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Coastal zones are highly dynamical systems affected by a variety of natural and anthropogenic forcing factors that include sea level rise, extreme events, local oceanic and atmospheric processes, ground subsidence, etc. However, so far, they remain poorly monitored on a global scale. To better understand changes affecting world coastal zones and to provide crucial information to decision-makers involved in adaptation to and mitigation of environmental risks, coastal observations of various types need to be collected and analyzed. In this white paper, we first discuss the main forcing agents acting on coastal regions (e.g., sea level, winds, waves and currents, river runoff, sediment supply and transport, vertical land motions, land use) and the induced coastal response (e.g., shoreline position, estuaries morphology, land topography at the land-sea interface and coastal bathymetry). We identify a number of space-based observational needs that have to be addressed in the near future to understand coastal zone evolution. Among these, improved monitoring of coastal sea level by satellite altimetry techniques is recognized as high priority. Classical altimeter data in the coastal zone are adversely affected by land contamination with degraded range and geophysical corrections. However, recent progress in coastal altimetry data processing and multi-sensor data synergy, offers new perspective to measure sea level change very close to the coast. This issue is discussed in much detail in this paper, including the development of a global coastal sea-level and sea state climate record with mission consistent coastal processing and products dedicated to coastal regimes. Finally, we present a new promising technology based on the use of Signals of Opportunity (SoOp), i.e., communication satellite transmissions that are reutilized as illumination sources in a bistatic radar configuration, for measuring coastal sea level. Since SoOp technology requires only receiver technology to

The first digital elevation model (DEM) of the Antarctic ice sheet derived from Global Navigation Satellite Systems‐Reflectometry (GNSS‐R) data from the UK TechDemoSat‐1 satellite is presented. This is obtained using 32 months of data from the mission. This opportunistic and inexpensive method is shown to produce encouraging results from the technology demonstration platform of TechDemoSat‐1, with median bias under 18 m and root‐mean‐square difference under 91 m when compared to the CryoSat‐2 1‐km DEM v1.0. Discrepancies between the two data sets are explored along with possible causes of such differences and potential improvements to further optimize this technique for future GNSS‐R missions.

Sea level is a very sensitive index of climate change since it integrates the impacts of ocean warming and ice mass loss from glaciers and the ice sheets. Sea level has been listed as an essential climate variable (ECV) by the Global Climate Observing System (GCOS). During the past 25 years, the sea level ECV has been measured from space by different altimetry missions that have provided global and regional observations of sea level variations. As part of the Climate Change Initiative (CCI) program of the European Space Agency (ESA) (established in 2010), the Sea Level project (SL_cci) aimed to provide an accurate and homogeneous long-term satellite-based sea level record. At the end of the first phase of the project (2010-2013), an initial version (v1.1) of the sea level ECV was made available to users (Ablain et al., 2015). During the second phase of the project (2014-2017), improved altimeter standards were selected to produce new sea level products (called SL_cci v2.0) based on nine altimeter missions for the period 19932015 (https://doi.org/10.5270/esa-sea_level_cci-1993_2015-v_2.0-201612; Legeais and the ESA SL_cci team, 2016c). Corresponding orbit solutions, geophysical corrections and altimeter standards used in this v2.0 dataset are described in detail in Quartly et al. (2017). The present paper focuses on the description of the SL_cci v2.0 ECV and associated uncertainty and discusses how it has been validated. Various approaches have been used for the quality assessment such as internal validation, comparisons with sea level records from other groups and with in situ measurements, sea level budget closure analyses and comparisons with model outputs. Compared with the previous version of the sea level ECV, we show that use of improved geophysical corrections, careful bias reduction between missions and inclusion of new altimeter missions lead to improved sea level products with reduced uncertainties on different spatial and temporal scales. However, there i

