Steric Sea Level Change Research Articles

Estimation of steric sea level variations from combined GRACE and Jason-1 data

We estimate the mean steric sea level variations over the 60°S–60°N oceanic domain for the recent period (from August 2002 to April 2006), by combining sea level data from Jason-1 altimetry with time-variable gravity data from GRACE. The observed global mean sea level change from satellite altimetry results in total from steric plus ocean mass change. As GRACE measurements averaged over the ocean represents the ocean mass change component only, the difference between GRACE and altimetry observations provides an estimate of the mean steric sea level. Two different sets of GRACE geoid solutions (the GRGS EIGEN-GL04 and the GFZ EIGEN-GRACE04S products) have been used. Each GRACE data set ranges over approximately 3 yr or more (August 2002–April 2006 for the GRGS geoids and February 2003–February 2006 for the GFZ geoids).We first focus on the seasonal variations. The two GRACE data sets agree very well in terms of mean annual mass variation, both in amplitude (approximately 7.3 mm equivalent sea level) and phase (maximum on October 14). For both time spans, Jason-1 sea level minus GRACE ocean mass has an annual amplitude of approximately 5.8 mm with a maximum on March 11. The latter signal compares well with the steric annual sea level estimated from the World Ocean 2004 climatology and the Ishii et al. [M. Ishii, M. Kimoto, K. Sakamoto, and S.I. Iwasaki, Steric sea level changes estimated from historical ocean subsurface temperature and salinity analyses, Journal of Oceanography, 62 (2) (2006) 155–170.] ocean temperature data. We also examine the interannual fluctuations of the Jason-1 minus GRACE sea level. The two resulting steric sea level time series (based on the two GRACE data sets) agree well. The inferred steric sea level curve exhibits an increasing trend during the last 3.5 yr (August 2002–April 2006), of the same order of magnitude as the 1993–2003 steric sea level trend computed with in situ hydrographic data. However, over the last 3.5 yr, we note a strong discrepancy between altimetry minus GRACE and in situ-based steric sea level trend, the latter exhibiting a negative slope. The cause for such a discrepancy is yet unknown but may be related to inadequate sampling of in situ ocean temperature measurements.

Spatiotemporal variability of interdecadal sea level oscillations in the North Pacific

The interdecadal variations in the North Pacific sea level field were investigated on the basis of two historical tide‐gauge data sets: long‐term sea level records spanning 100 years but spatially limited in the midlatitudes (the CL data set) and records from a wider area but with a data length of about 50 years (the HCL data set). A seesaw‐like sea level oscillation between the eastern and the western Pacific was extracted as the first mode of empirical orthogonal functions (EOF) from the CL data set. This mode shows a 22.5‐year period of variation that has continued throughout the 20th century. A similar variation was found in the HCL data set as the first mode of complex EOF (CEOF), though its dominant period was 14.5 years. This interdecadal sea level oscillation is the steric sea level change associated with the Pacific Inter‐Decadal Oscillation (PDO) in the upper ocean temperature field, because the temporal evolution and spatial structure in the interdecadal oscillation resemble those of the dominant modal pattern in the temperature field. However, this mode shows a standing wave characteristic in the midlatitudes rather than propagating, though the clockwise phase propagation around the subtropical gyre has been reported in the temperature field. Another interdecadal oscillation is found in the CEOF second mode, which was dominant in the 1960s when the first mode was relatively weak. The second mode shows a 15‐ to 16‐years period of variation and phase lags of 3–4 years at the subtropical stations relative to those in the midlatitudes.

Steric sea level changes estimated from historical ocean subsurface temperature and salinity analyses

