Large-scale Ocean Circulation Observed from Autonomous Profiling Floats

Abstract

The general circulation of the ocean plays a central role in the global transport of heat, freshwater, carbon, oxygen, nutrients, and other constituents. The ocean's large-scale circulation strongly influences the distribution of these important quantities and thus shapes both global climate and patterns of marine biological production. Observations of large-scale circulation, however, have traditionally been limited in both space and time because of the resource-intensive nature of shipboard observations and the sheer magnitude of the world's oceans. The focus of this dissertation is to examine the large-scale circulation of the ocean using observations from the Argo array of autonomous profiling floats, which have unprecedented resolution in both space and time. A novel multi-scale optimal analysis method is developed and applied in order to map the hydrographic and velocity data provided by the Argo floats. Using this method monthly estimates of absolute geostrophic velocities, potential temperature, and salinity in the upper 2000 decibars of the global ocean are computed for the period December 2004 to November 2010. These results are then combined with satellite observations of wind stress to assess the extent to which observed geostrophic transports are accurately predicted by Sverdrup balance, a simple but ubiquitous theory of the relationship between wind-forcing and ocean circulation. Within the uncertainties, good agreement is found over a large portion of the global ocean, namely the interior subtropics and tropics, while poorer agreement is found in the high latitudes and boundary regions. The meridional overturning circulation in the Southern Ocean, which plays an important role in the global climate system, is investigated using the computed absolute geostrophic velocity fields, together with eddy thickness fluxes also estimated from the Argo data. The resulting direct observations of the overturning circulation are examined in terms of spatial variability, and the relative contributions of the mean and eddy components of the overturning are determined. In addition to the scientific results found here, this dissertation demonstrates that measurements from the Argo array of profiling floats can be used to quantitatively estimate the large-scale circulation of the global ocean with unparalleled spatial and temporal resolution.