Sea level is an essential climate variable (ECV) that has a direct effect on many people through inundations of coastal areas, and it is also a clear indicator of climate changes due to external forcing factors and internal climate variability. Regional patterns of sea level change inform us on ocean circulation variations in response to natural climate modes such as El Niño and the Pacific Decadal Oscillation, and anthropogenic forcing. Comparing numerical climate models to a consistent set of observations enables us to assess the performance of these models and help us to understand and predict these phenomena, and thereby alleviate some of the environmental conditions associated with them. All such studies rely on the existence of long-term consistent high-accuracy datasets of sea level. The Climate Change Initiative (CCI) of the European Space Agency was established in 2010 to provide improved time series of some ECVs, including sea level, with the purpose of providing such data openly to all to enable the widest possible utilisation of such data. Now in its second phase, the Sea Level CCI project (SL_cci) merges data from nine different altimeter missions in a clear, consistent and well-documented manner, selecting the most appropriate satellite orbits and geophysical corrections in order to further reduce the error budget. This paper summarises the corrections required, the provenance of corrections and the evaluation of options that have been adopted for the recently released v2.0 dataset (https://doi.org/10.5270/esa-sea_level_cci-1993_2015-v_2.0-201612). This information enables scientists and other users to clearly understand which corrections have been applied and their effects on the sea level dataset. The overall result of these changes is that the rate of rise of global mean sea level (GMSL) still equates to ∼ 3.2 mm yr−1 during 1992–2015, but there is now greater confidence in this result as the errors associated with several of the corrections have be

We examine the issue of sustained measurements of sea level in the coastal zone, first by summarizing the long-term observations from tide gauges, then showing how those are now complemented by improved altimetry products in the coastal ocean. We present some of the progresses in coastal altimetry, both from dedicated reprocessing of the radar waveforms and from the development of improved corrections for the atmospheric effects. This trend towards better altimetric data at the coast comes also from technological innovations such as Ka-band altimetry and SAR altimetry, and we discuss the advantages deriving from the AltiKa Ka-band altimeter and the SIRAL altimeter on CryoSat-2 that can be operated in SAR mode. A case study along the UK coast demonstrates the good agreement between coastal altimetry and tide gauge observations, with RMSD's as low as 4 cm at many stations, allowing the characterization of the annual cycle of sea level along the UK coasts. Finally we examine the evolution of the sea level trend from the open to the coastal ocean along the Western coast of Africa, comparing standard and coastally-improved products. Different products give different sea level trend profiles, so the recommendation is that additional efforts are needed to study sea level trends in the coastal zone from past and present altimeters. Further improvements are expected from more refined processing and screening of data, but in particular from the constant improvements in the geophysical corrections.

This paper analyzes the availability and accuracy of coastal altimetry sea level products in the Strait of Gibraltar. All possible repeats of two sections of the Envisat and AltiKa ground-tracks were used in the eastern and western portions of the strait. For Envisat, along-track sea level anomalies (SLAs) at 18-Hz posting rate were computed using ranges from two sources, namely, the official Sensor Geophysical Data Records (SGDRs) and the outputs of a coastal waveform retracker, the Adaptive Leading Edge Subwaveform (ALES) retracker; in addition, SLAs at 1 Hz were obtained from the Centre for Topographic studies of the Ocean and Hydrosphere (CTOH). For AltiKa, along-track SLA at 40 Hz was also computed both from SGDR and ALES ranges. The sea state bias correction was recomputed for the ALES-retracked Envisat SLA. The quality of these altimeter products was validated using two tide gauges located on the southern coast of Spain. For Envisat, the availability of data close to the coast depends crucially on the strategy followed for data screening. Most of the rejected data were due to the radar instrument operating in a low-precision nonocean mode. We observed an improvement of about 20% in the accuracy of the Envisat SLAs from ALES compared to the standard (SGDR) and the reprocessed CTOH data sets. AltiKa shows higher accuracy, with no significant differences between SGDR and ALES. The use of products from both missions allows longer times series, leading to a better understanding of the hydrodynamic processes in the study area.

