The status of measurement of the Mediterranean mean dynamic topography by geodetic techniques

Abstract

We review the measurement of the mean dynamic topography (MDT) of the Mediterranean using ellipsoidal heights of sea level at discrete tide gauge locations, and across the entire basin using satellite altimetry, subtracting estimates of the geoid obtained from recent models. This 'geodetic approach' to the determination of the MDT can be compared to the independent 'ocean approach' that involves the use of in situ oceanographic measurements and ocean modelling. We demonstrate that with modern geoid and ocean models there is an encouraging level of consistency between the two sets of MDTs. In addition, we show how important geodetic MDT information can be in judging between existing global ocean circulation models, and in providing insight for the development of new ones. The review makes clear the major limitations in Mediterranean data sets that prevent a more complete validation, including the need for improved geoid models of high spatial resolution and accuracy. Suggestions are made on how a greater amount of reliable geo-located tide gauge information can be obtained in the future. IntroductionThis paper discusses the mean dynamic topography (MDT) of the Mediterranean and its measurement by geodetic techniques. The MDT is the amount by which the Mean Sea Surface (MSS) is further from the centre of the Earth than a surface called the geoid, which is an equipotential surface resulting from the Earth's spatially-varying gravity field, and which would correspond to the MSS in the absence of an ocean circulation. An accurate measurement of the MDT can, therefore, provide oceanographers with insight into the circulation. Conversely, knowledge of the circulation, obtained by a number of oceanographic techniques can be used to infer the MDT. 2Much is already known about the mean circulation of the Mediterranean and its variability (e.g. Pinardi and Masetti 2000) and, therefore, also about the spatial variation of its MDT. This insight has largely been obtained using in situ oceanographic measurements and ocean modelling. However, in recent years major advances have been made in geodetic techniques that have enabled the MDT to be determined, for the first time, as the difference between MSS and geoid to the accuracy required for oceanographic research (e.g. Bingham et al. 2014). These techniques have primarily involved satellite altimetry to provide the MSS over the open ocean and space gravity to provide the geoid, complemented at the coast with the use of tide gauges equipped with Global Positioning System (GPS) equipment (Woodworth et al. 2012). By studying the Mediterranean, particular lessons can be learned in the use of these techniques, notably by pointing to missing infrastructure and data limitations, which can also be relevant to discussion of the MDT in other data-sparse regions around the world.Models of the MDT in the Mediterranean have already been made by previous authors in various ways (e.g. Rio et al. 2014a). Consequently, the present paper focuses on reviewing how an MDT model can be compa...

A comparative assessment of coastal mean dynamic topography in Norway by geodetic and ocean approaches

“…We first note that geodetic and ocean MDTs agree on the $3-7 cm level. This is an encouraging observation, as similar studies for tide gauges along other coasts have shown an agreement between geodetic and ocean MDTs on the $6-14 cm level [e.g., Woodworth et al, 2012Woodworth et al, , 2015. Higginson et al [2015] got an agreement between geodetic and ocean MDTs of 2.3 cm along the east coast of North America; however, this number resulted from mean geodetic and ocean MDTs based on 7 geoid models and 11 ocean models, respectively.…”

“…Jayne [] as well as Woodworth et al . [] underline the importance of not mixing altimetry‐derived gravity information in a quasigeoid model for MDT estimation purposes, as some of the dynamic topography will blend into the quasigeoid model, corrupting the MDT estimate when combined with the ocean's time‐mean surface from altimetry or tide gauges. In this respect, there is a considerable difference between NMA2014 and EGM2008.…”

“…We first note that geodetic and ocean MDTs agree on the ∼3–7 cm level. This is an encouraging observation, as similar studies for tide gauges along other coasts have shown an agreement between geodetic and ocean MDTs on the ∼6–14 cm level [e.g., Woodworth et al ., ]. Higginson et al .…”

“…Woodworth et al . [] obtained an agreement of 6 cm between a geodetic MDT grid (computed by subtracting a DIR5‐based geoid from an altimetric MSS product) and an assimilated MDT grid in the Mediterranean. They further conclude that ∼5 cm is a likely general level of agreement between altimetric geodetic and ocean MDT grids.…”

“…The European Space Agency (ESA) gravimetric satellite mission Gravity and steady-state Ocean Circulation Explorer (GOCE) [Drinkwater et al, 2003] provides a global geoid with unprecedented detail and has significantly improved geodetic MDT determination. Presently, ocean and geodetic MDTs show an average agreement on subdecimetric level, with better agreement in the open ocean than along coastlines [e.g., Bingham et al, 2011;Albertella et al, 2012;Griesel et al, 2012;Johannessen et al, 2014;Higginson et al, 2015;Hughes et al, 2015;Woodworth et al, 2015].…”