Satellite Altimeter Observations Research Articles

Resolving the ubiquitous small-scale semi-permanent features of the general ocean circulation: a multiplatform observational approach

Abstract Based on twenty years of Argo and ship/animal-borne/glider hydrographic profile data, we derive a new high resolution hydrographic Atlas and associated circulation field for the oceans above 2000 dbar. Satellite altimetric observations are used to explicitly regress out eddy noise in the fit, greatly reducing one of the major sources of noise. Geostrophic shears are found from the fitted geopotential anomaly fields. Ekman velocities are estimated using satellite wind stresses. Both Argo trajectory observations at 1000 dbar and surface drifter observations are used to reference geostrophic shears derived from the Atlas hydrography. Surface drifter velocities are analyzed with an additional wind-friction term to remove the wind-related flow. Agreement between the surface geostrophic (referenced to Argo trajectories) and drifter-based surface velocity is high at both large and mesoscales, lending confidence to the derived geostrophic circulation fields. The Atlas reveals standing mesoscale eddies and meanders in western boundary systems, and the braided jet structure of the Antarctic Circumpolar Current. In the interior, the upper ocean flow consists of a highly baroclinic large-scale Sverdrup flow and smaller scale (~200 km width) semi-zonal jets, which are more barotropic (low vertical shear) and have an average zonal width of around 2000 km. These semi-zonal jets are globally ubiquitous - found in all basins pole-to-pole. The many permanent mesoscale features of the mean general circulation contrasts with that predicted by theories of the large-scale flow in simplified flat-bottomed domains. The new Atlas presents a new opportunity to benchmark modern high resolution ocean and climate models.

Evaluating westward and eastward propagating mesoscale eddies using a 1/10° global ocean simulation of CAS-LICOM3

This research evaluates the performance of CAS-LICOM3 (Chinese Academy of Science, State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics/Institute of Atmospheric Physics (LASG/IAP) Climate system Ocean Model, version 3) in simulating global coherent mesoscale eddies by comparison to satellite altimeter observations. The simulations of westward and eastward propagating eddies (WPEs and EPEs) and cyclonic and anticyclonic eddies (CEs and AEs) are separately analyzed. The results demonstrate that the simulated spatial-temporal variabilities in global mesoscale eddies agree roughly with the satellite observations. CAS-LICOM3 also reproduces the distinctive features between WPEs and EPEs or between CEs and AEs. However, some systematic biases are found. Globally, CAS-LICOM3 simulates a less frequent and weaker mesoscale eddy field than is observed. WPEs contribute more to these global biases than do EPEs. EPEs are relatively better reproduced than WPEs, exhibiting smaller underestimations and even overestimations in the energetic western boundary current and Antarctic circumpolar current regions. The simulation results for CEs resemble those of AEs, but AEs are comparatively less biased than CEs. These findings provide a basis for improving low-resolution and eddy-resolving ocean general circulation models (OGCMs) and developing submesoscale-resolving OGCMs.

Improved tidal estimates from short water level records via the modified harmonic analysis model

To fully resolve eight major tides from short-term records, classical harmonic analysis model usually infers unresolved constituents with the help of inference relationships from nearby long-term tide gauges. Our previous study developed a modified harmonic analysis model using the credo of smoothness (i.e., MHACS) which can achieve this without inference relationships. Via introducing the inherent natural links between major tides, MHACS breaks the restrictions of the Rayleigh criterion and requires only ∼9-day hourly records to resolve eight major tides. However, when data length is shorter than 9 days, the results of MHACS become problematic due to over-fitting. In this study, we introduce ridge regression to replace ordinary least squares (OLS) in the MHACS. Practical experiments on short-term hourly tide gauge records and satellite altimeter observations indicate that ridge regression can effectively eliminate meaningless mathematical artifacts obtained by OLS. The minimum length of records for MHACS to resolve eight major tides dramatically decreases from ∼210 h to ∼75 h as a result of using ridge regression. It is also found that ridge regression can notably reduce the uncertainties of tidal estimates from MHACS. Moreover, other modified harmonic analysis models such as NS_TIDE designed for river tides also suffer from over-fitting which can be solved by ridge regression in a similar way.