An analysis of spaceborne Global Positioning System reflectometry (GPS-R) data from the TechDemoSat-1 (TDS-1) satellite is carried out to image the ocean sea surface height (SSH). An SSH estimation algorithm is applied to GPS-R delay waveforms over two regions in the South Atlantic and the North Pacific. Estimates made from TDS-1 overpasses during a 6 month period are aggregated to produce SSH maps of the two regions. The maps generally agree with the global DTU10 mean sea surface height. The GPS-R instrument is designed to make bistatic measurements of radar cross section for ocean wind observations, and its altimetric performance is not optimized. The differences observed between measured and DTU10 SSH can be attributed to limitations with the GPS-R instrument and the lack of precision orbit determination by the TDS-1 platform. These results represent the first observations of SSH by a spaceborne GPS-R instrument.

Significant wave height (SWH) is mapped globally through satellite altimetry. SWH estimation is possible because the shape of a pulse-limited altimetric waveform depends on the sea state. The algorithm for SWH also depends on the width of the point target response (PTR) function. Particularly challenging for SWH detection are coastal data, due to land and calm water interference in the altimeter footprint, and low sea states, due to an extremely sharp leading edge in the waveform that is consequently poorly sampled. Here, Adaptive Leading Edge Subwaveform Retracker (ALES), a new algorithm for reprocessing altimetric waveforms, will be validated for SWH estimation in the German Bight. This challenging region presents both low sea state and coastal issues, and an extended network of buoys of the Bundesamt fuer Seeschiffahrt und Hydrographie is available for the in situ validation. Reprocessed data from Envisat, Jason-1, and Jason-2 missions are validated against the three offshore buoys. The in situ validation is applied both at the point nearest to the buoy and at all other points along track. The skill metrics is based on bias, standard deviation, slope of regression line, and number of cycles with correlation larger than 90%. We also tested the impact of the inclusion of two additional waveform samples that are provided in the Envisat Sensor Geophysical Data Records and the adoption of different values for the width of the PTR. Results show that near coast ALES estimations of SWH are generally better correlated with buoy data than standard processed products.

The seasonal sea level variations observed from tide gauges over 1900–2013 and gridded satellite altimeter product AVISO over 1993–2013 in the northwest Pacific have been explored. The seasonal cycle is able to explain 60–90% of monthly sea level variance in the marginal seas, while it explains less than 20% of variance in the eddy-rich regions. The maximum annual and semiannual sea level cycles (30 and 6 cm) are observed in the north of the East China Sea and the west of the South China Sea, respectively. AVISO was found to underestimate the annual amplitude by 25% compared to tide gauge estimates along the coasts of China and Russia. The forcing for the seasonal sea level cycle was identified. The atmospheric pressure and the steric height produce 8–12 cm of the annual cycle in the middle continental shelf and in the Kuroshio Current regions separately. The removal of the two attributors from total sea level permits to identify the sea level residuals that still show significant seasonality in the marginal seas. Both nearby wind stress and surface currents can explain well the long-term variability of the seasonal sea level cycle in the marginal seas and the tropics because of their influence on the sea level residuals. Interestingly, the surface currents are a better descriptor in the areas where the ocean currents are known to be strong. Here, they explain 50–90% of interannual variability due to the strong links between the steric height and the large-scale ocean currents.

The seasonal sea level variations observed from tide gauges over 1900-2013 and gridded satellite altimeter product AVISO over 1993-2013 in the northwest Pacific have been explored. The seasonal cycle is able to explain 60-90% of monthly sea level variance in the marginal seas, while it explains less than 20% of variance in the eddy-rich regions. The maximum annual and semiannual sea level cycles (30 and 6 cm) are observed in the north of the East China Sea and the west of the South China Sea, respectively. AVISO was found to underestimate the annual amplitude by 25% compared to tide gauge estimates along the coasts of China and Russia. The forcing for the seasonal sea level cycle was identified. The atmospheric pressure and the steric height produce 8-12 cm of the annual cycle in the middle continental shelf and in the Kuroshio Current regions separately. The removal of the two attributors from total sea level permits to identify the sea level residuals that still show significant seasonality in the marginal seas. Both nearby wind stress and surface currents can explain well the long-term variability of the seasonal sea level cycle in the marginal seas and the tropics because of their influence on the sea level residuals. Interestingly, the surface currents are a better descriptor in the areas where the ocean currents are known to be strong. Here, they explain 50-90% of interannual variability due to the strong links between the steric height and the large-scale ocean currents.

