The Study Of Thermodynamic Mechanism Of Global Sea Level Variation

Global sea level change is estimated using TOPEX/Poseidon (T/P) satellite altimeter measurements. Thermosteric and halosteric sea level are calculated from seawater temperature and salinity of Ishii (2005), steric sea level variation and its contribution to sea level at different time scales is evaluated. Six experiments are made by using POP model to stimulate the characters of seasonal sea level variation under different conditions of thermal forcing, and to discuss the thermal mechanism of the seasonal variation of global sea level. Conclusions could be drawn as below,In the Northern Hemisphere, the seasonal variation of sea level is mainly caused by variation of steric sea level, the amplitude and phase of seasonal variation of both of them are almost the same, steric contribution to sea level at seasonal time scale is up to 92%. Halosteric sea level is obvious decreasing from 2000 in the North Atlantic, and display significant seasonal characteristic in the North India Ocean. In long term, T/P sea level has a rising rate of 2.22 mm/yr, while steric sea level has a rising rate of 0.97 mm/yr, rising trend of steric sea level accounts for 44% of rising trend of T/P sea level, in which thermosteric sea level has a rising rate of 1.48 mm/yr, while halosteric sea level has a rising rate of -0.52 mm/yr.In the Southern Hemisphere, at seasonal time scale, phase of steric sea level is approximately identical with the one of T/P sea level, but amplitude of steric sea level is larger than the one of T/P sea level, which means non-steric and steric factors have opposite effects to sea level, and non-steric effect is significant. In long term, the rising rate of T/P sea level(3.85 mm/yr) is larger than that of steric sea level (1.30 mm/yr), which accounts for a smaller percentage of T/P sea level in the South Hemisiphere(34%) than the one in northern hemisphere, and in which thermosteric sea level has a rising rate of 1.1 mm/yr, while halosteric sea level has a rising rate of 0.21 mm/yr. ,In tropical area, both T/P and steric sea level display significant inter-annual variation, between 1997 and 1998 both of them decline obviously, which is embodied by the phenomena of the decline of the Pacific and the rising of the Atlantic and the Indian ocean,; there are also annual and semi-annual oscillation in tropical area. At the annual scale, T/P sea level is 3-4 months later than steric sea level in terms of phase, and has lager amplitude than steric sea level; at the semi-annual scale, however, T/P sea level is 2-3 months earlier than steric sea level, and also has lager amplitude than steric sea level.The linear rising rate of global mean T/P sea level is 2.89 mm/yr from 1993 to 2003, rising rate of thermosteric sea level is 1.22 mm/yr, while halosteric sea level is nearly zero; rising of steric sea level accounting for 42% of rising of T/P sea level; In the Pacific steric change is the major factor of sea level variation, among which the contribution in tropical Pacific steric is maximal (72%); In the North Atlantic, the magnitude of the rising rate of thermosteric(2.49 mm/yr)is the same as that of halosteric(-2.48 mm/yr), but their effect are opposite. The contributions of non-steric factor to sea level change in the Atlantic and tropical Indian Ocean southward region are larger than that of steric ones. In terms of variation of steric sea level, effect of thermo-steric is greater than the one of halosteric expect for in northern Atlantic.From 1993 to 2003, annual amplitude of steric sea level has approximately identical spatial pattern with T/P sea level, which is lager in the Northern Hemisphere than that in the Southern Hemisphere. The maximal amplitude appears near Kuroshio and its extension, which is 15 cm.T/P sea level from 1993 to 2003 is analyzed by EOF and the results show that the first mode is seasonal mode, which accounts for 24.2% of the variance; the second mode (18.1%) is the ENSO mode. The first mode of steric sea level (29.3%) is seasonal mode, and the secend mode is ENSO mode, which spatial-pattern is much different from the second mode of T/P sea level.Steric sea level from 1950 to 2003 have about 20-years prominent period, and there is an obvious rising trend from 1970. The global rising rate of thermosteric is 0.29 mm/yr, halosteric is 0.03 mm/yr, the rise of thermosteric in the Noth Atlantic is maximal, up to 0.78 mm/yr. Thermosteric sea level in the past 50 years is analyzed by EOF and it is found that the first mode (29.5%) is ENSO mode, and the second mode is PDO mode, which is related with subtropical circulation.Six experiments are made by using POP model to study effects of different sea surface thermal forcing on seasonal variation of sea level, and their results are compared with the characters of seasonal variations of T/P altimeter data and steric sea level from Ishii temperature and salinity data, it is found that seasonal variations of sea level and steric sea level are significant under forces of sea surface wind stress, sea surface heat flux and freshwater flux; their standard deviation has greater difference with the one of T/P sea level in mid-low latitudes, while has better coherence in mid-high latitudes. Sea water temperature will get lower, salinity will be seriously inaccurate and seasonal variation of sea level will be insignificant without forcing of sea surface heat flux. If sea surface heat flux do not have seasonal character, seasonal variation of sea level will also be insignificant, which indicates that seasonal variation of sea surface heat flux decides the one of sea level. Sea surface freshwater flux has some effects on seasonal variation of sea level, but not as much as sea surface heat flux; its seasonal variation has some effects on variation of sea level in certain individual area, but not much in the Pacific. If sea surface temperature is rising by 0.6℃, there is a significant effect on the seasonal variation of sea level in the high latitudes of the Northern Hemisphere.