Thermosteric Sea-level Changes Research Articles

Sea state contributions to thermosteric sea-level in high-resolution ocean-wave coupled simulations

This study examines the impact of wave-induced processes (WIPs) in modulating thermosteric sea-level changes, highlighting the need to include these processes in future sea-level rise assessments and climate projections. The impact of wave-induced processes on thermosteric sea-level changes is investigated using coupled ocean-wave simulations. These simulations include the effects of Stokes-Coriolis forcing, sea-state dependent surface stress and energy fluxes, and wave-induced mixing. The experiments use a high-resolution configuration of the Geesthacht COAstal Model SysTem (GCOAST), covering the Northeast Atlantic, the North Sea and the Baltic Sea. The GCOAST system uses the Nucleus for European Modelling of the Ocean (NEMO) ocean model to account for wave-ocean interactions and ocean circulation. It is fully coupled with the WAM spectral wind wave model. The aim is to accurately quantify the sea state contribution to thermosteric sea level variability and trends over a 26-year period (1992–2017). The ability of wave-ocean coupled simulations to reveal the contribution of sea state to sea level variability and surge is demonstrated. It is clear that wave-induced processes (WIPs) play a significant role in sea surface dynamics, ocean mixing (mixed layer thickness) and modulation of air-sea fluxes (e.g. heat flux) in both winter (10–20%) and summer (10%), which in turn affect thermosteric sea level variability. The North Atlantic (in summer) and the Norwegian Trench (in winter) show significant contributions (40%) to the thermosteric sea-level variability due to wave-induced processes. The influence of WIPs on thermosteric sea level trends in the North Atlantic is up to the order of 1 mm yr-1 in both winter and summer, in the open ocean and at the shelf break. Smaller contributions are observed over the shelf areas of the North Sea. This study underscores the crucial role of WIPs in modulating sea-level changes and highlights the importance of including these processes in future sea-level rise assessments and climate projections.

Simulated sea levels during 1948–2009 in a global ocean-sea ice model for OMIP

This paper assesses the ability of the State Key Laboratory of Atmospheric Science and Geophysical Fluid Dynamics of the Institute of Atmospheric Physics (LASG/IAP) Climate System Ocean Model version 3 (LICOM3) to simulate sea levels. For the time series of global changes, most of the global mean steric sea level (SSL) changes were dominated by thermosteric sea level (TSSL) changes in the upper 700 m, with a relatively small contribution from halosteric sea level (HSSL). LICOM3 can reproduce the observed dynamic sea level (DSL) in most regions to reflect the spatial patterns of regional changes. Deviations between LICOM3 and observations were mainly concentrated in the western boundary current and the tropical eastern Pacific. The departure in the western boundary current was primarily related to the model resolution. LICOM3 could not depict DSL changes in the high latitudes where the mesoscale eddies were active. The excessive negative trend in the tropical eastern Pacific was mainly caused by the trend error in the CORE-II forcing products. For the sea level variability, the analysis results of different DSL periods showed that the LICOM3 model could simulate interannual and decadal variability and the satellite altimeter period trend. At least 50 years of data were used to separate the decadal variability and long-term trend. The decadal variability had no significant effect on the 50-year long-term trend change in the DSL in the tropical western Pacific.Analysis of sea level component contributions showed that the changes in global time series, spatial patterns, and sea level variability mainly resulted from the change in TSSL. The TSSL changes in the upper 700 m could roughly reflect the TSSL changes for the whole depth, except for the North Atlantic and the Southern Ocean. In the Atlantic, the contribution of HSSL was comparable to and compensated by TSSL. When compared with reanalysis, the simulation generated differences in the Southern Ocean and North Atlantic. The linear trend of TSSL simulated by LICOM3 was much stronger than that simulated by reanalysis. The deviation of the Southern Ocean may be related to the CORE-II forcing product. The lack of observations in the Southern Ocean led to the uncertainty of the CORE-II forcing product.

