Core simulations complement the coupled climate and earth system models run for the. Over the deep ocean, the pvis determined by the combination of the water mass source, the overturning circulation, and mixing. The experiments underline the role of atmospheric variability for fluctuations of the largescale ocean circulation on time scales from years to decades, and they support the hypothesis that the mean overturning strength is controlled by the model representation of. Watermass transformation by sea ice in the upper branch of the southern ocean overturning.
Access to diabatic processes allows one to link c ant overturning across water masses, and finally its routing into the ocean interior to the thermodynamic processes air sea buoyancy fluxes. Water mass transformation in the southern ocean of a global isopycnal coordinate gcm robert marsh,a. The water sinks in deepwater formation regions in the nordic and labrador. Water mass formation, subduction, and the oceanic heat budget. Coordinated oceanice reference experiments cores were proposed by the wcrpclivar working group on ocean model development wgomd as a venue for comparing global oceansea ice models run under a common prescribed atmospheric state, with boundary fluxes computed via the same bulk formulae. It transports large amounts of water, heat, salt, car. The intent is to determine the relative roles of these water masses from the different source regions arctic ocean, nordic seas, and subpolar atlantic in the meridional overturning circulation. The model exhibits significant decadal variability in the. Transformation of deep water masses along lagrangian. Deep water and bottom waterthe waters that fill the deep parts of ocean basinsform when surface waters become dense and sink. Antarctic sea ice control on ocean circulation in present.
As the water masses cool in higher latitudes, their density increases causing them to sink. Recently, other climate scientists have become interested in the formation of water masses. As described in subsequent sections, and as illustrated in. It transports large amounts of water, heat, salt, car bon, nutrients and other. Meinen1, marlos goes1,2, shenfu dong1, hosmay lopez1,2, and stephen g. These limbs are linked by regions of overturning in the nordic. Cold water is known to sink to the region to the west and east of iceland dark blue arrows and drives the deep ocean circulation around the world. This twodimensional overturning is understood to involve. Ocean water massests diagrams and upper ocean waters. The atlantic meridional overturning circulation amoc is the zonallyintegrated component of surface and deep currents in the atlantic ocean. In contrast, all three deep water proxies, as well as the integrated 231 pa 230 th 0 ratio shift during the deglacial events starting at 14,600 years ago, invoke whole ocean change in both overturning rate and water mass source, similar to jumps in state via hysteresis behavior.
Movement of water masses is determined by ocean temperature, atmospheric wind forcing and freshwater input, all of which are vulnerable to global warming. Division for ocean circulation, southhampton oceanography centre, empress dock, southhampton so14 3zh, united kingdom. If the mixing process is described as simple diffusion, one needs a level of mixing commensurate with a diffusion. Strictly speaking, since surface ocean currents are not driven by thermohaline mechanics but by winds and to a much lesser degree, tides, the circulation of the ocean as a whole is often called the meridional overturning circulation. The imprint of southern ocean overturning on seasonal. Antarctic bottom water is an important water mass that forms on the antarctic continental shelf as a cold, dense residual brine during the formation of sea ice. Lumpkin r, speer k 2007 global ocean meridional overturning. This information is crucial to our understanding of the climate system because the extent to which the ocean will continue to be a heat and carbon reservoir depends. However, we will continue to use the term thermohaline when addressing deep water circulation. An oceanographic water mass is an identifiable body of water with a common formation history which has physical properties distinct from surrounding water.
It is characterized by a northward flow of warm, salty water in the upper layers of the atlantic, and a southward flow of colder, deep waters that are part of the thermohaline circulation. One of the most prominent ocean circulation systems is the atlantic meridional overturning circulation amoc. The thermohaline circulation is that part of the ocean circulation which is driven by fluxes of heat and freshwater across the sea surface and subsequent interior mixing of heat and salt. Water masses are formed as the result of climatic effects in specific regions. The upper part of the atlantic thermohaline circulation carries warm, upper ocean water through the tropics and subtropics toward the north while the deep part carries cold, dense polar water. Eddy stirring in the southern ocean raffaele ferrari. Another example is the flow driven by strong surface cooling of a previously warmer body of water, as occurs, for example, when a polynya opens up in sea ice buffoni et al. Water bodies with particular characteristics of temperature and salinity. These water masses are thought to be formed at sites. Variability of the meridional overturning circulation of. The formation of the oceans anthropogenic carbon reservoir. The mechanical process by which water masses are cycled throughout the ocean is known as moc.
