Satellite Altimeter Sea Surface Height Research Articles

Progress in research of submesoscale processes in the South China Sea

This paper overviews research progress in observation, theoretical analysis and numerical modeling of submesoscale dynamic processes in the South China Sea (SCS) particularly during recent five years. The submesoscale processes are defined according to both spatial and dynamic scales, and divided into four subcategories as submesoscale waves, submesoscale vortexes, submesoscale shelf processes, and submesoscale turbulence. The major new findings are as follows. (1) Systematic mooring observations provide new insights into the solitary waves (ISWs) and the typhoon-forced near-inertial waves (NIWs), of which a new type of ISWs with period of 23 h was observed in the northern SCS (NSCS), and the influences of background vorticity, summer monsoon onset, and deep meridional overturning circulation on the NIWs, as well as nonlinear wave-wave interaction between the NIWs and internal tides, are better understood. On the other hand, satellite altimeter sea surface height data are used to reveal the internal tide radiation patterns and provide solid evidence for that the ISWs in the northeastern SCS originate from the Luzon Strait. (2) Submesoscale offshore jets and associated vortex trains off the Vietnam coast in the western boundary of the SCS were observed from satellite chlorophyll concentration images. Spiral trains with the horizontal scale of 15–30 km and the spacing of 50–80 km were identified.(3) 3-D vertical circulation in the upwelling region east of Hainan Island was theoretically analyzed. The results show that distribution patterns of all the dynamic terms are featured by wave-like structures with horizontal wavelength scale of 20–40 km. (4) Numerical models have been used for the research of submesoscale turbulence. Submesoscale vertical pump of an anticyclonic eddy and the spatiotemporal features of submesoscale processes in the northeastern SCS are well modeled.

Improved Quantification of Global Mean Ocean Mass Change Using GRACE Satellite Gravimetry Measurements

AbstractGlobal mean ocean mass change derived from the Gravity Recovery and Climate Experiment (GRACE) gravity solutions generally agrees well with ocean mass change inferred from satellite altimeter sea surface height and Argo floats observations during the period January 2005 to December 2015. However, there is a systematic annual phase lag (~10°) between GRACE and Altimeter‐Argo estimates. This phase lag is attributed to the enforced mass conservation in GRACE gravity solutions, in which the ΔC00 coefficients (representing changes in total Earth mass) are set to zero. After a correct implementation of global mass conservation by removing global mean atmospheric mass from the GRACE solutions using atmospheric model predictions, the annual phase lag is nearly completely gone, yielding significantly improved agreement between GRACE and Altimeter‐Argo estimates. In addition, retaining GRACE ΔJ2 coefficients provides better GRACE ocean mass estimates at both seasonal and long‐term time scales.

The Ocean Mesoscale Regime of the Reduced-Gravity Quasigeostrophic Model

AbstractA statistical-equilibrium, geostrophic-turbulence regime of the stochastically forced, one-layer, reduced-gravity, quasigeostrophic model is identified in which the numerical solutions are representative of global mean, midlatitude, open-ocean mesoscale variability. Solutions are forced near the internal deformation wavenumber and damped linearly and by high-wavenumber enstrophy dissipation. The results partially rationalize a recent semiempirical stochastic field model of mesoscale variability motivated by a global eddy identification and tracking analysis of two decades of satellite altimeter sea surface height (SSH) observations. Comparisons of model results with observed SSH variance, autocorrelation, eddy, and spectral statistics place constraints on the model parameters. A nominal best fit is obtained for a dimensional SSH stochastic-forcing variance production rate of 1/4 cm2 day−1, an SSH damping rate of 1/62 week−1, and a stochastic forcing autocorrelation time scale near or greater than 1 week. This ocean mesoscale regime is nonlinear and appears to fall near the stochastic limit, at which wave-mean interaction is just strong enough to begin to reduce the local mesoscale variance production, but is still weak relative to the overall nonlinearity. Comparison of linearly inverted wavenumber–frequency spectra shows that a strong effect of nonlinearity, the removal of energy from the resonant linear wave field, is resolved by the gridded altimeter SSH data. These inversions further suggest a possible signature in the merged altimeter SSH dataset of signal propagation characteristics from the objective analysis procedure.

