Numerical Study On Eddy Kinetic Energy And Eddy Heat Transport In The South China Sea

Oceanic mesoscale eddies have typical spatial scales of tens to hundreds of kilometers and temporal scales of tens to hundreds of days, respectively. Most of the oceanic kinetic energy is contained in eddy kinetic energy (EKE), however, the eddies play a relatively minor role in the poleward heat transport. Recently, satellite observations show a rich pattern of mesoscale eddies embedded in the mean circulation in the South China Sea (SCS).The mesoscale eddy activity is investigated with newly re-processed satellite altimetry observations and hydrographic data. Instability theory analysis is applied to investigate the modulation mechanism of the EKE seasonal variation. Furthermore, Stammer’s theory on eddy heat transport (EHT) is adopted to discuss the characteristics of EHT in the SCS. The EKE level of basin-wide average shows a distinct seasonal cycle with the maximum in August-December and the minimum in February-May. In addition, the seasonal pattern of EKE in the basin is dominated by regions offshore central Vietnam (OCV), southwest of Taiwan (SWT), and southwest of Luzon (SWL), which are also the breeding grounds of mesoscale eddies in the SCS. Instability theory analysis suggests that the seasonal modulation of EKE is a manifestation of baroclinic instability. High eddy growth rates (EGR) are found in the active eddy regions. Vertical velocity shear in the upper ocean of50-500m depth is crucial for the growth of baroclinic instability, leading to seasonal EKE evolution in the SCS. The EHT calculated upon Stammer’s method results in a downgradient transport of heat in the SCS.An eddy-resolving regional ocean model is configured and reasonably well validated for the SCS. The vertical structure of EKE is investigated by using of a realistic simulation over the period of2000-2008. We find that the seasonality as well as meridional variation of EKE is largely confined to the surface and subsurface layers. It should be noted that even though the value of EKE is quite small in the deep layer, its integration over the deep waters is not negligible. In the vertical profile of basin-wide averaged EKE, there is a transportation point around200m, above which EKE decreases sharply from the surface. However, under that point, EKE varies slowly along with depth.The horizontal distribution, vertical structure and influence of thermocline on EHT are then investigated. Furthermore, we also discuss the mechanisms of EHT generation and examine why the method of Stammer fails in estimating the EHT in the SCS. The EHT is found to be strong in the western boundary current, while it is generally weak in the central basin. On the basin-scale, the eddy heat transport vector is anticyclonic, while the heat transport by the mean flow is cyclonic. We also found that the direction of EHT is opposite to that of the meridional temperature gradient. This is indicative of an upgradient tansport of heat by eddies. Therefore, Stammer’s method for estimating the EHT using altimetry data and the climatological temperature field fails to reproduce the model’s directly evaluated EHT in the SCS. Vertically, the EHT is largely confined to the surface and subsurface layers. The vertical profile of basin-wide averaged EHT indicates a direction reversal around the subsurface layer, under which the direction of EHT reverses to southward from northward above. The spatial variation of the effective depth of EHT (De) is also investigated, which is defined by the depth integrated EHT (DEHT) divided by EHT at the surface. Examination of vertical profiles of the simulated eddy reveals that, in the mixed layer, the temperature associated with mesoscale eddies is radically modified by the surface forcing, while the velocity field is not. The consequent enhanced misalignment of temperature and velocity anomalies leads to substantial modifications on the EHT across the seasonal thermocline.