Observations of oceanic potential vorticity and its relationship with other tracers

Abstract

Driven by interest in measuring the oceanic velocity field from space, sea surface temperature (SST) has been suggested as a proxy for potential vorticity (PV), which may then be inverted to give velocity. However, little is known about the relationships between PV and other water mass tracers, as these have not previously been thoroughly examined. In this thesis, the inter-relationships between PV, SST, potential temperature and salinity in three quite different frontal regions of the ocean are investigated. The regions studied were in the North-east Atlantic, the Sargasso Sea and the Bellingshausen Sea (Southern Ocean). The only earlier work known in this field was by Fischer et al. (1989), which found a near-linear relationship between PV and isopycnic potential temperature on a shallow isopycnal in the North Atlantic. This relationship was also evident in climatological values of PV and temperature in the North Atlantic. The results from the three regions considered in this thesis vary considerably, and are believed to be due to different frontal dynamics and water mass formation mechanisms. All the North-east Atlantic results are in close agreement, despite differences in measurement scales and the year of survey. The reasons for different relationships occurring are examined. Theories for setting PV and tracer values are investigated, and in particular the models of Woods (1985) and Spall (1995) are found to explain the relationships found in the North-east Atlantic and Sargasso Sea, respectively. However, a combination of these models, applied to different scales of motion, is needed to explain the Bellingshausen Sea results. Preliminary work is carried out using a one-dimensional computer model to follow the development of the relationship in the north-east Atlantic.

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