Variability Of Subsurface Ocean Temperature In Tropical Pacific And Its Relationship To ENSO Cycle

Due to the global warming and the increase of extreme weather phenomena, the climate variability has already become a central issue which was attended by the people all over the world. ENSO cycle was the strongest signal of climate variability in the interannual time scale and it could affect the weather around the world. Therefore, the study about ENSO cycle has important significance. Although the ENSO event's research had already made great progress, the process of ENSO event still couldn't be accurately forecasted. Therefore, our study was to analyses the variability of subsurface temperature and current in tropical Pacific Ocean and then deep research the mechanism of ENSO cycle.Based on the TOGA/TAO, SODA and NCEP datasets, the variability of ocean temperature and current was studied on the subsurface in tropical Pacific Ocean. The relationship between these factors and ENSO events was researched. Using the ocean model, the wind stress impacting on ocean subsurface factors was analyzed. The mainly results are as following:1. After the climate shift which occurred at the end of 1970s, the definition of western Pacific warm pool (WPWP) by SST≥28℃couldn't describe its basic character exactly. According to the analyses, we propounded that using the SST≥28.5℃defined the WPWP. The new definition could well reflect the character of WPWP not only before the climate shift but after that. The study about temperature in different depth of WPWP indicated that the anomalous temperature variability on subsurface (148m) was different from that on its upper and under layer. The ocean temperature on subsurface had the largest variation and the reverse trend of interdecadal variability to upper and under layers temperature. The interdecadal variability of subsurface ocean temperature in WPWP had relationship to the variability of PDO.2. An index to the equatorial Pacific sea temperature oscillation (EPOI) was defined to describe the ENSO phenomenon according to the ocean temperature on thermocline surface in Pacific Ocean. Comparing with the older ENSO indexes, the EPOI contained the information of ocean temperature on thermocline in eastern and western Pacific Ocean. Therefore, it could reflect the character of ENSO events in full especially in the interdecadal variability. Furthermore, the EPOI varied earlier than the ONI about two months. We could ahead forecast the ENSO event occurring using the EPOI. The EPOI could satisfy the research on ENSO cycle.3. We analyzed the anomalous equatorial undercurrent in Pacific Ocean by the method of Empirical orthogonal function (EOF). The results indicated that the previous two modes of equatorial undercurrent had the bigger variance contribution. The first mode's variance contribution was 30.75% and it mainly reflected the variability of eastern Pacific equatorial undercurrent. The second mode's variance contribution was 16.18% and it mainly reflected the variability of central Pacific equatorial undercurrent. These two main modes have obvious correlations with ENSO events. There had the biggest negative correlation between the first mode of undercurrent and ENSO when the first mode of undercurrent lagged the ENSO (r=-0.74). This indicated that eastern Pacific equatorial undercurrent weakened (strengthened) after the onset of warm (cold) ENSO events. The variation of central Pacific equatorial undercurrent had influence on the different pattern El Ni?o events. Before the onset of eastern pattern El Ni?o events, the central Pacific equatorial undercurrent singularly increased and then the signal of anomalous temperature quickly propagated to the eastern Pacific. Therefore the El Ni?o event was eastern pattern. Before the onset of central pattern El Ni?o events, the central Pacific equatorial undercurrent singularly decreased and then the signal of anomalous temperature accumulated on the central Pacific. Therefore the El Ni?o event was central pattern.4. In ENSO cycle, the depth of thermocline in 8oN-10oN Pacific was one of reasons that anomalous temperature signal transport to western Pacific along 8oN-10oN. Because the thermocline was very shallow in this area, the thermocline was like a hill across the eastern and western Pacific under the sea surface. Then the anomalous signal couldn't across this area from south (or north) to north (or south), so the signal in eastern Pacific could only transport to western Pacific along this area. On the contrary, the thermocline in the southern hemisphere couldn't block the signal's transportation from equator to the high latitude. There had been a marked change of signal intensity in the ENSO cycle. After the onset of ENSO events, the signal of 4-5 years hadn't reached 10oN. When the ENSO signal transported from eastern to western Pacific along 8oN-10oN, the signal intensity of 4-5 years increased. But the increased signal too weak to maintain the ENSO cycle. The ENSO cycle could maintained because there had other signal supplement from the middle and high latitude. 5. Modeling test indicated that the variability of wind stress in different area had different effect on the ocean factors. The wind stress in equatorial Pacific affected the ocean mainly through its zonal component. Its effect on the variation of subsurface ocean temperature was different in the eastern Pacific and western Pacific. But for the currents the difference was between the surface and subsurface. The variability of wind stress affecting the ocean in the northern hemisphere subtropical area was obviously different from in the equatorial area. The effect of subtropical wind stress on the subsurface ocean temperature was identical from eastern to western Pacific. But for the currents there had the contrary effects between eastern and western Pacific.