Numerical Analysis Of Seasonal Variation Characteristics Of The Mindanao Undercurrent

The western equatorial Pacific was called "water mass crossroads" due to theconfluence and existence there of several currents and water masses. During the pastdecade, people have steadily increased knowledge on the weatern equatorial PacificOcean circulation, especially the low-latitude western boundary currents (LLWBCs)in the Pacific Ocean. The LLWBCs played an important role in contributing toequatorial heat and mass budget, acting as important channels of mass, heat, and saltexchange between two hemispheres. The study on the undercurrents was aimed toobtain more acurate knowledge on the chatacteristics and vertical stucture of theLLWBCs. The discovery of two western boundary undercurrents, which are the NewGuinea Coastal Undercurrent (NGCUC) and the Mindanao Undercurrent (MUC), ledto better description of the vertical structure of the Pacific LLWBCs.The MUC was the important channel which connected the NGCUC and theNECC, and the key carrier which carryed sea water below surface in Southhemisphere flowing northward. So it is significative to study the MUC to figure outthe dynamic structure and change mechanism as well as the mass and heattransportations of the western tropical Pacific circulations.In this paper, the structure and the seasonal variability of the MUC have beenanalyzed using the OFES data, which is applied of the isopycnic-coordinatetransformation, the analysis of hydrological sections, T-S. The volume transports ofMUC has been calculated by the method of limiting potential density and salinity. Theseasonal variability resource and factors of the MUC have been studyed with thesensitive experiment of HYCOM. The results are showed as follows.(1)After analysing the OFES data with the method of analysis of hydrologicalsections and T-S, we found that the water mass which composed the MUC includedthe South Pacific Tropical Water (SPTW) and Antarctic Intermediate Water (AAIW).Besides SPTW and AAIW, a deep water mass whose center of potential density and salinity was27.7and34.64may be a composition of the MUC. In this paper, wecalled it High Potential Density Water (HPDW). When we calculated the volumetransports of MUC, we considered it as a part of the MUC.(2) The climatological monthly mean meridional velocities showed significantseasonal variabilities in both the quantity and the strength of the MUC velocity core.The quantities of the MUC velocity core varyed with months within the range of oneto five generally. Particularly, months with two cores dominated. The strength andextending area of the MUC velocity core also varyed significantly. The near-shorecore attained the largest velocities of up to10cm/s in November and December, withthe largest extending areas occurring in October. The off-shore core attained themaximum strength in April and May with the maximum nuclear velocities exceeding10cm/s and the largest extending area occurring in September. The deep velocity coreattained the largest velocity of5cm/s in March with the largest extending areaoccurring in the same month.(3) The method of integration with potential density and salinity limitation wasbetter to used to calculate the climatological monthly volume transports of the MUC,compared to the traditional depth integration method. The result of analysing theseasonal variation characteristics of the volume transports of the MUC showed thatthe volume transports of the MUC was the strongest in Spring, the second stronger inAutumn, and the weakest in Winter. It was simular that the volume transports of thethree parts of the MUC with the transports of the whole MUC, and it sugested thatboth the SPTW and the AAIW were the strongest in Spring and the weakest in Winter,however, the HPDW was the strongest in Spring and the weakest in Summer.(4) We designed a group of sensitive numerical experiments with HYCOM inorder to study the resource of the seasonal information of the MUC. The result of theexperiments showed that the seasonal variation of the whole MUC and the three partsof the MUC was effected by the seasonal variation of the wind with the east of140°Eand the west of140°E, however, the local wind, which is the wind of the west of140°E, had stronger effection on the seasonal variaton of the MUC. (5) We designed two groups of sensitive numerical experiments with HYCOMto research the possible factors which would have influence on the MUC. Theexperiments of closing the northward flowing channel showed that the MUC wasmainly effected by the MC, not the push of currents from Southern Ocean.Furthermore, the results of the experiments changing the strength of the MC showedthat the whole strength of the MUC becomed strong with the MC becoming strong.