Coastal Altimetry: Selected Case Studies from Asian Shelf Seas

Coastal Altimetry: Selected Case Studies from Asian Shelf Seas provides information on developments over the past decade in the processing of remotely sensed altimetry in coastal areas, with an overview of expected errors and where they stem from, along with remaining gaps in processing. Challenges covered include the retracking of the altimetric signal to account for land contamination, tropospheric water corrections, and tidal model improvements, along with the pros and cons of widely available products. Additional chapters provide recent research in the regional seas of Asia and cover variability, dynamics, predictability and prediction, impacts of extreme events, effects to ecosystems, and more.

This book offers readers a dataset that can illuminate our understanding of the propagation of planetary boundary waves that have a significant sea level signal in near coastal regions. As such, researchers and students who have a foundation in satellite altimetry and want to know the latest development of open ocean and coastal satellite altimetry, especially in Asian coastal regions, will benefit from this book.

• Presents the advancement of coastal altimetry technologies from various dedicated experts
• Includes case studies throughout to give real-life examples that can be implemented globally
• Provides chapters that include summaries of key points and an outlook to the future

• Language: English
• Publisher: Elsevier Science
• Release date: Jun 27, 2023
• ISBN: 9780323985710

Book preview

Chapter 1: Satellite altimetry and ocean circulation: from open ocean to the coast

Stefano Vignudelli ¹ , Mohd Fadzil Akhir ² , Zuraini Zainol ² , and Ku Nor Afiza Asnida Ku Mansor ²       ¹ Consiglio Nazionale delle Ricerche (CNR), Area delle Ricerca CNR S.Cataldo, Pisa, Italy      ² Institute of Oceanography and Environment, Universiti Malaysia Terengganu, Kuala Terengganu, Terengganu, Malaysia

Abstract

The advent of satellite altimeter missions and continuous data improvements has allowed an important scientific progress in the oceanography research field, particularly on large-scale ocean processes and circulations. Beginning with the first altimetry experiment in May 1973 that provided a general information on large-scale dynamics of the ocean, the advance in technology and processing in regional seas and near coasts has made possible detection and better characterization of mesoscale and small-scale features (e.g., eddies, boundary current, coastal upwelling, and sea-level rise).

Keywords

Asian shelf seas; Coastal altimetry; Ocean dynamics

1. Introduction

A number of remote-sensing techniques emerged as a key tool to gather information about the state of the ocean (Gower et al., 2011). A series of satellites have captured with unbelievable accuracy changes in sea level, ocean waves, currents, winds, temperature, salinity, suspended sediments (turbidity), chlorophyll, etc. (Aulicino et al., 2022). The data were obtained by the radar from satellite altimetry by measuring the distance from the satellite to the surface using a beam with a footprint of several kilometers (Birkett, 1998; Calmant et al., 2008; Eldardiry et al., 2022). Satellites have revolutionized our understanding of the oceans and now provide one of the most important records of how fast our climate is changing (Purkis and Klemas, 2011).

While open-ocean satellite-based observations have been improved and exploited for more than 3 decades, only in the last few years has there been growing interest in how to overcome the technical obstacles that were preventing the same information from being successfully retrieved in the coastal zone (Vignudelli end Benveniste, 2022). The coastal zone is of great societal importance, as population and economic activities are concentrated near the coastline. Rapid increases in the exploitation of coastal resources and global climate changes are making the coastal zone more vulnerable to human stresses and natural hazards. The coastal interface between land, sea, and air is a highly dynamic area that evolves in time and in space over a broad spectrum of scales in response to a number of drivers and pressures. Therefore, the availability of observations from sensors onboard satellites is an essential prerequisite for enhanced decision-making (Laignel et al., 2022).

Satellite altimetry is a legitimate component of the coastal ocean observing system (Cipollini et al., 2009). It provides a unique means to observe and monitor the marine coastal zone scale repeatedly in all weather and day/night conditions. The altimeter-derived information on sea level, sea state, currents would be extremely important for resolving the complex dynamics of the coastal ocean (Strub, 2001; Vignudelli et al., 2019), which is characterized by much finer scales of mesoscale variability and a variety of physical processes including the shelf circulation, coastal jets, offshore transport, tidal variability, wind-driven upwelling, outflow of rivers and estuaries, perturbations in the flow by abrupt bathymetric features, etc. Some ocean processes occur over time scales of seconds and space scales of meters (e.g., turbulence). Other processes such as tides occur over time scales of hours and space scales of kilometers. It is evident that many physical processes occur at scales not fully resolved by actual conventional altimeters alone, but if altimeter data are exploited in synergy with other data sources and modeling tools, this would contribute to significant improvements in their understanding.

