Steric Sea Level Variations Research Articles

Steric sea level variations inferred from combined Topex/Poseidon altimetry and GRACE gravimetry

Steric sea level variations inferred from combined Topex/Poseidon altimetry and GRACE gravimetry

Observing seasonal steric sea level variations with GRACE and satellite altimetry

Sea level rises and falls as the temperature and salinity of the water column varies, which is known as steric sea level. Sea level also changes as water mass is redistributed within the ocean or is added or removed. Satellite radar altimeters measure the combination of both effects, while the Gravity Recovery and Climate Experiment (GRACE) was designed to measure time variable gravity caused by movement of water mass. Theoretically, altimetry and GRACE data can be combined in order to compute the steric sea level variations. We test this by combining current GRACE and Jason 1 altimeter data and comparing against steric sea level observations. We will describe how to properly combine the altimetry and GRACE data, commenting on important corrections that need to be applied to each data type. Using empirical orthogonal function (EOF) analysis, we examine the leading modes of seasonal variability and find that using GRACE improves the ability to recover the dominant mode of steric sea level variability over using altimetry alone. The RMS error of the GRACE ocean mass variations is estimated to be about 2 cm of sea level at a 1000 km smoothing radius. Although this is larger than initially predicted from the GRACE mission, it is still significantly smaller than the recovered signal in several regions of the ocean.

Low-frequency Atlantic sea level variability

The control run of the Hadley Centre-coupled climate model (HadCM3) is used to establish the sources of multidecadal/centennial sea level (SL) variations in the northwest Atlantic. It is shown that variations in the sea level for the largest part of this area are related to variations in the thermohaline structure of the upper (≈500 m) part of the ocean. Temperature variations dominate steric sea level variations, while salinity variations are dominant only in the margins of the Labrador Sea and near the Mediterranean outflow. In the Labrador Sea, lower layers in the ocean also contribute to the variability in expansion/contraction of the water column. It is shown that along the North American East Coast, variations in the thermohaline structure of the water column are predominantly related to variations in the wind-driven circulation rather than the thermohaline circulation, which dominates the thermohaline structure in the central parts of the North Atlantic. In the Gulf Stream area and near the Labrador Current, sea level has on the multidecadal/centennial time scale a strong barotropic signal superposed on steric sea level variations.

Sea level in the Mediterranean Sea: The contribution of temperature and salinity changes

The steric sea level variability in the Mediterranean Sea is estimated from the Medar climatology. Temperature variations cause most of the overall steric sea level change in the upper 400 m. Between 1960 and the 1990s cooling of the upper waters of the Eastern Mediterranean caused reduction in the steric heights while after 1993 warming caused sea level to rise. The steric sea level changes in the upper waters of the Adriatic and the Aegean Sea are correlated with the North Atlantic Oscillation. The comparison between the steric sea levels and coastal tide‐gauges is unsatisfactory. This discrepancy questions both the practice of estimating basin‐wide sea level changes on point measurements and the use of steric height variations as measures of sea level variability in areas less sampled than the Mediterranean. Moreover the diverse behaviour of the Eastern Mediterranean at sub‐basin scales questions the meaningfulness of climatic basin averages.

Steric sea level variations during 1957–1994: Importance of salinity

Spatially averaged (50°S–65°N) temperature and salinity changes in the 0–3000 m layer during the 1957–1994 period resulted in a sea level rise at a mean rate of about 0.55 mm per year. About 10% of this rate is due to a decrease of the volume mean salinity. The magnitude of total steric sea level (TSSL) changes and the ratio of thermosteric and halosteric anomalies to TSSL anomaly are nonuniform geographically. Salinity effects are critically important to the TSSL changes in some regions of the ocean. For example, the thermosteric anomaly is nearly compensated by the halosteric anomaly in the subpolar North Atlantic. This fact will cause erroneous heat content estimates based on altimetric observations from space if a climatological salinity is assumed. Based on the present historical archive of salinity data, a decrease in global mean salinity has occurred. This increase of fresh water would cause sea level rise at a rate of 1.3 ± 0.5 mm/yr if the added water comes from sources other than floating sea ice.

Open‐ocean validation of TOPEX/POSEIDON sea level in the western equatorial Pacific

During the verification phase of the TOPEX/POSEIDON radar altimeter mission a rigorous open‐ocean validation experiment was conducted in the western equatorial Pacific Ocean. From August–September 1992 to February–March 1993, two Tropical Ocean and Global Atmosphere (TOGA) Tropical Atmosphere Ocean moorings at 2°S–156°E (1739 m depth) and 2°S–164.4°E (4400 m depth) were outfitted with additional temperature, salinity, and pressure sensors to measure precisely the dynamic height from the surface to the bottom at 5‐min intervals directly beneath two TOPEX/POSEIDON crossovers. Bottom pressure gauges and inverted echo sounders were deployed as well. A predeployment design study using full depth conductivity‐temperature‐depth casts, subsequently confirmed by postdeployment analyses, indicated this suite of instruments was capable of measuring sea surface height fluctuations to within 1–2 cm. The validation experiment also benefited from the comprehensive set of ocean‐atmosphere measurements that were made in the region during the TOGA Coupled Ocean‐Atmosphere Response Experiment intensive observation period of November 1992 to February 1993. The surface relative to bottom dynamic height fluctuations observed in situ during the 6–7 month experiment had a standard deviation of 5 cm with excursions of order ±15 cm. Energetic steric sea level variability was found to exist on short timescales of order hours to a few days, most notably, the quasi‐permanence of strong semidiurnal internal tides. Such internal tides were noted to induce changes in surface dynamic height with a standard deviation of 2 dynamic centimeters. At the shallower of the two sites, 2°S–156°E, a possible nonlinear rectification of the internal tide was observed occasionally to change the dynamic height by as much as 30 cm over less than an hour. On timescales longer than the 10–day repeat of the TOPEX/POSEIDON satellite, the low‐frequency fluctuations of dynamic height were related to interannual variations corresponding to the 1991–1993 El Niño‐Southern Oscillation, to the seasonal cycle, and to intraseasonal variations associated with the 40‐ to 60‐day oscillations of the equatorial zonal wind field. Instantaneous comparisons between the 1‐s TOPEX/POSEIDON altimeter retrievals and the 5‐min dynamic height were performed with regard to several tide models, the barotropic tide measured in situ, European Centre for Medium‐Range Weather Forecasts surface air pressure, and the surface air pressure measured in situ. Depending on the choice of altimeter and of the environmental corrections applied to the altimeter data, the rms differences between the satellite and the in situ measurements of sea level were as low as 3.3 cm at 2°S–156°E and 3.7 cm at 2°S–164.4°E. When additional satellite data in the general vicinity of the mooring are included and after the use of a 30‐day low‐pass filter, the satellite and in situ data were found to be highly correlated, with correlation coefficients of about 0.95 and rms differences around 1.8 cm.