Improving ocean tides by constraining the dynamic HAMTIDE model with altimetry an GRACE data

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The proposed effort of improving global ocean tide models through the assimilation of tidal information available from altimetry and GRACE data is of central concern for the SPP 1257 “Massentransporte und Massenverteilungen im System Erde” and is necessary to improve GRACE and GOCE gravity field modeling (to avoid aliasing). At the same time the effort will improve our knowledge about the role of tide in the general circulation of the ocean. Global ocean tide models describe short term ocean mass variations, quantify loading effects for stations on land, explain parts of the variations observed in Earth rotation, and are required for correcting altimetric sea surface heights and for GRACE and GOCE gravity field modeling (dealiasing). Equally important, tidal friction, mainly in the shallow seas, contribute to mixing the ocean, and exerts influence on the angular momentum and hence on the length of the day. Moreover, the internal wave drag caused by interaction of barotropic tidal currents with topography induces generation of the internal tides in the deep ocean. Such conversion of the mechanical energy into the internal tides is important for the abyssal ocean mixing to maintain the general ocean circulation (nearly steady-state process). Other important features of the tides are their influence on the solid Earth deformation (loading and self-attraction effect) and on the variation of the gravitational fields, consequently on the variation of the satellite orbits. Improving our understanding of ocean tides and expanding our insight into tidal currents, tidal energy transfer or dissipation is therefore an O(1)-problem to physical oceanography and the geodetic community alike. Improved estimates of ocean tides, beyond what we can learn from empirical tide models, can be obtained by constraining dynamical tide models by observed tidal information. The dataassimilative model HAMTIDE, which is used in this project, has already proven valuable and capable of modelling ocean tides during the first two funding periods of DAROTA. The high quality of the dynamical solution is obtained through an effective method by which the model is being constrained by surface tide information (water elevation defined by the altimetry as well as data inferring tidal currents). Results can be used for detecting rates and generation sites of the surface-to-internal tide conversion, finding regions of the tidally induced vertical mixing, and consequently influence on the climate, i.e., an important link from satellite signals to climate variations. In the next phase, the gravimetry (GRACE, GOCE) and altimetry (T/P, ICESat, CRYOSAT2) data will be used simultaneously as constraints in the assimilation process. This will improve tidal estimates not only in low and mid-latitudes where altimetry is available, but also in high latitudes where better tide models are needed to improve gravity field retrievals. The first result of this kind of data assimilation (with coarse resolution and few gravimetry data) is highly promising. Results will be used for an improved understanding of tidal dissipation (see also the figure on the title page and the attached draft paper Taguchi, Stammer and Zahel, 2010). Results will likewise be intercompared with all state of the art global tide models as part of the proposed effort.