Sea Surface Height Difference Research Articles

Assessment of reprocessed sea surface height measurements derived from HY-2A radar altimeter and its application to the observation of 2015–2016 El Niño

Haiyang-2A (HY-2A) is China’s first ocean dynamic environment satellite and the radar altimeter is one of its main payloads. One of the main purposes of the radar altimeter is to measure the sea surface height (SSH). The SSH determined from the altimeter range measurements includes some range and geophysical corrections. These corrections largely affect the accuracy of the SSH measurements. The range and the geophysical corrections are reprocessed and the altimeter waveforms in HY-2A sensor interim geophysical data set records (S-IGDR) are retracked from June 1, 2014 to June 14, 2014, and the accuracy of the reprocessed SSH measurements is evaluated. The methods of the range and geophysical corrections used to reprocess HY-2A altimeter data are validated by using these methods to reprocess the Jason-2 range and geophysical corrections and comparing the results with the range and geophysical corrections in Jason-2 geophysical dataset records (GDR) product. A crossover analysis is used to evaluate the accuracy of the reprocessed HY-2A SSH measurements. The standard deviation (STD) of the crossover SSH differences for HY-2A is around 4.53 cm while the STD of the SSH differences between HY-2A and Jason-2 is around 5.22 cm. The performance of the reprocessed HY-2A SSH measurements is significantly improved with respect to the SSH measurements derived from HY-2A interim geophysical dataset records (IGDR) product. The 2015–2016 El Nino has been the strongest El Nino event since 1997–1998. The range and the geophysical corrections in HY-2A IGDR are reprocessed and sea level anomalies are used to monitor the 2015–2016 El Nino. The results show that the HY-2A altimeter can well observe the 2015–2016 El Nino.

The Dominant Role of the East Siberian Sea in Driving the Oceanic Flow Through the Bering Strait—Conclusions From GRACE Ocean Mass Satellite Data and In Situ Mooring Observations Between 2002 and 2016

AbstractIt is typically stated that the Pacific‐to‐Arctic oceanic flow through the Bering Strait (important for Arctic heat, freshwater, and nutrient budgets) is driven by local wind and a (poorly defined) far‐field “pressure head” forcing, related to sea surface height differences between the Pacific and the Arctic. Using monthly, Arctic‐wide, ocean bottom pressure satellite data and in situ mooring data from the Bering Strait from 2002 to 2016, we discover the spatial structure of this pressure head forcing, finding that the Bering Strait throughflow variability is dominantly driven from the Arctic, specifically by sea level change in the East Siberian Sea (ESS), in turn related to westward winds along the Arctic coasts. In the (comparatively calm) summer, this explains approximately two thirds of the Bering Strait variability. In winter, local wind variability dominates the total flow, but the pressure head term, while still correlated with the ESS‐dominated sea level pattern, is now more strongly related to Bering Sea Shelf sea level variability.

REAPER: Reprocessing 12 Years of ERS-1 and ERS-2 Altimeters and Microwave Radiometer Data

Twelve years (1991–2003) of ERS-1 and ERS-2 altimetry data have been reprocessed within the European Space Agency (ESA) reprocessing altimeter products for ERS (REAPER) project using an updated, modern set of algorithms and auxiliary models. The reprocessed data set (identified as RP01) has been cross-calibrated against the reprocessed ENVISAT V2.1 data. The format of this reprocessed data set is network common data form (version 3). The new data set shows a clear improvement in data quality beyond that of previous releases. The product validation shows reduction of the mean standard deviation of the sea-surface height differences from 8.1 (previously available product) to 6.7 cm (RP01). This paper presents the details of how the reprocessing was conducted and shows selected results from the validation and quality-assurance processes. The major improvements of the REAPER RP01 data set with respect to the previous ESA ERS radar altimetry (RA) products are due to the use of four ENVISAT RA-2 retrackers, RA calibration improvements, new reprocessed precise orbit solutions, ECMWF ERA-interim model for meteorological corrections, new ionospheric corrections, and new sea state. The intent of this paper is to aid the reader in understanding the benefits of the new data set for their particular use-case.

