Sea Level Response Research Articles

Surface Ocean Warming Around Australia Driven by Interannual Variability and Long‐Term Trends in Southern Hemisphere Westerlies

AbstractThe ocean surface temperature and sea level response around Australia to both interannual variability as well as observed and projected changes in surface winds is presented. A hindcast ocean experiment shows interannual southward shifts in the Southern Hemisphere westerly winds drive ocean surface warming events in the South Australian Basin. Twenty‐first century climate and ocean projections in an ensemble of the fifth Coupled Model Intercomparison Project (CMIP5) models show that wind trends could play a role comparable in magnitude to that of radiative warming in driving surface ocean temperature change in this region. To evaluate the wind's role in these projected changes, we use an ocean experiment perturbed by the projected end of 21st century wind anomalies to show that the wind trends alone can generate approximately half of the ocean surface warming and sea level rise predicted by the CMIP5 models in the Tasman Sea.

Astronomical pacing of Late Cretaceous third- and second-order sea-level sequences in the Foz do Amazonas Basin

Because of their relatively reduced tectonic influence, post-rift sedimentary successions have a propensity to preserve climatically-driven cyclicity over long durations. Here we present an integrated cyclostratigraphic and sequence stratigraphic study of the post-rift Limoeiro sedimentary Formation (Fm) of the Foz do Amazonas Basin (offshore Brazil), which spans the entire Late Cretaceous epoch (almost 35 Myr long). The principal goal of the present study is to decipher very long (multi-Myr) sedimentary cyclicities and their potential origin(s) in order to delineate the main controlling factors of post-rift sediment sequences and packages. We used gamma-ray (GR) well-log data for cyclostratigarphy, and seismic data for sequence stratigraphy. Time-series analysis of GR data shows a rich series of Milankovitch frequency bands. In particular, long-period cyclicities (405 kyr, 2.4 Myr, 4.7 Myr and 9.5 Myr) are detected with high fidelity. Seismic and sequence stratigraphic interpretation shows a striking sea-level (SL) depositional sequence order, matching the 4.7 Myr orbital cyclicity inferred from cyclostratigraphy. Longer SL sequences interpreted in previous studies from the Limoeiro Fm closely match the 9.5 Myr GR related orbital cycles. Thus, we infer that the post-rift Limoeiro Fm was deposited continuously under astronomical forcing over the Late Cretaceous epoch, resulting in an extraordinary record of direct base- and sea-level responses to Milankovitch climatic forcing, including longer (multi-Myr) periodicities. The 4.7 Myr orbital component is recorded for the first time in SL sedimentary proxies, thus allowing here to update SL hierarchical orders. We suggest that third-order, and second-order and suborders SL sequences were most likely paced by long-period astronomical cycles (2.4 Myr eccentricity, and 4.7 and 9.5 Myr orbital cycles, respectively. • An integrated cyclo-seismic stratigraphy detects 2.4, 4.7 and 9.5 Myr orbital cycles. • 2.4, 4.7 and 9.5 Myr orbital cycles match third and second-order sea-level sequences. • The 4.7 Myr orbital cycle is highlighted for the first time in sea-level record.

On the potential of ensemble forecasting for the prediction of meteotsunamis in the Balearic Islands: sensitivity to atmospheric model parameterizations

This study investigates the potential of ensemble forecasting using full realistic high-resolution nested atmosphere–ocean models for the prediction of meteotsunamis in Ciutadella (Menorca, Spain). The sensitivity of model results to the parameterizations of the atmospheric model is assessed considering the ten most significant recent meteotsunami events for which observations are available. Different schemes adapted to high-resolution Weather Research and Forecasting model simulations were used for the representation of cumulus, microphysics, planetary boundary layer and longwave and shortwave radiations. Results indicate a large spread of the ensemble simulations in terms of the final magnitude of the meteotsunamis. While the modeling system is shown to be able to realistically trigger tsunamigenic atmospheric disturbances in more than 90% of the situations, the small-scale characteristics of these disturbances are significantly modified with the change of parameterizations, leading to significant differences in the magnitude of the simulated sea-level response. No preferred set of parameterizations can be identified that leads to either the largest or the most realistic magnitudes in the majority of situations. Instead, the performance of a given set of parameterizations is found to change with the meteotsunami event under consideration. Importantly, the observed magnitude of the extreme sea-level oscillations lies within the range of a nine-member ensemble in 70% of the cases. This ensemble approach would then allow to generate a realistic range of possibilities in the majority of events, thus improving the realism of meteotsunami predictions compared to single deterministic forecasts.

