Characteristics And Causes Of The Decadal Variability Of Thermocline Depth In The Tropical Pacific Ocean

Thermocline is defined as the water layer with the most vertical seawater temperature gradient in ocean, it’s an indicator which reflects important physical properties of the ocean temperature field. Thermocline stratified the upper layer of ocean which made it closely linked with other subdisciplines such as ocean circulation,internal wave, water mass and air-sea exchange. Thermocline is especially vital for ENSO which as the most important phenomenon of air-sea interaction in tropical Pacific ocean and the strongest signal of annual variation of global climate.The traditional El Ni?o called Eastern Pacific El Ni?o. However, in the last two decades, nontraditional El Ni?o were observed which called Central Pacific El Ni?o.At the same time, the characteristics of ENSO such as strength, period and propagation both have decadal variation. While the occurrence, development and extinction of ENSO both have closed relationship with thermocline. So the research of thermocline depth offer theoretical basis for forecasting the trend of ENSO with the global warming.The study of the decadal variation of thermocline in the tropical Pacific ocean has be made some achievements, but previous studies are mainly based on analysis of space distribution characteristics. The characteristics and mechanism of the decadal variation of thermocline in the tropical Pacific Ocean are investigated using the SODA and NCEP/NCAR reanalysis datasets. There also a small amount of research on the SST and MLD. At last we try to forecast the change trend of thermocline depth in the next few decades. Conclusions as follows:(1). The change in thermocline depth in the tropical Pacific after the 1976-77 climate shift is obvious. The thermocline depth in the eastern tropical Pacific, close to Peru coast, becomes deeper after the climate shift but is shallowed in the equatorialPacific 10°S-10°N. The zone where thermocline depth shallowed is not symmetrical about the equator. Additionally, The change in thermocline depth has seasonal characteristics: the center of the change in thermocline depth moves from the east tropical Pacific to the west with season as its magnitude increases.(2). The change in wind in the tropical Pacific ocean includes two parts: one is the westerly anomaly at the equator and the other one is the wind anomaly toward each pole off the equator. The westerly anomaly drive the warm water from the western to the eastern, which made the thermocline depth shallowed in western while deeper in the eastern. The wind anomaly toward each pole off the equator form wind divergence at the equator which produce upwelling and the thermocline depth become shallower. The divergence field, as same as thermocline, is not symmetrical about the equator. Additionally, two parts of wind anomaly both has seasonal characteristics: the position of the most significant change moves from the eastern Pacific ocean to the western with season as its intensity enhancement. So two parts of wind anomaly as mentioned above are the causes of the decadal variation of thermocline depth.(3). The decadal change of SST out phase of MLD: SST is warmer in the eastern and western Pacific ocean while MLD is shallower; SST is colder in the middle Pacific ocean while MLD is deeper. The change of SST is mainly influenced by the wind anomaly while the MLD is not only effected by the wind anomaly but also the SST anomaly. The decadal change of SST may affect the Walker circulation which lead to the decadal change of thermocline depth. Similarly, SST and MLD have seasonal characteristics but both of them are not associated with thermocline depth.(4). The change trendency of Ni?o Index and thermocline depth is opposite in1967-1997. Thermocline depth becomes shallower while Ni?o Index becomes increased. We calculate the trend of thermocline depth and Ni?o Index in the next few decades by using CMIP5 mode data, the results are diverse. There seven modes are accord with the observed result, four of therm show that thermocline would be deeper and ENSO would be weakened.