Slow Down of the Gulf Stream during 1993\u20132016

The Gulf Stream, the main heat-carrier from low to high latitudes in the North Atlantic Ocean, influences the climate and weather in the northern hemisphere. In this study we determine and analyze the position, speed, and width of the Gulf Stream (GS) from 80°W–50°W using satellite altimeter sea surface height (SSH) measurements to examine the possible link between changes in the strength of the GS and coastal sea levels along the U.S. East Coast. During our 24-year study period (1993–2016), the GS experienced a southward shift east of 65°W after passing the New England Seamount chain. This southward shift was accompanied by a weakening of the GS, associated with an increase in SSH to the north of the GS. West of 70°W, however, we found no statistically significant trends in the GS properties, consistent with results based on in situ measurements. This lack of a trend to the west fails to support a direct link between a long-term slowdown of the GS west of 70°W and sea level rise acceleration along the U.S. East Coast, though a slowdown of the GS east of 65°W may contribute to sea level rise. It is also possible that heat carried to the region by the GS may be responsible for these observed sea level changes.

Shear dispersion and delayed propagation of temperature anomalies along the Norwegian Atlantic Slope Current

Using satellite altimetric sea surface height (ADT) data, we search for propagation of hydrographic anomalies along the Norwegian Atlantic Slope Current (NwASC) from the Svinøy section in the south to the Fram Strait in the north. Our analyses indicate that ADT anomalies, related to low-frequency temperature variations, propagate downstream with speeds of about 2 cm s. Notably, this speed is nearly an order of magnitude slower than the speed of the NwASC, which in agreement with previously estimated propagation speeds of hydrographic anomalies along the flow. A conceptual tracer advection model, consisting of a thin current core interacting with an adjacent slow moving reservoir, is introduced to examine temperature anomaly propagation along the NwASC. It is shown that shear dispersion effects, resulting from cross-stream eddy mixing and velocity shear, can qualitatively explain the observed delayed propagation of hydrographic anomalies: low-frequency temperature anomalies move downstream with an effective velocity that corresponds to a mean velocity across the entire Atlantic Water layer, rather than the speed of Norwegian Atlantic Slope Current.

Detecting small seamounts in AltiKa repeat cycle data

We present a technique of stacking repeat cycles of satellite altimeter sea surface height profiles that lowers the noise and improves the resolution of small seamounts. Our approach differs from other studies because it uses the median (not the mean) of the stacks, which suppresses outliers. Seamounts as small as 720 m tall are easily detected in stacked 40 Hz AltiKa data profiles, and a 500 m tall seamount is perceptible. Noise variance decreases with an increase in the number of cycles stacked, and RMS noise dips below 2 cm when 11 or more cycles are stacked. Coherence analyses between geoid height and bathymetry show that full wavelengths down to about 10 km can be resolved. Comparisons of study areas with and without seamounts find that signal from small seamounts lies in the ~10–28 km waveband. A simple Gaussian band-pass filter based on the seamount waveband passes signals that can be used in seamount detection studies. Such studies may find seamounts <2 km tall that are predicted to be abundant on the ocean floor.

Suitability of satellite sea surface salinity data for use in assessing and correcting ocean forecasts

Abstract Near-surface salinity data from the Forecasting Ocean Assimilation Model (FOAM) system are used to understand various characteristics of satellite sea surface salinity (SSS) data from SMOS. The errors in the model fields are first estimated by comparing them to near-surface Argo salinity measurements, with RMS errors of less than 0.2 pss over most of the global oceans, except for regions of high variability in SSS such as boundary current regions and areas of large precipitation or river run-off. Regional biases are generally less than 0.05 pss but some regions such as the Antarctic Circumpolar Current and the region to the north of the Gulf Stream extension have larger biases. Various different processing versions of the SMOS data are assessed, including different temporal and spatial averaging, and the daily 1° resolution SMOS differences to FOAM are approximately 3–10 times larger than Argo-FOAM differences. The spatial information in the SMOS data is also assessed by comparing spatial gradients in the satellite SSS data with those calculated from other datasets including satellite sea surface temperature (SST) and satellite altimeter sea surface height (SSH) data, as well as with the model's gradients. The SMOS data contain information about the underlying ocean dynamics in the summer months, in agreement with the SSH data, which are not present in the satellite SST data or in the model's SSS fields. This demonstrates that the data contains useful information which could be used to correct the model through data assimilation.

