Study On The Structure Property And Variation Mechanism Of The North Pacific Meridional Overturning Circulation

The North Pacific meridional overturning circulation (NPMOC) is an importantleakage of the meridional material and energy exchange. The NPMOC and itsvariation play an important role in the North Pacific meridional heat transport, theupper ocean thermohaline structure and the climate change. Therefore, it is significantto study the structure property and variation of the North Pacific meridionaloverturning circulation, which is the basis of understanding the North Pacific upperthermohaline structure variation and the global climate change, and is the urgent needof the short-term climate prediction and the oceanic security guarantee, and is alsoone of the scientific issues of general interest on the present international oceanic andclimate studies.In this work, the three-dimensional structure and seasonal and interannualvariability of the North Pacific (060N) meridional overturning circulation areextensively analyzed, and their formation causes and variation mechanism are alsodiscussed with combining the monthly mean SODA (Simple Ocean Data Assimilation)data from1958to2007and the NEMO model simulation. The major results are asfollows.1. The NPMOC displays four cells: two clockwise meridional cells (TC, STC)and two anticlockwise meridional cells (DTC, SPC). Among the cells, the TC is astrong clockwise meridional cell located in the upper200m depth and centered at50mdepth in the low-latitude ocean. The overturning range of the TC is located at175E100W. The STC is a weaker clockwise meridional cell centered at50m depthwhich moves seasonally between7N-18N. It is also located in the upper200mdepth with an overturning range of155E120W. The DTC is a weakeranticlockwise meridional cell in the below100m depth between3N and15N and itsoverturning range is located at160E100W. In contrast to the TC and STC, theDTC is not highly symmetrical, because there is a weak northward flow below500m depth. The SPC is the weakest anticlockwise meridional cell in the upper200m depthbetween35N and50N. Both the data analysis and the NEMO model test show thatthe zonal wind stress is a main factor driving the TC, STC, and DTC, while the heattransport also has some effect on them.2. The DTC, the TC and the STC are all of very strong seasonal variations. As tothe DTC, the southward transport is strongest in fall and weakest in spring. For theTC, the northward transport is strongest in winter and weakest in spring, while thesouthward transport is strongest in fall and weakest in spring, which is associated withthe strong southward flow of the DTC in fall. As to the STC, the northward transportis strongest in winter and weakest in summer, while the southward transport isstrongest in summer and weakest in spring.3. Both the data analysis and the NEMO model test show that the seasonalvariation of the wind stress plays an important role in the seasonal variation of the TCand STC southward and northward transports and the southward transport of the DTC,while the seasonal variation of the heat flux and the freshwater flux has weakerinfluence on them.4. The first two modes of the meridional streamfunction field in the North Pacificexplain56.8%and29.8%of the total variance, and display two N-S tilting modes. Inthe first mode (PC1), the TC, STC, and DTC are in phase on annual time scales. Inthe second mode, the TC and STC are out of phase on the interannual scales, whereasthe DTC may be complex in its interannual variability.5. The TC, the STC and the DTC are all of very strong interannual variationsassociated with ENSO. All the northward, southward, upward and downwardtransport branches of the TC weaken in El Ni os and strengthen in La Ni as, but theyrespond to ENSO in different time. The TC northward transport has about1monthleading before the ENSO. However, the other three transport branches have14-month lag behind the ENSO. As to the STC, except the upward transport, thenorthward and downward transports strengthen in most El Ni os and weaken in mostLa Ni as, while the southward transport weaken in most El Ni os and strengthen in most La Ni as. These three branches all have about3~4-month lag behind the ENSO.The DTC northward and southward transports decrease during strong El Ni os andincrease during strong La Ni as, while its upward and downward transports haveweaker interannual variation.6. The southward and northward transports of both the TC and STC and the DTCsouthward transport only forced by the wind stress are all of very strong interannualvariations, which are in agreement with the results forced by the wind stress, heat andfreshwater flux. But the branch transports forced by both the heat and freshwater fluxall have weak interannual variations. Therefore, it can be concluded that the windstress is the main factor of the interannual variation of NPMOC, while the heat andfreshwater flux play less role in the interannual variation.