Dissertations from ProQuest

Title

The circulation and ventilation of thermocline and intermediate waters in the North Pacific Ocean

1991

Article

Degree Name

Doctor of Philosophy (Ph.D.)

First Committee Member

Rana A. Fine, Committee Chair

Second Committee Member

Donald B. Olson, Committee Member

Abstract

This dissertation uses chemical tracer data, hydrographic data, and simple models to examine the processes responsible for the ventilation of North Pacific thermocline and intermediate waters on decadal time scales. After two decades since the major bomb input, tritium is found to be well mixed within both the eastern and western portions of the subtropical gyre, on outcropping isopycnals. Between the eastern and western portion of the basin a gradient exists with higher tritium values in the eastern region. Penetration of tritium to isopycnals which do not outcrop suggests that NPIW is formed in the northwestern portion of the subpolar gyre and advected as part of the wind driven circulation. The presence of excess tritium in the Alaskan Gyre, and the salinity distribution suggest an additional ventilation source. The residence time for North Pacific Intermediate Water (NPIW) in the subpolar gyre is found to be less than fourteen years. The residence time for deeper water is found to be more than twenty years. Two mechanisms which could further ventilate NPIW within the Alaskan Gyre are investigated. First, scaling arguments are used to suggest that during the winter sufficient energy can be added to force NPIW to outcrop within the Alaskan Gyre. A simple model calculation shows that the excess tritium in the Alaskan Gyre can be accounted for by temporally and spatially limited outcrops (10's of days and several hundred km's in diameter). Second, the linear nature of a salinity vs $\sigma\sb\theta$ plot from the Alaskan Gyre, suggests that cross isopycnal mixing occurs within the Alaskan Gyre. Simple scaling arguments show that it takes only 8.5 months to diffuse water between 26.60 and 26.80 $\sigma\sb\theta$ within the Alaskan Gyre, whereas it takes over 4 years to do it outside of the Alaskan Gyre. Finally, numerical experiments are carried out using a simple one layer model. Tritium, CFC-11, and an idealized age tracer are added to simulate the formation of NPIW. The model results predict that NPIW is further ventilated within the Alaskan Gyre. In addition, model results predict that on isopycnals $\ge$26.80$\sigma\sb\theta$ horizontal advection determines the tracer distribution in the subpolar region and horizontal diffusion determines the tracer distribution in the subtropical region.

Keywords

Physical Oceanography