Extending the applications of satellite altimetry to the coastal zone requires validated, quality controlled data. We present a validation study in the Gulf of Cadiz (SW Iberian Peninsula), an area of relevant social, economic, and strategic importance. We compare against in-situ data seven years (Dec. 2002–Jan. 2010) of significant wave height (SWH) measurements by the Envisat RA-2 altimeter from two sources: 1) standard geophysical data records (GDR, 1 Hz) and 2) the processor developed in the COASTALT (development of radar altimetry data processing in the coastal zone) project (18 Hz). The comparison is made along two Envisat passes (one descending, i.e., north to south, one ascending). For the descending pass (land to ocean transition) the COASTALT processor improves SWH retrieval in the coastal fringe. In particular, the COASTALT SWH product displays accuracies in the sub-coastal strip (12–20 km from the coastline) of a very similar magnitude to those further offshore, representing a clear improvement over GDR. The bias and standard deviation of the difference with regard to the in-situ measurements in the coastal fringe is about 60% and 40% lower, respectively, when COASTALT data are used instead of the standard GDR product. For the ascending pass, the differences in the ocean to land transition are less marked, probably due to the altimeter keeping a good lock on the sea surface until relatively close proximity to the coastline. This validation case shows that this new coastal-oriented product gets closer to the shoreline than before, while also making available higher-resolution along-track estimates.

The scientific community involved in the research and development of applications of satellite altimetry in the coastal zone met for the 6th Coastal Altimetry Workshop. Research in coastal altimetry is going through exciting times: Experimental data sets for the coastal zone are now available, such as those from the French PISTACH project for the Jason-2 satellite and from the European Space Agency (ESA)–funded COASTALT project for the Envisat satellite, and many diverse applications were shown at the workshop. These range from observing coastal currents to monitoring river and lake levels, ice margins, and storm surges to gravity mapping in coastal zones.

The relation between physical and biological processes affecting the Mediterranean Sea surface layer was investigated through univariate Empirical Orthogonal Function (EOF) decompositions of remotely sensed chlorophyll-a (CHL), sea surface temperature (SST) and Mediterranean Absolute Dynamic Topography (MADT) weekly time series (1998–2006). As part of the analysis, the Data INterpolating Empirical Orthogonal Functions (DINEOF) technique was successfully applied to CHL images. Results from the single EOFs, along with a cross-correlation analysis, identified physical–biological interactions at both short (weeks to months) and long (years) temporal scales, and from meso- to basin-scales. Phytoplankton biomass abundance and the sea surface thermal stratification show a strong inverse relationship at seasonal and sub-basin scales. At a regional scale, the spring bloom space–time variability is related to the intensity and spatial extent of the deep water formation process and especially to its pre-conditioning phase. At interannual and sub-basin scales, a gradual decline of the phytoplankton biomass in the whole central Mediterranean occurs with a delay of one year relative to the decrease of the cyclonic circulation in the eastern basin, and the northward displacement of the Algerian current. Regionally, the phytoplankton biomass and the surface heat content anomalies associated with extreme atmospheric anomalies (such as the cold 1998–1999 winter and the summer 2003 heat wave) show a significant correlation with a ~ 5-month time lag.

