Altimeter-derived Sea Level Anomalies Research Articles

Intra-seasonal sea level variability along the west coast of India.

The importance of local versus distant forcing is studied for the wind-driven intra-seasonal (30-120 day) sea level anomaly (SLA) variations along the west coast of India. Significant correlations of altimeter-derived SLA on the west coast are found with the mid-basin SLA east of Sri Lanka and SLA as far as Sumatra and the equator, with increased lags, connecting with the remote forcing from the equator in the form of reflected Rossby waves. The highest correlations between SLA on the west coast and winds are found with the winds at the southern tip of India. Coherence calculations help to identify the importance of a narrow band (40-60 day) for the interactions of winds with the intra-seasonal SLA variations. A multivariate regression model, along with the coherences within this narrower band, suggest the lags of SLA on the west coast with winds to range from 0 to 2 days with the local forcing to 11-13 days with the forcing along south east coast of India. Hovmöller diagrams illustrate the propagation of signals by estimating phase speed for Rossby waves (57 cm/s) across the Indian Ocean from Sumatra and Coastal Trapped Waves (CTWs) along the west coast of India (178 cm/s). Propagation from the south-east coast of India is not as robust as Rossby waves from Sumatra.

Automated system for near-real time modelling and prediction of altimeter-derived sea level anomalies

This paper serves as a presentation of a novel geoinformation system and a dedicated service, jointly named as Prognocean and based at the University of Wrocław (Poland), that aim to predict Sea Level Anomaly (SLA) maps and publish them online. The system works in near-real time and is updated daily. The data are provided by the Archiving, Validation and Interpretation of Satellite Oceanographic data (AVISO), and the time series processed by Prognocean is delivered by various altimetric satellites. The emphasis is put on gridded SLA maps, also known as MSLA, which are provided as Delayed Time (DT) and Near-Real Time (NRT) daily products. The daily sampling interval, however, does not coincide with typical repeat cycles of altimetric satellites and is obtained through reprocessing produced by AVISO. The two-module infrastructure forms the system. The first module is responsible for the near-real time communication with AVISO to download the most recent MSLA data and acquire the corrected data when the geophysical corrections have been available. The second module forms the main engine which does data processing, modelling, forecasting, statistical quality control and finally generates products as maps. The online service, however, publishes the products online every day. The above-mentioned components and infrastructure are described in detail. The performance of the system was evaluated using at least 150 predicted MSLA maps, available after half year of computations carried out in near-real time. We identified a few regions of imperfect performance of our prognoses and found that they spatially correspond to the mouth of the Amazon River and locations of key mesoscale eddies, the vast majority of which being nonlinear and hence unmodelled in our experiment.

Dynamic height anomaly from Argo profiles and sea-level anomaly from satellite altimetry: a comparative study in the Indian Ocean

Altimeter-derived sea-level anomaly (SLA) has been compared with Argo-derived dynamic height anomaly (DHA) in the Indian Ocean. The anomalies have been found to agree quite well in the region above 10°S. The agreement is improved when climatological salinity is replaced by Argo salinity. Seasonal variation of correlation of DHA computed with Argo salinity and SLA has also been studied. The reasons for the poor correlation could be attributed to the fact that there is a barotropic (non-steric) component of the SLA, not captured by DHA, which is a purely steric effect. It has been also found that the impact of halosteric component on dynamic height is quite high in certain regions of the Arabian Sea because of (i) low-salinity water of southern region advected into the region by Somali and North Equatorial Current, (ii) upwelled water of low salinity during south-west monsoon along the Arabian coast and (iii) precipitation and river run-off from rivers in the eastern Arabian sea contributing to the local formation of low-salinity water. In the Bay of Bengal, the factors responsible for the large impact of halosteric component are precipitation and river influx in the northern part. In the south-eastern Indian Ocean, Indonesian Throughflow is the governing factor for this impact.