Revisiting the pole tide for and from satellite altimetry

Satellite altimeter sea surface height observations include the geocentric displacements caused by the pole tide, namely the response of the solid Earth and oceans to polar motion. Most users of these data remove these effects using a model that was developed more than 20 years ago. We describe two improvements to the pole tide model for satellite altimeter measurements. Firstly, we recommend an approach that improves the model for the response of the oceans by including the effects of self-gravitation, loading, and mass conservation. Our recommended approach also specifically includes the previously ignored displacement of the solid Earth due to the load of the ocean response, and includes the effects of geocenter motion. Altogether, this improvement amplifies the modeled geocentric pole tide by 15 %, or up to 2 mm of sea surface height displacement. We validate this improvement using two decades of satellite altimeter measurements. Secondly, we recommend that the altimetry pole tide model exclude geocentric sea surface displacements resulting from the long-term drift in polar motion. The response to this particular component of polar motion requires a more rigorous approach than is used by conventional models. We show that erroneously including the response to this component of polar motion in the pole tide model impacts interpretation of regional sea level rise by $$\pm $$ 0.25 mm/year.

Comparing the steric height in the Nordic Seas with satellite altimeter sea surface height

In this study the steric height anomaly which is calculated from the hydrological data (EN3) is compared with the sea level anomaly derived from satellite altimetry in the Nordic Seas. The overall pattern of steric height is that it is higher in the margin area and lower in the middle area. The extreme values of steric height linear change from 1993 to 2010 occur in the Lofoten Basin and off the Norwegian coast, respectively. Such a distribution may be partly attributed to the freshening trend of the Nordic Seas. The correlation between SLA (sea level anomaly) and SHA (steric height anomaly) is not uniform over the Nordic Seas. The time series of SLA and SHA agree well in the Lofoten Basin and northern Norwegian Basin, and worse in the northern Norwegian Sea, implying that the baroclinic effect plays a dominant role in most areas in the Norwegian Sea and the barotropic effect plays a dominant role in the northern Norwegian Sea. The weaker correlations between SLA and SHA in the Greenland and Iceland Seas lead a conclusion that the barotropic contribution is significant in these areas. The area-mean SHA over the entire Nordic Seas has similar amplitudes compared with the SLA during 1996–2002, but SHA has become lower than SLA, being less than half of SLA since 2006.

A long-term volume transport time series estimated by combining in situ observation and satellite altimeter data in the northern South China Sea

We deployed five pressure-recording inverted echo sounders (PIES) along a satellite altimeter track across the continental slope of the northern South China Sea (NSCS) from October 2012 to July 2014, and obtained a time series of volume transport (VT PIES ) across the sec- tion from their records. Applying the empirical relation- ship between VT PIES and the satellite altimeter sea surface height anomaly difference across the section, we obtained a time series of volume transport (VT NSCS ) over 22 years from 1992 to 2014. The VTNSCS shows a small mean value of −1.6 Sv (1 Sv = 10 6 m 3 s −1 ) (i.e., toward the south- west), but a significant seasonal reversal and mesoscale eddy induced fluctuations ( −11.8 to 19.7 Sv). The monthly mean of VTNSCS over 22 years shows a maximum (3.6 Sv) in July and a minimum (−7.3 Sv) in December. This is the first long time series of volume transport for the NSCS based on in situ data.

Global heat and salt transports by eddy movement

Oceanic mesoscale eddies contribute important horizontal heat and salt transports on a global scale. Here we show that eddy transports are mainly due to individual eddy movements. Theoretical and observational analyses indicate that cyclonic and anticyclonic eddies move westwards, and they also move polewards and equatorwards, respectively, owing to the β of Earth's rotation. Temperature and salinity (T/S) anomalies inside individual eddies tend to move with eddies because of advective trapping of interior water parcels, so eddy movement causes heat and salt transports. Satellite altimeter sea surface height anomaly data are used to track individual eddies, and vertical profiles from co-located Argo floats are used to calculate T/S anomalies. The estimated meridional heat transport by eddy movement is similar in magnitude and spatial structure to previously published eddy covariance estimates from models, and the eddy heat and salt transports both are a sizeable fraction of their respective total transports.

