Cheinway Hwang

TOPEX/Poseidon observations of mesoscale eddies over the Subtropical Countercurrent: Kinematic characteristics of an anticyclonic eddy and a cyclonic eddy

[1] Relative dynamic heights and geostrophic fields were derived from TOPEX/Poseidon altimetry data and then used to track mesoscale eddies over the Subtropical Countercurrent (STCC). The radii, centers, vorticities, shearing deformation rates, stretching deformation rates, divergences, and center velocities of all identified eddies over the STCC were determined using a model that assumes constant velocity gradients. Most eddies are concentrated in a zonal band near 22� N, and there is an interannual variation in the number of eddies. A case study was made for a cyclonic eddy and an anticyclonic eddy, with time series of eddy kinematic parameters computed. Both eddies survive for � 220 days and propagate westward along over 22� N–24� N to reach the Kuroshio Current east coast of Taiwan, where the eddies were dissipated and in turn affected the Kuroshio Current in many ways. Sea surface temperature data and drifter data confirm the existence of these two eddies. The radii of both eddies vary and their shapes are mostly elliptical during propagation. The anticyclonic eddy propagated almost westward with oscillating north-south components, and the mean speed is 8.3 km/day. The cyclonic eddy moved southwestward before reaching 130� E and then moved northwestward, with a mean speed of 7.6 km/day. The propagations of these two eddies are basically consistent with the standard theory of eddy propagation but with larger speeds. The propagating direction could be altered while passing steep bottom topography or merging with the other eddies. INDEX TERMS: 4520 Oceanography: Physical: Eddies and mesoscale processes; 4576 Oceanography: Physical: Western boundary currents; 4512 Oceanography: Physical: Currents; 4594 Oceanography: Physical: Instruments and techniques; 4556 Oceanography: Physical: Sea level variations;

Adjustment of relative gravity measurements using weighted and datum-free constraints

A program in FORTRAN 90 is developed which can adjust relative gravity measurements and solve for gravimeter parameters using the weighted constraint and datum-free constraint models. The weighted constraint model is chosen when there exist reliable a priori gravity values for use as supplementary-data, or when it is required that a gravity network be attached to an existing gravity network of a higher order. The datum-free model uniquely determines relative gravity values among all stations without the need of a fixed gravity value, thus it is suitable for detecting relative gravity change. The optional solve-for gravimeter parameters include drift coefficients, and coefficients of the long wavelength and periodic components of calibration function. The program can also detect outliers in observations using the τ-test method. A set of relative gravity data in Taiwan was used to test this program using five different choices of command-line arguments. The results show that there are no outliers ih these data and that the estimated reading accuracy of the LaCoste and Romberg G meter (serial number 838) is about 0.02 mgal and the gravimeter drift rate is 0.9 mgal, month. The coefficients of the long wavelength and periodic components of calibration function are statistically equal to zero. The result from the datum-free solution is used to detect gravity variation due to the 1999 Chi-Chi earthquake, concluding that only one station experiences a significant change.

Lake Level Variations Monitored With Satellite Altimetry Waveform Retracking

Over lake shores, altimetric waveforms are generally contaminated by lands, rough lake surfaces, and lag effects of the altimeters automatic gain control. To improve altimeter ranging accuracy and in turn to get better surface height measurement, contaminated waveforms should be retracked against geophysical corrections. In this paper, an improved threshold retracker (ITR) is developed to retrack waveforms over lakes. ITR considers not only the physical characteristics of the reflecting surface, but also the stochastic feature of waveform, and two new retrackers, the N-Beta function model, and the N-5-Beta function model, are also put forward to develop the waveform retracking program of this study. TOPEX/POSEIDON waveforms over Hulun Lake in the North China are retracked to monitor the temporal lake level variations. A comparison with the in situ hydrological data indicates ITR is very efficient to monitor the lake level variations with the retracked altimetric data. The result of our study shows accurate seasonal level variations and the descending trend of Hulun Lake.