Kinematics of global mean thermosteric sea level during 1993–2019

Abstract Because oceans cover 71% of Earth’s surface, ocean warming, consequential for thermal expansion of sea water, has been the largest contributor to the global mean sea level rise averaged over the 20 th and the early 21 st century. This study first generates quasi-observed monthly globally averaged thermosteric sea level time series by removing the contributions of global mean sea level budget components, namely, Glaciers, Greenland, Antarctica, and Terrestrial Water Storage from satellite altimetry measured global sea level changes during 1993–2019. A baseline kinematic model with global mean thermosteric sea level trend and a uniform acceleration is solved to evaluate the performance of a rigorous mixed kinematic model. The model also includes coefficients of monthly lagged 60 yearlong cumulative global mean sea surface temperature gradients and control variables of lunisolar origins and representations for first order autoregressive disturbances. The mixed kinematic model explains 94% (Adjusted R 2)1 of the total variability in quasi-observed monthly and globally averaged thermosteric time series compared to the 46% of the baseline kinematic model’s Adjusted R 2. The estimated trend, 1.19±0.03 mm/yr., is attributed to the long-term ocean warming. Whereas eleven statistically significant (α = 0.05) monthly lagged cumulative global mean sea surface temperature gradients each having a memory of 60 years explain the remainder transient global mean thermosteric sea level changes due to the episodic ocean surface warming and cooling during this period. The series also exhibit signatures of a statistically significant contingent uniform global sea level acceleration and periodic lunisolar forcings.

Global thermosteric sea level change contributed by the deep ocean below 2000 m estimated by Argo and CTD data

Due to an insufficient amount of observational data, little is known about the temperature variation in the global abyssal ocean and its contribution to sea level change. In this study, we estimated the monthly seawater temperature below 2000 m from 2005–2015 by combining Array for Real-time Geostrophic Oceanography (Argo) and conductivity-temperature-depth (CTD) data. For this purpose, we divided the global ocean into 38 regions according to the CTD temperature profile properties. For each region, we used CTD data to construct a mathematical model to describe the relation of temperatures between 1000–2000 m and those below 2000 m. We then used this mathematical model and the Argo data to reconstruct the temperature profile below 2000 m and evaluate its thermostatic contribution to sea level change. We discovered that the deep ocean below 2000 m contributed 0.12 ± 0.03 mm/yr to the mean global thermosteric sea level rise from 2005–2015. Our result was confirmed by geodetic observations, i.e., the rate of thermosteric sea level rise calculated using altimeter, Gravity Recovery and Climate Experiment (GRACE), and Argo data was approximately 0.15 ± 0.41 mm/yr. The contribution of each ocean layer generally decreased with depth, and there was an obvious fluctuation between approximately 4000–5000 m, interpreted as being caused by geothermal heating of the ocean floor.

The sea level response to ice sheet freshwater forcing in the Community Earth System Model

We study the effect of a realistic ice sheet freshwater forcing on sea-level change in the fully coupled Community Earth System Model (CESM) showing not only the effect on the ocean density and dynamics, but also the gravitational response to mass redistribution between ice sheets and the ocean. We compare the ‘standard’ model simulation (NO-FW) to a simulation with a more realistic ice sheet freshwater forcing (FW) for two different forcing scenario’s (RCP2.6 and RCP8.5) for 1850–2100. The effect on the global mean thermosteric sea-level change is small compared to the total thermosteric change, but on a regional scale the ocean steric/dynamic change shows larger differences in the Southern Ocean, the North Atlantic and the Arctic Ocean (locally over 0.1 m). The gravitational fingerprints of the net sea-level contributions of the ice sheets are computed separately, showing a regional pattern with a magnitude that is similar to the difference between the NO-FW and FW simulations of the ocean steric/dynamic pattern. Our results demonstrate the importance of ice sheet mass loss for regional sea-level projections in light of the projected increasing contribution of ice sheets to future sea-level rise.

Detection and attribution of global mean thermosteric sea level change

AbstractChanges in sea level are driven by a range of natural and anthropogenic forcings. To better understand the response of global mean thermosteric sea level change to these forcings, we compare three observational data sets to experiments of 28 climate models with up to five different forcing scenarios for 1957–2005. We use the preindustrial control runs to determine the internal climate variability. Our analysis shows that anthropogenic greenhouse gas and aerosol forcing are required to explain the magnitude of the observed changes, while natural forcing drives most of the externally forced variability. The experiments that include anthropogenic and natural forcings capture the observed increased trend toward the end of the twentieth century best. The observed changes can be explained by scaling the natural‐only experiment by 0.70 ± 0.30 and the anthropogenic‐only experiment (including opposing forcing from greenhouse gases and aerosols) by 1.08 ± 0.13(±2σ).