Sea-level responses to rapid sediment erosion and deposition in Taiwan

Numerous studies have shown that sediment deposition can perturb sea level by several meters over millennial timescales by modifying the gravity field, crustal elevation, and sediment thickness. Relatively few studies have focused on the complementary role of erosion on sea-level change despite its effects on the same quantities, partly because many rapidly eroding mountains are too far from shorelines to strongly perturb sea level at the coast. Taiwan, a mountainous island eroding rapidly within tens of km of the coast, offers an opportunity to investigate the joint influences of rapid onshore erosion and rapid offshore deposition on sea-level change. Here we develop a sediment loading history for Taiwan since the previous interglacial (∼120 ka) by compiling published erosion and deposition rate measurements and by applying a geometric marine sediment deposition and compaction model for sites without deposition rate measurements. We use the resulting sediment redistribution history to drive sea-level responses in a gravitationally self-consistent sea-level model. Our simulations show that the effects of rapid onshore erosion outweigh the effects of rapid offshore deposition along Taiwan's east coast. Along the east coast of Taiwan, sediment redistribution induces rapid sea-level fall, a response that differs in sign from the coastal sea-level rise induced by rapid sediment redistribution in many other river systems around the world. The spatial extent of the modeled sea-level fall is sensitive to the Earth model, particularly the effective elastic thickness of the lithosphere, a sensitivity that we describe in further detail in the Discussion. These results suggest that sediment redistribution could have generated sea-level changes of >10m on the east coast of Taiwan since 10 ka and >100m since 120 ka. This can account for some of the discrepancy between observed and modeled paleo-sea-level marker elevations, which reduces estimates of tectonically driven rock uplift rates inferred from the elevation differences between paleo-sea-level markers and modeled sea level. This highlights the importance of accounting for erosional unloading in interpretations of paleo-sea-level reconstructions and associated estimates of tectonically driven uplift rates.

Higher Sea Levels at Hawaii Caused by Strong El Niño and Weak Trade Winds

AbstractHawaii experienced record-high sea levels during 2017, which followed the 2015 strong El Niño and coincided with weak trade winds in the tropical northeastern Pacific. The record sea levels were associated with a combination of processes, an important contributing factor of which was the persistent high sea level (~10 cm above normal) over a large region stretching between Hawaii and Mexico. High sea levels at Mexico are known to occur during strong El Niño as the coastal thermocline deepens. Planetary wave theory predicts that these coastal anomalies propagate westward into the basin interior; however, high sea levels at Hawaii do not occur consistently following strong El Niño events. In particular, Hawaii sea levels remained near normal following the previous strong El Niño of 1997. The processes controlling whether Hawaii sea levels rise after El Niño have so far remained unknown. Atmosphere-forced ocean model experiments show that anomalous surface cooling, controlled by variable trade winds, impacts sea level via mixed layer density, explaining much of the difference in Hawaiian sea level response after the two recent strong El Niño events. In climate model projections with greenhouse warming, more frequent weak trade winds following El Niño events are expected, suggesting that the occurrence of high sea levels at Hawaii will increase as oceanic anomalies more often traverse the basin.

Coastal GIA processes revealed by the early to middle Holocene sea-level history of east China

