Efficient representation of the hydrographic structure of the north Atlantic Ocean and aspects of the circulation from objective methods

Abstract

The general theme of this thesis is the study of systematic mathematical techniques for determining the ocean circulation from classical hydrographic data. Two aspects of this theme are analyzed. The first is finding an efficient representation of hydrographic structure so as to make it most useful and informative. The second is application of inverse methods to the data to determine ocean circulation. Both subjects are examined in the North Atlantic Ocean. The efficient representation is examined in terms of empirical orthogonal functions (EOFs) among the variations in vertical hydrographic profiles. The data used are of a new set of high quality hydrography, all obtained in the early 1980s. Common EOFs are examined among temperature, salinity, oxygen, phosphate, silicate, and nitrate. The EOFs identify a fundamental simplicity in the spatial distributions of t hese properties. Although the volume of numbers involved in the raw data is large, the significant degrees of freedom are only six in space and two among the six properties; temperature and salinity are represented by one mode, while the nutrients by another. The modal structure reflects some underlying simplicity in ocean physics. EOFs form a quantitative basis from which models of the ocean's hydrographic structure can be constructed for various degrees of complexities. As for the second aspect, two applications of inverse methods are explored on small regional scales. The first problem addressed concerns the circulation inside a 12° square located in the eastern basin over the axis of the Mediterranean Water tongue. The study is based on an ocean model constructed by mapping the modes identified in the first half of the thesis over the entire North Atlantic Ocean. A combination of box model inverse and β-spiral method is used to determine the geostrophic reference level velocities. The circulation consists of an anticyclonic circulation near the surface, which is part of the eastern half of the wind-driven subtropical gyre. The flow at depth is weak, and is a cyclonic circulation around the core of the Mediterranean Water tongue. In the second inverse problem, we examine a decaying warm-core ring. Observations of a warm-core ring are used to formulate a model for diagnosing the physics of ring change over a two month period. About 30 hydrographic casts and acoustic doppler current measurements are used to generate estimates of an equivalent radially symmetric ring with radial contrasts of stratification, temperature, salinity, azimuthal velocity, angular momentum, and potential vorticity. A series of related models are inverted for the ring circulation and mixing coefficients. The circulation is insensitive to the model details, is well-resolved, and is a radial outflow and upwelling. Eddy coefficients are only partially resolved; determining the mixing with any degree of confidence appears to require a much more elaborate data set than the one available.