An historical objective analysis of subsurface temperature and salinity was carried out on a monthly basis from 1945 to 2003 using the latest observational databases and a sea surface temperature analysis. In addition, steric sea level changes were mainly examined using outputs of the objective analyses. The objective analysis is a revised version of Ishii et al. and is available at 16 levels in the upper 700 m depth. Artificial errors in the previous analysis during the 1990s have been worked out in the present analysis. The steric sea level computed from the temperature analysis has been verified with tide gauge observations and TOPEX/Poseidon sea surface height data. A correction for crustal movement is applied for tide gauge data along the Japanese coast. The new analysis is suitable for the discussion of global warming. Validation against the tide gauge reveals that the amplitude of thermosteric sea level becomes larger and the agreement improves in comparison with the previous analysis. A substantial part of local sea level rise along the Japanese coast appears to be explained by the thermosteric effect. The thermal expansion averaged in all longitudes from 60°S to 60°N explains at most half of recent sea level rise detected by satellite observation during the last decade. Considerable uncertainties remain in steric sea level, particularly over the southern oceans. Temperature changes within MLD make no effective contribution to steric sea level changes along the Antarctic Circumpolar Current. According to statistics using only reliable profiles of the temperature and salinity analyses, salinity variations are intrinsically important to steric sea level changes in high latitudes and in the Atlantic Ocean. Although data sparseness is severe even in the latest decade, linear trends of global mean thermosteric and halosteric sea level for 1955 to 2003 are estimated to be 0.31 ± 0.07 mm/yr and 0.04 ± 0.01 mm/yr, respectively. These estimates are comparable to those of the former studies.

Linear trends of zonally averaged thermosteric, halosteric, and total steric sea level for individual ocean basins and the world ocean, (1955–1959)–(1994–1998)

We present estimates of the linear trends of zonally averaged fields of thermosteric, halosteric, and steric sea level by pentads (1955–1959)–(1994–1998) for individual ocean basins and the world ocean. The Atlantic is characterized by density compensating linear trends in the thermosteric and halosteric components of steric sea level change. Northward of approximately 45°N the halosteric trend acts to increase steric sea level and the thermosteric trend acts to decrease sea level. Southward of 45°N these two terms are reversed in sign. Similar to the Atlantic, the Indian Ocean is characterized by density compensating trends at most latitudes, a major exception being 28°S–38°S. The Pacific is different from the Atlantic and Indian oceans. The two components act in concert to change sea level in the same way with the exception of the 34°N–45°N and 22°S–38°S regions.

How nonlinearities in the equation of state of seawater can confound estimates of steric sea level change

The process of mixing water masses that have differing temperatures, salinities, and pressures creates water that is typically denser than the average density of the source waters. Thus mixing the ocean, either along isopycnals or vertically, tends to decrease global sea level, even when no heat or salt is added to the ocean. This effect is small in the global ocean, but regional effects can be substantial. Two recommendations emerge from this study. First, numerical predictions of steric sea level rise should be designed to minimize sensitivity to erroneous isopycnal or vertical/diapycnal mixing rates that might over or under mix water masses. Second, retrospective analyses of steric sea level rise in historic data should avoid using gridded temperature and salinity fields to compute steric sea level change, since mapping smooths data in the same way that mixing does.

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.

Contributions of hydrological processes to sea level change

We estimate the global mean sea level (GMSL) change using TOPEX/Poseidon satellite radar altimeter measurements and investigate possible contributions from water mass redistribution within the global hydrological cycle using a few numerical models. We examine the global mean sea level change at seasonal, interannual, and long-term time scales. The atmospheric and hydrological models include the ECMWF operational atmospheric model and the NCEP/NCAR reanalysis system. The World Ocean Atlas 1998 and over 19 years’ satellite sea surface temperature observations are used to evaluate steric mean sea level change at different time scales. Both hydrological cycle and steric change provide important contribution to seasonal GMSL change. At interannual time scales, atmospheric water vapor variation shows good correlation to the altimeter observation and can introduce observable changes in the mean sea level. The snow water over Greenland and Antarctica estimated from the ECMWF model also shows encouraging interannual variability during the 1997/1998 El Niño period. Preliminary results show that thermal effect is a major but not a dominant contributor to long-term sea level rise, indicating that snow and ice melting associated with global warming may play an important role in driving GMSL rise.

Freshwater and Heat Changes in the North and South Pacific Oceans between the 1960s and 1985–94

Comparisons of hydrographic conditions in the North and South Pacific Oceans in the 1960s and 1985‐94 have been made along five World Ocean Circulation Experiment sections. Below the seasonal mixed layer, statistically significant temporal differences in salinity and temperature have been detected in the water masses that occur in the upper 2000 dbar of the water column. These water mass property differences have been used to estimate the freshwater and heat storage trends in the Pacific over the study period. Along 248N, 108N, and 178S, where either North Pacific Intermediate Water or Antarctic Intermediate Water is present, the upper waters have increased in salinity, while the intermediate and deep waters have decreased in salinity. Although the depthintegrated salinity changes observed along these sections are small, the regional redistribution of freshwater associated with the water mass changes is significant and implies significant redistribution of surface freshwater fluxes over the Pacific. Heat loss has occurred along 478N and 178S, but significant warming has occurred along 248 and 108N, giving the Pacific a net heat gain of 1.79 3 108 Jm 22. The resulting steric sea level change for the area in the Pacific between 608N and 31.58S over the roughly 20-yr study period is estimated to be a rise of 0.85 mm yr21, consistent with those in existing literature, but larger than that estimated from numerical models reported in the Intergovernmental Panel on Climate Change Second Assessment Report.

