| Seasonally varying circulation gyres and their interannual variations are prominent in the South China Sea and are important to the local climatic system. In this thesis, seasonal and interannual variations are examined using a three-dimensional primitive-equation model driven by daily wind and sea surface temperature derived from the NCEP/NCAR 40-year reanalysis project. Model outputs are first analyzed using vertical normal mode decomposition to examine the baroclinic circulation in the basin. Three types of circulation patterns in the first baroclinic mode are identified and show agreement with those reported in literature: a southern anticyclonic gyre in summer, a central gyre during transition months, and a strong cyclonic gyre over the entire basin in winter. Spatial and temporal variations of these gyres in the sea surface height (SSH) field are further examined using Empirical Orthogonal Functions (EOF). SSH variations consist mainly of two modes, corresponding well to the first two modes of the wind stress curl. Mode 1, which is associated with a southern gyre, shows symmetric seasonal reversal. Mode 2, which contributes to a central gyre, is responsible for the asymmetric seasonal and interannual variations.; Description of the circulation is further improved by assimilating the TOPEX/POSEIDON satellite altimeter data into the model. The dynamical inference method is used to project the SSH variations to the variation in the subsurface density field. The projected subsurface water properties are then nudged into the model as a Newtonian relaxation term. With data assimilation, the root mean square (RMS) errors between the simulated and the observed SSH fields are one-half to one-third of that in the case without data assimilation. Data assimilation not only reproduces the seasonal patterns in earlier studies but also resolves mesoscale highs and lows. Circulation under climatological conditions between late 1993 and mid-1994 is contrasted with that during the El Niño event of late 1994 to mid-1995. During the latter period, currents at 50 m depth either weaken or disappear, while temperature at 65 m. shows significant warming. This study shows that the circulation and upper-ocean temperature follow closely the El Niño event in the equatorial Pacific Ocean. |
