Sea Level Anomaly

Satellite altimetry is a fundamental component of ocean observation. Although devised for open ocean measure-ments, its extension to the coastal zone is rapidly evolving, enlarging the already large number of applications relying on satellite altimetry data. In this context the ...

An Ocean General Circulation Model (OGCM) of the tropical Pacific in which combined TOPEX/Poseidon and ERS sea level anomalies are assimilated over January 1994 through July 1999, is used to investigate equatorial wave characteristics during the intense 1997-1999 El Niño-La Niña event. Near the equator, the linear vertical modes are estimated at each grid point of the OGCM simulation with and without assimilation. Consistently with an increase of the vertical gradient within the thermocline and a rise of the thermocline depth in the eastern basin, the assimilation results in an increased contribution of the higher-order baroclinic modes in the eastern basin and a decreased contribution of the first baroclinic mode in the western Pacific for the zonal current variability. For pressure, the first baroclinic mode contribution is reduced whereas the higher-order modes contribution is weakly impacted. Kelvin and first-meridional Rossby waves are then derived for the first two more energetic baroclinic modes in the simulation with assimilation. Kelvin waves of both modes constructively contribute to the strong warming observed in 1997, with the first (second) baroclinic mode being more energetic than the second (first) baroclinic mode in the early (mature) stage of the warming. Kelvin waves of both modes reflect as first meridional Rossby waves at the eastern boundary (reflection efficiency of ˜95%) and contribute to push back the warm pool westward. The reversal of the warming is apparently initiated by the second baroclinic mode contribution which controls the position of the 28°C isotherm in the surface layer in the far eastern Pacific from January 1998. At the western boundary, reflections of Rossby waves take place for both modes with an estimated total efficiency of ˜50% at 165°E. This suggests that, in our model, the delayed oscillator theory is not applicable for explaining the reversal from warm to cold conditions in 1998 while the zonal advective feedback was at work. More generally, the study suggests that it is necessary to take into account the vertical structure of the ocean when interpreting altimetric data, which can be done through an assimilation experiment.

Three methods for automatic detection of mesoscale coherent structures are applied to Sea Level Anomaly (SLA) fields in the South Atlantic. The first method is based on the wavelet packet decomposition of the SLA data, the second on the estimation of the Okubo-Weiss parameter and the third on a geometric criterion using the winding-angle approach. The results provide a comprehensive

In order to guide physical investigations, characterize the structure ofmodel biases, and assess the impact of numerical and physical choices, DRAKKARsimulations are evaluated against altimetric sea-level anomalies (AVISO: 1993-present,see Poster by Penduff et al), and temperature/salinity profiles (ENACT/ENSEMBLES:1956-present). Model outputs are first sub-sampled like actual observations to buildaltimetric and hydrographic "synthetic observations". Real and synthetic observations andmore integral quantities (mixed layer depth cycle, heat/salt contents, etc.) are thencompared over various regions, periods, and timescales. This study presents themethods and a validation of DRAKKAR simulations with respect to hydrography.

Oceanographic applications using satellite altimeter data become very challenging when leaving the deep ocean for the coastal regions. Close to the coast, altimeter observations are often of lower quality for a number of reasons, including land contamination of the satellite footprints or inaccurate resolution of the corrections of the high frequency ocean response to tidal and atmospheric loading. This paper presents a new processing toolbox, called X-TRACK, to derive improved altimeter products, such as sea surface heights (SSH), mean sea surface heights (MSSH) or sea level anomalies (SLA), dedicated for coastal applications. Starting from classical geophysical data records (GDR) products, particular attention is made to recover a maximum amount of exploitable data (dedicated data editing, interpolation of missing corrective terms). Where possible, local modelling of the high frequency response of the ocean to the tidal and atmospheric loading is applied instead of standard, globa...

Data from satellite altimeters and from a 13-month deployment of in situ instruments are used to determine an empirical relationship between sea-level anomaly difference (SLA) across the Kuroshio in the East China Sea (ECS-Kuroshio) and net transport near 28°N. Applying this relationship to the altimeter data, we obtain a 12-year time series of ECS-Kuroshio transport crossing the C-line (KT). The resulting mean transport is 18.7 ± 0.2 Sv with 1.8 Sv standard deviation. This KT is compared with a similarly-determined time series of net Ryukyu Current transport crossing the O-line near 26°N southeast of Okinawa (RT). Their mean sum (24 Sv) is less than the mean predicted Sverdrup transport. These KT and RT mean-flow estimates form a consistent pattern with historical estimates of other mean flows in the East China Sea/Philippine Basin region. While mean KT is larger than mean RT by a factor of 3.5, the amplitude of the KT annual cycle is only half that of RT. At the 95% confidence level the transports are coherent at periods of about 2 years and 100–200 days, with RT leading KT by about 60 days in each case. At the annual period, the transports are coherent at the 90% confidence level with KT leading RT by 4–5 months. While the bulk of the Kuroshio enters the ECS through the channel between Taiwan and Yonaguni-jima, analysis of satellite altimetry maps, together with the transport time series, indicates that the effect of mesoscale eddies is transmitted to the ECS via the Kerama Gap southwest of Okinawa. Once the effect of these eddies is felt by the ECS-Kuroshio at 28°N, it is advected rapidly to the Tokara Strait.

A 1/6° numerical simulation is used to investigate the vertical structure of westward propagation between 1993 and 2000 in the North Atlantic ocean. The realism of the simulated westward propagating signals, interpreted principally as the signature of first-mode baroclinic Rossby waves (RW), is first assessed by comparing the simulated amplitude and zonal phase speeds of Sea Level Anomalies (SLA) against TOPEX/Poseidon-ERS satellite altimeter data. Then, the (unobserved) subsurface signature of RW phase speeds is investigated from model outputs by means of the Radon Transform which was specifically adapted to focus on first-mode baroclinic RW. The analysis is performed on observed and simulated SLA and along 9 simulated isopycnal displacements spanning the 0-3250 m depth range. Simulated RW phase speeds agree well with their observed counterparts at the surface, although with a slight slow bias. Below the surface, the simulated phase speeds exhibit a systematic deceleration with increasing depth, by a factor that appears to vary geographically. Thus, while the reduction factor is about 15-18% on average at 3250 m over the region considered, it appears to be much weaker (about 5-8%) in the eddy-active Azores Current, where westward propagating structures might be more coherent in the vertical. These results suggest that the often-made normal-mode assumption of many WKB-based extended theories that the phase speed is independent of depth might need to be revisited. They also suggest that the vertical structure of westward propagating signals could significantly depend on their degree of nonlinearity, with the degree of vertical coherence possibly increasing with the degree of nonlinearity.

The performance of coastal altimetry over a wide continental shelf is assessed using multiple-year ocean current observations by moored Acoustic Doppler Current Profilers (ADCP) and high-frequency (HF) radar on the West Florida Shelf. Across-track, surface geostrophic velocity anomalies, derived from the X-TRACK along-track sea level anomalies are compared with the near surface current vector components from moored ADCP observations at

Ocean Sci., 6, 269–284, 2010 www.ocean-sci.net/6/269/2010/ © Author(s) 2010. This work is distributed under the Creative Commons Attribution 3.0 License. ... Impact of global ocean model resolution on sea-level variability with ... T. Penduff1,2, M. Juza1, L. Brodeau1, GC ...