The effects of temporal changes in the marine geoid on estimates of the ocean dynamic topography is being investigated. Influences from changes of land hydrology, ice sheets, Post-Glacial Rebound (...

Recent geoids from the Gravity Recovery and Climate Experiment (GRACE) and the Gravity field and steady state Ocean Circulation Explorer satellite mission (GOCE) contain useful short-scale information for the construction of a geodetic ocean mean dynamic topography (MDT). The geodetic MDT is obtained from subtracting the geoid from a mean sea surface (MSS) as measured by satellite altimetry. A gainful use of the MDT and an adequate assessment needs an optimal filtering. This is accomplished here by defining a cutoff length scale d(max) for the geoid and applying a Gaussian filter with half-width radius r on the MDT. A series of MDTs (GRACE, GOCE, and combined satellite-only (GOCO) solutions) is tested, using different sets of filter parameters d(max) and r. Optimal global and regional dependent filter parameters are estimated. To find optimal parameters and to assess the resulting MDTs, the geostrophic surface currents induced by the filtered geodetic MDT are compared to corrected near-surface currents obtained from the Global Drifter Program (GDP). The global optimal cutoff degree and order (d/o) d(max) (half-width radius r of the spatial Gaussian filter) is 160 (1.1 degrees) for GRACE; 180 (1.1-1.2 degrees) for 1st releases of GOCE (time-and space-wise methods) and GOCO models; and 210 (1.0 degree) for 2nd and 3rd releases of GOCE and GOCO models. The cutoff d/o is generally larger (smaller) and the filter length smaller (larger) for regions with strong, small-scale (slow, broad scale) currents. The smallest deviations from the drifter data are obtained with the GOCO03s geoid model, although deviations of other models are only slightly higher.

Ocean bottom pressure variability is analyzed from three monthly products available from (1) the Gravity Recovery and Climate Experiment (GRACE), (2) sterically corrected altimetry, and (3) from a forward run of the German part of the Estimating the Circulation and Climate of the Ocean (GECCO‐2) model. Results lead to an approximate error estimate for each of the ocean bottom pressure (OBP) maps under the assumption of noncorrelated errors among the three products. The estimated error maps are consistent with the misfits of individual fields against OBP sensor data, with the caveat that a general underestimation of the signal strength, as a common, correlated error in all products, cannot be recovered by the method. The signal‐to‐noise ratio (SNR) increases in all products, when a 3 month running mean filter is applied. Using this filter, we estimate globally averaged errors of 8.6, 11.1, and 5.7 mm of equivalent water height for GRACE, nonsteric altimetry, and GECCO2, respectively. Based on resulting uncertainties, a new OBP product is being produced by merging all three data sets. When validated with bottom pressure observations this new OBP product has a 20% increased SNR compared to the best individual product (GECCO2‐ref). Estimated total ocean mass variations explain a considerable part of OBP variability with a SNR above 1 in most of the ocean. In some regions the nonuniform part is weaker than the estimated error. However, most dynamic ocean models are designed to reproduce only the nonuniform, dynamic, OBP variability, but do not accurately describe total mass variability.

In the Nordic seas, we combine a computation of absolute surface current flow derived from geodetic data with in situ historical hydrographic data to estimate the absolute volume, heat, and salt transports as a function of depth. Our mean dynamic topography (MDT) is calculated from marine, airborne and satellite gravimetry, combined with satellite altimetry, using a new algorithm called the iterative combination method (ICM). Residual noise in the gravimetric geoid is the limit on MDT resolution and is suppressed using a Gaussian filter with a width at half‐peak amplitude of 59 km. Detailed and coherent flow paths for surface geostrophic currents are clearly identified. ICM MDT was used as fixed boundary condition to transform historical hydrography into absolute estimates of volume, heat, and salt transport, replacing the assumption of an isobaric surface at a predetermined depth. For the inflow of Atlantic Water (potential temperature Θ > 6°C) through the Faroe‐Shetland Channel into the Nordic seas, we obtain time‐averaged fluxes between 1993 and 1996 of 3.5 Sv (volume), 121 TW (heat), and 124 × 106 kg s−1 (salt), very close to reported observations from acoustic Doppler current profiler moorings and conductivity‐temperature‐depth data. For the Svinøy section, we obtain a northward transport of Atlantic Water (S > 35.0, T > 5.0°C) of 3.9 Sv in the eastern branch of the Norwegian Atlantic Current comparable with reported measurements of 4.2 Sv. Similarly good agreement is found for the Hornbanki and Iceland‐Faroe Ridge sections and for monitoring Atlantic Water outflow across the Barents Sea Opening to the Arctic shelf.

The variability of steric height in the Nordic Seas is analyzed on seasonal, interannual, and decadal timescales using a comprehensive data set of temperature and salinity observations for the second half of the twentieth century. Results from a regional Ocean General Circulation Model (OGCM) are used to assess the reliability of the averaging and the temporal interpolation of the inhomogeneous distributed observation data. The annual cycle explains only a minor part of the monthly variability for most of the region. The analysis on interannual to decadal timescales confined to the Norwegian Sea displays a clear rising trend starting at the end of the 1960s with particularly strong changes along the Barents Sea opening (6 to 7 cm). Moreover, a general freshening is found for the entire Norwegian Sea. In addition, a north‐south dipole of the thermal component of the steric height variability is identified. This dipole elevates the general rising trend along the Barents Sea opening and reduces it in the southern Norwegian Sea. The bulk of the interannual variability in steric height is governed by variations in the local meridional wind stress that determine the relative distribution of Atlantic and Arctic waters in the Norwegian Sea. In addition, reduced heat loss to the atmosphere strongly correlates with the North Atlantic Oscillation winter index. This, in turn, may in particular explain the large steric height increase found at the Barents Sea opening.