The ECMWF Ocean Analysis System: ORA-S3

Abstract A new operational ocean analysis/reanalysis system (ORA-S3) has been implemented at ECMWF. The reanalysis, started from 1 January 1959, is continuously maintained up to 11 days behind real time and is used to initialize seasonal forecasts as well as to provide a historical representation of the ocean for climate studies. It has several innovative features, including an online bias-correction algorithm, the assimilation of salinity data on temperature surfaces, and the assimilation of altimeter-derived sea level anomalies and global sea level trends. It is designed to reduce spurious climate variability in the resulting ocean reanalysis due to the nonstationary nature of the observing system, while still taking advantage of the observation information. The new analysis system is compared with the previous operational version; the equatorial temperature biases are reduced and equatorial currents are improved. The impact of assimilation in the ocean state is discussed by diagnosis of the assimilation increment and bias correction terms. The resulting analysis not only improves the fit to the data, but also improves the representation of the interannual variability. In addition to the basic analysis, a real-time analysis is produced (RT-S3). This is needed for monthly forecasts and in the future may be needed for shorter-range forecasts. It is initialized from the near-real-time ORA-S3 and run each day from it.

The ECMWF Ocean Analysis System: ORA-S3

A new operational ocean analysis/reanalysis system (ORA-S3) has been implemented at ECMWF. The reanalysis, started from 1 January 1959, is continuously maintained up to 11 days behind real time and is used to initialize seasonal forecasts as well as to provide a historical representation of the ocean for climate studies. It has several innovative features, including an online bias-correction algorithm, the assimilation of salinity data on temperature surfaces, and the assimilation of altimeter-derived sea level anomalies and global sea level trends. It is designed to reduce spurious climate variability in the resulting ocean reanalysis due to the nonstationary nature of the observing system, while still taking advantage of the observation information. The new analysis system is compared with the previous operational version; the equatorial temperature biases are reduced and equatorial currents are improved. The impact of assimilation in the ocean state is discussed by diagnosis of the assimilation increment and bias correction terms. The resulting analysis not only improves the fit to the data, but also improves the representation of the interannual variability. In addition to the basic analysis, a real-time analysis is produced (RT-S3). This is needed for monthly forecasts and in the future may be needed for shorter-range forecasts. It is initialized from the near-real-time ORA-S3 and run each day from it.

Offshore limit of coastal ocean variability identified from hydrography and altimeter data in the eastern Arabian Sea

In this communication, we describe a hitherto-unknown offshore limit to the coastal ocean variability signatures away from the continental shelf in the eastern Arabian Sea, based on hydrographic observations and satellite altimeter (TOPEX/Poseidon) information. Vertical sections of temperature from two contrasting seasons—south-west (SW) and north-east (NE) monsoons—when projected together are found to intersect offshore at 350–400 km from the coast along a transect in the eastern Arabian Sea during 1996. Formation of such intersections along transects from south to north in the entire eastern Arabian Sea is noticed in the hydrographic data of 1987–88 also. Existence of these intersections has further been authenticated by the altimeter-derived sea level anomalies during 1992–97. It is proposed that these intersections arise due to the westward propagation of Rossby waves from the coastal Kelvin wave guide in the eastern Arabian Sea where alternate upwelling and downwelling processes occur during the contrasting seasons of SW and NE monsoons. We interpret these intersections as the offshore limit of variability of the water properties in the coastal region. Indications are that areas of high productivity in the coastal regions overlap a major portion of the width of intersection implying extension of the upwelling features towards offshore requiring revised estimates of primary productivity and carbon budgeting.

Kinetic Energy Variability in the North Indian Ocean Using a Numerical Model

The North Indian Ocean exhibits profound impact of variation in lower tropospheric winds. In the present study climatological monthly winds are used to force a nonlinear reduced gravity model of the North Indian Ocean to simulate climatological surface circulation and sea level anomaly for all 12 months of the year. The sea level anomalies agree reasonably well with satellite altimeter derived sea level anomalies. The model successfully simulates the varying eddy structure and current pattern of the North Indian Ocean. Finally, the kinetic energy variation in the North Indian Ocean with special reference to equatorial region and the boundaries is analyzed in detail.