Vertical tectonic crustal movements along the Japanese coastlines inferred from late Quaternary and recent relative sea-level changes

Observed relative sea-level (RSL) changes during the past ∼130 kyr make it possible to evaluate the vertical crustal movement due to several tectonic processes such as the subduction of oceanic plates. Here we infer average rates of tectonic crustal movement along the Japanese coastlines on three typical timescales of ∼50 yr, ∼6 kyr and ∼125 kyr based on tide gauge and Holocene RSL observations and the altitudes of marine terraces formed at the Last Interglacial (LIG) phase at ∼125 ka. The rates on a timescale of ∼50 yr are derived from tide gauge data, thermosteric sea-level changes due to thermal expansion of the oceans and predictions due to the glacio-hydro isostatic adjustment (GIA) for the last deglaciation and also recent melting of the mountain glaciers and Greenland and Antarctic ice sheets. Those for ∼6 kyr and ∼125 kyr are based on the RSL observations and the predictions by GIA modelling, considering uncertainties for temporal changes in eustatic sea-level for the mid- to late-Holocene and LIG phase. The inferred rates for ∼50 yr are significantly different from those for ∼125 kyr in most sites, particularly for sites along the coastline from eastern Hokkaido to northeastern Japan, Shikoku and south Kyushu facing the Pacific Ocean. In these regions, the rates for ∼125 kyr and ∼50 yr are positive (uplift) and negative (subsidence), respectively. Also, the observed RSL changes at ∼6 ka are consistent with the inferred RSL changes using the rates for ∼125 kyr and GIA-predictions in many sites, but inconsistent with those for ∼50 yr in most sites except for a few sites. These results suggest that the rates on a timescale of ∼50 yr are not representative of the tectonic crustal movements for timescales longer than ∼6 kyr in most sites along the Japanese coastlines. However, the inferred rates on these timescales may be useful in discussing the recurrence of megathrust earthquake with its interval of ∼1 kyr such as the 2011 earthquake off the Pacific coast of Tohoku.

Twentieth century sea-level rise inferred from tide gauge, geologically derived and thermosteric sea-level changes

Relative sea-level (RSL) changes at thirteen sites derived from tide gauge and/or salt-marsh sediment sequences, with information for RSL changes during at least the past ∼200 years, are used to infer the rates and causes of the global sea-level rise in the twentieth century (pre-satellite era) by incorporating spatially non-uniform thermosteric sea-level change and recent estimates for the melting of mountain glaciers and both polar ice sheets. The main advantage of the method adopted here is that we can avoid a model-dependence in the GIA (glacial isostatic adjustment) correction and any steady tectonic effect depending on each location can be corrected for. We first estimate the acceleration of sea-level rise in the twentieth century at each RSL observation site, and then evaluate the residuals between the sea-level acceleration and thermosteric sea-level rise. The spatially non-uniform residuals are examined to infer the rates of RSL rise due to the melting of mountain glaciers and polar ice sheets. The comparison between the residuals and the predicted rates for the melting wholly supports the estimates for the melting by the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC 2007 Report), and does not require a significant melting from the Greenland ice sheet. However, the possible equivalent sea-level rise for mountain glaciers and/or Antarctic ice sheet may be ∼0.3 mm yr−1 larger than the preferred estimate by IPCC 2007 Report (∼0.7 mm yr−1), and the total one is ∼1.0 mm yr−1. The additional melting for ∼0.3 mm yr−1 is due to the comparison mainly for Brest with long tide gauge records of ∼200 years. Also, the present study indicates that the global mean thermosteric sea-level rise before the rapid sea-level acceleration occurred at 1990 is ∼0.3 mm yr−1.

Depth-dependent temperature change contributions to global mean thermosteric sea level rise from 1960 to 2010

The dependency of global mean thermosteric sea level changes to temperature at different depths down to 700m is investigated from 1960 to 2010 using two separate gridded temperature datasets, and compared with reconstructed estimates of sea level change. The rates of thermosteric sea level changes are closely correlated with those of reconstructed sea level changes with correlation coefficients larger than 0.8, but the former has smaller amplitudes than the latter, indicating contributions to total sea level change from processes other than upper ocean temperature changes examined here. Most of the net thermosteric sea level rise (~92%) can be attributed to temperature changes of the upper ocean (0–300m), but an intriguing temporal lag is found between thermal anomalies of the upper (0–300m) and lower (300–700m) layers in the historical temperature datasets, suggestive of a time-lag associated with heat penetrating from the surface into deeper layers of the ocean. Results of global mean thermosteric sea level estimates from the two different temperature datasets are found to be consistent with each other in time and in depth.

