Quantifying ocean data from hydrographic data

Abstract

The relationship between the general circulation of the ocean and, alongisopycnal and vertical mixing is explored. Firstly, advection down isopycnal tracer gradients is related to mixing in specific regions of the ocean. Secondly, a general inverse method is developed for estimating both mixing and the general circulation. Two examples of down gradient advection are explored. Firstly the region of Mediterranean outflow in the North Atlantic. Given a known transport of warm salty water out of the Mediterranean Sea and the mean hydrography of the eastern North Atlantic, the vertical structure of the along-isopycnal mixing coefficient, K, and the vertical mixing coefficient, D, is revealed. Secondly, the Southern Ocean Meridional Overturning Circulation, SMOC, is investigated. There, relatively warm salty water is advected southward, along-isopycnals, toward fresher cooler surface waters. The strength and structure of the SMOC is related to K and D by considering advection down along-isopycnal gradients of temperature and potential vorticity. The ratio of K to D and their magnitudes are identified. A general tool is developed for estimating the ocean circulation and mixing; the tracer-contour inverse method. Integrating along contours of constant tracer on isopycnals, differences in a geostrophic streamfunction are related to advection and hence to mixing. This streamfunction is related in the vertical, via an analogous form of the depth integrated thermal wind equation. The tracer-contour inverse method combines aspects of the box, beta spiral and Bernoulli methods. The tracer-contour inverse method is validated against the output of a layered model and against in-situ observations from the eastern North Atlantic. The method accurately reproduces the observed mixing rates and reveals their vertical structure.