Response of the North Atlantic dynamic sea level and circulation to Greenland meltwater and climate change in an eddy-permitting ocean model

The response of the North Atlantic dynamic sea surface height (SSH) and ocean circulation to Greenland Ice Sheet (GrIS) meltwater fluxes is investigated using a high-resolution model. The model is forced with either present-day-like or projected warmer climate conditions. In general, the impact of meltwater on the North Atlantic SSH and ocean circulation depends on the surface climate. In the two major regions of deep water formation, the Labrador Sea and the Nordic Seas, the basin-mean SSH increases with the increase of the GrIS meltwater flux. This SSH increase correlates with the decline of the Atlantic meridional overturning circulation (AMOC). However, while in the Labrador Sea the warming forcing and GrIS meltwater input lead to sea level rise, in the Nordic Seas these two forcings have an opposite influence on the convective mixing and basin-mean SSH (relative to the global mean). The warming leads to less sea-ice cover in the Nordic Seas, which favours stronger surface heat loss and deep mixing, lowering the SSH and generally increasing the transport of the East Greenland Current. In the Labrador Sea, the increased SSH and weaker deep convection are reflected in the decreased transport of the Labrador Current (LC), which closes the subpolar gyre in the west. Among the two major components of the LC transport, the thermohaline and bottom transports, the former is less sensitive to the GrIS meltwater fluxes under the warmer climate. The SSH difference across the LC, which is a component of the bottom velocity, correlates with the long-term mean AMOC rate.

Studies of the Canadian Arctic Archipelago water transport and its relationship to basin‐local forcings: Results from AO‐FVCOM

AbstractA high‐resolution (up to 2 km), unstructured‐grid, fully coupled Arctic sea ice‐ocean Finite‐Volume Community Ocean Model (AO‐FVCOM) was employed to simulate the flow and transport through the Canadian Arctic Archipelago (CAA) over the period 1978–2013. The model‐simulated CAA outflow flux was in reasonable agreement with the flux estimated based on measurements across Davis Strait, Nares Strait, Lancaster Sound, and Jones Sounds. The model was capable of reproducing the observed interannual variability in Davis Strait and Lancaster Sound. The simulated CAA outflow transport was highly correlated with the along‐strait and cross‐strait sea surface height (SSH) difference. Compared with the wind forcing, the sea level pressure (SLP) played a dominant role in establishing the SSH difference and the correlation of the CAA outflow with the cross‐strait SSH difference can be explained by a simple geostrophic balance. The change in the simulated CAA outflow transport through Davis Strait showed a negative correlation with the net flux through Fram Strait. This correlation was related to the variation of the spatial distribution and intensity of the slope current over the Beaufort Sea and Greenland shelves. The different basin‐scale surface forcings can increase the model uncertainty in the CAA outflow flux up to 15%. The daily adjustment of the model elevation to the satellite‐derived SSH in the North Atlantic region outside Fram Strait could produce a larger North Atlantic inflow through west Svalbard and weaken the outflow from the Arctic Ocean through east Greenland.

The global mean sea surface model WHU2013

The mean sea surface (MSS) model is an important reference for the study of charting datum and sea level change. A global MSS model named WHU2013, with 2′ × 2′ spatial resolution between 80°S and 84°N, is established in this paper by combining nearly 20 years of multi-satellite altimetric data that include Topex/Poseidon (T/P), Jason-1, Jason-2, ERS-2, ENVISAT and GFO Exact Repeat Mission (ERM) data, ERS-1/168, Jason-1/C geodetic mission data and Cryosat-2 low resolution mode (LRM) data. All the ERM data are adjusted by the collinear method to achieve the mean along-track sea surface height (SSH), and the combined dataset of T/P, Jason-1 and Jason-2 from 1993 to 2012 after collinear adjustment is used as the reference data. The sea level variations in the non-ERM data (geodetic mission data and LRM data) are mainly investigated, and a combined method is proposed to correct the sea level variations between 66°S and 66°N by along-track sea level variation time series and beyond 66°S or 66°N by seasonal sea level variations. In the crossover adjustment between multi-altimetric data, a stepwise method is used to solve the problem of inconsistency in the reference data between the high and low latitude regions. The proposed model is compared with the CNES-CLS2011 and DTU13 MSS models, and the standard derivation (STD) of the differences between the models is about 5 cm between 80°S and 84°N, less than 3 cm between 66°S and 66°N, and less than 4 cm in the China Sea and its adjacent sea. Furthermore, the three models exhibit a good agreement in the SSH differences and the along-track gradient of SSH following comparisons with satellite altimetry data.

