Mesoscale eddies in the upper ocean are ubiquitous coherent rotating structures with radial scales ranging from 20-100 kilometers and vertical extension to more than 1000 m. They play a key role in the transport and mixing of heat, momentum, and tracers across the World Ocean, they moderate the physical-biological interaction and co-variability and are also strongly influencing air-sea interaction. Already back in 1890´ies Helland-Hansen and Nansen reported on the presence of puzzling waves in the Norwegian Sea. They regarded these waves as evidence of ocean variability associated with eddies embedded in the inflowing Atlantic Current (Helland-Hansen and Nansen, 1909) and considered their existence to be highly important for ocean variability, and in significant need of further investigation. However, even 100 years later, it is fair to say that the complete quantitative understanding of their role in the variability of the coupled ocean-atmosphere system as well as for the ecosystem is yet to be further explored.
Today multi-sensor satellite observations manifest the surface expressions of mesoscale eddy features in the upper ocean on a regular basis, while Argo profiling floats every 10-days can observe the corresponding vertical water mass structures and biogeochemical properties within the eddies down to 2000 m. In addition, forced numerical ocean models with and without data assimilation resolve mesoscale eddy features allowing the 3-dimensional thermo-dynamic structure of the eddies to be investigated from the onset of the generation mechanisms to the decay. As such a 3-dimensional (3D) eddy study based on sensor-synergy, colocation and co-analyses with ocean models is highly timely and feasible. In this study, we capitalize on the growing amount of Sentinel-1 SAR data colocated with multi-mission Sentinel-3 radar altimeter and optical data and Argo profiling floats to study the evolution of the coupled physical-biogeochemical structures of eddies in the Lofoten Basin within the Norwegian Sea from January to June for the years 2016-2022. The presentation will focus on results and findings derived form:
• Co-variability analyses of the 2D surface expression of the mesoscale eddies using satellite sensor synergy.
• Data-driven approach to advance the 2D to 3D characterisation of mesoscale eddies from colocation and co-variability analyses of satellite observations and Argo profiling floats.

Coastal ocean areas are very dynamic regions subject to strong anthropogenic pressure (e.g. industry, tourism, renewable energies, population). Satellite data constitute a unique tool that allows one to study and monitor these areas at a unique spatial and temporal resolution. While “traditional” satellites like Sentinel-3 provide daily temporal resolution, the sensors onboard these satellites do not measure at the necessary high spatial resolution to resolve complex coastal dynamics. High spatial resolution sensors, like MSI onboard Sentinel-2 (10 m resolution), are able to resolve these small scales, but their temporal revisit time is far from optimal (about 5 days). Both high spatial resolution datasets and traditional ones are hindered by the presence of clouds, resulting in a large amount of missing data. The complementarity of these two datasets (Sentinel-2 and Sentinel-3) will be exploited to derive a gap-free, super-resolution dataset of total suspended matter in the Belgian coast (North Sea), with the spatial resolution of Sentinel-2 and the temporal resolution of Sentinel-3. The approach used is based in DINEOF (Data Interpolating Empirical Orthogonal Functions), a technique to reconstruct missing data that can also retain the spatial scales of the highest resolution dataset (Sentinel-2 in this case). The final, high spatial and temporal resolution dataset will be used to assess the influence of ocean dynamics in the transport of suspended sediments in the study region. The combination with ocean surface currents from a hydrodynamic model will help in assessing how the use of this variable can help in understanding the distribution of ocean colour variables in a dynamic region like the North Sea.

Synthetic aperture radar (SAR) offers the possibility to observe the sea surface circulation with very high spatial resolution. These high resolution observations are particularly relevant in coastal areas and shelf seas where the spatial and temporal scales are shorter than in the open ocean. SAR has been providing valuable information on sea surface winds and waves for many decades. During the last decade, a new application of SAR measurements based on the analysis of the Doppler shift has emerged [1][2]. The SAR Doppler shift is directly related to the sea surface motion, thus in principle direct measurements of surface currents is possible. It is however a challenging problem in practice due to the wave contribution to the observed Doppler shift, which must be accurately estimated and removed [3][4].