Hydrological and oceanic excitations to polar motion andlength-of-day variation

SUMMARY Water mass redistributions in the global hydrosphere, including continental water storage change and non-steric sea level change, introduce variations in the hydrological angular momentum (HAM) and the oceanic angular momentum (OAM). Under the conservation of angular momentum, HAM and OAM variations are signi¢cant excitation sources of the Earth rotational variations at a wide range of timescales. In this paper, we estimate HAM and OAM variations and their excitations to polar motion and length-of-day variation using soil moisture and snow estimates and non-steric sea level change determined by TOPEX/Poseidon satellite radar altimeter observations and a simpli¢ed steric sea level change model. The results are compared with the variations of polar motion and LOD that are not accounted for by the atmosphere. This study indicates that seasonal continental water storage change provides signi¢cant contributions to both polar motion and LOD variation, especially to polar motion X, and the non-steric sea level change is responsible for a major part of the remaining excitations at both seasonal scale and high frequencies, particularly in polar motion Y and LOD. The good correlation between OAM contributions and the remaining excitations shows that large-scale non-tidal mass variation exists in the oceans and can be detected by TOPEX/Poseidon altimeter observations.

Seasonal sea level change from TOPEX/Poseidon observation and thermal contribution

Seasonal steric sea-level change due to temperature variation in the mixing layer is assessed using space-measured sea-surface temperature data and historical in situ temperature measurements. The results are compared with TOPEX/Poseidon satellite altimeter measurement at different large spatial scales. It is indicated that thermal effect accounts for much of the observed seasonal variability, especially when averaging over zonal regions. Some regional seasonal patterns of sea-level anomalies in the tropical oceans are well represented by the thermal model prediction. Systematic differences are shown between TOPEX/Poseidon observation and thermal contribution at a 1–2 cm level. The potential causes for these differences are discussed, including water mass exchanges among the atmosphere, land, and oceans, and error sources in the steric result and geophysical corrections applied in TOPEX/Poseidon data.

Geophysical contributions to satellite nodal residual variation

Even‐degree zonal gravitational variations due to mass redistribution within the Earth's system, especially in the atmosphere, hydrosphere, and oceans will lead to variations in the nodal precession rate of satellite orbit. The accurately measured nodal variation for LAGEOS 1 provides a means to study planetary‐scale mass redistributions and gravitational variations from the space. In this paper, we investigate atmospheric and hydrologic contributions to the LAGEOS 1 nodal changes using barometric pressure, soil moisture, and snow accumulation values from data‐assimilating numerical models. Oceanic effects are estimated from nonsteric sea level change determined by TOPEX/Poseidon satellite radar altimeter observation and a simple model for steric sea level changes. The results are compared with the LAGEOS 1 nodal change time series observed by satellite laser ranging. At annual and semiannual time scales, the atmosphere and hydrosphere provide significant contributions. The atmosphere provides broadband excitation of nodal changes at intraseasonal timescales. Seasonal and intraseasonal nontidal oceanic effects are also significant. General agreement between predicted and observed nodal precession rate residuals is improved relative to earlier studies, in part because of the better estimation of hydrological effects and new assessment of nontidal oceanic effects.

