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Geostrophic Currents in the Northern Nordic Seas from a Combination of Multi-Mission Satellite Altimetry and Ocean Modeling

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Geostrophic Currents in the Northern Nordic Seas from a Combination of Multi-Mission Satellite Altimetry and Ocean Modeling Earth Syst. Sci. Data, 11, 1765–1781, 2019 https://doi.org/10.5194/essd-11-1765-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. Geostrophic currents in the northern Nordic Seas from a combination of multi-mission satellite altimetry and ocean modeling Felix L. Müller1, Denise Dettmering1, Claudia Wekerle2, Christian Schwatke1, Marcello Passaro1, Wolfgang Bosch1, and Florian Seitz1 1Deutsches Geodätisches Forschungsinstitut, Technische Universität München, Arcisstraße 21, 80333 Munich, Germany 2Climate Dynamics, Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bussestraße 24, 27570 Bremerhaven, Germany Correspondence: Felix L. Müller ([email protected]) Received: 14 June 2019 – Discussion started: 26 June 2019 Revised: 15 October 2019 – Accepted: 23 October 2019 – Published: 26 November 2019 Abstract. A deeper knowledge about geostrophic ocean surface currents in the northern Nordic Seas supports the understanding of ocean dynamics in an area affected by sea ice and rapidly changing environmental con- ditions. Monitoring these areas by satellite altimetry results in a fragmented and irregularly distributed data sampling and prevents the computation of homogeneous and highly resolved spatio-temporal datasets. In order to overcome this problem, an ocean model is used to fill in data when altimetry observations are missing. The present study provides a novel dataset based on a combination of along-track satellite-altimetry-derived dynamic ocean topography (DOT) elevations and simulated differential water heights (DWHs) from the Finite Element Sea ice Ocean Model (FESOM) version 1.4. This innovative dataset differs from classical assimilation methods because it substitutes altimetry data with the model output when altimetry fails or is not available. The combination approach is mainly based on a principal component analysis (PCA) after reducing both quan- tities by their constant and seasonal signals. In the main step, the most-dominant spatial patterns of the modeled differential water heights as provided by the PCA are linked with the temporal variability in the estimated DOT from altimetry by performing a principal component synthesis (PCS). After the combination, the annual signal obtained by altimetry and a constant offset are re-added in order to reference the final data product to the al- timetry height level. Surface currents are computed by applying the geostrophic flow equations to the combined topography. The resulting final product is characterized by the spatial resolution of the ocean model around 1 km and the temporal variability in the altimetry along-track derived DOT heights. The combined DOT is compared to an independent DOT product, resulting in a positive correlation of about 80 %, to provide more detailed information about short periodic and finer spatial structures. The derived geostrophic velocity components are evaluated by in situ surface drifter observations. Summarizing all drifter observations in equally sized bins and comparing the velocity components shows good agreement in spatial patterns, magnitude and flow direction. Mean differences of 0:004 m s−1 in the zonal and 0:02 m s−1 in the meridional component are observed. A direct pointwise comparison between the combined geostrophic velocity components interpolated onto the drifter locations indicates that about 94 % of all residuals are smaller than 0:15 m s−1. The dataset is able to provide surface circulation information within the sea ice area and can be used to support a deeper comprehension of ocean currents in the northern Nordic Seas affected by rapid environmental changes in the 1995–2012 time period. The data are available at https://doi.org/10.1594/PANGAEA.900691(Müller et al., 2019). Published by Copernicus Publications. 1766 F. L. Müller et al.: Geostrophic currents in the northern Nordic Seas 1 Introduction Besides observation-based ocean circulation products, model simulations provide information about the ocean dy- Water mass flowing northward and southward through the namics. In general, their resolution is much better than these Greenland Sea and Fram Strait represents the major path- of observations; however, they rely on the underlying math- ways of the bidirectional water exchange between the Arc- ematical or physical formulations, which naturally contain tic Ocean and the global conveyor belt. Most of the water simplifications and suffer from deficiencies in process de- mass is transported via the northward-flowing West Spitsber- scriptions. Ocean models differ in spatio-temporal resolu- gen Current (WSC) and the southward-flowing