The main limitation of spaceborne SAR for monitoring geophysical processes and particularly fast evolving ocean dynamics is the long revisit time (~12 days). In order to overcome this limitation, data from three satellite-borne SAR sensors are combined in this study, namely Sentinel-1A, Sentinel-1B and TanDEM-X. Sentinel-1 is a conventional single-antenna SAR while TanDEM-X is an along-track interferometric SAR based on a formation of two satellites (TerraSAR-X and TanDEM-X). In addition, the two systems (Sentinel-1 and TanDEM-X) differ in the operating frequency (C-band and X-band) and in the imaging mode (TOPS and stripmap).

The aim of this study is to investigate the potential of spaceborne SAR to monitor the inflow/outflow circulation between the Baltic Sea and the North Sea through the narrow Danish straits using a time series of SAR derived Doppler shift. The Danish straits experience alternating inflow and outflow events. These events are irregular and predominantly driven by changes in atmospheric forcing [5]. In this study, the potential of SAR monitoring surface circulation is demonstrated using two months of opportunistic data (June and July 2020) covering the Danish strait (Fehmarn Belt). This time period is constrained by the availability of coincident (Sentinel-1 and TanDEM-X) data covering the area of interest. Note that TanDEM-X is not an ocean-dedicated mission, thus acquisitions suitable (with small along-track baseline) for ocean current retrieval are sporadic. The Doppler shift is estimated from Sentinel-1 using the Doppler anomaly analysis method, while for TanDEM-X, it is estimated from the interferometric phase.

By combining the three satellites, a time sampling interval varying between 1-3 days is achieved depending on availability and quality of the data. The study demonstrates that there is a good agreement between the radial velocities derived from Sentinel-1 and TanDEM-X, provided both datasets are calibrated over land and the time delay between acquisitions is below ~20 min. The residual difference is due to the wave-induced Doppler shift. It is also shown that the satellite SAR derived velocities agree reasonably well, provided that the wave-induced Doppler shift is compensated for, with the CMEMS analysis product BALTICSEA_ANALYSIS_FORECAST_PHY_003_006.

The study also investigates the relationship between the surface current along the Fehmarn Belt, the sea surface wind and the sea level as an attempt to understand the main drivers of the surface flow. First, there is a high variability in the duration of inflow/outflow from several days to hours. The longest and the shortest durations are one day and 10 days, respectively. Second, it is found that the surface current is predominantly in the east-to-west direction (outflow). Third, there is a clear relationship between the local wind and the surface current. This relationship is stronger in the outflow situation. There is also a noticeable relationship between the surface current and the sea level gradient. This relationship is stronger in the inflow situation. This indicates a probably larger contribution of the sea level gradient driven by the large-scale atmospheric forcing to the inflow than to the outflow. Though these observations agree with previous studies [5], it is however difficult to draw firm conclusions on the driving force from these limited dataset, additional data is required to verify these suggestions. Finally, it is also found that the observed surface circulation carries small-scale features that are mainly due to bathymetric modulation and wave-current interaction, and that these features vary depending on the direction of the flow.

The major limitation of the existing single-antenna SAR systems is that only the radial component of the surface current can be retrieved. This hampers the comparison and validation of the measured radial current to other sensors/data, e.g. HF radar, and complicates the assimilation of the measurements to numerical ocean models. Moreover, the actual wave-induced Doppler compensation is based on the coarse resolution wind direction typically provided by numerical weather prediction models. The Earth Explorer 11 SEASTAR candidate mission [6] will provide the total current vector owing to the innovative dual-squinted-antenna configuration. In addition, SEASTAR will simultaneously provide the total surface wind vector, which will be used to estimate and compensate for the wave-induced Doppler shift. Finally, its flexible revisit time will contribute to higher sampling of fast ocean dynamic processes.

References

[1] B. Chapron, F. Collard, and F. Ardhuin, “Direct measurements of ocean surface velocity from space: Interpretation and validation,” Journal of Geophysical Research, vol. 110, Mar 2005.

[2] R. Romeiser, H. Breit, M. Eineder, H. Runge, et al., "Current measurements by SAR along-track interferometry from a Space Shuttle," in IEEE
Transactions on Geoscience and Remote Sensing, vol. 43, no. 10, pp. 2315-2324, Oct. 2005

[3] Martin, A. C. H., C. Gommenginger, J. Marquez, S. Doody, et al., Wind-wave-induced velocity in ATI SAR ocean surface currents: First experimental evidence from an airborne campaign, J. Geophys. Res. Oceans, 121, 1640–1653, 2016.