In order to examine relative sea-level responses to the postglacial ice-volume change and the glacio-hydro isostatic adjustments (GIA), this study investigated the inner part of the Hangzhou Bay, east China, a tectonically relatively stable far-field location, and reconstructed the early to middle Holocene sea-level history. This investigation has established the elevational relationship between modern saltmarsh-mudflat and tidal levels based on diatom analysis for sea-level indicative meaning estimates, produced 17 high-quality sea-level index points, and simulated GIA processes for the study site. These results reveal that the relative sea level rose from −38.3 ± 1.6 m in c. 10,000 cal a BP to the present height by c. 7000 cal. a BP, and the average rate of sea-level rise decreased gradually from 19.6 ± 2.6 mm/a to 2.3 ± 1.5 mm/a during the 3000 years. This period of sea-level history was punctuated by two episodes of accelerated rise around 8200 and 7500 cal a BP. The relative sea level rose to 0.8 ± 1.4 m above msl by c. 6500 cal. a BP, followed by a gradual fall back to the present height at 4500 cal a BP, implying a different response to the potential additional ice melting between 7000 and 4000 cal a BP. A comparison of the sea-level histories between the inner and outer Hangzhou Bay indicates the coastal levering effect due to the marine inundation of the continental shelves. A further comparison between sea-level data from China and Malay Peninsula reveals different GIA effects between the Cathaysia-Yangtze Blocks and the Sundaland Block.

Self‐Consistent Ice Mass Balance and Regional Sea Level From Time‐Variable Gravity

AbstractMeasurements of time‐variable gravity from the Gravity Recovery and Climate Experiment (GRACE) and the GRACE Follow‐on (GRACE‐FO) missions are an invaluable tool for monitoring changes in the mass of the Earth's glaciated regions. We improve upon estimates of glacier and ice sheet mass balance from time‐variable gravity by including instantaneous spatiotemporal variations in sea level. Here, a least squares mascon technique is combined with solutions to the sea level equation to iteratively correct the GRACE/GRACE‐FO data for the induced sea level response on a monthly basis. We find that variations in regional sea level affect ice sheet mass balance estimates in Greenland by approximately 4% and in Antarctic by approximately 5%. Since 2002, the Greenland ice sheet has been losing mass at an average rate of 263 ± 23 Gt/yr, and the Antarctic ice sheet has been losing mass at average rates between 90 ± 52 and 122 ± 53 Gt/yr depending on the rate of glacial isostatic adjustment. The mass losses from both ice sheets represent an increase of 15.6 ± 2.0 mm to global mean sea levels since 2002.

Constraints on global mean sea level during Pliocene warmth.

Reconstructing the evolution of sea level during past warmer epochs such as the Pliocene provides insight into the response of sea level and ice sheets to prolonged warming1. Although estimates of the global mean sea level (GMSL) during this time do exist, they vary by several tens of metres2-4, hindering the assessment of past and future ice-sheet stability. Here we show that during the mid-Piacenzian Warm Period, which was on average two to three degrees Celsius warmer than the pre-industrial period5, the GMSL was about 16.2 metres higher than todayowingto global ice-volume changes, and around 17.4 metres when thermal expansion of the oceans is included. During the even warmer Pliocene Climatic Optimum (about four degrees Celsius warmer than pre-industrial levels)6, our results show that the GMSL was 23.5 metres above the present level,with an additional 1.6 metres from thermal expansion. We provide six GMSL data points, ranging from 4.39 to 3.27 million years ago, that are based on phreaticovergrowths on speleothems from the western Mediterranean (Mallorca, Spain). This record is unique owing to its clear relationship to sea level, its reliable U-Pb ages and its long timespan, which allows us to quantify uncertainties on potential uplift. Our data indicate that ice sheets are very sensitive to warming and provide important calibration targets for future ice-sheet models7.

Uncertainty in satellite estimates of global mean sea-level changes, trend and acceleration