On the spatial structure and temporal variability of poleward transport between Scotland and Greenland

AbstractThe flow north of warm subtropical water though the northeastern Atlantic is known to have many pathways that vary over time. Here we use a combination of upper ocean current measurements between Greenland and Scotland near 60°N and satellite altimetry to examine the space‐time variability of poleward transport. The high‐resolution scans of currents in the top 400 m show that the Reykjanes Ridge serves as a very effective separator of flow toward the Nordic and Labrador Seas, respectively. Whereas the Labrador Sea branch exhibits two mean flows to the north on the western slope of the Reykjanes Ridge, the eastern branch flows north in roughly equal amounts over the deep Maury channel and east of Hatton Bank including the Slope Current. There is also a well‐defined southward flow along the eastern slope of the Reykjanes Ridge. The satellite altimetric sea surface height (SSH) data show good overall agreement with geostrophically determined ‐level difference from the repeat ADCP sections (1999–2002), but are unable to resolve the fine structure of the topographically defined mean circulation. The altimetric data show that variations in poleward flow west and east of the Reykjanes Ridge are strongly anticorrelated. They further reveal that the two eastern subbranches also exhibit anticorrelated variability, but offset in time with respect to the Labrador Sea branch. Remarkably, all these variations cancel out for the entire Greenland‐Scotland section leaving a gradual decrease in sea‐level difference of about 0.06 m over the 1993 to the end of 2010 observation period.

Low‐frequency western Pacific Ocean sea level and circulation changes due to the connectivity of the Philippine Archipelago

Interannual‐to‐decadal sea level and circulation changes associated with the oceanic connectivity around the Philippine Archipelago are studied using satellite altimeter sea surface height (SSH) data and a reduced gravity ocean model. SSHs in the tropical North Pacific, the Sulu Sea and the eastern South China Sea (ESCS) display very similar low‐frequency oscillations that are highly correlated with El Niño and Southern Oscillation. Model experiments reveal that these variations are mainly forced by the low‐frequency winds over the North Pacific tropical gyre and affected little by the winds over the marginal seas and the North Pacific subtropical gyre. The wind‐driven baroclinic Rossby waves impinge on the eastern Philippine coast and excite coastal Kelvin waves, conveying the SSH signals through the Sibutu Passage‐Mindoro Strait pathway into the Sulu Sea and the ESCS. Closures of the Luzon Strait, Karimata Strait, and ITF passages have little impacts on the low‐frequency sea level changes in the Sulu Sea and the ESCS. The oceanic pathway west of the Philippine Archipelago modulates the western boundary current system in the tropical North Pacific. Opening of this pathway weakens the time‐varying amplitudes of the North Equatorial Current bifurcation latitude and Kuroshio transport. Changes of the amplitudes can be explained by the conceptual framework of island rule that allows for baroclinic adjustment. Although it fails to capture the interannual changes in the strongly nonlinear Mindanao Current, the time‐dependent island rule is nevertheless helpful in clarifying the role of the archipelago in regulating its multidecadal variations.

The role of mindanao dome in the variability of the Pacific North equatorial current bifurcation

The Mindanao Dome (MD) features prominent oceanic variability and is located geographically close to the bifurcation latitude Y (b) of the Pacific North Equatorial Current. In this study, the role of the MD in the variability of Y (b) is examined with 20 years of satellite altimetric sea surface height (SSH) data and a 1.5-layer linear Rossby wave model. It is shown that the seasonal variations of surface Y (b) are related to not only the SSH fluctuations near the bifurcation point (bifurcation box; 125A degrees-130A degrees E, 12A degrees-15A degrees N) but also those outstanding in the MD region (MD box; 127A degrees-132A degrees E, 6A degrees-9A degrees N). The impact of the MD SSH changes is significant when the bifurcation point stays at southerly latitudes during February-September, which hinders (delays) the southward leap (northward retreat) of Y (b) in April-May (July-August) and thus leads to the asymmetry of the mean Y (b) seasonal cycle (with a positive skewness of gamma = +0.64). Such asymmetry also shows year-to-year variations depending on yearly mean Y (b) value. A southerly yearly mean Y (b) involves larger contribution of the MD and thus causes larger asymmetry of Y (b) seasonal cycle. At interannual and longer timescales, the MD acts to amplify the fluctuations of the bifurcation. It is responsible for about 20 % of the total low-frequency Y (b) variances and plays an important role in the 0.12A degrees A year(-1) southward trend of Y (b) in the past two decades. The impact of the MD on Y (b) changes is becoming increasingly significant at various timescales such as the bifurcation point migrating southward in recent years.