Large radiational S2 tides in the East China Sea

S2 ocean tide, as one of the most important semi-diurnal constituents, is composed of astronomical S2 tide originated from gravitational forcing and radiational S2 tide derived from solar radiation. Although there are lots of researches which have explored the features of radiational S2 tides in the globe, less known and less discussed, are the radiational S2 tides in the East China Sea due to lack of water level observations. In present study, radiational S2 tides in the East China Sea are estimated and discussed via the combination of 39 coastal tide gauges and EOT20 tidal model derived from multi-satellite altimeter records. We find that radiational S2 tides obtained from tide gauges and satellite-derived EOT20 model are roughly consistent in the East China Sea: (1) The largest radiational S2 amplitude can be as high as 14 cm. (2) Radiational and astronomical S2 tidal amplitudes are large in the coastal zones but small in the open sea. (3) Radiational S2 amplitudes are ∼12% of astronomical S2 amplitudes while phase differences between radiational and astronomical S2 tides are ∼114°. The inconsistency between coastal tide gauges and EOT20 model in coastal zones should be induced by the temporal evolution of tides and the inaccuracy of satellite altimeter observations in this region. Furthermore, the methods used in the East China Sea can be easily applied to other sea areas, except diurnal resonant sea areas where the smooth nature of semi-diurnal admittances may be corrupted by nonlinear interactions between anomalously large diurnal tides.

Performance of ERA5 wind speed and significant wave height within Extratropical cyclones using collocated satellite radar altimeter measurements

ABSTRACT Similar in strength to hurricanes, Extratropical Cyclones (ECs) are responsible for innavigable sea states, coastal inundation and erosion, and subsequent destruction to coastal infrastructure. Across modern operational wave models, there exists a known systematic underestimation of wave heights during these extreme events. Using a global database of EC storm tracks and 36 years of satellite altimeter data, we examine EC structure and assess model performance through storm centered composite analyses of significant wave height ( H s ) and U10 wind speed ( | U 10 | ). Through the collocation of satellite altimeter observations with a state-of-the-art reanalysis product (ERA5), we investigate model performance with respect to | U 10 | and H s within ECs of varying intensities. By rotating our data reference frames, we align all storm directions to account for asymmetry in EC wind fields and subsequent increased wave growth This rotation results in the organization of the strongest wind speed and H s , as well as trends in the ERA5 performance. A characteristic EC radii is calculated and used to normalize data coordinates in the storm centered reference frame, which highlights the organization of EC structures. Performance metrics are then compared within different EC quadrants to explore the relationship between wind forcing accuracy and underestimation of H s .

Sea Surface Height Response to Tropical Cyclone From Satellite Altimeter Observations and SAR Estimates

Sea Surface Height Response to Tropical Cyclone From Satellite Altimeter Observations and SAR Estimates

A modified tidal harmonic analysis model for short-term water level observations

Tidal constants of major semi-diurnal and diurnal constituents are essential for understanding tidal features and long-term tidal prediction. Widely-used classical harmonic analysis (CHA) model requires at least half-year hourly sea level records to resolve eight major semi-diurnal and diurnal constituents based on the Rayleigh criterion. When the analyzed records are short (typically shorter than one month), most previous studies adopt tidal inference method to infer more constituents in the CHA model. The performance of tidal inference method depends on the accuracy of given inference relationships. In this paper, we propose a modified harmonic analysis model on the basis of the credo of smoothness (i.e. MHACS) which indicates inherent natural links between major astronomical diurnal and semi-diurnal constituents. In the CHA, tidal constituents are independent of each other and estimated individually. However, in the MHACS, major constituents are connected via smooth functions of frequency (linear or quadratic functions) indicated by the credo of smoothness. MHACS aims to determine such smooth functions via least squares and then calculate all major tides simultaneously. A series of ideal and practical experiments are conducted to examine the effectiveness of MHACS. It is found that MHACS can derive relatively accurate tidal constants of eight major tides from short-term hourly tide gauge records and satellite altimeter observations. As a general and understandable method, MHACS is easy to realize and can be widely used to analyze diverse tidal signals in the global ocean except diurnal resonant regions where semi-diurnal tidal admittances may be distorted by nonlinear interactions. Furthermore, it should be noted that widely-used tidal inference method can be considered as a special case of MHACS when tidal admittances are assumed as constant functions of frequency.