Abstract. Satellite altimetry missions now provide more than 25 years of accurate, continuous and quasi-global measurements of sea level along the reference ground track of TOPEX/Poseidon. These measurements are used by different groups to build the Global Mean Sea Level (GMSL) record, an essential climate change indicator. Estimating a realistic uncertainty in the GMSL record is of crucial importance for climate studies, such as assessing precisely the current rate and acceleration of sea level, analysing the closure of the sea-level budget, understanding the causes of sea-level rise, detecting and attributing the response of sea level to anthropogenic activity, or calculating the Earth's energy imbalance. Previous authors have estimated the uncertainty in the GMSL trend over the period 1993–2014 by thoroughly analysing the error budget of the satellite altimeters and have shown that it amounts to ±0.5 mm yr−1 (90 % confidence level). In this study, we extend our previous results, providing a comprehensive description of the uncertainties in the satellite GMSL record. We analysed 25 years of satellite altimetry data and provided for the first time the error variance–covariance matrix for the GMSL record with a time resolution of 10 days. Three types of errors have been modelled (drifts, biases, noises) and combined together to derive a realistic estimate of the GMSL error variance–covariance matrix. From the latter, we derived a 90 % confidence envelope of the GMSL record on a 10 d basis. Then we used a least squared approach and the error variance–covariance matrix to assess the GMSL trend and acceleration uncertainties over any 5-year time periods and longer in between October 1992 and December 2017. Over 1993–2017, we have found a GMSL trend of 3.35±0.4 mm yr−1 within a 90 % confidence level (CL) and a GMSL acceleration of 0.12±0.07 mm yr−2 (90 % CL). This is in agreement (within error bars) with previous studies. The full GMSL error variance–covariance matrix is freely available online: https://doi.org/10.17882/58344 (Ablain et al., 2018).

Challenges and research priorities to understand interactions between climate, ice sheets and global mean sea level during past interglacials

Quaternary interglacials provide key observations of the Earth system's responses to orbital and greenhouse gas forcing. They also inform on the capabilities of Earth system models, used for projecting the polar ice-sheet and sea-level responses to a regional warmth comparable to that expected by 2100 C.E. However, a number of uncertainties remain regarding the processes and feedbacks linking climate, ice-sheet and sea-level changes during past warm intervals. Here, we delineate the major research questions that need to be resolved and future research directions that should be taken by the paleoclimate, sea-level and ice-sheet research communities in order to increase confidence in the use of past interglacial climate, ice-sheet and sea-level reconstructions to constrain future predictions. These questions were formulated during a joint workshop held by the PAGES-INQUA PALSEA (PALeo constraints on SEA level rise) and the PAGES-PMIP QUIGS (QUaternary InterGlacialS) Working Groups in September 2018.

Atmospheric circulation changes and their impact on extreme sea levels around Australia

Abstract. Projections of sea level rise (SLR) will lead to increasing coastal impacts during extreme sea level events globally; however, there is significant uncertainty around short-term coastal sea level variability and the attendant frequency and severity of extreme sea level events. In this study, we investigate drivers of coastal sea level variability (including extremes) around Australia by means of historical conditions as well as future changes under a high greenhouse gas emissions scenario (RCP 8.5). To do this, a multi-decade hindcast simulation is validated against tide gauge data. The role of tide–surge interaction is assessed and found to have negligible effects on storm surge characteristic heights over most of the coastline. For future projections, 20-year-long simulations are carried out over the time periods 1981–1999 and 2081–2099 using atmospheric forcing from four CMIP5 climate models. Changes in extreme sea levels are apparent, but there are large inter-model differences. On the southern mainland coast all models simulated a southward movement of the subtropical ridge which led to a small reduction in sea level extremes in the hydrodynamic simulations. Sea level changes over the Gulf of Carpentaria in the north are largest and positive during austral summer in two out of the four models. In these models, changes to the northwest monsoon appear to be the cause of the sea level response. These simulations highlight a sensitivity of this semi-enclosed gulf to changes in large-scale dynamics in this region and indicate that further assessment of the potential changes to the northwest monsoon in a larger multi-model ensemble should be investigated, together with the northwest monsoon's effect on extreme sea levels.

PALeo constraints on SEA level rise (PALSEA): Ice-sheet and sea-level responses to past climate warming

Here we summarize the motivation and issues surrounding the responses of ice sheets and sea level to past climate warming as part of the PALeo constraints on SEA level rise (PALSEA) working group. Papers in this special issue of Quaternary Science Reviews focus on the timescale of glaciations during the late Pliocene, the magnitude of ice-sheet fluctuations and volume leading up to and during the last glacial maximum, the timing and persistence of ice-sheet impacts on deglacial and future relative sea-level change, and relative sea-level change during peak interglacial climate. A more dynamic cryosphere is noted under both late Pliocene and last glacial cycle climate conditions, while relative sea-level changes during the last deglaciation appear to correspond closely with individual ice-sheet deglaciation. Lastly, relative sea-level change during peak interglacial conditions may have fluctuated by as much as a meter, although the sources of such variability (Greenland, Antarctica or elsewhere) remain elusive.

