Analyzing the Effect of Ocean Internal Variability on Depth-Integrated Steric Sea-Level Rise Trends Using a Low-Resolution CESM Ensemble

Abstract

Abstract:Ocean heat uptake is a key indicator of climate change, in part because it contributes to sea-level rise. Quantifying the uncertainties surrounding ocean heat uptake and sea-level rise are important in assessing climate-related risks. Here, comprehensive global climate model ensembles are used to evaluate uncertainties surrounding decadal trends in depth-integrated global steric sea-level rise due to thermal expansion of the ocean. Results are presented against observational estimates, which are used as a guide to the state of recent literature. The first ensemble uses the Community Earth System Model (CESM), which samples the effects of internal variability within the coupled Earth system including contributions from the sub-surface ocean. We compare and contrast these results with an ensemble based on the Coupled Model Intercomparison Project Phase 5 (CMIP5), which samples the combined effects of structural model differences and internal variability. The effects of both internal variability and structural model differences contribute substantially to uncertainties in modeled steric sea-level trends for recent decades, and the magnitude of these effects varies with depth. The 95% range in total sea-level rise trends across the CESM ensemble is 0.151 mm·year −1 for 1957-2013, while this range is 0.895 mm·year −1 for CMIP5. These ranges increase during the more recent decade of 2005-2015 to 0.509 mm·year −1 and 1.096 mm·year −1 for CESM and CMIP5, respectively. The uncertainties are amplified for regional assessments, highlighting the importance of both internal variability and structural model differences when considering uncertainties surrounding modeled sea-level trends. Results can potentially provide useful constraints on estimations of global and regional sea-level variability, in particular for areas with few observations such as the deep ocean and Southern Hemisphere.

The Effect of Internal Variability on Ocean Temperature Adjustment in a Low‐Resolution CESM Initial Condition Ensemble

“…We present results from two climate change ensemble experiments using the low-resolution version of CESM (Shields et al, 2012), introduced by Sriver et al (2015), Hogan and Sriver (2017), and Vega-Westhoff and . Each ensemble uses the same preindustrial control simulation with constant preindustrial radiative forcing, which was spun up with a full-coupled atmosphere-ocean component, until the deep ocean had reached approximate dynamic equilibrium (by year 4200).…”

“…Each ensemble uses the same preindustrial control simulation with constant preindustrial radiative forcing, which was spun up with a full-coupled atmosphere-ocean component, until the deep ocean had reached approximate dynamic equilibrium (by year 4200). This method reduces the effect of model ocean drift from affecting our estimates of ocean variables including the deep ocean (Hogan & Sriver, 2017).…”

“…Since 1950, the ocean has absorbed 80 to 90% of the Earth's radiation imbalance due to anthropogenic forcing (Intergovernmental Panel on Climate Change, 2014; Levitus et al, 2000Levitus et al, , 2005Marshall & Zanna, 2014). This uptake has influenced the rate and surface pattern of atmospheric warming but also has important implications for the distribution of heat within the ocean (Garuba & Klinger, 2016) and sea level rise trends (Hogan & Sriver, 2017;Kuhlbrodt & Gregory, 2012), as thermal expansion is a proxy for temperature changes in the ocean. The ocean is able to affect climate through its high heat capacity relative to the surrounding land (Clark et al, 2002).…”

“…Climate models are often evaluated to provide information on changes in the deep ocean, and studies have found that ocean heat uptake contributes considerably to climate model spread (Bóe et al, 2009). Here we focus on quantifying the time scales of adjustment in the ocean using fully coupled, comprehensive model ensemble runs simulated by the Community Earth System Model (CESM; Sriver et al, 2015;Hogan & Sriver, 2017;Vega-Westhoff & Sriver, 2017;Haugen et al, 2018; -https://journals.ametsoc.org/doi/abs/ 10.1175/JCLI-D-17-0782.1). We evaluate ocean internal variability in temperature as a proxy for ocean adjustment and assess this variability on both global-and basin-scale spatial regions, and throughout different ocean depths.…”

The Role of Climate Sensitivity in Upper‐Tail Sea Level Rise Projections

“…For simplicity, we approximate the contributions from ocean thermal expansion and land water storage as spatially uniform, and we do not consider changes in dynamic sea surface height. The combined effect of these neglected contributions can be expected to produce first-order differences in local sea level in many areas of the globe (e.g., Hogan & Sriver, 2017;Slangen et al, 2014). Thus, the sea level change maps presented here are meant to capture large-scale behavior driven primarily by land ice mass redistribution, and are not meant as a substitute for local process-based modeling.…”