrinsic length scale (qT3κTN8)1/4 is shown to be superior. Including the present laboratory convecting layer thicknesses and those observed in oceans and lakes, where layer thickness ranges from 0.01 to 1000 m, the parameterization is updated as H=C(Rρ-1)2(qT3κTN8)1/4, where C=38.3 for the bottom convective layer and 10.8 for the subsequent layers. Different prefactors are proposed to be attributed to different convective instabilities induced by different boundary conditions. © 2016. American Geophysical Union. All Rights Reserved.


Wang F.,Chinese University of Hong Kong | Huang S.-D.,Chinese University of Hong Kong | Zhou S.-Q.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology | Xia K.-Q.,Chinese University of Hong Kong
Journal of Geophysical Research: Oceans | Year: 2016

Motivated by a desire to understand the geothermal heating effects on ocean circulation, a large-scale circulation generated and sustained by thermal forcing at the surface subject to a small amount of heating from the bottom boundary is investigated through laboratory experiments. Despite its idealization, our experiments demonstrate that the leading order effect of geothermal heating is to significantly enhance the abyssal overturning, in agreement with the findings in ocean circulation models. Our experiments also demonstrate that geothermal heating cannot influence the poleward heat transport due to the strong stratification in the thermocline. Our study further reveals that the ratio of geothermal-flux-induced turbulent dissipation to the dissipation due to other energies is the key parameter determining the dynamical importance of geothermal heating. This quantity explains why the impact of geothermal heating is sensitive to the deep stratification, the diapycnal mixing, and the amount of geothermal flux. Moreover, it is found that this dissipation ratio may be used to understand results from different studies in a consistent way. © 2016. American Geophysical Union.


Chen X.,University of Chinese Academy of SciencesBeijing China | Qiu B.,University of Hawaii at Manoa | Du Y.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology | Chen S.,University of Hawaii at Manoa | Qi Y.,Chinese Academy of SciencesGuangzhou China
Journal of Geophysical Research: Oceans | Year: 2016

Interannual and longer timescale variations of the North Equatorial Countercurrent (NECC) in the western Pacific are investigated using the multidecade (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 137°E 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 Niño events. Unlike the extratropical 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 an interdecadal timescale and is characterized by NECC's 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. © 2016. American Geophysical Union.


Liao X.,University of Chinese Academy of Sciences | Zhan H.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology | Du Y.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology
Journal of Geophysical Research: Oceans | Year: 2016

Using satellite-derived and in situ data, the wind-driven potential new production (nitrate supply) for the 300 km wide coastal band in two upwelling regions of the western Arabian Sea (AS) during the southwest monsoon is estimated. The upward nitrate flux to the euphotic zone is generally based on the physical processes of coastal transport (Ekman transport and geostrophic transport) and offshore Ekman pumping. The coastal geostrophic current in the western AS influences the upwelling intensity and latitudinal distributions of nitrate supply. The Oman and Somalia upwelling regions have similar level of potential new production (nitrate supply) during the summer monsoon, while the satellite estimates of primary production off Oman are 2 times greater than those off Somalia. The much higher potential f-ratio in the Somalia upwelling region indicates that the primary production could be limited by availability of other macronutrients (e.g., silicate). The correlation analysis of the primary production and the aerosol optical thickness shows that the Oman upwelling region displays a stronger coupling between the atmospheric deposition and the phytoplankton abundance. The high summertime dust levels in the atmosphere are suggested to contribute to the high primary production in the Oman upwelling region. © 2016. American Geophysical Union. All Rights Reserved.


Zeng L.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology | Liu W.T.,Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena | Xue H.,University of Chinese Academy of Sciences | Xiu P.,University of Chinese Academy of Sciences | Wang D.,State Key Laboratory of Tropical OceanographySouth China Sea Institute of Oceanology
Journal of Geophysical Research C: Oceans | Year: 2014

Newly available sea surface salinity (SSS) data from the Aquarius together with in situ hydrographic data are used to explore the spatial and temporal characteristics of SSS in the South China Sea (SCS). Using in situ observations as the reference, an evaluation of daily Aquarius data indicates that there exists a negative bias of 0.45 psu for the version 3.0 data set. The root-mean-square difference for daily Aquarius SSS is about 0.53 psu after correcting the systematic bias, and those for weekly and monthly Aquarius SSSs are 0.45 and 0.29 psu, respectively. Nevertheless, the Aquarius SSS shows a reliable freshening in the SCS in 2012, which is larger than the Aquarius uncertainty. The freshening of up to 0.4 psu in the upper-ocean of the northern SCS was confirmed by in situ observations. This freshening in 2012 was caused by a combined effect of abundant local freshwater flux and limited Kuroshio intrusion. By comparing the Kuroshio intrusion in 2012 with that in 2011, we found the reduction as a relatively important cause for the freshening over the northern SCS. In contrast to the northern SCS, reduced river discharge in 2012 played the leading role to the saltier surface in the region near the Mekong River mouth with respect to 2011. © 2014. American Geophysical Union. All Rights Reserved.

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