Table of contents Preface.- Chapter 1: Altimetry Missions: past, present and future.- Chapter 2: From Research to Operations: The USDA Global Reservoir and Lake Monitor.- Chapter 3: User requirements in the coastal ocean for satellite altimetry.- Chapter 4: Re-tracking altimeter waveforms near the coasts: A review of retracking methods and some applications to coastal waveforms.- Chapter 5: Range and geophysical corrections in coastal regions - and implications for mean sea surface determination.- Chapter 6: Tropospheric corrections for coastal altimetry.- Chapter 7: Surge Models as Providers of Improved "Inverse Barometer Corrections" for Coastal Altimetry Users.- Chapter 8: Tide Predictions in Shelf and Coastal Waters: Status and Prospects.- Chapter 9: Post-processing altimeter data toward coastal applications and integration into coastal models.- Chapter 10: Coastal Challenges for Altimeter Data Dissemination and Services.- Chapter 11: In situ Absolute Calibration and Validation - A link from coastal to open-ocean altimetry.- Chapter 12: Introduction and assessment of improved coastal altimetry strategies: case-study over the North Western Mediterranean Sea.- Chapter 13: Satellite Altimetry Applications in the Caspian Sea.- Chapter 14: Satellite Altimetry Applications in the Black Sea.- Chapter 15: Satellite Altimetry Applications in the Barents and White seas.- Chapter 16: Satellite Altimetry Applications off the Coasts of North America.- Chapter 17: Evaluation of Retracking Algorithms over China and Adjacent Coastal Seas.- Chapter 18: Satellite altimetry for geodetic, oceanographic and climate studies in the Australian region.- Chapter 19: Lakes studies from satellite altimetry.- Chapter 20: The Future of Coastal Altimetry.

The advent of high accuracy satellite altimetry in the 1990's brought the first global view of ocean dynamics, which together with a global network of supporting observations brought a revolution in understanding of how the ocean works [1]. At present a constellation of flying satellite missions routinely provides sea level anomaly, sea winds, sea surface temperature (SST), ocean colour, etc. with mesoscale resolution (50km to 100km, 20 to 150 days) on a near global scale. Concurrently, in situ monitoring is carried out by surface drifters, Argo floats, moorings, sea gliders as well as ship-borne CTD (Conductivity-Temperature-Depth) and XBT (Expendable Bathythermograph) (to measure profiles of temperature and salinity), and ADCP (Acoustic Doppler Current Profiler) (to measure current velocity profiles). This global observational system allowed observational oceanography to develop into an essentially quantitative science. This became possible because 1) the accuracy and volume of observations exceeded critical values, 2) numerous studies demonstrated good agreement between independent datasets, and critically 3) the data resolution crossed the threshold of revealing much of the mesoscale in two-dimensions, when previously it was only revealed in one-dimension along satellite ground tracks [2] with wide gaps in between. Fortuitously computing power kept pace allowing basin scale numerical ocean models to cross the threshold of revealing the mesoscale around the turn of the century [3]. The mesoscale is characterized by the most energetic motions and strong nonlinear interactions, issuing in a more complex range of phenomena. Below in Sect. 1 we present some highlights of this development. In Sect. 2, we describe future prospects, and Sect. 3 provides a reminder of the importance of maintaining continuity of high-quality observations. We conclude in Sect. 4 with discussion of integrating optimizing the observing system. The benefits to society from development of quan

The present study focuses on the spatial and temporal variability of interactions between physics and biogeochemistry during the Rossby wave passage in the South Atlantic Ocean. The Rossby wave signature in sea level anomalies (SLA) and surface chlorophyll a concentration anomalies (CHLA) is analyzed using remotely sensed data from 1997 to 2006. Wavelengths between 400 and 1100 km, with westward propagating speeds up to 7.5 cm.s−1, are observed. Using a theoretical model, three processes (meridional advection of surface chlorophyll a concentrations, uplifting of subsurface chlorophyll a maximum, and upwelling of nutrients) are likely to explain the chlorophyll a Rossby wave signature. A statistical assumption allows quantifying the relative importance of each process. Three zones are identified. The Subtropical Gyre is the only area where the contribution of the uplifting process reaches 20%. North and south of this gyre, the meridional advection process is responsible for an important part (around 60%) of the observed chlorophyll a signals. The temporal variability of this dominant process is studied using the phase relationships between CHLA and SLA and the surface meridional chlorophyll a gradient. A seasonal meridional shift (4°) is shown on both data sets on the area of negative meridional gradient. At 30°S–31°S, a clear seasonal cycle is observed in both data sets for the whole studied period, except in 2003 and 2004 where both data sets do not follow the usual seasonal cycle. These particular years can be related to anomalies in large scale atmospheric circulation over the South Atlantic Ocean.