2. Evolution of satellite altimetry application in ocean circulation studies

The launch of satellite altimeter mission, which focused on sea surface height (SSH) data in the 90s, has become a game changer in the oceanography research field. Prior to that, knowledge of the ocean circulation has been pieced together from scattered observations, which proved to be inadequate, especially when dealing with large-scale ocean processes and circulation. Globally, limited nature of hydrographic and in situ data has caused large-scale ocean circulation and its variability to be restricted both in space and time. However, with simultaneous operation of TOPEX/Poseidon (T/P) and European Remote Sensing Satellite-2 (ERS-2) missions since 1995, the estimation of the scales of ocean surface circulation processes has been made possible with an unprecedented resolution (Lázaro et al., 2005).

Earlier study of ocean circulation has been generally focused on large-scale dynamics. However, the presence of altimeter measurements has improved our understanding on the dynamics and thermodynamics of gyre circulation boundary currents by providing a synoptic overview of the current systems. The higher temporal data have allowed better understanding of the interannual variations and allowed scientists to quantify heat transport, seasonal and interannual variations in eddy kinetic energy (EKE).

In general, ocean current motion is mostly hydrostatic that is determined by pressure difference. Since pressure is related to SSH, scientists use SSH to calculate geostrophic current from altimetry data. Earlier work with altimeter data comes from the large-scale ocean dynamics that involves wide open-ocean basins involving large boundary current system such as California Current, Gulf Stream, Kuroshio Current, and many more.

The western boundary current understanding was further improved by radar altimeter as it provides both the dynamics and thermodynamics characteristics of the system. Observations of SSH anomalies from the T/P radar altimeter (Fu and Smith, 1996) since 1992 suggest that there are large interannual-to-decadal variations in the structure of these current systems (Qiu, 2000; Vivier et al., 2002).

From literature, Strub and James (2000) are among the pioneer scientists that combined the altimeter height fields with the sea surface temperature (SST) of six years altimeter data from three satellites; the Geosat data from the exact repeat mission (ERM), the TOPEX altimeter data and European Remote Sensing Satellite-1 (ERS-1) data, to examine the seasonal evolution of the large-scale circulation in the California Current System, along with its eddy characteristics. An almost similar technique was applied by Lazaro et al. (2005), who have analyzed an eight-year time-series merged ERS-2 and T/P satellite altimeter data to describe the seasonal and interannual variability of the ocean circulation around the Cape Verde Archipelago in the northeast Atlantic Ocean.

Heat transport by the boundary current is one of the important aspects that determine the air–sea interaction climate variability. The ability of altimetry data to observe low-frequency spatial variability change in SSH has allowed researchers to study the interannual dramatic events such as El Nino Southern Oscillation (ENSO) and Indian Ocean dipole, in which SST, SSH, and wind velocity could be used to analyze the various mechanisms responsible for these interannual changes. For example, Picaut et al. (2002) used strong ENSO data from 1997–1998 and demonstrated the importance of the surface current driven by west wind burst in carrying warm water to central Pacific that is responsible for the formation of El Nino system (Fu et al., 1994).

Mesoscale eddies are one of the most important features within the ocean circulation dynamics. Through the understanding of mesoscale eddies, the knowledge of boundary current circulation is usually improved as well. Satellite altimetry has provided a unique contribution to the global observation by providing the seasonal and interannual variations in the variability and intensity of this mesoscale feature. Moreover, improved resolution derived from the SSALTO/Data Unification and Altimeter Combination System (DUACS) merged T/P (resp. Jason-1) and ERS (resp. ENVISAT) datasets has provided new insight into eddy dynamics and its roles in the ocean circulation and heat transport.

The earliest work on mesoscale eddy using altimetry data was made by Stammer (1998), and later work by him (Stammer, 2008) has enable the estimation of the meridional eddy heat and salt transport for the global ocean using T/P altimeter data. The study provides important results on strong poleward eddy heat and salt transport occurring in the energetic western boundary currents: the Gulf Stream, the Kuroshio, and the Agulhas Current. In the equatorial region, eddy heat and salt flux are northward in the Pacific and southward in the Indian Ocean, while equatorward eddy transport occurs between 5 degrees and 20 degrees latitude.

