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A modelling study of the 1976 climate regime shift in the North Pacific Ocean

University of British Columbia

During the 1976 climate regime shift, the Aleutian low deepened. Many studies have shown that the oceanic shift was mainly driven by the atmospheric dynamics. In this thesis numerical studies to investigate the oceanic interdecadal variation as the result of the 1976 climate shift have been carried out with the GFDL OGCM. Two interdecadal experiments are performed. One is forced by monthly wind stress climatology for 1976- 1988 and relaxed to monthly SST climatology from the same time period. The other is forced by monthly wind stress climatology for 1952-1975 and monthly SST climatology from the same time period. The same sea surface salinity monthly climatology data is used throughout all the experiments. The simulated ocean circulations exhibit significant interdecadal variations. The gyre circulations are spun up both in the subarctic region and the subtropical region, except in the northern and eastern part of the Gulf of Alaska where the gyral circulation is weakened. The large scale surface cooling in the central North Pacific and the warming along the west coast of North America are simulated. The intensified surface Ekman drifts are very important in generating the thermal variation near the surface, especially in mid and high latitudes. The deepening of the Aleutian Low generates stronger surface divergence in the subarctic area and stronger convergence in the subtropic. Consequently the upwelhng in the subarctic region is increased, with a maximum increase of 108% at a depth of 60 m. The downwelling in the subtropical area is enhanced as well. Along the coast of North America above 40°7Y, there is an increase in downwelling, which is consistent with the weakening of the Alaskan gyre. The significant variations in upwelling in the subarctic and downwelling in subtropical regions are confined approximately above a few hundred meters depth. By working with the large vertical temperature gradients, they produce large scale subsurface thermal variation, with the cooling in the subarctic and warming in the subtropic. The depth range of the subsurface thermal variation is limited to above 650 m depth. The meridional circulation and heat transport are changed consequently. In low latitudes the poleward heat transport is increased. In the mid latitudes the equatorward heat transport is increased and mainly contributed by the meridional component, though the gyre and diffusion components are not negligible. The heat transport in high latitudes, poleward and very small, is increased. The estimation of the salinity effect on the current studies shows that a change in surface salinity forcing fields by an amount of 0.0817ppt in subarctic area does not have significant impacts on the results of this study.

Thesis/Dissertation

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2009-04-30

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http://hdl.handle.net/2429/7758

Oceanography

University of British Columbia

UBCV

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