Steric Sea Level in the Los Alamos POP Code—Non-Boussinesq Effects

ABSTRACT The recent availability of accurate global ocean surface elevation data from satellites permits intercomparison with model results, and therefore raises the question of the accuracy of the computed sea level in ocean models. Most models, even those that incorporate a free surface, make use of the Boussinesq approximation—that is, they assume an incompressible fluid and conserve volume rather than mass. This results in an error in predicting steric sea level changes. The Los Alamos POP model, a free-surface Bryan-Cox type of ocean model developed for parallel computers, is in this category. We generalize the POP (Parallel Ocean Program) model to account for non-zero dilatation (non-zero velocity divergence—the primary non-Boussinesq effect) due to a variety of causes, such as expansion due to thermal and salinity mixing, as well as compressibility. We then compute both local and global changes of the sea level due to these effects and conclude that overall global dynamic effects are entirely negligible. There are some small local effects such as a very small increase in the Antarctic Circumpolar Current transport, for example. We also observe a global mean sea level decrease at a rate on the order of a half-metre per Century. While in this case this is most likely due to the ocean relaxing toward equilibrium (the computation is not initiated from a state of equilibrium), this does indicate that mean ocean expansion/contraction may be Significant from the point of view of climate and ought to be included in models.

Comparison of TOPEX/POSEIDON altimetry and in situ sea level data at Sao Tome Island, Gulf of Guinea

In situ sea level data from a shallow pressure tide gauge (mean depth 3.21 m) at Sao Tome Island, Gulf of Guinea, and altimeter data from TOPEX/POSEIDON (T/P) were analyzed and compared. The Texas model, a T/P‐derived tide model, is used for the tidal correction of altimeter data. Sea level anomaly time series from both data sets were low‐pass filtered by using a Gaussian recurrent interpolation scheme, suppressing high‐frequency fluctuations of periods much less than 2 months. According to the 1992–1993 Sao Tome tide gauge data, a clear seasonal signal was observed both in 1992 and in 1993, presenting two sea level maxima in February and October and two minima in June and December. These sea level variations are consistent with the known seasonal mean sea level cycle of the area, most of which can be explained by steric sea level changes due to the seasonal cycle of upper 500‐m water column properties in the Gulf of Guinea. On the basis of the 60‐day low‐passed tide gauge time series, significant year to year variations were observed as well, with a 6.3‐cm sea level difference in February (+8.1 cm in 1992 versus +1.8 cm in 1993) and a 4.4‐cm difference in October (+4.6 cm in 1992 versus +9.0 cm in 1993). Minimum sea levels, occurring just prior to the upwelling season (July–August), were almost the same for both years (−9 to −10 cm) but lasted much longer in 1992 (2 months) than in 1993 (2 weeks). During the 15‐month period of concomitant observations, from October 1992 to December 1993, these seasonal and year to year sea level variations are remarkably well reproduced by the T/P sea level time series, with an rms difference of 2.2 cm and a correlation coefficient of 0.88. This result in the equatorial Atlantic is consistent with other intercomparisons in the Pacific and Indian Oceans. This excellent recovery of the oceanic signal is undoubtedly the result of unprecedented high precision of T/P measurements and the reliable T/P‐derived tide model that is used in the present study.

Sea level variations induced by heating and cooling: An evaluation of the Boussinesq approximation in ocean models

In this paper a sigma coordinate ocean model is modified to remove the commonly used Boussinesq approximation so that the effect of thermal expansion is exactly included in the basic equations in order to cope with the seasonal heating cycle and the detection of climate change through variation in sea level height. Tests are performed to evaluate the differences between Boussinesq and non‐Boussinesq calculations under different heating and cooling conditions and different model domains. For an idealized case of a flat bottom, shallow ocean basin without wind forcing, simulations of a warm eddy show that the non‐Boussinesq dynamics have only a minor effect on the baroclinic current field. However, vertically averaged velocities, though small compared with the baroclinic velocities, are cyclonic for the Boussinesq calculation and anticyclonic for the non‐Boussinesq calculation. The results indicate that global or closed basin Boussinesq models should be able to simulate most of the observed steric sea level changes on seasonal or climate timescales, when corrected by a spatially uniform, time‐dependent factor calculated from the volume‐averaged density change. The seasonal variation of the globally averaged sea level calculated from climatological data is small, about 1 cm. Variations in steric sea level in regional models, both Boussinesq and non‐Boussinesq, may differ from those of global models owing to the unknown transport across their boundaries associated with the local heating and cooling. A spatially uniform, time‐dependent correction, similar to that associated with thermal expansion, is proposed to account for transport across open boundaries of regional models. Variations of sea level obtained from a Boussinesq model of the Atlantic Ocean approximate the seasonal signal due to the heating/cooling cycle of each hemisphere as observed by satellite altimeter data.