SARAL/AltiKa Wet Tropospheric Correction: In-Flight Calibration, Retrieval Strategies and Performances

The SARAL/AltiKa mission is a complement of the Jason altimeter series. A two-channels (23.8 GHz and 37 GHz) microwave radiometer (MWR) is combined to the altimeter in order to correct the altimeter range for the excess path delay (referred as WTC for wet tropospheric correction. First, the in-flight calibration of AltiKa MWR is assessed from a systematic comparison to other radiometers using a complete set of metrics (comparison to simulations and over geophysical targets). Then the “mixed” empirical approach successfully used for Envisat shows nonoptimal performances for the WTC retrieval. In order to find the potential sources of issues, this method is compared to a purely empirical relationship established between measured brightness temperatures (TB) and altimeter backscattering coefficient () on one hand and modeled WTC on the other hand. Various retrieval configurations for both AltiKa MWR and advanced microwave radiometer (AMR) on Jason-2, are for the first time systematically compared with respect to their performances against the variance of sea surface height differences at crossovers. Finally, the issues on the “mixed” approach are attributed to the differences between simulated and measured at Ka-band. Now, a configuration of the empirical approach proved to have performances closed to what is initially expected with the “mixed” approach.

Comparison of Regression Algorithms for the Retrieval of the Wet Tropospheric Path

This paper addresses the subject of the regression models used for the wet troposphere path delay correction for range measurements by satellite radar altimeters. The objective of this study is twofold: 1) to find which regression method is better suited for the retrieval between a neural network algorithm and a log-linear regression and 2) to determine whether the use of the altimeter backscattering coefficient at Ka- or Ku-band can substitute for the use of the radiometer brightness temperature at 18 GHz as an input for the retrieval. Several configurations of algorithms, including those used in the operational processing of altimetry missions such as Jason-1 or Envisat, are built and compared on the same learning and test database to determine which retrieval strategy is more appropriate. The importance of each input is analyzed and the performances of the different algorithms are assessed in terms of bias and standard of the errors and also in terms of their geographical distribution and correlations with other environmental variables. Their performances are then assessed on Jason-2 radiometer measurements using the criterion of variance in sea-surface height differences at crossovers. The study shows that the neural network formalism is better suited for the retrieval of the wet tropospheric path delay than the log-linear regression. In terms of variable selection, better results were obtained when the brightness temperature at 18 GHz was used instead of the backscattering coefficient. Overall, the best results are obtained with the combination of a three-channel radiometer and a neural network algorithm.

Nonstationary Internal Tides Observed Using Dual-Satellite Altimetry

AbstractDual-satellite crossover data from the Jason-2 and Cryosat-2 altimeter missions are used in a novel approach to quantify stationary and nonstationary tides from time-lagged mean square sea surface height (SSH) differences, computed for lags from 1 to 1440 h (60 days). The approach is made feasible by removing independent estimates of the stationary tide and mesoscale SSH variance, which greatly reduces the sampling error of the SSH statistics. For the semidiurnal tidal band, the stationary tidal variance is approximately 0.73 cm2, and the nonstationary variance is about 0.33 cm2, or 30% of the total. The temporal correlation of the nonstationary tide is examined by complex demodulation and found to be oscillatory with first 0 crossing at 400 h (17 days). Because a significant fraction of the time-variable mesoscale signal is resolved at time scales of roughly 150 h by the present constellation of satellite altimeters, the results suggest that it may be feasible to predict the nonstationary tide from modulations of the resolved mesoscale, thus enhancing the efficacy of tidal corrections for planned wide-swath altimeters such as the Surface Water and Ocean Topography (SWOT) mission.

Indonesian throughflow proxy from satellite altimeters and gravimeters

AbstractThe Indonesian throughflow (ITF) from the Pacific to the Indian Ocean plays an important role in global ocean circulation and climate. Yet, continuous ITF measurement is difficult and expensive. The longest time series of in situ measurements of the ITF to date were taken in the Makassar Strait, the main pathway of the ITF. Here we have demonstrated a plausible approach to derive the ITF transport proxy using satellite altimetry sea surface height (SSH), gravimetry ocean bottom pressure (OBP) data, in situ measurements from the Makassar Strait from 1996 to 1998 and 2004 to 2011, and a theoretical formulation. We first identified the optimal locations of the correlation between the observed ITF transport through the Makassar Strait and the pressure gradients, represented by the SSH and OBP differences between the Pacific and Indian Oceans at a 1° × 1° horizontal resolution. The optimal locations were found centered at 162°E and 11°N in the Pacific Ocean and 80°E and 0° in the Indian Ocean, then were used in the theoretical formulation to estimate the throughflow. The proxy time series follow the observation time series quite well, with the 1993–2011 mean proxy transport of 11.6 ± 3.2 Sv southward, varying from 5.6 Sv during the strong 1997 El Niño to 16.9 Sv during the 2007 La Nina period, which are consistent with previous estimates. The observed Makassar mean transport is 13.0 ± 3.8 Sv southward over 2004–2011, while the SSH proxy (for the same period) gives an ITF mean transport of 13.9 ± 4.1 Sv and the SSH + OBP proxy (for 2004–2010) is 15.8 ± 3.2 Sv.