[4] A. Elyouncha, L. E. B. Eriksson, R. Romeiser, and L. M. H. Ulander, “Measurements of sea surface currents in the Baltic Sea region using spaceborne along-track InSAR,” IEEE Transactions on Geoscience and Remote Sensing, vol. 57, no. 11, pp. 8584–8599, Nov 2019.

[5] Hans Ulrich Lass, Wolfgang Matthäus, General Oceanography of the Baltic Sea, In book: State and Evolution of the Baltic Sea, 1952-2005: A Detailed 50-Year Survey of Meteorology and Climate, Physics, Chemistry, Biology, and Marine Environment, 2008

[6] C. Gommenginger, B. Chapron, A. Hogg, Christian E. Buckingham, et al., SEASTAR: A Mission to Study Ocean Submesoscale Dynamics and Small-Scale Atmosphere-Ocean Processes in Coastal, Shelf and Polar Seas, Frontiers in Marine Science, vol 6, 2019

Funding

This work was funded by the Swedish National Space Agency (SNSA) contract dnr 214/19.

Monitoring ocean circulation at high resolution in both space and time is of paramount importance for under-standing and modelling the ocean-atmosphere climate system, especially in coastal areas. Spaceborne radar al-timeters have been used to successfully monitor ocean circulation on a global scale ( > 30km) in the deep ocean when the geostrophic approximation is generally valid. The ocean structures seen in high-resolution satellite measurements at meso (10-100km) and sub-mesoscale ( < 10km) are ubiquitous but little is known about their dynamics. During the last two decades, many studies have highlighted the key role played by the ocean sub-mesoscale in air-sea interactions, upper-ocean mixing and ocean vertical transport and the importance of ageo-strophic circulation in these processus. Understanding these smaller currents is critical to drive scientific under-standing of the exchanges of gas, heat, and momentum between the atmosphere and the ocean, and have im-portant implications for forecasting models and climate projections.
SEASTAR is an Earth Explorer 11 candidate mission, which aims to observe ocean submesoscale dynamics and small-scale atmosphere-ocean processes in coastal, shelf and polar seas by providing simultaneous measurements of current and wind vectors at 1km resolution with accuracy of respectively 10cm/s and 2m/s. OSCAR (Ocean Surface Current Airborne Radar) is the demonstrator for this satellite concept, and is in development at the Euro-pean Space Agency (ESA) in the frame of its preparatory activities for ocean surface current retrieval with Meta-sensing as a prime contractor. OSCAR system will be representative of a satellite mission concept, observation parameters and accuracies directly relate to a potential satellite mission.
OSCAR is a Ku-band (13.5 GHz) three-look direction SAR system with Along-track Interferometric (ATI) SAR and scatterometric capabilities. It is tailored to the measurement of 2D ocean surface motion and wind retrieval. The OSCAR features an along-track interferometric (ATI) baseline for two fields of view that are squinted 45° fore and aft from the broadside direction. Each of these views provide an interferometric measurement and these two views are angularly separated by 90 degrees, ensuring two independent measurements of the ocean surface movement velocity and eventually enabling retrieval of the 2D ocean surface movement vector. In addition, a broadside antenna will ensure measurements in the zero-Doppler direction to improve the retrieval sensitivity to wind direction, which is critical to retrieve an accurate ocean surface current. Indeed, the motion sensed by the microwave radar (after correcting for navigation and geometry) has two components: the total ocean surface cur-rent – consisting of all currents contributing to actual horizontal transport of water – and an unwanted measure-ment bias associated with wind-waves (known as wind-wave induced artifact surface velocity — WASV; see Martin et al., 2016). The WASV is understood to be mainly caused by the phase velocity of the surface scatterers responsible for the microwave backscatter (e.g. Bragg waves) and the effect of the orbital motion of longer ocean waves and is at first order function of the wind direction. Fully polarimetric observations are possible and the instrument is designed to be flexible in terms of the operational parameters and configuration, including the ATI baseline length, which is adjustable to be fully representative of the potential future spaceborne mission.
A dedicated system performance analysis tool has been implemented in order to derive all the instrument and ob-servation specifications to meet the requirements on the ocean surface observables. The key performances, which are driving the instrument are a measurement accuracy of 2D ocean surface motion of 5 cm/s (for velocities be-low 50cm/s and wind speed > 3m/s).
The performance tool is based only on numerical models without assumptions and calculates the reflected power and the Doppler characteristic of each backscattering cell of the illuminated sea surface in dependency on the ra-dar properties, the acquisition geometry, and the flight parameters. The performance of the instrument for the baseline design achieves a swath width larger than 2km in case of wind speed of 3m/s. The noise equivalent sigma zero (NESZ) is between -30dB and -45dB and the radiometric resolution is better than 0.1dB. The veloci-ty accuracy that the system can obtain is fulfilled from look angles lower than 20 degrees up to look angles above 63 degrees, resulting in more than 3km of swath available.
A first airborne verification of the instrument was performed in July 2021 by means of a test flight over land. The results confirmed a high SNR and coherency close to 1 for all channels (broadside and squinted). Internal calibration data showed the high stability of the instrument, well within requirements. The validation is continuing this winter with airborne data acquisitions over corner reflectors, flat land and over water.
A functional test campaign over ocean is planned in spring 2022 in the Iroise Sea to verify the end-to-end func-tionality and performance of the fully integrated instrument, including internal calibration capabilities, and of the processor by means of on-ground testing and flight tests.