Global extreme wave estimates and their sensitivity to the analysed data period and data sources

In the absence of wave measuring buoys operating over extended periods, wave hindcast data or satellite observations are indispensable for estimating global extreme wave heights. However, the results may depend on the analysed wind wave sources and the period's length. The sensitivity of the estimated extreme significant wave heights (SWH) to the analysed data sources and periods is investigated in this study. Global extreme wave heights are estimated using ECMWF Reanalysis v5 data (ERA5), global wave hindcast developed based on Simulating WAves Nearshore forced by the Japanese 55-year Reanalysis (SWAN-JRA55), satellite altimeter observations, and long-term wave buoy measurements. Both Annual Maximum fitting to the Generalized Extreme Value Distribution (AM-GEV) and Peaks Over Threshold fitted to the Generalized Pareto Distribution (POT-GPD) models are used. The results show that the global extreme SWH estimates considerably depend on the analysed data sources. The relative differences observed between the analysed data sources are >20% in large parts of the world. Thus, the relative differences in extreme SWH are mainly lower by increasing the analysed data periods. However, they can reach 30% and are more critical using AM-GEV. Besides, by comparing the extreme values from reanalysis and hindcast wave data to those from long-term wave measurements, underestimations of up to 2 m are observed for a return period of 100 years in the North-West Atlantic and North-East Pacific.

Projected 21st Century Wind‐Wave Climate of Bass Strait and South‐East Australia: Comparison of EC‐Earth3 and ACCESS‐CM2 Climate Model Forcing

AbstractThis study analyses the past and future wave climate of Bass Strait and south‐east Australia by comparing two high‐resolution regional wave climate models (RWCMs), respectively forced by the surface wind speed of EC‐Earth3 and ACCESS‐CM2 global circulation models (GCMs) under two emission scenarios (SSP1‐2.6 and SSP5‐8.5). The two RWCMs are verified against long‐term buoy and satellite observations as well as hindcast model data. The comparison with long‐term satellite altimeter observations shows good agreement between the long‐term model climate and the observations. Both RWCMs project a future increase in swell, approaching the coast of Tasmania and mainland Australia, generated from distant storms in the Southern Ocean. However, different projected wave height spatial distributions are found in the coastal regions, affected by local synoptic weather events and by the coarse resolution of the GCMs winds. We also show that the long‐term wave climate in the domain is impacted by the Southern Annular Mode. As low‐pressure systems are projected to move southward, especially in the SSP5‐8.5 scenario, the swells approaching south‐east Australia are projected to be longer and more energetic in the future. The combination of more energetic swell systems and a counterclockwise rotation in peak wave direction, projected by both RWCMs, may influence the coastal equilibrium, impacting critical coastal locations.

Contrasting trends in short-lived and long-lived mesoscale eddies in the Southern Ocean since the 1990s

Mesoscale eddies play an important role in the transport of heat, carbon, and nutrients in the Southern Ocean. Previous studies have documented an increasing intensity of the Southern Ocean eddy field during recent decades; however, it remains unclear whether the mesoscale eddies with different lifetimes have different temporal variations. Using satellite altimeter observations from 1993 to 2020, we found that the increasing trend in the intensity of eddies is dominated by long-lived eddies (with lifetimes ⩾ 90 d), whose amplitude has increased at a rate of ∼2.8% per decade; the increase is concentrated downstream of topography. In contrast, short-lived eddies (with lifetimes < 90 d) do not appear to have a significant trend in their amplitudes since the early 1990s. An energy conversion analysis indicates that the increased baroclinic instabilities of the mean flows associated with topography are responsible for the amplitude increase of the long-lived eddies. This study highlights the need for a better understanding of the changes in mesoscale eddies owing to their importance in the transport of heat, carbon, and nutrients.