A study of the Kuroshio's seasonal variabilities using an altimetric-gravimetric geoid and TOPEX/POSEIDON altimeter data

The first year of TOPEX/POSEIDON (T/P) altimeter data were used to study the seasonal variabilities of the Kuroshio Current. The intercomparison between the T/P and gauge sea levels at selected tide gauge stations around Taiwan shows that the two have a correlation of about 0.9 at the immediate vicinity of the deep ocean, and both show an annual cycle with an amplitude of 15 cm. A 3′ × 3′ geoid for the Western Pacific was constructed by least squares collocation using gravity anomalies and sea surface gradients derived from Seasat, Geosat, ERS 1, and T/P altimetry. To account for the oceanographic signal in the altimeter data, we derived a covariance function for the gradients of the sea surface topography (SST) based on a global averaging concept and a spherical harmonic expansion of the Levitus SST to degree 50. The accuracy of the geoid ranges from 5 to 40 cm. The SST values derived from the geoid and the data along T/Ps descending tracks were fitted to the hyperbolic functions in a two-step procedure to find the Kuroshios seasonal axes and the parameters that describe its characteristics. The mean variabilities in the Kuroshios path, maximum velocity, baroclinic transport, and width are 22 km, 19 cm, 8 Sv, and 27 km, respectively. The averaged percentage variability of all quantities is 25%. All the variabilities are relatively large near the northeast coast of Taiwan and the starting point of the Kuroshio Extension. The large-scale circulations over the Western Pacific were obtained by median filtering the SST. At the 1334-km spatial scale, the Pacifics subtropical gyre is clearly visible, and it shows different features over the four seasons and its kinetic energy shows possible correlations with the determined parameters. Over the South China Sea, a warm ring with a radius of 300–400 km and a center at 15°N, 113°E was detected in the spring and summer.

Results from parallel observations of superconducting and absolute gravimeters and GPS at the Hsinchu station of Global Geodynamics Project, Taiwan

few mm a � 1 of horizontal and vertical motions around HS. The calibration factor and driftingrateofT48are � 75.96±0.07mGalV � 1 and0.2±0.7mGala � 1 (1mGal=10 � 8 ms � 2 ). Both the SG and absolute gravity records contain trends of about 2–3 mGal a � 1 . The ocean tide gravity effects (OTGEs) were estimated from NAO.99b, FES2004, and CSR4.0, and their amplitudes agree with the SG observations at the submicroGal level, but their phasesdifferfromtheobservationsupto10.TheNewtonianeffectofoceantidecontributes 20% to the total OTGE at HS, and it is larger at islands in the Taiwan Strait. The inelastic body tide model of Dehant et al. (1999) is more consistent with the SG observations than the elastic model. Modeled gravity-atmosphere admittances based on an exponential distribution of air mass explain well the observed admittances. The average gravityatmosphere admittance during typhoons is 30% larger than that in a nontyphoon time. A list of coseismic gravity changes from T48 caused by earthquakes over 2006–2007 is given for potential studies of fault parameters. The modeled effects of atmospheric pressure, groundwater, soil moisture, and polar motion explain the FG5 observed gravity trend to 1.1 mGal a � 1 . Seasonally, the groundwater-induced gravity change contributes the most

A subwaveform threshold retracker for ERS-1 altimetry: A case study in the Antarctic Ocean

Based on a correlation analysis method, a subwaveform threshold retracker is developed and coded in FORTRAN for satellite altimetry to determine the leading edge and retracking gate, and to improve the precision of sea surface heights (SSHs) and gravity anomalies (GAs). Using ERS-1/ERM waveforms, the subwaveform threshold retracker outperforms full-waveform threshold retrackers at the tide gage Port Station. A direct comparison between retracked SSHs and in situ SSHs is made at tide gage Port Station. Here the subwaveform retracking improves SSH precision from 0.241 to 0.193m, yielding an improvement percentage (IMP) of 20%. Using ERS-1/GM waveforms, the subwaveform threshold retracker outperforms the Beta-5 and full-waveform threshold retrackers over the Bellingshausen and Amundsen Seas (BAS) in the Antarctic Ocean. The standard deviations of raw and retracked SSHs are 0.157 and 0.070 and 1.836 and 0.220m over the ice-free and ice-covered oceans, corresponding to IMPs of 54.4% and 88%, respectively. Use of retracking improves the precision of GAs by up to 46.6% when comparing altimeter-derived and shipborne GAs.

