The Science

The Slab Ocean Model (SOM) is an approximation of a fully Dynamic Ocean Model (DOM). Unlike a DOM that can simulate changes in ocean circulation, transports, and full-depth temperature and salinity, the SOM is designed to capture temperature changes in the well-mixed ocean surface layer and its interaction with the dynamic atmosphere and sea ice components, while ocean heat transports prescribed are fixed at a climatological value. SOM has been widely used in climate modeling studies due to its fast equilibration time (e.g., two decades compared to hundreds of years in the DOM) and its ability to accurately simulate equilibrium climate and global surface temperature response to CO₂ doubling, compared to DOM in the same host model. Scientists from Pacific Northwest National Laboratory led the implementation, evaluation, and application of the SOM in the Energy Exascale Earth System Model version 2 (E3SMv2). The newly developed E3SMv2-SOM reproduced key aspects of the equilibrium baseline climate (surface temperature, precipitation, sea ice volume and extent) of the fully coupled E3SMv2 simulations using the DOM.

The Impact

The SOM capability is developed in E3SMv2 and applied to study the model sensitivity to low and robust ocean heat transport (OHT) strengths using a set of E3SMv2-SOM simulations with different prescribed OHT values. Results show that the E3SMv2 model has a large sensitivity to OHT changes due to its strong response in marine low-level clouds to sea surface temperature changes in the Southern Ocean, also known as the Antarctic Ocean. The SOM implementation adds a new capability to the E3SM hierarchical modeling framework, especially for climate sensitivity studies with different ocean heat transport perturbations and climate-forcing changes.

Summary

SOM can simulate the evolution of the ocean surface temperature and upper ocean heat content reasonably well, at a fraction of the cost of a climate model with explicit ocean dynamics. It saves computational cost because of the ability of the model to reach equilibrium in a fast time (a scale of 20 simulated years), compared to a significantly slower time (a scale of hundreds of simulated years) that is needed for a full ocean model to equilibrate. SOM is thus a useful tool for evaluating a model’s climate sensitivity (i.e., global average surface air temperature reached after all the effects of CO₂ doubling on the climate have found an equilibrium) at much cheaper computation cost, and for hierarchical climate model studies. This study describes the implementation and evaluation of SOM in the E3SMv2. The E3SMv2-SOM is evaluated by comparing its climate simulation to the model’s full version. SOM reproduces the baseline unperturbed climate of the full E3SMv2, as well as its climate sensitivity. SOM is further used to test the sensitivity of the baseline climate simulated by the E3SM model to ocean heat transport strengths. The results show that E3SMv2 has a large surface temperature sensitivity to ocean heat transport changes, particularly over the Southern Ocean. This large sensitivity occurs due to responses in marine low-level clouds, which cause changes in shortwave radiation reaching the surface and enhance the surface temperature changes. Atmosphere heat transport also responds to and compensates for ocean heat transport changes and, as a result of the large temperature response in the Southern Ocean, this compensation is greater there. The E3SMv2-SOM framework also shows that increases in ocean heat transport reduce the equilibrium surface temperature response to CO₂ doubling.

PNNL Contacts

Hailong Wang, Pacific Northwest National Laboratory, hailong.wang@pnnl.gov

Ruby Leung, Pacific Northwest National Laboratory, ruby.leung@pnnl.gov

Oluwayemi Garuba, Pacific Northwest National Laboratory, oluwayemi.garuba@pnnl.gov

Funding

This research has been supported by the Department of Energy (DOE) Office of Science Regional and Global Model Analysis program area as part of the High Latitude Application and Testing of Earth System Models project and the Water Cycle and Climate Extremes Modeling project. The research computing resources of the National Energy Research Scientific Computing Center (NERSC), DOE Office of Science User Facility operated under Contract No. DE-AC02-05CH11231.