A satellite altimetry data assimilation approach to optimise sea state estimates from vessel motion

Estimates of directional wave spectra and related parameters can be obtained from ship motion data through the wave-buoy analogy approach. The fundamental input is the response amplitude operator (RAO), which translates ship response into a wave energy spectrum. While ship motion is routinely measured on ocean going vessels, the RAO is not directly available and it is approximated using ship hydrodynamic models. The lack of publicly available details of hull geometry and loading conditions can results in significant inaccuracy of this operator. Considering the reliability of remotely sensed wave height, here we propose an assimilation technique that uses satellite altimeter observations to calibrate the RAO and minimise its uncertainties. The method is applied to estimate sea state conditions during the Antarctic Circumnavigation Expedition by converting motion response of the icebreaker Akademik Tryoshnikov as recorded by the on-board inertial measurement unit. Comparison against concurrent sea state observations obtained from a marine radar device shows a good agreement for a variety of parameters including significant wave height, wave periods and mean wave direction.

Characteristics of Mesoscale Eddies in the Vicinity of the Kuroshio: Statistics from Satellite Altimeter Observations and OFES Model Data

Mesoscale eddies propagate westward in the northwestern Pacific Ocean and interact with the Kuroshio in the vicinity of the western boundary of the ocean. However, the processes affecting the eddy properties and the detailed structure of the eddies when they encounter the Kuroshio remain unclear. In this study, we analyze the statistics of the eddy properties around the Kuroshio using 25 years of satellite altimeter data and the eddy-resolving OFES model product. The spatial compositions of the eddies in the northwestern Pacific show that, as the eddies propagate westward, their radius and amplitude decrease sharply when they approach the Kuroshio region. The radius, amplitude, and kinetic energy of the eddies reaching the Kuroshio region decay much faster during their lifespan compared with the eddies in the interior of the Pacific Ocean. Furthermore, the three-dimensional structure of the eddies obtained from the OFES model data shows that the maximum temperature anomalies in the cyclonic and anticyclonic eddies occur at ~300 m, and the maximum depth reduces as a result of the interaction between the eddies and the main Kuroshio current.

Seasonal variability of eddy kinetic energy in the East Australian current region

The East Australian Current (EAC) is an important western boundary current of the South Pacific subtropical Circulation with high mesoscale eddy kinetic energy (EKE). Based on satellite altimeter observations and outputs from the eddy-resolving ocean general circulation model (OGCM) for the Earth Simulator (OFES), the seasonal variability of EKE and its associated dynamic mechanism in the EAC region are studied. High EKE is mainly concentrated in the shear-region between the poleward EAC southern extension and the equatorward EAC recirculation along Australia's east coast, which is confined within the upper ocean (0-300 m). EKE in this area exhibits obvious seasonal variation, strong in austral summer with maximum (465±89 cm² s-²) in February and weak in winter with minimum (334±48 cm² s-²) in August. Energetics analysis from OFES suggests that the seasonal variability of EKE is modulated by the mixed instabilities composed of barotropic and baroclinic instabilities confined within the upper ocean, and barotropic instability (baroclinic instability) is the main energy source of EKE in austral summer (winter). The barotropic process is mainly controlled by the zonal shear of meridional velocities of the EAC southern extension and the EAC recirculation. The poleward EAC southern extension and the equatorward EAC recirculation are synchronously strengthened (weakened) due to the local positive (negative) sea level anomalies (SLA) under geostrophic equilibrium, and the barotropic instability dominated by zonal shear is enhanced (slackened), which results in a high (low) level of EKE in the EAC region.

Seasonal Variation of Intra-Seasonal Eddy Kinetic Energy along the East Australian Current

By using satellite altimeter observations and the eddy-permitting Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2), the seasonal variation of eddy kinetic energy (EKE) along the East Australian Current (EAC) is investigated. Both observations and ECCO2 outputs indicate active intra-seasonal EKE along the EAC path. The ECCO2 result reveals that the intra-seasonal EKE is mainly concentrated in the upper 500 m layer, and shows a prominent seasonal cycle, strong in austral summer and weak in austral winter. Eddy energy budget diagnosis reveals that the evolution of EKE is controlled by barotropic instability of the mean EAC. The seasonal variation of baroclinic instability is opposite to the barotropic instability variation, but of a much smaller magnitude. Further analysis indicates that the seasonal cycle of mesoscale signals in this region is related to the transport variability of the EAC.