This view ignored the pivotal role of the southern ocean so in controlling both the stratification and the overturning of deep water masses 1921. The ventilated ocean patrick haertel and alexey fedorov geology and geophysics, yale university, new haven, connecticut. Atlantic meridional overturning circulation wikipedia. In contrast, all three deep water proxies, as well as the integrated 231 pa 230 th 0 ratio shift during the deglacial events starting at 14,600 years ago, invoke wholeocean change in both overturning rate and water mass source, similar to jumps in state via hysteresis behavior.
The temperature tsalinity s diagram relative conservative 3. This diapycnal mixing is dominant in the mixed layer and near the bottom rintoulandnaveiragarabato20,thelatter,dueto the interaction with the topography mashayek et al. Assessment of southern ocean water mass circulation and. What we dont understand, however, are the mechanisms that control overturning strength and how and why the overturning will change in the decades ahead. The meridional overturning circulation is a system of surface and deep currents encompassing all ocean basins. The shallow overturning circulation of the indian ocean. Hemming3 the meridional overturning circulation is a system of surface and deep currents encompassing all ocean basins. Antarctic intermediate water was colder by 34 c and could be traced as. As water is removed from the surface it carries not only heat and salinity anomalies to great depths, but. Global meridional overturning circulation inferred from a. A climate regulator ocean circulation has profound impacts on the.
Tamsitt v, abernathey rp, mazloff mr, wang j, talley ld 2018 transformation of deep water masses along lagrangian upwelling pathways in the southern ocean. Yeager3 1noaa atlantic oceanographic and meteorological laboratory, miami, fl, usa, 2cooperative institute for marine and. As shown schematically in figure 1, the zonalmean southern ocean overturning water mass circulation is thought to involve an upper and lower branch. Recent work has suggested a weakening of the atlantic meridional overturning circulation amoc. It is thought that the volume of sea ice exiting the arctic into the greenland sea influences the rate of sinking. Watermass transformation by sea ice in the upper branch. Role of the overturning circulation in determining the.
Because southern ocean dynamics are influenced by the southern hemisphere westerly winds sww, changes in the westerly wind forcing could significantly affect the circulation and mixing of water masses in this important location. The most vigorous thermohaline circulation in the ocean today is in the atlantic ocean where the overturning is often likened to a giant conveyor belt. Interior watermass variability in the southern hemisphere. In these examples, thermohaline fluxes at the ocean boundary. Ocean state estimate, a stateoftheart data assimilation that incorporates millions of ocean and ice observations. Coastal upwelling brings deep waters to the surface where currents direct these masses towards the poles. Over the upper ocean, a combination of ventilation and eddy stirring determines the pv, which are usually associated with open and closed contours, respectively. Particularly in the upper ocean water masses undergo property changes in response to atmospheric conditions, as indicated in figure 5. Pdf watermass transformation by sea ice in the upper. Chapter 4, potential for abrupt change in the atlantic. Middle holocene expansion of pacific deep water into the. The major role of the southern ocean and its complex processes highlight the intertwined nature of the different components of the moc, which is simplified in an illustration in figure 2. Finally, skinner, elderfield, and hall present a synthesis of deep water temperature, oxygen isotope and carbon measurements to shed light on links between overturning. The north atlantic ocean is in a state of reduced overturning.
Ocean circulation, it is the relative spatial variability of and the large. We investigate the formation process and pathways of deep water masses in a coupled iceocean model of the arctic and north atlantic oceans. The simulation of the southern ocean water mass overturning will then be investigated. This is evidence for two different ocean circulation responses. As the largest ocean basin, the pacific has the strongest eastwest variations in upper water masses. The oceans global overturning circulation regulates the transport and storage of heat, carbon and nutrients. The north atlantic ocean is in a state of reduced overturning d. These exchanges create water masses with different. In fact, most of what physical oceanographers know today about the deep ocean circulation comes from tracking such water properties. Synchronous deglacial overturning and water mass source. There, surface waters do not become dense enough to sink more than a few hundred meters.
The southern ocean is a key region for the overturning and mixing of water masses within the global ocean circulation system. On the stability of the atlantic meridional overturning. Sidedness of divergence as a key to understanding southern. Contribution of topographically generated submesoscale. Formation and pathways of north atlantic deep water in a. Water mass, body of ocean water with a distinctive narrow range of temperature and salinity and a particular density resulting from these two parameters. Modeling the water masses of the atlantic ocean at the. We then use the watermass transformation framework10 to compare the relative roles of atmospheric, seaice, and glacial freshwater. Water mass transformation in the southern ocean of a. On the driving processes of the atlantic meridional.
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