Bathymetric Influence on the Coastal Sea Level Response to Ocean Gyres at Western Boundaries

AbstractIt is our aim with this paper to investigate how the presence of a continental shelf and slope alters the relationship between interior ocean dynamics and western boundary (coastal) sea level. The assumption of a flat-bottomed basin with vertical sidewall at the coast is shown to hide the role that depth plays in the net force acting on the coast. A linear β-plane theory is then developed describing the transmission of sea level over variable depth bathymetry as analogous to the steady advection–diffusion of a thermal fluid. The parameter , relating the friction parameter r to the bathymetry depth H and width , is found to determine the contribution of interior sea level to coastal sea level, with small giving maximum penetration and large maximum insulation. In the small (infinite friction) limit the frictional boundary layer extends far offshore, and coastal sea level tends toward the vertical sidewall solution. Adding simple stratification produces exactly the same result but with reduced effective depth and hence enhanced penetration. Penetration can be further enhanced by permitting weakly nonlinear variations of thermocline depth. Wider and shallower shelves relative to the overall scales are also shown to maximize penetration for realistic values of . The theory implies that resolution of bathymetry and representation of friction can have a large impact on simulated coastal sea level, calling into question the ability of coarse-resolution models to accurately represent processes determining the dynamic coastal sea level.

The Tropical Indian Ocean decadal sea level response to the Pacific Decadal Oscillation forcing

The tropical Indian Ocean sea level displayed decadal variations in response to Pacific Decadal Oscillation (PDO). Contrasting patterns of decadal oscillation in sea level is found during the opposite phases of PDO especially in the thermocline ridge region of the Indian Ocean (TRIO; 50°E–80°E; 15°S–5°S). Epochal mean sea level rise is observed over the TRIO region during the cold phase of PDO (1958–1977), whereas epochal mean sea level fall is observed during the warm phase of PDO (1978–2002). Analysis reveals that the decadal variability in the sea level pattern in the TRIO region is in accordance with the PDO phase shifts and is primarily caused by changes in the surface forcing over the Indian Ocean as a response to PDO. The changes in the large scale Walker circulation over the tropical Indian Ocean region during the different phases of PDO support our hypothesis. The winds and wind stress curl variations associated with these large scale circulation changes are primarily inducing the observed regional decadal sea level variability over TRIO. The decadal forcing through Indonesian Through Flow (ITF; oceanic channel) however did not show any significant impact on the TRIO sea level variability. Further Ocean General Circulation Model (OGCM) sensitivity experiments are carried out to understand the mechanisms and the possible contribution of the Pacific Ocean through oceanic pathways, in the decadal variability of the TRIO sea level. It is noted that wave propagation from the Pacific Ocean to the Indian Ocean through ITF region has contributed to the sea level variations in the eastern Indian Ocean. But on the decadal time scale, tropical Indian Ocean/TRIO sea level is unaffected by decadal variability in the ITF. Moreover, the ITF contribution to the decadal sea level variability in the Indian Ocean is found to be significant only in the region south of 20°S.

Arctic Sea Level and Surface Circulation Response to the Arctic Oscillation

AbstractThe Arctic Oscillation (AO) is the leading mode of extratropical northern hemisphere atmospheric variability, affecting surface pressure, winds, temperature, and precipitation. Here we use an altimeter sea level record spanning 2003–2014, covering the ice‐covered and ice‐free ocean, to examine the influence of the AO on Arctic sea level and surface geostrophic circulation. AO‐driven alongshore wind anomalies drive cross‐shelf Ekman transport and opposing barotropic sea level anomalies between the shelf seas and deep basins of the Arctic Ocean, with maximum sea level anomaly differences across the shelf‐break of ~3 cm per unit AO index. This pattern of sea level variability generates topographically steered (generally along‐shelf) current anomalies of around 0.5 cm/s per unit AO index. AO‐driven wind variability modulates surface currents associated with Atlantic and Pacific water inflow, with opposing inflow anomalies between the Barents Sea Opening and Bering Strait.

