Seasonal responses of monthly mean sea levels in the Bohai Sea to hydrometeorological forcing and their double screening regression models

-In this paper, the maximum entropy spectral, the cross-spectral and the frequency response analyses are madeon the basis of the data of monthly mean sea levels at coastal stations in the Bohai Sea during 1965-1986. The results show that the annual fluctuations of the monthly mean sea levels in the Bohai Sea are the results of the coupling response of seasonal variations of the marine hydrometeorological factors. Furthermore, the regression prediction equation is obtained by using the double screening stepwise regression analysis method . Through the prediction test , it is proved that the obtained results are desirable.

Period components in the monthly mean sea level variations in the Pacific Ocean

To study the period components in the monthly mean sea level variations in the Pacific Ocean, this paper analyses the monthly mean series of 236 tide gauge stations in the Pacific on the basis of the improved analytical method of the monthly mean sea level period signals. The results show that the oscillation periods of the sea level series are not fixed, but vary with stations. This paper also discusses the existences of the quasi-2 years atmospheric oscillation period, the 3～7 a period related to the El Nino events, sunspots, double sunspots and the 25～30 a overlength oscillation periods. The corresponding amplitudes and initial phase angles as well as the linear trend coefficient are obtained by using significant period components plus the linear trend to fit the monthly mean sea level series. A conspicuous features of the results are found,i. e.,taking 25°～30°S as the axial line in the Pacific, the annual lags gradually increase to the north and the south respectively, depicting a variation trend of partial symmetry. But in China seas, the north and south distributional trend of the annual period lags is almost opposite to those in most parts of the Pacific Ocean. In the low-latitude zone where El Nino events occur or impose more significant effects, the ratio of the sum of amplitudes of various periods within 2～7 a to the annual amplitude is equal to or more than 1. 0, while that of the sum of amplitudes of periods larger than 8 a to the annual amplitude is equal to or more than 0. 5. This suggests that the dominant role of annual amplitude in the monthly mean sea level series of this region has declined and that, correspondingly, there exist significant period components of El Nino events as well as the long-period oscillations of quasi-10 a and over 10 a. In the high-latitude zone, off China, in North America north of 40°N and the coast of Japan, the annual period oscillation occupies a dominant position in the various period components.

Research on the spectral analysis and testmethod of period signals in monthly mean sea level

This paper summarizes the general methods,existing problems and their causes of the period analysis for the monthly mean sea level and points out that it is the key to the analysing period signals and forecasting the change trend of the monthly mean sea level that the periods of the signals are selected reasonably. As there are often many period signals in these series, nonlinear effects exist between pairs of period signals. In order to avoid the false periods that may be introduced due to the effects of side lobes and the periods with statistical phase significance coherence that may be introduced due to the effects of nonlinear effects and their restraint to other period signals, the maximum entropy spectral analysis and the corresponding significance period test may be performed repeatedly on the basis of the bispectrum analysis and meanwhile the most significant period component is filtered out by the least square filtering method, i. e., the method of the significance period analysis with mixed spectra modeled by a nonlinear system is adopted and the signal periods approaching the reality are selected one by one. The examples of the bispectrum analysis, the signal period analysis by mixed spectra and the fitting parameters for combined period components with linear trend in the time series of monthly mean sea level are given in this paper.

Improving the understanding of the influencing factors on sea level based on wavelet coherence and partial wavelet coherence

Relationship between sea level change and a single climate indicator has been widely discussed.However,few studies focused on the relationship between monthly mean sea level(MMSL)and several key impact factors,including CO_(2) concentration,sea ice area,and sunspots,on various time scales.In addition,research on the independent relationship between climate factors and sea level on various time scales is lacking,especially when the dependence of climate factors on Nino 3.4 is excluded.Based on this,we use wavelet coherence(WC)and partial wavelet coherence(PWC)to establish a relationship between MMSL and its influencing factors.The WC results show that the influence of climate indices on MMSL has strong regional characteristics.The significant correlation between Southern Hemisphere sea ice area and MMSL is opposite to that between Northern Hemisphere sea ice area and MMSL.The PWC results show that after removing the influence of Nino 3.4,the significant coherent regions of the Pacific Decadal Oscillation(PDO),Dipole Mode Index(DMI),Atlantic Multidecadal Oscillation(AMO),and Southern Oscillation Index(SOI)decrease to varying degrees on different time scales in different regions,demonstrating the influence of Nino 3.4.Our work emphasizes the interrelationship and independent relationship between MMSL and its influencing factors on various time scales and the use of PWC and WC to describe this relationship.The study has an important reference significance for selecting the best predictors of sea level change or climate systems.

An analysis on the mean sea level change of the seas adjacent to Korea

Monthly changes of sea level recorded on the seas adjacent to Korea (the Huanghai Sea, the East ChinaSea and the East As) are investigated. The major influences on the spatial and temporal variation of mean sea level arequantitatively identified. A set of modes of monthly air pressure variation over the Huanghai Sea, East China Sea andEast Sea for the period of 1978-1992 is obtained. Each monthly air pressure distribution can be precisely defined bylinear combination of these modes. Hence, the set of air pressure series can be replaced by a set of time varying coefficents, where each coefficient describes the contribution of a particular mode to a given air pressure distribution. A selected set of the modal coefficients is then added to a multiple Fegresion model, consisting of a trend, monthly wind stress and tidal term, in an attempt to represent the effect of meteorological variations on monthly mean sea level on the seas adjacent to Korea. It is found that although the model may account for over 90% of the observed mean sea level variance, there still remains a high correlation between the residuals, hence identifying a regional variation for further study.