Processes explaining increased ocean dynamic sea level in the North Sea in CMIP6

Ocean dynamic sea level (ODSL) is expected to be one of the major contributors to sea level rise in the North Sea during the 21st century. This component is defined as the spatial sea level anomaly due to ocean currents, wind stresses and local thermosteric and halosteric effects. Climate models from CMIP5 and CMIP6 show a large spread, as well as an increase between CMIP5 and CMIP6 North Sea ODSL projections. In this study, we apply linear regression models on CMIP5 and CMIP6 data to get a better understanding of the processes that influence ODSL change in the North Sea. We find that neither global surface air temperature nor global mean thermosteric sea level can reproduce ODSL projections based on a linear relation in CMIP6, whereas this was the case for CMIP5. Including the strength of the Atlantic meridional overturning circulation (AMOC) as an additional predictor enables us to reproduce long-term changes in ODSL for both ensembles. The sensitivity to the AMOC increased in CMIP6, which points to a difference in model dynamics between CMIP5 and CMIP6, and a more important role of the deep ocean. To investigate this further, we analyse mixed layer depth data in the North Atlantic. We find that models with a relatively deep mixed layer in the Greenland Sea over the period 1985-2004, project larger rise in ODSL in the North Sea for both CMIP5 and CMIP6. This implies that the location of deep water formation in the North Atlantic potentially influences ODSL in the North Sea. The number of these models increased from CMIP5 to CMIP6, again pointing to a different sensitivity to larger scale processes, potentially explaining the difference between the two ensembles

Oceanic heat transport into the Arctic under high and low CO2 forcing

Enhanced ocean heat transport into the Arctic is linked to stronger future Arctic warming and polar amplification. To quantify the impact of ocean heat transport on Arctic climate, it is imperative to understand how its magnitude and the associated mechanisms change in other climate states. This paper therefore assesses the ocean heat transport into the Arctic at 70∘N for climates forced with a broad range of carbon dioxide concentration levels, ranging from one-fourth to four times modern values. We focused on ocean heat transports through the Arctic entrances (Bering Strait, Canadian Archipelago, and Nordic Seas) and identified relative contributions of volume and temperature to these changes. The results show that ocean heat transport differences across the five climate states are dominated by heat transport changes in the Nordic Seas, although in the warmest climate state heat transport through the Bering Strait plays an almost equally important role. This is primarily caused by changes in horizontal currents owing to anomalous wind responses and to differential advection of thermal anomalies. Changes in sea ice cover play a prominent role by modulating the surface heat fluxes and the impact of wind stresses on ocean currents. The Atlantic meridional overturning circulation and its associated heat transport play a more modest role in the ocean heat transport into the Arctic. The net effect of these changes is that the poleward ocean heat transport at 70∘N strongly increases from the coldest climate to the warmest climate state

Impact of slowdown of Atlantic overturning circulation on heat and freshwater transports

Recent measurements of the strength of the Atlantic overturning circulation at 26°N show a 1 year drop and partial recovery amid a gradual weakening. To examine the extent and impact of the slowdown on basin wide heat and freshwater transports for 2004–2012, a box model that assimilates hydrographic and satellite observations is used to estimate heat transport and freshwater convergence as residuals of the heat and freshwater budgets. Using an independent transport estimate, convergences are converted to transports, which show a high level of spatial coherence. The similarity between Atlantic heat transport and the Agulhas Leakage suggests that it is the source of the surface heat transport anomalies. The freshwater budget in the North Atlantic is dominated by a decrease in freshwater flux. The increasing salinity during the slowdown supports modeling studies that show that heat, not freshwater, drives trends in the overturning circulation in a warming climate

Impact of slowdown of Atlantic overturning circulation on heat and freshwater transports

Recent measurements of the strength of the Atlantic overturning circulation at 26°N show a 1 year drop and partial recovery amid a gradual weakening. To examine the extent and impact of the slowdown on basin wide heat and freshwater transports for 2004–2012, a box model that assimilates hydrographic and satellite observations is used to estimate heat transport and freshwater convergence as residuals of the heat and freshwater budgets. Using an independent transport estimate, convergences are converted to transports, which show a high level of spatial coherence. The similarity between Atlantic heat transport and the Agulhas Leakage suggests that it is the source of the surface heat transport anomalies. The freshwater budget in the North Atlantic is dominated by a decrease in freshwater flux. The increasing salinity during the slowdown supports modeling studies that show that heat, not freshwater, drives trends in the overturning circulation in a warming climate

Calibration of basal melt on past ice discharge lowers projections of Antarctica’s sea level contribution

