A study of the ocean's water masses using data and models

Water masses are collections of water parcels that share similar histories and fate. The validity and value of the concept of a water mass depends on scale, spatial and temporal, as well as purpose. In spite of pragmatic and conceptual limitations, this dissertation shows that the treatment of the ocean as composed of a finite set of water masses can provide a practical analytical framework in which to describe aspects of the ocean’s physical and biogeochemical structure. In particular, the case is made that, given the current biases of ocean models, model diagnostics developed in a water mass reference frame are useful tools to identify, understand and attribute causes for the specific biases. Water masses also provide an intuitive basis for analyzing the response of ocean models to many types of perturbations, be they numerical experiments or analogues of real climatic phenomena. This dissertation is divided in three parts. The first part provides a detailed account of the ocean’s water mass system. The second part utilizes the information gathered in part I to develop an analytical framework that is used to evaluate global coupled model simulations. Finally, part III investigates a methodology designed to account for the confounding role of natural variability for the purpose of measuring the change in the oceanic anthropogenic carbon distribution from repeat hydrography sampling programs. Part I A synthesis of the ocean’s major water masses is presented that compares and contrasts the water masses of the Northern and Southern polar, subpolar and subtropical regions. The world’s most prominent bottom waters, deep waters, intermediate waters and central waters are reviewed sequentially, including a discussion of formation mechanisms, thermohaline and biogeochemical properties, variability, formation rates and their role in ocean circulation or climate. Water masses originating at high polar latitudes are discussed first, with a comparative description of the freshwater and sea ice conditions in the Arctic and Antarctic regions. Low salinity intermediate waters from subpolar regions are described next, followed by high salinity intermediate waters that originate from evaporation basins. The central waters that ventilate the thermocline are discussed last, including a presentation of the five types of mode waters. One recurrent theme of the water mass synthesis presented here is that overflows, entrainment at the overflows, water mass formation and export is often impacted critically by small bathymetric features, such as the Maud Rise in the Weddell Sea, the Orphan Knoll in the Labrador Sea, the depth and width of continental shelves or the presence of canyons cutting through the shelf break. Mode and intermediate water formation are critically dependent on the winds and on the cycling of the mixed layer. Some water masses, such as Eastern Subtropical Mode Waters in the North Pacific, can also depend critically on peculiarities of the seasonal cycle of the buoyancy flux, making these waters an interesting potential early warning diagnostic for climate change. As water mass formation often requires cold winter air bursts, small zonal or meridional shifts in the winds can have a dramatic impact on the formation characteristics of water masses.