The role of large-scale circulation on eddy propagation was also well demonstrated in the study of Isoguchi and Kawamura (2003) by using the merged T/P and ERS datasets. In the study, they documented the eddies along the western boundary region of the Subarctic North Pacific were advected by the time-dependent Sverdrup wind-driven current. This is very important as it shows that when two eddies exist in the same region, they show coherent pattern that will influence the main current circulation dynamics.

In large ocean basins such as South Indian Ocean, there are many eddies formation in a single region. Their propagations behaviors usually form in a particular fashion. Fang and Morrow (2003) analyzed the anticyclonic warm core eddies for six years from 1995–to 2000 and found that long-lived warm-core eddies could be tracked for a period longer than six months and propagated as far as 1500 km. Most of the time, eddies steered by bathymetry and it decays after three months and turns slower over time.

Mesoscale eddy dynamics in marginal seas provide another beneficial overview. Chen et al. (2009) used 15 years of datasets from different mission (ERS-1/2, T/P, Jason-1, and ENVISAT) to analyze the variability of the EKE in the South China Sea (SCS). The study managed to characterize EKE strength based on different region. They also found that the EKE structure is the consequence of the superposition of different variability components. First, interannual variability is important in the SCS; second, the EKE shows different trends in different regions. It turns out that the seasonal cycle is the most obvious timescale affecting EKE variability, but interannual effect caused by El Nino or any sporadic events can easily affect the EKE variability.

Smaller-scale current system in marginal seas also benefited from the satellite altimetry data. For example, Fang et al. (2006) used 11-year satellite altimeter SSH anomaly data from January 1993 to December 2003, aiming to observe the spatial and temporal variations of the SCS surface circulation through Empirical Orthogonal Function analysis. Meanwhile, in the northern part of the East Sea, Japan, gridded multisatellite sea-level anomalies (SLAs) data that include all of standard corrections generated by the SSALTO multimission ground segment/DUACS (SSALTO/DUACS) were employed by Kim et al. (2021) to estimate the properties of mesoscale thermodynamic features in this area.

Although satellite altimeter data were more applicable in the open ocean, somehow the use of higher frequency altimeter measurements allows the observation of the variability of the large shallow shelf region in Sunda Shelf. Earlier works try to move away from shallow region as it reduces the efficiency of calculating geostrophic current from SSH data. Through an interesting work in the east coast of Peninsular Malaysia, which involved a compilation of 19-year multimission satellite altimeter using the Radar Altimeter Database System (RADS), Pa'suya et al. (2014) were able to document the sea surface circulation pattern over the coast under two different monsoon seasons, which surprisingly well agreed with the measured field data.

Apart from general current circulation, satellite altimeter data have been widely applied in coastal upwelling studies. Through the merged eight-year of ERS and T/P SSH climatology data, Xie et al. (2003) found an anticyclonic circulation off the coast of South Vietnam, which functions to advect the cold coastal water offshore, inducing upwelling formation in this area. Meanwhile, in the Gulf of Thailand, Sojisuporn et al. (2010) succeed to document two upwelling areas: one in the inner gulf and the other one at the western side of the gulf entrance, through the computation of total geostrophic circulation by utilizing the World Ocean Database and TP and ERS-2 altimetry data. A formation of cyclonic eddy that triggered an upwelling formation in the Andaman Sea was also managed to be captured by Buranapratheprat et al. (2010) through the analysis of geostrophic current anomaly from the satellite altimeter data.

Apart from the oceanographic features explained earlier, there are other various important oceanographic features that also benefited from satellite altimeter, for instance, ocean tides where T/P data have allowed physical oceanographer to better understand the tidal energy dissipation and internal tide, especially some longstanding questions about tidal energetics (Xu et al., 2019). Meanwhile, another important feature is planetary waves that cross the ocean and interact with other ocean circulation, that is, Rossby waves and Kelvin waves. These waves are only being predicted without proper evidence until the altimetry data were available.