Spatial–temporal patterns and driving mechanisms of semiannual variations in the Philippine Sea

Satellite altimetric sea surface height (SSH) measurements from 1992 through 2010 are used to explore the oceanic semiannual variations in the Philippine Sea (PS). Pronounced semiannual SSH variations are detected within two zonal bands. One lies east of Luzon Strait (19°–22°N) in the northern PS, while the other is southeast of Mindanao coast (4°–7°N) in the southern PS. In the two near-coast boxes where semiannual harmonic amplitude exceeds 4cm, the northern box (127°–133°E, 19°–22°N) and the southern box (127°–133°E, 4°–7°N), semiannual changes contribute considerably to the total annual SSH variance by 12% and 17%, respectively. Despite prominent SSH variability, the bifurcation latitude of the North Equatorial Current (NBL) exhibits weak fluctuations with a peak-to-peak difference of only 0.3° on semiannual time scale. While the in-phase annual SSH variations between the two boxes work together to enhance annual NBL changes, their out-of-phase semiannual SSH variations offset each other in driving the NBL displacements. Further analysis with a 11/2-layer reduced-gravity model forced by ECMWF wind stress reveals that the observed semiannual SSH variations are primarily driven by local wind forcing in the far western Pacific. Rossby wave signals propagating from the eastern/central Pacific contribute much less due to along-path dispersion and cancellation. Semiannual signals of wind field in the northern PS reflect mainly the semiannual changes of the Asian Monsoon system, while those in the southern PS arise from the combined effects of Monsoon transition and the annual migration of the intertropical convergence zone (ITCZ).

Effects of stencil width on surface ocean geostrophic velocity and vorticity estimation from gridded satellite altimeter data

This paper examines the effect of “stencil width” on surface ocean geostrophic velocity and vorticity estimated from differentiating gridded satellite altimeter sea surface height products. In oceanographic applications, the value of the first derivative at a central grid point is generally obtained by differencing the sea surface heights at adjacent grid points. This is called a “three‐point stencil centered difference”. Here the stencil width is increased from three to five, seven, and nine points, using well‐known formulae from the numerical analysis literature. The discrepancies between velocities computed with successive stencils decreases with increasing stencil width, suggesting that wide stencil results are more reliable. Significant speed‐dependent biases (up to 10–20%) are found between results computed from three‐point stencils versus those computed from wider stencils. The geostrophic velocity, and the variance of geostrophic velocity, are underestimated with thin stencils. Similar results are seen in geostrophic velocities computed from high‐resolution model output. In contrast to the case when three‐point stencils are used, wider stencils yield estimates of the anisotropy of velocity variance that are insensitive to the differences in grid spacing between two widely used altimeter products. Three‐point stencils yield incorrect anisotropies on the 1/4° anisotropic AVISO grid; we recommend the use of 7‐point stencils. Despite the demonstrated inadequacies of the three‐point stencils, the conclusions of earlier studies based on them, that the zonally averaged midlatitude eddy kinetic energy field is nearly isotropic, are found to pertain also with wider stencils. Finally, the paper also examines the strengths and limitations of applying noise‐suppressing differentiators, versus classic centered differences, to altimeter data.

Internal tides around the Hawaiian Ridge estimated from multisatellite altimetry

[1] Satellite altimetric sea surface height anomaly (SSHA) data from Geosat Follow-on (GFO) and European Remote Sensing (ERS), as well as TOPEX/Poseidon (T/P), are merged to estimate M2 internal tides around the Hawaiian Ridge, with higher spatial resolution than possible with single-satellite altimetry. The new estimates are compared with numerical model runs. Along-track analyses show that M2 internal tides can be resolved from both 8 years of GFO and 15.5 years of ERS SSHA data. Comparisons at crossover points reveal that the M2 estimates from T/P, GFO, and ERS agree well. Multisatellite altimetry improves spatial resolution due to its denser ground tracks. Thus M2 internal tides can be plane wave fitted in 120 km × 120 km regions, compared to previous single-satellite estimates in 4° lon × 3° lat or 250 km × 250 km regions. In such small fitting regions the weaker and smaller-scale mode 2 M2 internal tides can also be estimated. The higher spatial resolution leads to a clearer view of the M2 internal tide field around the Hawaiian Ridge. Discrete generation sites and internal tidal beams are clearly distinguishable, and consistent with the numerical model runs. More importantly, multisatellite altimetry produces larger M2 internal tidal energy fluxes, which agree better with model results, than previous single-satellite estimates. This study confirms that previous altimetric underestimates are partly due to the more widely spaced ground tracks and consequently larger fitting region. Multisatellite altimetry largely overcomes this limitation.