Phasing of ice-sheet and sea-level responses to past climate change

Phasing of ice-sheet and sea-level responses to past climate change

Dynamic Ice Sheet and Sea Level Response to Past Climate Change

PALSEA2 Workshop; Playa del Carmen, Quintana Roo, Mexico, 6–9 November 2017

A dual model for emulation of thermosteric and dynamic sea-level change

Future thermosteric and dynamic sea-level changes are often projected by process-based climate models. Emulation of such computationally expensive models helps enable model intercomparison over a range of forcing scenarios and thus enables additional analysis of sea-level rise projection uncertainty. Current emulation methods use linear response functions to estimate global mean sea-level response. Here, we introduce a novel dual model to emulate global mean thermosteric sea-level rise that incorporates short- and long-term responses to climate forcing. This nonlinear response function outperforms existing linear response functions over six illustrative general circulation models and the four representative concentration pathways. To emulate dynamic sea-level projections, we introduce a linear pattern scaling model that relates regional sea-level changes to global mean thermosteric sea-level rise. Pattern scaling is shown to reproduce strongly forced sea-level trends. Our results demonstrate effective emulation of global and regional sea-level rise, which can facilitate the consideration of sea-level rise projection uncertainty critical to the analysis of sea-level rise hazard.

Meteotsunami (“Marrobbio”) of 25–26 June 2014 on the Southwestern Coast of Sicily, Italy

A major tsunami-like event, locally known as ‘marrobbio’, impacted the southwestern coast of Sicily on 25–26 June 2014. The event was part of a chain of hazardous episodes in the Mediterranean and Black seas during the last week of June 2014 resulting from an anomalous atmospheric system (“tumultuous atmosphere”) propagating eastward over the region. The synoptic patterns and vertical structure of the atmosphere over Sicily at the time of the event indicate that atmospheric wave ducting was responsible for the generation of tsunamigenic air pressure disturbances that produced especially high sea level responses (“meteotsunamis”) at certain sites along the Sicilian coast. The strongest sea level oscillations were observed at Mazara del Vallo, where a 1-m meteotsunami bore, propagating upstream in the Mazaro River, was generated. The combined effects of external resonance (Proudman resonance on the western Sicilian shelf) and internal resonant conditions (bathymetric and topographic characteristics of specific sites) were found to be the key factors that caused the meteotsunami (marrobbio phenomenon) on the coast of Sicily and the meteobore at Mazara del Vallo.

Awareness of sea-level response under climate change on the coast of Ghana

In response to climate change, coastal communities are expected to experience increasing coastal impacts of sea-level rise (SLR). Strategies formulated and implemented to curb these impacts can thus be more effective if scientific findings on the response to climate change and SLR impacts on coastal communities are taken into consideration and not based merely on the need for coastal protection due to physical coastal erosion. There is also the need to determine the level of awareness of sea-level rise and responses in coastal communities to improve adaptation planning. This study assesses the impact of future erosion on the coastal land cover of Ghana. This assessment estimates approximately 2.66 km2, 2.77 km2, and 3.24 km2 of coastal settlements, 2.10 km2, 2.20 km2 and 2.58 km2 of lagoons, 1.39 km2, 1.46 km2 and 1.71 km2 of wetlands to be at risk of inundation by the year 2050 based on coastal erosion estimates for the 2.6, 4.5 and 8.5 Representative Concentration Pathways (RCPs) used in the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC). This study also assesses the level of awareness of respondents to SLR on the coast of Ghana and explores the availability and level of integration of scientific knowledge of SLR into coastal adaptation strategies in Ghana. Assessment of the awareness of SLR responses to the changing climate in Ghana is made through semi-structured interviews at national, municipal/district and coastal community scales. Although settlements may be inundated based on the coastal erosion estimates, coastal dwellers interviewed cherish their proximity to the sea and are determined to maintain their occupancy close to the sea as spatial location influences their source of livelihood (fishing). Respondents lack knowledge/understanding of SLR, as the majority of household interviewees attributed the rise or fall in sea level to God. Respondents from Ngiresia alleged that the ongoing coastal sea defence project in their community has led to increased malaria cases.