Observed three-dimensional structure of a cold eddy in the southwestern South China Sea

Abstract

[1] The dynamic structure of an ocean eddy in the eddy-abundant South China Sea has rarely been captured by measurements and has seldom been discussed in the literature. In the present study, in situ current and hydrographic measurements from a weeklong cruise and concurrent satellite altimeter observations were utilized to examine the three-dimensional structure and physical properties of a cold eddy in the southwestern South China Sea. The underlying forcing mechanism for the formation of this cyclonic cold eddy was found to be tightly associated with the recirculation in a coastal baroclinic jet that had separated off the Vietnamese coast. The eddy was significantly influenced by a coexisting, anticyclonic warm eddy in the separated jet. With relatively steady intensity and radius, the cold eddy endured for two weeks after its swift formation in late August and prior to its quick dissipation in mid-September. This cold eddy was horizontally and vertically heterogeneous. Asymmetric currents with much stronger magnitude were found on its southeastern flank, next to the warm eddy, where a front in the pycnocline was responsible for the sharp decrease in the cold eddy's intensity in the water below. The distributions of temperature, vorticity, and vertical velocity in the cold eddy were spatially asymmetric and not overlapping. The intensity of the cold eddy gradually decreased with the depth and the eddy extended downward for more than 250 m with a vertically tilted central axis. The upward velocities around the center of the eddy and the downward velocities to the southwest and to the east of the center jointly formed the upward domes of isotherms and isohalines in the central part of the cold eddy.

Seasonal variations of sea–air CO<sub>2</sub> fluxes in the largest tropical marginal sea (South China Sea) based on multiple-year underway measurements

“…In spring, offshore high pCO 2 values of 420 to 463 µatm were mostly detected in domain C in April 2005 (Table 5). In summer and autumn, most relatively high pCO 2 values of 420 to 440 µatm (Table 5) were associated with the cyclonic cold eddies observed during our August-September 2007 cruise in the southwestern SCS (Chen et al, 2010;Hu et al, 2011). It should also be noted that, in the four domains, very low sea surface pCO 2 values of 280 to 320 µatm were only observed in domain A and mostly in summer (Table 3).…”

“…Domain C was located between 7 and 18 • N and 110 and 117 • E. As the deep basin of the SCS, its upper mixed layer is shallow all the year round. Both mesoscale eddies and the Mekong River plume have impacts on the southwestern part of this domain during the southwest monsoon period (e.g., Chen et al, 2010;Hu et al, 2011). However, regionally averaged primary production and sea surface chlorophyll concentrations vary within limited ranges (Tan and Shi, 2009;Sasai et al, 2013).…”

Progress of regional oceanography study associated with western boundary current in the South China Sea

“…Hu et al [74] analyzed the three-dimensional characteristics of eddies along the SCSWBC in September 2007 using the current and temperature observation data. The authors pointed out that the formation of the cyclonic eddy north of the Vietnam offshore jet is related to the recirculation caused by the baroclinic instability of the jet.…”

Comparison and validation of global and regional ocean forecasting systems for the South China Sea

“…Eddies in the NSCS have attracted increasing attention over recent decades. Much work has been done based on the combination of satellite observations and in situ hydrographic data (Wang and Chern, 1987;Li et al, 1998a;Chu et al, 1999;Wang et al, 2003;Hu et al, 2011;Nan et al, 2011b), or numerical models (Wu and Chiang, 2007;Xiu et al, 2010;Zhuang et al, 2010). Some work has focused on the statistical characteristics of eddies in the SCS, but eddies are greatly different from each other, causing different criteria for eddy identification to be employed by different literature Xiu et al, 2010;Du et al, 2014).…”