3.1Oceandynamics
Ocean dynamics
Satellite altimetry has revolutionized our understanding of the variability and dynamics of the ocean. A decade of measurements of the sea surface height has led to new insight about processes in the ocean interior, its density structure and associated velocity field. Until now mostly temporal anomalies have been exploited with only little references to a geoid model. With the anticipated precise geoid and absolute sea surface dynamical height surfaces we hope for a similarly successful advance in our understanding of absolute structure of and transport in the ocean.
Physical oceanography and marine geodesy
Recently the interest of oceanographers in satellite altimetry and space geodesy has increased dramatically, primarily through the enormous success of the TOPEX/POSEIDON altimetric mis-sion in studying ocean phenomena. Until now this progress was mostly limited to time-varying currents and other transient phenomena. However, the combination of a highly accurate altimetric sea surface height field with improved geoid models anticipated from GRACE and GOCE will revolutionize our skill in
observ-ing the ocean dynamic sea surface topography and associated surface currents. This implies that for the first time in the history of oceanog-raphy there is the possibility to ob-tain the absolute oceanic current field with a sufficient precision that allows the investigation of many long standing oceanographic prob-lems, including the investigation of strong and narrow boundary cur-rents, the absolute eddy field and its interaction with the mean flow, with the topography and flow insta-bility processes. In particular, the new knowledge will help to deter-mine oceanic transports with an ac-curacy that is required to enhance our understanding of the oceans role in climate variability and in the global hydrologic cycle.
Equal-O
ceanD
ynamicsBenefits
− Direct observation of the dynamic sea surface topography with cm-precision is key to determine the ocean circulation.
− For the first time, oceanic mass variations become observable with global coverage.
− Measurements of sea ice thickness and changes in continental ice cover allow a better understanding of forces driving the ocean circulation.
challenges
− Assimilation of satellite gravity and altimetry data together with oceanographic in-situ data into ocean circulation models.
− Recovery of time-varying top-to-bottom absolute ocean current fields.
− Separation of contributions to sea level rise due to thermal expansion, ice melting, oceanic mass redistribution, and vertical land motion and simulating those effects properly in climate models.
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ly important, it will enable us to identify heat and mass variations in the ocean, regionally and globally, to identify long-term changes and to improve numerical simulations of climate change.
Physical oceanography and marine geodesy have had a long symbiotic history through the joint problem of determining the marine geoid: While for the geodesist, the geoid height is a funda-mental description of the shape of the Earth, to the oceanographer it is a reference surface neces-sary for computing the oceanic circulation and ocean transports from altimetric and in situ ocean observations. Many other branches of both sciences overlap as well, including the study of tides, mean sea level, Earth rotation and polar motion, and global and regional sea level rise and fall.
As a result there exists an intimate relation between both disciplines that brings the determina-tion of the ocean circuladetermina-tion and of ocean transport of mass and heat to the forefront of studying and understanding the system ,,Earth“. Since Wunsch and Gaposchkin (1980) initially laid out the framework for a combined estimation of the Earth‘s geoid and the ocean circulation, enormous progress has been made in observing the ocean using satellite altimetry and in determining the Earth‘s gravity field. To a large extent, this progress is due to the success of the TOPEX/Posei-don altimetric mission. At the same time, ocean modelling also improved substantially, mostly through improvements in forcing conditions and through increased model resolution, i.e., through advances in computer technology. Lastly, uncertainties in marine geoid estimates have been re-duced over the last decades to well below one meter on the scales resolved by ocean models.
This uncertainty will be further reduced by orders of magnitude during the next few years based on GRACE and GOCE data. Despite the recent exciting success, the elements of a modern geo-detic/oceanographic symbiosis remain the same. Because the sea surface nearly, but not quite, coincides with the geoid, slopes of the sea surface relative to the geoid imply measurable oceanic velocities. As the sea surface slopes relative to the geoid are less than one meter in thousands of kilometres, small errors in estimates of the slopes imply large erroneous oceanic mass and proper-ty fluxes. Thus, comparatively crude oceanic circulation estimates can provide relatively accurate estimates of the geoid height slopes. New approaches to combine altimetric and geodetic obser-vations with in situ ocean data and with ocean models can therefore lead to significant progress in oceanography and geodesy.
A measured dynamic sea surface topography is shown in Figure 3.1.1 as a combination of satellite altimetry and a geoid model from GRACE. Please notice the contour interval of only 10 centime-tres. Until today the varying sea ice coverage makes it impossible to derive stable estimates of the mean topography at high latitudes.
Ocean models, despite being not perfect and error-prone in many aspects, began to show encour-aging degree of realism in several aspects (Griffies, 2000). Ocean state estimation aims to fur-ther improve those models by bringing them into consistency with ocean data. The goal of those
„ocean syntheses“ is to obtain the best possible description of the changing ocean and to estimate the atmospheric forcing fields that are in agreement with the ocean observations. At the same time, the method identifies model components that need improvements, including ocean mixing parameters, and produces guidelines to improved oceanic observing systems (e.g., Schröter and Wunsch, 1986). See Stammer et al. (2002) and Stammer (2004) for a detailed discussion.
Increased attention to the problem of a simultaneous determination of the marine geoid and the ocean circulation arose with the preparation and launch of the high accuracy geodetic missions CHAMP, GRACE and GOCE. GRACE will provide gravity field information with an accumu-lated accuracy of 2 cm down to scales of approximately 150 km and on time scales from months to the duration of the mission (cf. Figure 2.8). The GOCE mission will resolve stationary gravity field structures down to scales of approximately 70 km.
Wunsch and Stammer (2003) provide a revised discussion of the critical requirement for improve-ments of the joint estimation problem. A successful combination of altimetric data with in situ observations and an ocean model will determine the oceanic flow field that is in agreement with
3.1Oceandynamics
Figure 3.1.1: Mean sea surface as the difference between the mean sea surface height from altimetry (CLS_
SHOM98.2) and the geoid EIGEN_GRACE. The unprecedented accuracy of the GRACE mission allows for the first time the calculation of a realistic mean dynamic topography which is relevant for oceanography.
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observations of all types and will also provide an estimate of the marine geoid that is similarly in agreement with geodetic observations and known ocean dynamics. The estimates of the ocean flow field contain estimates of mass and heat transport and help to quantify interaction between stationary and time-dependent currents. This for the first time opens the possibility to obtain the absolute oceanic current field with a precision, necessary to approach long standing oceanograph-ic and geodetoceanograph-ic problems. Jointly with altimetry and ocean modelling, the new gravity data will enable us in particular to obtain a better understanding of the time-mean circulation.
The mutual connections involved in the process of jointly evaluating geodetic and oceanograph-ic information will be outlined in the following. For that purpose, we will first discuss how im-proved GRACE and GOCE estimates of the geoid will advance estimates of the ocean circulation.
Special emphasis is laid on oceanic transports and fluxes and related climate problems. The ne-cessity of using oceanography to de-alias satellite gravity measurements is described. We will subsequently summarize how improved ocean estimates will feed back into the geoid estimation procedure. On global scale, the problem is complex and involves various different components of the Earth system. As an example, global sea level rise is associated with global ocean warm-ing, but also with redistribution of mass from the cryosphere, the land or the atmosphere into the ocean. Understanding global sea level rise thus involves understanding global mass balances and glacial melting, among others (Chapter 3.2). At the same time, all those processes as well as the ocean circulation affect Earth angular momentum changes. It is obvious, therefore, that the ocean does play an important role as a link of otherwise isolated Earth components. Ocean modelling and data assimilation can thus help understanding all individual components in a mutually con-sistent way.