Seafloor secrets revealed

Satellite data reveal formerly unknown tectonic structures [Also see Report by Sandwell et al.] The trenches and ridges on Earths seafloor are shaped by tectonic processes such as seafloor spreading and plate subduction. Detailed knowledge of seafloor tectonics is lacking in many areas. The most comprehensive data come from satellite altimeters, which use the strength and waveform of the radar signal returned from the sea surface to determine the tectonic properties of the underlying seafloor. On page 65 of this issue, Sandwell et al. (1) present the latest global marine gravity and depth data from altimeter missions CryoSat-2 and Jason-1. The data reveal buried tectonic structures, for example, in the Gulf of Mexico and the South Atlantic Ocean, that help to elucidate past tectonic processes.

Spherical harmonic analysis and synthesis using FFT: Application to temporal gravity variation

FFT and complex algebra-based methods of spherical harmonic analysis and synthesis are presented. Two computer programs in FORTRAN are developed based on the methods. Both general and special cases are discussed. Special cases involve the analyses of gravity changes of the hydrological origin and the atmospheric origin. Functionals of the Earths gravity field such as gravity anomaly and geoidal height can also be computed via synthesis. Thermal-corrected sea level anomaly from TOPEX/Poseidon and atmospheric pressure from ECMWF are used to compute changes of geopotential coefficients due to oceanic and atmospheric mass redistributions. Interesting phenomena in the changes of geopotential coefficients have been identified. The two computer programs can facilitate analyses and syntheses of gravity products from satellite missions such as GRACE.

SHALLOW-WATER GRAVITY ANOMALIES FROM SATELLITE ALTIMETRY : CASE STUDIES IN THE EAST CHINA SEA AND TAIWAN STRAIT

Abstract The differences between two global satellite altimeter‐derived gravity anomaly grids over the East China Sea and the Taiwan Strait are investigated and the causes of the differences are discussed. Difference of gravity anomaly is correlated with tide model error, standard deviation of sea surface heights (SSHs) and ocean depth. We present two new SSH‐derived observations of altimetry (differenced height and height slope) for gravity derivation. An iterative method for removing altimeter data outliers is used, and differenced height is found to be most sensitive to this method. For altimeter‐gravity conversion, we used three altimeter‐derived observations, i.e., deflection of the vertical, differenced height and height slope, in two conversion methods, i.e., the least‐squares collocation (LSC) and the inverse Vening Meinesz method. Use of differenced heights in LSC produces the best result when comparing altimeter‐derived and shipborne gravity anomalies. Use of land gravity data in the vicinity of coasts enhances the accuracy of altimeter‐derived gravity anomalies.

Marine Gravity Anomaly from Satellite Altimetry: a Comparison of Methods over Shallow Waters

Gravity anomalies over shallow waters are useful in many geodetic and geophysical applications. This work compares three methods of gravity anomaly derivation from altimetry over shallow waters near Taiwan: (1) compute gravity anomalies by LSC using along-track, differenced geoidal heights and height slopes, (2) compute gravity anomalies by least-squares collocation (LSC) using altimeter-derived along-track deflections of vertical (DOV), and (3) grid along-track deflections of vertical by LSC and then compute gravity anomalies by the inverse Vening Meinesz formula. A nonlinear filter with outlier rejection is applied to along-track data. We used altimeter data from Seasat, Geosat, ERS-1, ERS-2 and TOPEX/POSEDION missions. Retracked ERS-1 waveforms are shown to improve the accuracy of estimated gravity anomalies. For the three methods, the RMS differences between altimetry-derived gravity anomalies and shipborne gravity anomalies are 9.96 (differenced height) and 10.26 (height slope), 10.44 and 10.73 mgals, respectively. The RMS differences between shipborne gravity anomalies with gravity anomalies from retracked and non-retracked ERS-1 waveforms are 11.63 and 14.74 mgals, indicating retracking can improve the accuracy.