Changes to the Canadian Arctic Archipelago Sea Ice and Freshwater Fluxes in the Twenty-First Century under the Intergovernmental Panel on Climate Change A1B Climate Scenario

A coupled ocean and sea-ice pan-Arctic model forced by the Intergovernmental Panel on Climate Change A1B climate scenario is used to study the evolution of ice and ocean surface conditions within the Canadian Arctic Archipelago (CAA) during the twenty-first century. A summer ice-free CAA is likely by the end of our simulation. Sea ice undergoes significant changes from the mid-2020s to the mid-2060s in both concentration and thickness. The simulation shows a shrinking of 65% and a thinning of 75% in summer over the 40 years, resulting in a partially open Northwest Passage by the 2050s. However, ice in central Parry Channel might increase due to a decrease in export from April to June, linked to a reduced cross-channel sea surface height (SSH) gradient, before melting thermodynamically. On a larger scale, the central CAA throughflow will experience a significant decrease in both volume and freshwater transport after 2020, which is related to the change in the SSH difference between the two ends of Parry Channel, particularly the lifting of SSH in Baffin Bay. With a lower albedo, a warmer ocean is simulated, particularly in summer. The sea surface salinity within the CAA demonstrates a strong decadal oscillation without a clear trend over the entire simulation. A north–south pattern, separated by Parry Channel, is also found in the changes of ocean temperature and salinity fields due to different ice conditions.

Long-term variability of the Kuroshio transport east of Taiwan and the climate it conveys

The Kuroshio transport (KT) was established using satellite altimetry data from 1993 to 2010 via three methods: the geostrophic relationship (GR), the empirical relationship (ER), and the transfer function (TF). The relationships were built on the observed KT from the World Ocean Circulation Experiment moored current meter array east of Taiwan (referred to as PCM-1) and its surrounding sea surface height (SSH) difference. Using a modeled 10-year climatology run of the KT, we verify the three methods and associated characteristics in long-term applications. The GR could not capture high-frequency variability of less than four months well. The KT approximated by the ER had a similar variation with a weaker magnitude and the transport was underestimated. Although the TF can establish signals from all available frequency domains, results suggest that the observational period of PCM-1 may be too short to build a reliable TF. The KT estimated by both the GR and the TR was subsequently used to analyze the long-term variability of the KT in the PCM-1 line. The annual averaged KT was influenced by the northern branch of the North Equatorial Current that was affected primarily by El Niño-Southern Oscillation events. The anomaly of the KT was dominated by mesoscale eddies east of Taiwan resulting from the influence of the West Pacific (WP) teleconnection pattern. Correlation analysis confirmed that the long-term KT in the PCM-1 line conveys not only the ENSO signal but the WP teleconnection, which connects the tropic and extratropic dynamics together.

Can sea surface height be used to estimate oceanic transport variability?

The relation between the sea surface height and the meridional transport across a zonal section at 26.5°N in the North Atlantic is studied by using an eddy resolving ocean state estimate simulated with the Massachusetts Institute of Technology general circulation model. It is shown that the correlation between the zonal sea surface height difference and transport can be substantially increased if the steric height contribution from the seasonal thermocline is removed. The latter explains a substantial part of sea surface height variability, but its effect on transport is weak. It is also found that the zonal steric height difference correlates well with the transport after the contribution of the seasonal thermocline has been removed. There is a similar agreement (with correlation coefficient of 0.63 for the full signal and 0.89 for the mean seasonal cycle) between the meridional transport and steric height based on observations from the Rapid Climate Change (RAPID) project.

Sea State Bias in Radar Altimetry Revisited

Sea state bias (SSB) is calculated anew for each radar altimeter. The input data are sea surface height (h) differences, separated in time by at most one repeat cycle, or the difference between measured h and a time-mean sea surface, an approach called here “differences from the mean” (DFM). In order to release data to users soon, early estimates of SSB are computed from short time series. This work has three objectives: 1) show that the DFM approach introduces spurious values in the estimated SSB, 2) quantify the SSB error caused by short time series, and 3) estimate SSB for OSTM.