In dynamic regions of the open ocean, coastal zones and sea-ice margins, complex and beautiful patterns of sea surface temperature (SST) are a surface expression of fluid motion within the ocean. Global SST analyses rely on a mix of observing systems, dominated by infrared remote sensing platforms using 'meteorological' sensors. These instruments' native resolution is typically between 0.5 km and 5 km, varying with viewing geometry. The feature resolution in global SST analyses after filling of observational gaps does not generally preserve variability on scales of 10 km or smaller, even when data are presented on finer grids.

Infrared-based SST imagery at native resolution (L2) does preserve 1 km scales. In operational products, however, strong thermal gradients are often masked by cloud-detection procedures. Where SST fronts and filaments are imaged, their location and shape may be clear, but not all SST retrieval procedures used operationally accurately preserve the magnitude of small-scale temperature differences. Although retrievals can be designed to avoid this, many available L2 SST products underestimate the strength of fronts, often significantly (by up to 50%). Such underestimation has implications for understanding phenomena such as the interaction of ocean fronts with the atmospheric boundary layer.

In addition to the ~1 km infrared imagers that collect global data, coastal zones are imaged in thermal wavelengths by imagers with resolution of the order of 100 m. Examples include the Landsat series and ECOSTRESS, with varying capacities for SST determination depending on channels and their calibration. The temporal repeat of these data sources is generally lower. However, this category of infrared imaging is due to expand with missions planned in coming years, and it will be important that these return data for coastal zones and marginal ice zones worldwide.

The series of Sea and Land Surface Temperature Radiometers (SLSTRs on Sentinel 3a to d) from are planned to be followed by missions giving enhanced continuity, including finer thermal spatial resolution and additional wavelengths. The improved fidelity in the < 10 km scale representation of SST variability arising from this enhanced capability will be presented, both in terms of the impact of resolution on cloud detection and imaging and in terms of the quantitative accuracy of SST contrasts. The need for and potential for finer-resolution coastal zone SST analyses will be discussed in relation to current and prospective thermal remote sensing capability.

High-resolution satellite images of sea surface temperature and ocean colour reveal an abundance of ocean fronts, swirls, vortices and filaments at horizontal scales below 10 km that permeate the global ocean, especially near mesoscale jets and eddies, in coastal seas and close to sea ice margins. Numerous research studies and high-impact scientific publications confirm the key role of submesoscale processes in air-sea interactions, upper-ocean mixing, lateral transports and vertical exchanges with the ocean interior. Small-scale processes also visibly dominate in coastal, shelf seas and polar seas - regions of disproportionally high strategic and societal value as hosts to numerous human activities and natural resources.

This talk will review some of the evidence about the fundamental role of small-scale ocean surface dynamics in the Earth System and outline the scientific case for new observations from space to characterise these important phenomena. The contribution will recall the science drivers and objectives of the SEASTAR candidate mission currently under study in Phase 0 of Earth Explorer 11 to initiate dialogue with the wider research community to refine our understanding of observational needs and future uses of SEASTAR observations.