Spatial and temporal scales of variability in Tropical Atlantic sea surface salinity from the SMOS and Aquarius satellite missions

Taking advantage of the spatially dense, multi-year time series of global Sea Surface Salinity (SSS) from two concurrent satellite missions, the spatial and temporal decorrelation scales of SSS in the Tropical Atlantic 30°N–30°S are quantified for the first time from SMOS and Aquarius observations. Given the dominance of the seasonal cycle in SSS variability in the region, the length scales are calculated both for the mean and anomaly (i.e. seasonal cycle removed) SSS fields. Different 7–10 days composite SSS products from the two missions are examined to explore the possible effects of varying resolution, bias corrections and averaging characteristics. With the seasonal cycle retained, the SSS field is characterized by strongly anisotropic spatial variability. Homogeneous SSS variations in the Tropics have the longest zonal scales of over ~ 2000 km and long temporal scales of up to ~ 70–80 days, as shown by both SMOS and Aquarius. The longest meridional scales, reaching over ~ 1000 km, are seen in the South Atlantic between ~ 10°–25°S, most discernible in Aquarius data. The longest temporal scales of SSS variability are reported by both satellites to occur in the North-West Atlantic region 15°–30°N, at the Southern end of the Sargasso Sea, with SSS persisting for up to 150–200 days. The removal of the seasonal cycle results in a noticeable decrease in the spatio-temporal decorrelation scales over most of the basin. Overall, with the exception of the differences in the South Atlantic, there is general agreement between the spatial and temporal scales of SSS from the two satellites and different products, despite differences in individual product calibration and resolution characteristics. These new estimates of spatio-temporal decorrelation scales of SSS improve our knowledge of the processes and mechanisms controlling the Tropical Atlantic SSS variability, and provide valuable information for a wide range of oceanographic and modelling applications

Modeling Envisat RA-2 waveforms in the coastal zone: case-study of calm water contamination

Radar altimeters have so far had limited use in the coastal zone, the area with most societal impact. This is due to both lack of, or insufficient accuracy in the necessary corrections, and more complicated altimeter signals. This paper examines waveform data from the Envisat RA-2 as it passes regularly over Pianosa (a 10 km2 island in the NW Mediterranean). Forty-six repeat passes were analysed, with most showing a reduction in signal upon passing over the island, with weak early returns corresponding to the reflections from land. Intriguingly one third of cases showed an anomalously bright hyperbolic feature. This feature may be due to extremely calm waters in the Golfo della Botte (northern side of the island), but the cause of its intermittency is not clear. The modelling of waveforms in such a complex land/sea environment demonstrates the potential for sea surface height retrievals much closer to the coast than is achieved by routine processing. The long-term development of altimetric records in the coastal zone will not only improve the calibration of altimetric data with coastal tide gauges, but also greatly enhance the study of storm surges and other coastal phenomena

Simultaneous ocean surface current and wind vectors retrieval with squinted SAR interferometry: Geophysical inversion and performance assessment

Simultaneous measurements of ocean surface current and wind vectors at the ocean submesoscale (O [1–10 km]) are needed to improve our understanding of upper ocean mixing, air-sea interactions, ocean biophysical processes and large-scale oceanic transports. A new satellite mission concept called SEASTAR aims to do just that. The concept is a Ku-band along-track interferometric Synthetic Aperture Radar (SAR) system with two squinted beams pointing ±45° from broadside and incidence angles around 30°. The paper presents an inversion strategy to retrieve simultaneously ocean surface current and wind vectors and reports on the performance obtained with different wind/current conditions and instrument configurations. Results are based on numerical simulations using a Bayesian approach and existing geophysical model functions (GMFs) of the microwave Normalized Radar Cross Section (NRCS) and Doppler shift. Using the baseline two-look instrument configuration and realistic instrument noise figures (radiometric resolution: kp = 5 and 12%; Δdf = 2 and 5 Hz), the root-mean square errors (RMSE) of the retrieved current and wind vectors are typically better than [0.1 m/s, 10°] for current and [0.5 m/s, 5°] for wind. This inversion setup yields four ambiguous solutions within a current range of ∼1 m/s. The addition of dual polarization (VV, HH) capability helps to discriminate these ambiguities. The retrieval performance depends weakly on geophysical parameters such as wind speed, current velocity or current direction, but is sensitive to wind direction because of its strong effect on current retrieval through the wind-wave induced artifact surface velocity (WASV). Larger retrieval errors are obtained when the wind is aligned with one of the antenna line-of-sight (LoS) directions, although errors remain typically below [0.2 m/s, 25°] for current and [0.5 m/s, 15°] for wind. Improving the retrieval performance regardless of wind direction could be achieved either with lower noise figures on σ0, or with higher incidence angles, or by including an additional third-look direction in azimuth (e.g. to achieve a configuration similar to Metop/ASCAT scatterometers) as per the SEASTAR mission concept submitted to EE10

First multi-year assessment of Sentinel-1 radial velocity products using HF radar currents in a coastal environment