Meddies and Their Sea Surface Expressions: Observations and Theory

Abstract The sea surface expressions of Mediterranean Water eddies, known as “meddies,” are observed in satellite data, and their main characteristics are measured. Satellite altimeter observations of surface expressions are detected over the meddies observed in situ using the MEDTRANS meddy dataset (1950–2013). In this study 209 observed meddy cores in the North Atlantic Ocean, selected over the period of the 22 years of sea surface height measurements with satellite altimetry (1993–2013), were analyzed. Results show relatively good agreement between the theoretical estimates of the meddy surface signals as reported by Bashmachnikov and Carton and the measured surface expressions. It was found that, on average, the theoretical results underestimate the measured sea surface elevations of the meddy surface expressions by a factor of 2. Although the variability of the measured expressions is reasonably well described by the combination of meddy core and the ocean background parameters of the theoretical expression, we cannot define a single individual parameter of the meddy core, which chiefly shapes the magnitude of the meddy surface signal. Interestingly, the overall distribution of characteristics of meddy surface expressions in the Atlantic shows that the sea level anomalies formed by meddies intensify westward, growing both in magnitude and radius. This opposes the expected theoretical decrease of meddy surface signals due to a known progressive decay of the meddy cores with distance from their generation region at the Iberian continental slope. This observed tendency is attributed to meddy interaction with the upper-ocean currents and other eddies (in particular in the region of the North Atlantic Current and Azores Current) that are not considered by the theory.

Anomalous 18.61-Year Nodal Cycles in the Gulf of Tonkin Revealed by Tide Gauges and Satellite Altimeter Records

Understanding nodal tidal characteristics is essential for accurate long-term tidal prediction. Observational nodal evolution of tides is mainly based on tide gauge records in coastal areas which are limited in time and space, thus impeding coherent determinations of basin-wide patterns of tidal variability. In this paper, we indicate the potential of satellite altimeter data to investigate 18.61-year nodal modulations of main constituents in the Gulf of Tonkin. Three tide gauges and multi-source satellite altimeter observations (TOPEX/Poseidon, Jason1, Jason2, and Jason3) revealed that 18.61-year nodal cycles in tidal amplitudes have noticeable deviations from the equilibrium tidal theory in the Gulf of Tonkin. In general, M2 and N2 nodal modulations are anomalously larger than theoretical values while K2, K1, and O1 nodal modulations are noticeably smaller than theoretical values. Compared to point-based tide gauges, satellite altimeter records can provide basin-wide features of nodal modulations of main constituents. Although overlapping geographical blocks are applied to eliminate the effect of tidal alias originated from long-period sampling intervals, the estimation of nodal cycles of minor constituents are still questionable. Nevertheless, the methods described here provide a strong foundation for future research on time-varying tidal dynamics using the combination of tide gauges and satellite altimeter data.

Global Mesoscale Ocean Variability from Multiyear Altimetry: An Analysis of the Influencing Factors

Abstract Sea surface slope (SSS) responds to oceanic processes and other environmental parameters. This study aims to identify the parameters that influence SSS variability. We use SSS calculated from multiyear satellite altimeter observations and focus on small resolvable scales in the 30–100-km wavelength band. First, we revisit the correlation of mesoscale ocean variability with seafloor roughness as a function of depth, as proposed by Gille et al. Our results confirm that in shallow water there is statistically significant positive correlation between rough bathymetry and surface variability, whereas the opposite is true in the deep ocean. In the next step, we assemble 27 features as input variables to fit the SSS with a linear regression model and a boosted trees regression model, and then we make predictions. Model performance metrics for the linear regression model are R2 = 0.381 and mean square error = 0.010 μrad2. For the boosted trees model, R2 = 0.563 and mean square error = 0.007 μrad2. Using the hold-out data, we identify the most important influencing factors to be the distance to the nearest thermocline boundary, significant wave height, mean dynamic topography gradient, and M2 tidal speed. However, there are individual regions, that is, the Amazon outflow, that cannot be predicted by our model, suggesting that these regions are governed by processes that are not represented in our input features. The results highlight both the value of machine learning and its shortcomings in identifying mechanisms governing oceanic phenomena.