Overview and Update of the Sea State Bias Corrections for the Jason-2, Jason-1 and TOPEX Missions

We present a Jason-2 sea state bias (SSB) model from the first year of GDR products along with updates of Jason-1, TOPEX-A, and TOPEX-B solutions. Our re-evaluation of the Jason-1 SSB focused on data from the new version (C) of the GDR while for TOPEX, the updated solutions benefited from the availability of both a set of new precise orbits that are consistent with Jason-2 and an updated set of state-of-the-art geophysical corrections. Therefore, derived from homogeneous sea surface height (SSH) data to minimize geographically correlated SSH differences between missions, these empirical SSB models contribute to the consistency between Jason-2 and Jason-1 SSH data and between TOPEX and Jason-1 data during their respective tandem phase and thus to the stability of the whole altimetry system over the past 17-year period.

Sea Surface Height Signals as Indicators for Oceanic Meridional Mass Transports

Abstract Numerical models are used to test whether the sea surface height (SSH) can be used as an indicator for the variability of Atlantic meridional oceanic mass transports. The results suggest that if the transports over the western boundary current region and those in the eastern part of the basin are considered separately, significant correlations (0.3–0.9) are found between zonal SSH differences and the meridional transports in the top 1100 m. Much weaker correlations are found for the basinwide transport, which corresponds to the surface branch of the meridional overturning circulation (MOC). For the eastern and western branches of the meridional transport, combining the SSH signal with the baroclinic structure obtained from Rossby wave theory enables calculation of a quantitative estimate of the transport variability in the top 1100 m. The results of the method are less convincing for the variability of the MOC. The reason for this is that even small relative errors in the variability of the eastern and western branches can be large compared with the MOC variability. These errors project onto the sum of the eastern and western transports and therefore onto the surface branch of the MOC. Nevertheless, being able to infer transport anomalies from SSH signals in the eastern and western parts of the Atlantic might prove useful in interpreting MOC observations from the U.K. Natural Environment Research Council Rapid Climate Change (RAPID) mooring array at 26°N, which show a large subannual variability that is mainly due to changes at the western boundary. Transports inferred from the SSH could help to identify the origin of this variability and whether transport anomalies propagate into the western boundary region from the basin interior or from other latitudes.

Decadal phase change in large‐scale sea level and winds in the Indo‐Pacific region at the end of the 20th century

Satellite observations of sea surface height (SSH) and wind stress for the period of 1993–2006 reveal a near‐coherent large‐scale decadal variability in much of the Indo‐Pacific region with a phase change at the turn of the 20th century. Trade wind variations in the tropical Pacific and South Indian Ocean are anti‐correlated with each other as are SSH differences across these two basins, implying anti‐correlated variation of the subtropical cells in the two oceans. Decadal changes in large‐scale SSH in the extra‐tropics are mostly associated with well‐defined patterns of wind stress curl indicating a near‐coherent decadal variation in the strength of subtropical and subpolar gyres. Together, these variations reflect a linkage in the circulation of the Pacific and Indian Oceans via atmospheric and oceanic bridges. The phase change in the tropical Pacific tends to occur earlier than elsewhere, suggesting a potential role of the tropical Pacific in regulating decadal variability of the entire region.

Regional Dynamic and Steric Sea Level Change in Response to the IPCC-A1B Scenario

