Preliminary Study On Pacific's Available Gravitational Potential Energy And Decadal Variability

Available Gravitational Potential Energy (AGPE) in the different regions of the Pacific was calculated from WOA98 and assimilated ocean model data by using Huang's method for reference state. Seasonal, interannual and decadal variability of AGPE in different cases are studied as well as its probable causes. Moreover, based on FOAM model and existent theory, we preliminarily studied the mechanism of decadal variation in the tropical Pacific.Result calculated with WOA98 shows that the AGPE of the Pacific takes up only less than 0.01% of Pacific's total gravitational potential energy. Also, AGPE in tropical Pacific is larger than that in middle latitude regions. The middle-scale AGPE distributed mostly in KOE and where NEC(SEC) and NECC(SECC) meet. The front of density is the probable reason for the large value in these areas.Seasonal variability of AGPE in the northern Pacific is totally different from that in the southern Pacific. The net heat flux caused by the cycle of Sun radiation is the most probable reason of the difference. Moreover, may be influenced by the SEC, the tropical Pacific's AGPE seasonal variability is similar to that in southern Pacific. On interannual and decadal time scales, AGPE in the tropical Pacific shows similar variation with AGPE in the whole Pacific, is out of phase with PDO index. Regarding the regime shift in the 1970s, the mean value after the shift of AGPE in the tropical Pacific as well as that of the whole Pacific is smaller than those before the shift. While the mean value of AGPE in northern Pacific after the shift obviously increased, which is totally different from that of the tropical Pacific.In the tropical Pacific, wind stress has an obvious decadal change around 1977 which caused the adjustment of thermocline in tropical Pacific. Besides, the cooling of subsurface and warming of the surface strengthened the thermocline. These changes increased the stability of tropical Pacific then decreased the AGPE in this region. In the northern Pacific, the vast decrease of temperature, from surface to the depth of approximate 500m increased the in-situ density of upper water. The density increase of upper water reduced the gravitational stability, then increased the AGPE. As for the whole Pacific, the position of mass center decreased after the regime shift. Assuming that the surface of the ocean is fixed, then the lower mass center means more dense water moved downward, or the upper water became lighter, both of which increased the stability of the whole Pacific and decreased the AGPE in the whole Pacific. Varaition of Wind stress, thermocline, temperature structure and mass center may be the probable cause of decadal variation of AGPE in different regions of the Pacific.Based on the results of FOAM, the coupling of ocean and atmosphere in the northern Pacific has less influence on decadal variability in the tropical Pacific than the coupling of ocean and atmosphere in the tropical Pacific. Considering the track of water particles at the bottom of the mixed layer, water particles from the southern Pacific could reach the equatorial Pacific while little water from the northern Pacific could arrive at the equatorial Pacific. Moreover, based on Giese's work, which described the propagation of subsurface temperature anomaly from the southern Pacific to the equatorial Pacific, we verified the propagation from middle-latitude AGPE.The innovative aspects of this work are shown below. Combining the ocean assimilation data, we calculated the AGPE time series in different regions of Pacific. Meanwhile, the variation characteristics of AGPE were also analyzed. We also explored the probable reason for the decadal variation of AGPE in the Pacific. Based on the numerical model and track of water particles, we studied the contribution of the southern and northern Pacific to the tropical Pacific. One theory which explained the decadal variation in tropical Pacific was also verified from the aspect of AGPE.