Direct sensing of total ocean surface currents with microwave Doppler signals is a growing topic of interest for oceanography, with relevance to several new ocean mission concepts proposed in recent years. Since 2014, the spaceborne C-band SAR instruments of the Copernicus Sentinel-1 (S1) mission routinely acquire microwave Doppler data, distributed to users through operational S1 Level-2 ocean radial velocity (L2 OCN RVL) products. S1 L2 RVL data could produce high-resolution maps of ocean surface currents that would benefit ocean observing and modelling, particularly in coastal regions. However, uncorrected platform effects and instrument anomalies continue to impact S1 RVL data and prevent direct exploitation. In this paper, a simple empirical method is proposed to calibrate and correct operational S1 L2 RVL products and retrieve two-dimensional maps of surface currents in the radar line-of-sight. The study focuses on the German Bight where wind, wave and current data from marine stations and an HF radar instrumented site provide comprehensive means to evaluate S1 retrieved currents. Analyses are deliberately limited to Sentinel-1A (S1A) ascending passes to focus on one single instrument and fixed SAR viewing geometry. The final dataset comprises 78 separate S1A acquisitions over 2.5 years, of which 56 are matched with collocated HF radar data. The empirical corrections bring significant improvements to S1A RVL data, producing higher quality estimates and much better agreement with HF radar radial currents. Comparative evaluation of S1A against HF radar currents for different WASV corrections reveal that best results are obtained in this region when computing the WASV with sea state rather than wind vector input. Accounting for sea state produces S1 radial currents with a precision (std of the difference) around 0.3 m/s at ∼1 km resolution. Precision improves to ∼0.24 m/s when averaging over 21 × 27 km2, with correlations with HF radar data reaching up to 0.93. Evidence of wind-current interactions when tides and wind align and short fetch conditions call for further research with more satellite data and other sites to better understand and correct the WASV in coastal regions. Finally, 1 km resolution maps of climatological S1A radial currents obtained over 2.5 years reveal strong coastal jets and fine scale details of the coastal circulation that closely match the known bathymetry and deep-water coastal channels in this region. The wealth of oceanographic information in corrected S1 RVL data is encouraging for Doppler oceanography from space and its application to observing small scale ocean dynamics, atmosphere and ocean vertical exchanges and marine ecosystem response to environmental change

Reduced ascending/descending pass bias in SMOS salinity data demonstrated by observing westward-propagating features in the South Indian Ocean

The European Space Agency (ESA) Soil Moisture and Ocean Salinity (SMOS) satellite has been providing data, including sea surface salinity (SSS) measurements, for more than five years. However, the operational ESA Level 2 SSS data are known to have significant spatially and temporally varying biases between measurements from ascending passes (SSSA) and measurements from descending passes (SSSD). This paper demonstrates how these biases are reduced through the use of SSS anomalies. Climatology products are constructed using SMOS Level 2 data to provide daily, one-degree by one-degree climatologies separately for ascending and descending passes using a moving window approach (in time and space). The daily, one-degree products can then be averaged to provide values of climatological SSS at different spatial and/or temporal resolutions. The averaged values of the SMOS climatology products are in good general agreement with data from the World Ocean Atlas 2013. However, there are significant differences at high latitudes, as well as in coastal and dynamic regions, as found by previous studies. Both the mean and standard deviation of the differences between data from ascending passes and data from descending passes for the anomalies are reduced compared with those obtained using the original salinity values. Geophysical signals are clearly visible in the anomaly products and an example is shown in the Southern Indian Ocean of westward-propagating signals that we conclude represent the surface expression of Rossby waves or large-scale non-linear eddies. The signals seen in salinity data agree (in speed) with those from sea surface temperature and sea surface height and are consistent with previous studies

A new daily quarter degree sea level anomaly product from CryoSat-2 for ocean science and applications

The European Space Agency launched CryoSat-2 as the first European ice mission in 2010. Its advanced altimeter met primary objectives concerned with sea ice thickness and ice sheets. The value of Cryosat-2 data over global oceans was recognised, and operational products were developed via the CryoSat Ocean Processor (COP). The novel orbit of CryoSat-2 results in a denser coverage of sample points compared to other satellite altimeters. The National Oceanography Centre Sea Level Anomaly (NOCSLA) gridded product is based on interpolating Geophysical Ocean Products (GOP) using weights in space and time. GOP represents the highest quality operational ocean data. NOCSLA is a daily, ¼° sea level anomaly product covering non-coastal oceans between [60°N 60°S] and January 2011 to October 2020. The paper presents the methodology and scientific applications of NOCSLA. Oceanographic features observed are compared against products from other missions, including Rossby waves and El Niño signals. Results show good agreement with other products, confirming the value of Cryosat-2 data for ocean science and applications

Ocean Surface Current Airborne Radar (OSCAR): a new instrument to measure ocean surface dynamics at the sub-mesoscale

Oceans form Space V Symposium, 24-28 october 2022, Venice, Italy.-- 2 pages, 3 figuresThe ocean interacts with the atmosphere, land and ice on multiple spatial scales including fine submesoscales that are often observed in high resolution optical images. Little is known about their dynamics however. SeaSTAR is an innovative satellite mission concept that proposes to address this gap by mapping ocean current and wind vectors at 1 km resolution. In this paper, we present the OSCAR instrument - an airborne demonstrator of the SeaSTAR concept - and the first results from a scientific campaign over the Iroise Sea in May 2022. The capabilities of OSCAR are demonstrated against ground truth data with very promising first results. These results open the door to using OSCAR as a scientific tool to provide unique 2D synoptic views of ocean and atmosphere dynamics at km-scalesThis work was supported by ESA/ESTEC Contract Number 4000116410/16/NL/BJ for the OSCAR development and ESA/ESTEC contract number 400017623/22/NL/IA for the campaign over Iroise SeaPeer reviewe