Seasonal thermocline variations

Surface water temperatures are extremely variable, obviously influenced by location and season. The minimum temperature found in polar latitudes approaches the freezing point of nearly —2 °C. Equatorial oceanic waters can reach 30 °C. Temperature variations with depth are far from consistent. In a region where mixing prevails, as observed especially in the surface waters, a layer forms with a relatively uniform temperature. The zone immediately beneath normally exhibits a sharp change in temperature, known as the thermocline. The thermocline in the ocean extends down to about 1000 m within equatorial and temperate latitudes. It acts as an important boundary in the ocean, separating the surface and deep layers and limiting mixing between these two reservoirs. 

Vertical distributions of several zooplankton parameters such as the biomass, taxonomic composition and size exhibit considerable seasonal and regional variations. Rapid decreases in the biomass (by one to two orders of magnitude) occur within the thermocline. When the subsurface 02 concentration falls below 0.1 mil-1, the extent of vertical migration, and hence the difference between... 

Theoretically, the observed SSH seasonal cycle represents the variation in the mean thermocline in the SCS, where the response to Ekman pumping is concerned. The circulation in the NSCS is relatively stable. However, the SSCS circulation has a strong annual variation. One reasonable interpretation is that the seasonal signal in the SSCS is much stronger than that in the NSCS. In winter, there exists a cyclonic vorticity center over both the SSCS and NSCS. 

The thermocline was ventilated in the NSCS, which was accompanied by the occurrence of potential vorticity with a specific circulation pattern. The thermocline ventilation is a seasonal phenomenon in the SCS actually. The potential vorticity in winter has a high value center and its distribution looks like a thin and fiat ellipsoid. Around the edge of this center, a horizontal circulation movement can be tracked. Water subducted into a thermocline from the mixed layer moves southward in the path of the cyclone along the edge of a seasonal potential vorticity pool. Research shows that signals in terms of the monthly temperature increment seem to come from the intrusion of the Kuroshio into the SCS. These show that seasonal variation in the SCS can deeply effect the thermocline. Thus, a study of the SCS upper circulation should include thermocline dynamics. Moreover, research into the dynamics of the thermocline and the mixed layer can help interpret the seasonal variation in the SCS circulation. 

---