Modeling waves over the Changjiang River Estuary using a high-resolution unstructured SWAN model

A high resolution unstructured SWAN model has been implemented for the Changjiang River Estuary (CRE). Five different winds are adopted to assess their quality over the CRE, including ERA5, CFSv2, GFS, CCMP-NRT and a newly available high resolution (9 km) wind product from APRCP (Asia-Pacific Regional Coupled Prediction System). The performance of the five winds, together with four wind input source functions is evaluated by comparing with satellite altimeter observations and multiple in-situ observations. Systemic difference is presented in the four input source functions, with the JANSSEN and KOMEN packages outperformingWST in terms of significant wave height (Hs) over the CRE. The default ST6 has comparative skill as JANSSEN during fair weather conditions but tends to predict higher Hs during extreme weather events. All winds have the tendency to underestimate the altimeter observed Hs during fair weather conditions. Part of the negative bias attributes to the overestimation of altimeter observations, while the boundary conditions only play a minor role, given a relatively large model domain extending to about 135° E. The CFSv2/GFS wind has the best performance during fair weather conditions, but overestimates the extreme wave height during typhoon events. The ERA5, on the other hand, underestimates Hs during both fair and extreme weather conditions. In terms of extreme waves, CCMP-NRT generally agrees best with observations. The high resolution APRCP behaves significantly different from other wind forcings and shows poorer performance in wave simulation. But high resolution winds have the advantage in resolving the detail wind structure and show better performance in reproducing the high frequency wave fluctuations adjacent to the CRE. In addition, the deviation due to different wind forcings is more distinct than the different input source functions. Nevertheless, given a certain wind forcing, calibrations would further improve the model performance. This study provides a good start to build a hindcast and forecast system for the CRE region.

Arctic sea level variability from high-resolution model simulations and implications for the Arctic observing system

Abstract. Two high-resolution model simulations are used to investigate the spatiotemporal variability of the Arctic Ocean sea level. The model simulations reveal barotropic sea level variability at periods of &lt; 30 d, which is strongly captured by bottom pressure observations. The seasonal sea level variability is driven by volume exchanges with the Pacific and Atlantic oceans and the redistribution of the water by the wind. Halosteric effects due to river runoff and evaporation minus precipitation ice melting/formation also contribute in the marginal seas and seasonal sea ice extent regions. In the central Arctic Ocean, especially the Canadian Basin, the decadal halosteric effect dominates sea level variability. The study confirms that satellite altimetric observations and Gravity Recovery and Climate Experiment (GRACE) could infer the total freshwater content changes in the Canadian Basin at periods longer than 1 year, but they are unable to depict the seasonal and subseasonal freshwater content changes. The increasing number of profiles seems to capture freshwater content changes since 2007, encouraging further data synthesis work with a more complicated interpolation method. Further, in situ hydrographic observations should be enhanced to reveal the freshwater budget and close the gaps between satellite altimetry and GRACE, especially in the marginal seas.

Intrinsic and Wind-Driven Decadal Variability of the Kuroshio Extension in Altimeter Observations

In this work we use satellite altimeter observations to study the mechanism of decadal variability of the Kuroshio Extension (KE), with special attention on jet-eddy energy transfer, and on the relationship between the wind-driven sea surface height anomalies (SSHAs) and those directly driven by intrinsic oceanic processes including jet and eddies. It is shown that energy feedback between the jet and mesoscale eddies can maintain the decadal oscillation of the KE. The wind-driven SSHAs are broad-scale and very weak compared to the intrinsic variability. Physically they can potentially trigger delayed responses of the latter by modulating vorticity advection from upstream but the statistical significance is low. KE perturbations resulting from the intrinsic variability, on the other hand, could feedback onto the wind-driven SSHAs by inducing anomalous basin-scale wind stress. The KE jet is thus an integrated system involving the jet and the eddies, possibly feeding back to, and paced by, wind stress anomalies.