Deep Signatures of Southern Tropical Indian Ocean Annual Rossby Waves*

Abstract The southern tropical Indian Ocean contains a striking forced annual Rossby wave studied previously using satellite altimeter sea surface height data, surface wind fields, expendable bathythermograph ocean temperature data, and models. Here, the deep reach of this wave and its velocity are analyzed using density–depth profiles and 1000-dbar horizontal drift data from Argo. Significant annual cycles in isopycnal vertical displacements and zonal velocity persist to the deepest pressures to which Argo data can be mapped reliably in the region, 1600–1900 dbar. Phase propagation of the annual cycle of the directly measured zonal velocities at 1000 dbar suggests a zonal wavelength of about 6000 km—about the length of the deep basin in which the wave is found—and a westward phase speed of ~0.2 m s−1. Apparent upward phase propagation in isopycnal vertical displacements suggests energy propagation downward into the abyss. This pattern is clearer when accounting for both the potential and kinetic energy of the wave. The largest zonal current associated with this wave has a middepth maximum that decays rapidly up through the pycnocline and less rapidly with increasing depth, suggesting a first-vertical-mode structure. The anomalous zonal volume transport of this annually reversing current is ~27 × 106 m3 s−1 across 80°E in mid-November. The peak zonal velocity of 0.06 m s−1 implies a maximum zonal excursion of about 600 km associated with the wave over an annual cycle.

Altimeter Data Evaluation in the Coastal Gulf of Maine and Mid-Atlantic Bight Regions

Satellite altimeter sea surface height anomaly (SSHA) data accuracy and applications are rapidly maturing over the open ocean, but more attention is required where continental shelves and marginal seas are concerned. Refined radar range measurement correction approaches are needed to enhance the quality of coastal altimeter SSHA data and to enhance the potential impact that altimeter data can play in improved ocean circulation modeling and prediction. This study provides a regional assessment of satellite altimeter SSHA data from the Jason-2, Jason-1, and TOPEX/Poseidon data sets collected over the Northwest Atlantic coastal region of the United States. The validation approach follows recent studies in numerous shelf regions to address changes from standard or default data quality flagging scheme that can enhance satellite altimeter SSHA data recovery, optimal choice of de-aliasing corrections in deriving subtidal sea level time series, and validation of altimeter data using in situ tide gauge and current measurement data sets. Spatial analyses show the impact of data filtering and corrections from the coastline out onto the slope. Revised handling of the data provides a 5–10% increase in availability (from about 70% up to 80% for waters beyond 50 km from shore). The RMS differences between tide gauge and surface layer current measurements at the 60 day time scale are 3–4 cm and 6 cm/s, respectively, and of magnitude consistent with several recent coastal altimeter data validation efforts.

Progress of North Pacific mode water research in the past decade

This article reviews the progress in research on North Pacific mode waters of the past decade from the physical oceanographic perspective. The accumulation of satellite altimeter sea surface height data, the rapid growth of the Argo profiling float array, and the advancement in eddy-resolving ocean general circulation models have greatly improved the traditional views on the mode waters that were formed prior to the 1990s based on the analyses of historical temperature/salinity data. Areas where significant progress was made include: (1) descriptions of the mode waters’ distributions and properties with fine spatial scales, particularly in their formation regions in winter where observational data had been insufficient; (2) clarifications of the mode waters’ formation and subduction processes relating to the large-scale mean circulation, as well as to the time-varying mesoscale eddy field; (3) impacts of the mode waters’ circulation and dissipation processes on the climate and biogeochemical processes; and (4) dynamic versus thermodynamic causes underlying the mode waters’ decadal changes. In addition to the review, future directions for mode water research are also presented.