Study On The Variability And Mechanisms Of The Thermohaline Structure In The Western Pacific Warm Pool

As the largest warm and fresh water mass in the global ocean, the Western Pacific Warm Pool(WPWP) has abundant precipitation, which makes the thermocline deeper than the halocline, and thus the barrier layer forms there. It is the barrier layer that makes the WPWP display a particular vertical thermohaline structure. From the vertical distribution of temperature and salinity and using the 28℃ isotherm as its definition, the WPWP can be divided into three layers: mixed layer(ML), barrier layer(BL) and deep layer(DL). The ML and BL can influence exchanges of energy, momentum and mass between ocean and atmosphere, the distribution of net heat fluxes, movement of sea surface warm water and ENSO. So, in order to understand the variability of the WPWP, improve the ability to simulate the WPWP and predict ENSO, it is really meaningful to detect the variation characteristics and mechanisms of the WPWP. Therefore, based on the monthly temperature and salinity data during 1950-2011, this paper analyzed the seasonal variability and mechanisms of the WPWP three-dimensional structure, and examined the seasonal and interannual variabilities and mechanisms in the WPWP thermohaline structure. The main results are as follows.1. The WPWP three-dimensional structure has a strong but inconsistent seasonal variability, while its variation mechanisms are also different. The zonal displacement is mainly induced by the zonal wind, South Equator Current(SEC) and net heat fluxes in the equatorial Pacific, and the meridional movements result from the zonal winds, zonal currents and net heat fluxes, while the zonal wind, zonal currents, vertical current and net heat fluxes are all favorable to the WPWP vertical variability.2. ΔT =- 0.4 ° C is more suitable to calculate the ML and BL thickness(MLT and BLT) in the WPWP using the monthly-averaged temperature and salinity data. The MLT and BLT have a similar seasonal variability, which are thicker in spring and autumn and thinner in winter and summer, while the DL thickness(DLT) is thicker in spring and autumn and thinner in summer. The temperatures in ML and BL are higher in spring and autumn and lower in winter and summer, while the temperature in DL is higher in spring and lower in summer. Salinity in three layers is higher in winter and lower in summer. The analysis results show that the seasonal variabilities of MLT and BLT are mainly controlled by zonal wind, SEC, North Equatorial Counter Current(NECC), net heat fluxes and precipitation, while the seasonal variabilities of temperature are mainly controlled by NECC, SEC and net heat fluxes, and the seasonal variabilities of salinity might be mainly controlled by STC.3. The MLT, BLT and DLT fields in WPWP are mainly composed by two leading modes with similar temporal periods and different spatial patterns. Of them, the first modes suggest a coincident oscillation in the whole area, while the second modes have an anti-phase oscillation. The first modes of their temperature fields suggest that the temperatures in the central and eastern WPWP have strong interannual variability and long-term trend and undertake a regime shift about 1985, while the second modes indicate that the temperatures in the western WPWP have strong interannual variability and long-term trend and undertake a regime shift about 1995. The first modes of salinity fields suggest that the salinity in the central WPWP has interannual variability and decadal variability and undertakes a regime shift about 1977, while the second modes indicate that the salinity in the northwest WPWP have interannual variability and undertake a regime shift about 1985.4. The MLT first mode is mainly controlled by zonal wind, SEC, net heat fluxes and precipitation, and its second mode by zonal wind in northwest WPWP and SEC. The mechanisms for BLT are different in different area. For the first mode, the BLT south(north) of the equator is mainly influenced by zonal wind, SEC and precipitation(SEC and NECC), while as to the second mode, the BLT northwest(southeast) WPWP is mainly controlled by zonal wind(precipitation). The first(second) mode of DLT is mainly influenced by SEC(Equatorial Undercurrent, EUC). The two modes of ML and BL(DL) temperature fields are primarily controlled by net heat fluxes, zonal winds and SEC(SEC and EUC). The first(second) modes of ML and BL salinity fields are controlled by zonal wind, SEC and precipitation(zonal wind, NECC, SEC and precipitation). The two modes of DL salinity field are mainly affected by SEC and EUC.