Analyses, Simulation And Prediction Of The Temporal And Spatial Variations Of The Themocline In The China Seas And The Northwestern Pacific Ocean

The thermocline reflects the ocean temperature field's important physics characteristics, and has important influence on underwater communication, submarine activity as well as fishery farming and fishing. In this study, dstributions and seasonal variations of the thermocline in the China Seas and Northwestern Pacific Ocean (NWP) (110oE-140o E, 10oN-40oN) were studied with an improved method for identifying the thermocline from the historical data from 1930 through 2002 (totally 510143 profiles).Meanwhile,based on the Princeton Ocean Model (POM), hydrologic- al structure in the China Seas, particularly along the southeast Chinese coast, is simulated to find the affection of the ocean hydrological environment to the inversion thermocline. Empirical Orthogonal Function (EOF) analyse was used to get the main eigenvector fields of historical temperature for the China Seas and NWP, then the nice temperature profile is reconstructed based on the in situ temperature data and the thermocline is rapidly estimated.The main results of analyseing the historical data can be summarized as follows: The thermocline has obvious seasonal variations in the study area north of 20°N influenced by the strong annual cycles of SST and wind. The thermocline is basically shallower and stronger in summer while deeper and weaker in winter. However, it is thinner in spring and thicker in autumn. There is not the thermocline along the western coasts of the Bohai Sea (BS), Yellow Sea (YS) and northern East China Sea (ECS) from December to March, and in some areas of NWP from January to March. The thermocline is infrequent along the shoal areas of the western and eastern YS and in the vicinity of the Taiwan Strait affected by the wind and tidal mixing. Seasonal variation of the thermocline strength is generally enhanced in the shelf area with averaged seasonal amplitude of about 0.31°C/m that is much larger than that in the deep area (about 0.05°C/m). While the amplitudes of seasonal variations of the thermocline depth and thickness are larger in the deeper area than those in the shelf area. It reveals little seasonal variations in the study area south of 20°N. The inversion thermocline is found in winter and spring (Oct.-May). It appears in the southeast Chinese coast with the longest lasting period and the highest occurrence probability resulting from the Yangtze River diluted water and the Taiwan Warm Current (TWC). In the areas west and south of Korean Peninsula and the northern coast of the Shandong Peninsula, the emergence and disappearance of the inversion thermocline there are all accorded with the onset and decay of the YS Warm Current (YSWC). The multi-thermocline exists in the North Equatorial Current (NEC) and the Tsushima Current (TC) areas all the year round. The multi-thermocline reveals obvious seasonal variations in the branches of the Kuroshio in the YS, ECS and the South China Sea (SCS). In the central YS, the occurrence probability is higher in spring than in summer and autumn. In the western ECS, the multi-thermocline mainly appears in summer. The occurrence probability of the multi-thermocline is larger in winter and spring than in summer and autumn in the northern SCS. These different seasonal variations are mainly influenced by the variations of the SST and the horizontal advection of the surface warm water carried by the wind-driven currents.Based on the Princeton Ocean Model (POM), hydrological structure in the China Seas, particularly along the southeast Chinese coast, is simulated. The results show that the seasonal changes and expansion of the Yangtze River diluted water in summer better meet the actual results. The main distributions and seasonal variations of the circulation, temperature, salt and the inversion thermocline in the BS, YS and ECS are well simulated. There are no inversion themocine in the entire study area without Yangtze River and Yellow River; therefor the diluted water plays a leading role to the formulation of the inversion thermocline. Meanwhile the increasing discharges of the Yangtze River, TWC and Kuroshio make the occurrence probability, gradient and depth of the inversion themocline in the southeast China Seas increase too. The Yangtze River plays the major role to the formulation of the inversion themocline in autumn and winter, and make the inversion thermocline transfer southeastward, however the TWC and Kuroshio make it transfer northwestward in early spring.By analyzing the long period cycle of the strong thermocline in the YSCWM and ECSCE, we can find that there are 3.8yrs and 18.9yrs period oscillations of the thermocline gradient in the YSCWM area in summer. These changes were mainly the responses to the variations of the atmospheric temperature in the summer and former winter in the East Asian. In the ECSCE area, the interannual oscillation of the thermocline gradient with about 3.7-yr period in summer (stronger in El Nino yrs) is well correlated with that of local wind stress. The transition from weak to strong thermocline gradient in the ECSCE during the mid-1970s is consistent with the change of the Kuroshio volume transport in summer.The results of the prediction of vertical temperature structure show that the accumulative variance of the first four eigenvector fields reaches 95%, and the vertical distribution of the reconstructed temperature is most stable using the in situ temperature near the surface to get the coefficient. The model test is operated using the CTD data from the ECS, SCS and the areas around Taiwan Island. The reconstructed profiles have a high correlation with the observed ones and reaching the 95% confidence level. The average error between the reconstructed profiles in these three area and the observed ones were 0.69℃, 0.52℃, 1.18℃. This shows this statistical model can estimate the temperature profile vertical structure well. Comparing the thermocline characteristics from the reconstructed profiles and historical data respectively, the results in the ECS show that the upper thermocline boundary, lower thermocline boundary and the gradient average absolute errors are 1.51m, 1.36m and 0.17℃/m respectively, and the average relative errors of them are 24.7%, 8.9% and 22.6% respectively. Although the relative errors of thermocline depth and gradient there are large, the average error is small. In the SCS, average absolute errors of the upper thermocline boundary, lower thermocline boundary and the gradient are 4.1m, 27.7m and 0.007℃/m respectively, while the average relative errors of them are 16.1%, 16.8 and 9.5% respectively that are all <20%. Although the average absolute error of the thermocline lower bound is larger, but contrast to the spatial scale of average depth of the thermocline lower bound (168 m), the average relative error is only 16.8%. Therefor the model can estimate the thermocline well.Based on the analyses and prediction of the thermocline, a system that can estimate the thermocline quickly is developed, which is one of the most systematic research results about the thermocline analyses and prediction so far.