AbstractThis paper analyzes regional sea level changes in a climate change simulation using the Max Planck Institute for Meteorology (MPI) coupled atmosphere–ocean general circulation model ECHAM5/MPI-OM. The climate change scenario builds on observed atmospheric greenhouse gas (GHG) concentrations from 1860 to 2000, followed by the International Panel on Climate Change (IPCC) A1B climate change scenario until 2100; from 2100 to 2199, GHG concentrations are fixed at the 2100 level. As compared with the unperturbed control climate, global sea level rises 0.26 m by 2100, and 0.56 m by 2199 through steric expansion; eustatic changes are not included in this simulation. The model’s sea level evolves substantially differently among ocean basins. Sea level rise is strongest in the Arctic Ocean, from enhanced freshwater input from precipitation and continental runoff, and weakest in the Southern Ocean, because of compensation of steric changes through dynamic sea surface height (SSH) adjustments. In the North Atlantic Ocean (NA), a complex tripole SSH pattern across the subtropical to subpolar gyre front evolves, which is consistent with a northward shift of the NA current. On interannual to decadal time scales, the SSH difference between Bermuda and the Labrador Sea correlates highly with the combined baroclinic gyre transport in the NA but only weakly with the meridional overturning circulation (MOC) and, thus, does not allow for estimates of the MOC on these time scales. Bottom pressure increases over shelf areas by up to 0.45 m (water column equivalent) and decreases over the Atlantic section in the Southern Ocean by up to 0.20 m. The separate evaluation of thermosteric and halosteric sea level changes shows that thermosteric anomalies are positive over most of the World Ocean. Because of increased atmospheric moisture transport from low to high latitudes, halosteric anomalies are negative in the subtropical NA and partly compensate thermosteric anomalies, but are positive in the Arctic Ocean and add to thermosteric anomalies. The vertical distribution of thermosteric and halosteric anomalies is highly nonuniform among ocean basins, reaching deeper than 3000 m in the Southern Ocean, down to 2200 m in the North Atlantic, and only to depths of 500 m in the Pacific Ocean by the end of the twenty-first century.

Leveling the Sea Surface Using a GPS-CatamaranSpecial Issue: Jason-1 Calibration/Validation

In the framework of the TOPEX/Poseidon and Jason-1 CNES-NASA missions, two probative experiments have been conducted at the Corsica absolute calibration site in order to determine the local marine geoid slope under the ascending TOPEX/Poseidon and Jason-1 ground track (No. 85). An improved determination of the geoid slope was needed to better extrapolate the offshore (open-ocean) altimetric data to on-shore tide-gauge locations. This in turn improves the overall precision of the calibration process. The first experiment, in 1998, used GPS buoys. Because the time required to cover the extended area with GPS buoys was thought to be prohibitive, we decided to build a catamaran with two GPS systems onboard. Tracked by a boat at a constant speed, this innovative system permitted us to cover an area of about 20 km long and 5.4 km wide centered on the satellites' ground track. Results from an experiment in 1999 show very good consistency between GPS receivers: filtered sea-surface height differences have a mean bias of −0.2 cm and a standard deviation of 1.2 cm. No systematic error or distortions have been observed and crossover differences have a mean value of 0.2 cm with a standard deviation of 2.7 cm. Comparisons with tide gauges data show a bias of 1.9 cm with a standard deviation of less than 0.5 cm. However, this bias, attributable in large part to the effect of the catamaran speed on the waterline, does not affect the geoid slope determination which is used in the altimeter calibration process. The GPS-deduced geoid slope was then incorporated in the altimeter calibration process, yielding a significant improvement (from 4.9 to 3.3 cm RMS) in the agreement of altimeter bias determinations from repeated overflight measurements.

Annual Rossby waves in the Arabian Sea from TOPEX/POSEIDON altimeter and in situ data

Sea-surface height data acquired by the TOPEX/POSEIDON satellite over the Arabian Sea from October 1992 to October 1998 are analyzed. Strong seasonal fluctuations are found between 61 and 101N, which are mainly associated with westward propagating annual Rossby waves radiated from the western side of the Indian subcontinent and that are continuously forced by the action of the wind-stress curl over the central Arabian Sea. An analysis of hydrographic data acquired during August 1993 and during January 1998 at 81N in the Arabian Sea reveals the existence of first- and second-mode annual Rossby waves. These waves, which can be traced as perturbations in the density fields, have wavelengths of 12 � 10 3 and 4.4 � 10 3 km as well as phase velocities of 0.38 and 0.14 m/s, respectively. The waves are associated with a time-dependent meridional overturning cell that sloshes water northward and southward. Between 581 and 681E in the central Arabian Sea, we found a Rossby-wave induced transport in the upper 500 m of about 10 Sv southward in August 1993 and northward in January 1998. Below 2000 m, there was still a northward transport of 3.2 Sv in August 1993 and a southward transport of 4.8 Sv in January 1998. A comparison of steric height differences between August 1993 and January 1998 calculated from the observed density fields as well as calculated from the reconstructed density fields using first- and second-mode annual Rossby waves agree quite well with the corresponding sea-surface height differences. Implications resulting from the reflection of annual Rossby waves, like fluctuations of the western boundary currents, are discussed. r 2002 Elsevier Science Ltd. All rights reserved.