East Green- tions, forcing the model background and underlying math- land Current (EGC). More than 60 % of the total water trans- ematical formulations. Recent developments focus on so- port is based on geostrophic movements, caused for example called unstructured ocean models, allowing for locally highly by water density and sea level elevation variations (Rudels, refined spatial resolutions (Danilov, 2013), while keeping a 2012). coarser resolution in other regions of the Earth (e.g., Finite Geostrophic currents (GCs) can be directly derived from Element Sea ice Ocean Model in Wang et al., 2014, or Model measurements of the dynamic ocean topography (DOT) with for Prediction Across Scales, Ocean model – MPAS-Ocean respect to the Earth’s gravity field and rotation and the Corio- – in Ringler et al., 2013). One of the unstructured models is lis force involved. In contrast to hydrographic pressure, tem- the Finite Element Sea ice Ocean Model version 1.4 (FES- perature and salinity observations, collected by irregularly OMv1.4) described by Wang et al.(2014). In the following distributed in situ data (e.g., ARGO floats or ship-based mea- text, FESOMv1.4 is abbreviated by FESOM. surements), satellite altimetry is the only possibility for ob- For the northern Nordic Seas, an eddy-resolving con- taining spatially and temporally homogeneous information figuration has been developed, enabling the simulation of about the global geostrophic circulation. In situ sampling small-scale eddies down to 1 km (Wekerle et al., 2017). Be- platforms can deliver high-resolution measurements, but in sides total ocean current velocities including wind-driven polar regions their availability is limited due to sparse spatial and geostrophic components, FESOM includes sea surface coverage and challenging environmental conditions. How- heights with respect to the bottom ocean topography, which ever, especially in sea ice areas, even geostrophic ocean cur- can be also seen as an estimation of the dynamic ocean to- rents derived by altimetry suffer from irregular sampling and pography. Applying the gradient to these differential water data gaps. Furthermore, the generation of a dataset requires elevations leads to the computation of simulated geostrophic some sort of interpolation or gridding techniques, which currents. In contrast to observational based data, models cause smoothing effects and a coarser spatio-temporal res- show consistent spatio-temporal resolutions and enable in- olution. Moreover, in open-ocean regions, beyond the sea vestigations of ocean surface currents under the sea ice layer. ice edge, the spatial coverage of altimetry data is sparse However, they are limited to a fixed defined mathematical due to the along-track acquisition geometry with constant background and function as an assumption of the reality. and fixed orbit patterns. Hence, studies are limited to long- The current publication aims to present an innovative com- term means (e.g. Farrell et al., 2012) or to satellite altime- bined data product based on the advantages of both simulated try missions dedicated to sea ice conditions (e.g., CryoSat- and observed datasets. In contrast to other commonly used 2; Kwok and Morison, 2015, and ICESat; Kwok and Mori- datasets or assimilation methods, the introduced product is son, 2011). Nevertheless, monthly DOT estimates have been mainly focused on the observational side by filling in mod- generated and published by Armitage et al.(2016) using eled DOT elevations where altimetry data are missing or cor- DOT observations derived from long-term satellite altime- rupted. Several investigations and consistency checks have try. Furthermore, Armitage et al.(2017) presented a dataset been made by Müller et al.(2019), concluding with good based on a 12-year altimetry observation (from 2003 to agreement of simulated and observed DOT in terms of the 2014) of geostrophic currents at a monthly time frame on most-dominant seasonal signals and spatial patterns aiming ◦ ◦ a 0:75 × 0:25 longitude–latitude regular data grid up to a at a combination of the temporal variability provided by al- ◦ latitudinal limit of 81.5 N. The authors created a dataset timetry along-track derived DOT elevations with simulated which combines satellite-altimetry observations from ice- spatio-temporally homogeneous DOT heights of the model. covered and open-ocean regions. Further publicly available The combined dataset obtained is characterized by the spa- geostrophic ocean current products based on observational tially homogeneous resolution of the model and the temporal data from satellite altimetry only and in combination with in variability in altimetry-derived DOT elevations. This enables situ buoys (e.g. Rio et al., 2014) are provided, for example, further studies of geostrophic surface currents in sea ice re- by the GlobCurrent project and by the Copernicus Marine
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