Looking into the development of satellite altimetry in recent years, the technology has now been established as essential datasets for open-ocean and sea-level studies. Additionally, new technology can now support diverse applications in the coastal zone with improve accuracy. Classical study of large-scale global ocean circulation, mesoscale eddies, and SSH in open seas has now been added with higher-resolution features in a localized area such as coastal current, coastal sea-level changes, and storm surge.

One of the most important advancements that satellite altimetry made toward the ocean circulation study is the new coastal altimetry datasets that has turn another gear in altimetry contributions toward ocean circulation studies. One of the most important advancements that coastal altimetry bring forward is the ability of the data increase and close to resolution and provide reliable data close to the coast. Earlier researchers focus their usage of altimetry data only on the mesoscale variability dynamics that is more than 10 km scale and ocean feature that is far from the coastal region.

There are many efforts by altimetry community to improve data accuracy in coastal region, and numbers of important altimetry product have been made available. These products receive major improvement by providing correction in many aspects, for instance, geophysical correction using robust median-based editing criterion for ionosphere, and Loess filter for sea state bias, as used by X-Track processor (Birol and Delebecque, 2014). Meanwhile, some product use waveform classification and design of an adaptive retracker that can be applied to a variety of waveforms. As an example, one of the coastal altimetry applications using retracking algorithms are implemented and tested using Jason-1 around China coastal seas. The data analysis is using SLA differences between ascending and descending passes that are calculated at crossover points and in situ SSH measurements from tide gauge stations.

Another study from Australia added huge number of observational data parameters to utilize altimetry data with the intention to improve the quality of altimeter SSH data in coastal regions, while observing and understanding the structure and variability of the major boundary current systems, and estimating sea-level changes. In the study by Deng (2011), altimeter data are synergistically used with oceanic data such as drifting buoys, in situ data, and coastal tide gauges. The result has successfully presented an integration of altimetric and in situ data with a high-resolution computer model, providing a simulation of the sea-level changes.

Coastal altimetry is important in oceanography aspect because it is one of the key remote-sensing technologies in the coastal zone. This is even more important when we look at the impact of changing climate toward the coastal region. At this stage, coastal altimetry has been actively explored by altimetry community, and it will continue to make an important contribution to a number of key strategic research areas if more oceanographers and coastal marine scientist involve in the development of coastal altimetry data in exploring more scientific gaps in coastal region.

3. Technology advancements in coastal altimetry

Figs. 1.1–1.3 show the development of the satellite radar altimetry constellation for history, recent, and future of altimetry mission. Every mission has been providing beneficial information for an international user community to obtain ocean data particularly. The high precision of satellite altimetry has revolutionized especially for researchers to understand the earth and its oceans. Since 1986, altimetry starts to provide the data, but the experiment of the altimetry was started May 1973 to February 1974 named Skylab (Jet Propulsion Laboratory of California [JPL], 2022) under Earth Resources Experiment Package. This first space-borne mission was designed and launched to dissolve the issues of atmosphere and ocean problems. For oceanography, the main purpose is to determine the position of the ocean's surface reference toward the center of the Earth to precisely measure SST, explore the dynamic of the ocean such as current circulation, upwelling event, high productivity area, describe the erosion, river runoff, and others. During the experiment, the data from Skylab were validated with ground truth data.

Figure 1.1  Timeline for previous satellite altimetry.

Figure 1.2  Timeline for current satellite altimetry.

Figure 1.3  Timeline for future satellite altimetry.

Skylab observations were found to be useful in studying the oceans. The SST was measured more accurately using an infrared spectrometer after atmospheric effects were corrected to an accuracy of ± 1K using data analysis in two selected wavelengths sensed by the instrument. Aside from being able to observe depths of up to 18 m, the multispectral scanner was used to obtain data that could be used to describe the temperature distribution toward current circulation and upwelling. Researchers validated theories and techniques that will be used in the future to observe sea ice, sea state, and winds over the ocean on a global scale during the Skylab mission. Skylab attempts to understand the interaction of the atmosphere with the land and ocean surfaces in the atmosphere section. Skylab's success in the atmosphere can be seen in the development of meteorological satellites that use the microwave spectrum to measure surface winds over the oceans for weather forecasting as well as shipping. The instrument on the Geodynamics and Earth Ocean Satellite (GEOS-3) has been upgraded based on the knowledge gained by Skylab, and a more accurate model will be used on the Seasat (JPL,