This report is a quantitative study of gravity field and geoid, mean sea surface, mean ocean dynamic topography and tides for the Arctic Ocean, for a large range of existing and new models. The report gives quantitative assessments of errors and error covariances of the different fields, mainly for use of understanding total errors in the mean sea surface (MSS). The error studies are especially directed towards the CryoSat sea-ice freeboard processor, where the MSS is the basic reference surface, especially in order to quantify if this processor will benefit from adaptive “smart” interpolation, where the varying error covariances of the MSS are taken into account. As part of the studies a new Arctic gravity field model and geoid has been derived, based on all available terrestrial and airborne gravity data and GRACE satellite data, augmented with gravity field information from ICESat. Rigorous error and covariance of studies of the computed geoid model have been done, showing that the new Arctic geoid is accurate to better than 10 cm over most of the region. Mean dynamic ocean topography is estimated from remote sensing by combining ERS and ICESat altimetry with the geoid, and compared to four different oceanographic models. The comparison shows large differences between the different oceanographic models, and a reasonable agreement to the results from remote sensing. This part of the study illustrates the potential of future radar altimetry missions such as Cryosat for determining ocean dynamic topography and its temporal changes, in addition to the sea ice freeboard heights. A number of tidal models are intercompared, and compared to tide gauge data in the Canadian Arctic. The presence of sea-ice is found to damp the tidal amplitudes and it appears that tidal errors will be a major regionally-dependent error source in sea surface determination for CryoSat. The error covariances and model comparisons are used in a “smart” adaptive estimator scheme, where a linear interpolator operator, similar to the future CryoSat freeboard estimator, is used to quantify errors of the interpolator. Some suggestions for utilization of GOCE data are included in the report, as well as recommendations and a future outlook. The work described in this report was done under ESA Contract. Responsibility for the contents resides in the author or organisation that prepared it. Authors: R. Forsberg, H. Skourup, O. B. Andersen, P. Knudsen, S. W. Laxon, A. Ridout, J. Johannesen, F. Siegismund, H. Drange, C. C. Tscherning, D. Arabelos, A. Braun, and V. Renganathan NAME OF ESA STUDY MANAGER Mark Drinkwater Division: Mission Science Division Directorate: Earth Observations Programmes ESA BUDGET HEADING

Existing studies on recent global warming are almost exclusively based on environmental data from the Earth's surface. Seasonal information on the effects of climate change on subsurface settings of mid to high latitudes is extremely scarce. Here, we present the first temperature proxy record from bottom (c. 50 m) water settings of the North Sea employing the oxygen isotope composition of ocean quahog shells. Results indicate that delta(18)O(aragonite) measured across shells of Arctica islandica can provide reliable estimates (+/- 0.25 to +/- 0.4 degrees C) of the ambient bottom water temperatures. Over the period AD 1880-2001, warming trends in bottom waters are of the order of 0.042 to 0.138 degrees C/decade. Apparently, the annual maximum -temperature trend shows a twofold increase over the past four decades (0.236 degrees C/decade) while the minimum-temperature trend has remained relatively stable (0.042 degrees C/decade). During the same time interval, however, annual maximum temperatures at the sea surface quadrupled. Shell oxygen-isotope-derived winter temperatures also provide a proxy for the winter North Atlantic oscillation index (WNAO). Some 28 to 50% of the variability in minimum temperatures below the thermocline can be explained by changes of the WNAO. Our new toot enables testing and verification of climate models prior to the 20th century greenhouse forcing. Copyright (c) 2005 Royal Meteorological Society.

Using data from the NCEP/NCAR reanalysis for the 40 yr period from 1958 to 1997, a wind statistic for the North Sea is derived. This analysis includes consideration of the monthly mean wind speed and decadal variability in angular distribution. The wind density function is introduced, which combines frequency of occurence and mean wind speed for a given direction, The North Sea is subdivided into 3 sectors. Changes in the wind density function for the 3 regions are analysed for 2 seasons, October to January and February to March. The annual mean wind speed for the North Sea shows a rising trend of similar to 10 % during the last 40 yr, mainly restricted to the period from October to March. For October to January, west-southwesterly wind directions are enhanced in the last 3 decades compared to the period from 1958 to 1967 for the whole North Sea area. The last decade from 1988 to 1997 is outstanding: The duration of typical winter wind conditions, with high wind speeds and the prevalence of west-southwesterly winds, is extended from October to January in the first 3 decades towards February and March in the last one. The wind density functions for both seasons investigated and all of the 3 sectors resemble each other in the last decade. In the first 3 decades an enhancement of southerly wind directions is found in the northern part of the North Sea, northward of 59 degreesN. This trend is broken in the last decade when west-southwesterly directions for the whole North Sea area predominate.