The timescale and extent of thermal expansion of the global ocean due to climate change

Abstract. With recently improved instrumental accuracy, the change in the heat content of the oceans and the corresponding contribution to the change of the sea level can be determined from in situ measurements of temperature variation with depth. Nevertheless, it would be favourable if the same changes could be evaluated from just the sea surface temperatures because the past record could then be reconstructed and future scenarios explored. Using a single column model we show that the average change in the heat content of the oceans and the corresponding contribution to a global change in the sea level can be evaluated from the past sea surface temperatures. The calculation is based on the time-dependent diffusion equation with the known fixed average upwelling velocity and eddy diffusivity, as determined from the steady-state limit. In this limit, the model reduces to the 1966 Munk profile of the potential temperature variation as a function of depth. There are no adjustable parameters in the calculation and the results are in good agreement with the estimates obtained from the in situ data. The method allows us to obtain relevant timescales and average temperature profiles. The evaluation of the thermosteric sea level change is extended back to the beginning of accurate sea surface temperature records. The changes in sea surface temperature from 1880 until the present time are estimated to have produced a thermosteric sea level rise of 35 mm. Application to future IPCC scenarios gives results similar to the average prediction of more complex climate models.

The influence of the North Atlantic Oscillation on the sea-level around the northern European coasts reconsidered: the thermosteric effects

The thermosteric contribution of the North Atlantic Oscillation (NAO) to the North Sea sea-level for the winter period is investigated. Satellite sea surface temperature as well as in situ measurements are used to define the sensitivity of winter water temperature to the NAO as well as to determine the trends in temperature. The sea surface temperature sensitivity to the NAO is about 0.85 degrees C per unit NAO, which results in thermosteric sea-level changes of about 1-2 cm per unit NAO. The sensitivity of sea surface temperatures to the NAO is strongly time-dependent. Model data from a two-dimensional hydrodynamic tide+surge model are used in combination with the estimated thermosteric anomalies to explain the observed sea-level changes and, in particular, the sensitivity of the datasets to the NAO variability. The agreement between the model and the observed data is improved by the inclusion of the thermosteric effect.

Contribution of thermal expansion to present-day sea-level change revisited

We investigate the thermosteric (i.e., due to temperature only) sea-level change over the last 50 years using two global ocean temperature data sets recently published ( Levitus et al., 2000a [Levitus, S., Stephens, C.M., Antonov, J.I., Boyer, T.P., 2000a. Yearly and year-season upper ocean temperature anomaly fields, 1948–1998, pp. 23, U.S. Gov. Printing Office, Washington, DC] and Ishii et al., 2003 [Ishii, M., Kimoto, M., Kachi, M., 2003. Historical ocean subsurface temperature analysis with error estimates, Monthly Weather Rev., 131, 51–73]). These data sets which provide gridded temperatures, down to 3000 m and 500 m respectively, are based on interpolation schemes of raw historical profiles over 1950–1998. We find that the two data sets compare well over 1950–1990, both in terms of thermosteric sea-level trends and global mean. Some difference is noticed however beyond 1990, due to differences in the raw temperature data processing. Analyses based on ‘Empirical Orthogonal Function’ show that the interannual variability of the thermosteric sea level is dominated by the signatures of El Niño Southern Oscillation, Pacific Decadal Oscillation and influenced by North Atlantic Oscillation. As a result, regional thermosteric sea-level trends are not stationary on a century time scale and have a typical lifetime on the order of a decade. In terms of global mean, the rate of thermosteric sea-level change computed over 10-year windows displays high variability, with values reaching up to three times the 40-year (1950–1990) average at some periods. Even negative values are noticed at other periods. One important consequence is that the pattern of sea-level trends derived from Topex/Poseidon altimetry over 1993–2003, which is mainly caused by thermal expansion, is very likely a non-permanent feature. Thus past and future extrapolation based on this 10-year altimetry pattern should be considered with caution.