Abstract. Antarctic mass loss is the largest contributor to uncertainties in sea level projections on centennial timescales. In this study the contribution of Antarctica’s ice discharge to future sea level changes is computed with ocean thermal forcing from 14 earth system models and linear response functions from 16 ice sheet models for three greenhouse gas emission scenarios. Different than in previous studies, basal melt was calibrated on observed Antarctic ice discharge rather than on basal melt itself with an iterative approach. For each model combination, a linear and quadratic melt dependency were calibrated both regionally (in five Antarctic sectors) and at the continental scale. Projections using all model combinations show that the variation in basal melt computation methods affect the projected sea level more than the scenario variations (SSP1-2.6 to SSP5-8.5). After calibration, a high number of model pairs still underestimated ice discharge in hindcasts over 1979–2017. Therefore top 10 % best-performing model combinations were selected for each method. A comparison between these model selections shows that the quadratic melt parameterisation with Antarctic-wide calibration performs best in reproducing past ice discharge. We conclude that calibration of basal melt on past ice discharge combined with model selection makes projections of Antarctic ice discharge (more) consistent with observations over the past four decades. Moreover, calibration of basal melt on past ice discharge results in lower basal melt sensitivities and thus lower projections of Antarctica’s sea level contribution than estimates of previous multi-model studies

Robust estimates for the decadal evolution of Agulhas leakage from the 1960s to the 2010s

Agulhas leakage, the transport of warm and salty waters from the Indian Ocean into the South Atlantic, has been suggested to increase under anthropogenic climate change, due to strengthening Southern Hemisphere westerly winds. The resulting enhanced salt transport into the South Atlantic may counteract the projected weakening of the Atlantic overturning circulation through warming and ice melting. Here we combine existing and new observation- and model-based Agulhas leakage estimates to robustly quantify its decadal evolution since the 1960s. We find that Agulhas leakage very likely increased between the mid-1960s and mid-1980s, in agreement with strengthening winds. Our models further suggest that increased leakage was related to enhanced transport outside eddies and coincided with strengthened Atlantic overturning circulation. Yet, it appears unlikely that Agulhas leakage substantially increased since the 1990s, despite continuously strengthening winds. Our results stress the need to better understand decadal leakage variability to detect and predict anthropogenic trends

The acceleration of sea-level rise along the coast of the Netherlands started in the 1960s

The global acceleration of sea-level rise (SLR) during the 20th century is now established. On the local scale, this is harder to establish as several drivers of SLR play a role, which can mask the acceleration. Here, we study the rate of SLR along the coast of the Netherlands from the average of six tide gauge records covering the period 1890-2021. To isolate the effects of the wind field variations and the nodal tide from the local sea-level trend, we use four generalised additive models (GAMs) which include different predictive variables. From the sea-level trend estimates, we obtain the continuous evolution of the rate of SLR and its uncertainty over the observational period. The standard error in the estimation of the rate of SLR is reduced when we account for nodal-tide effects and is reduced further when we also account for the wind effects, meaning these provide better estimates of the rate of SLR. A part of the long-term SLR is due to wind forcing related to a strengthening and northward shift of the jet stream, but this SLR contribution decelerated over the observational period. Additionally, we detect wind-forced sea-level variability on multidecadal timescales with an amplitude of around 1 cm. Using a coherence analysis, we identify both the North Atlantic Oscillation and the Atlantic Multidecadal Variability as its drivers. Crucially, accounting for the nodal-tide and wind effects changes the estimated rate of SLR, unmasking an SLR acceleration that started in the 1960s. Our best-fitting GAM, which accounts for nodal and wind effects, yields a rate of SLR of about 1.72.21.3 mm yr-1 in 1900-1919 and 1.51.91.2 mm yr-1 in 1940-1959 compared to 2.93.52.4 mm yr-1 in 2000-2019 (where the lower and upper bounds denote the 5th and 95th percentiles). If we discount the nodal tide, wind and fluctuation effects and assume a constant rate of SLR, then the probability (p value) of finding a rate difference between 1940-1959 and 2000-2019 of at least our estimate is smaller than 1 %. Consistent with global observations and the expectations based on the physics of global warming, our results show unequivocally that SLR along the Dutch coast has accelerated since the 1960s

Supporting data: "Uncertainty in sea level rise projections due to the dependence between contributors"

<p>These files contain the data analyzed in Le Bars 2018. The paper is available on EarthArXiv (https://eartharxiv.org/uvw3s/) and was submitted to Earth's Future.</p> <p>The NetCDF files contain the Probability Density Functions output from the Probabilistic Sea Level Projection (PSLP) model version 1.</p> <p>Simulations are:<br> IPCC1: The control IPCC AR5 simulation<br> IPCC2: The same but assuming independence between sea level contributors<br> IPCC3: The same but assuming correlation of 1 between sea level contributors<br> Prob1: The control simulation from the probabilistic model<br> Prob2: Assuming independence<br> Prob3: Assuming correlation of 1 between sea level contributors<br> Prob4: Low dependence case<br> Prob5: High dependence case<br> Prob6 to Prob9: Sensitivity experiments replacing each contributor by its expected value.</p> <p>The matrices of Spearman correlation for year 2100 for all experiments are called: <br> SpearmanCorr_namelist*_*.txt</p> <p>The Table*.txt files contain the data used to make tables of sea level percentiles in the paper.</p> <p>The pdf files contain the figures used in the paper and additional pannels not included in the paper.</p> <p>Reference:<br> Le Bars, D. (2018, March 8). Uncertainty in sea level rise projections due to the dependence between contributors. http://doi.org/10.17605/OSF.IO/UVW3S</p