Slab-ocean model
The slab-ocean model is a simplified, computer-generated representation of the ocean used in climate simulations where the ocean is depicted as a stationary layer of water, typically 50 to 100 meters deep. This model, sometimes called a "simple mixed-layer model," calculates sea surface temperatures based on surface heat interactions without accounting for ocean currents or vertical movements. While it can simulate seasonal changes in upper-ocean heat storage, it cannot accurately represent dynamic ocean processes, such as currents or upwelling, which are critical for understanding real ocean behavior.
In climate research, slab-ocean models serve to estimate how the climate system responds to various forcing factors, such as increased atmospheric carbon dioxide or melting ice. Their design allows for quicker simulations and adjustments to reach equilibrium, typically within 20 to 30 model years, which contrasts with more complex models that take centuries to stabilize. However, because they omit significant ocean dynamics, results may show unrealistic sea surface temperature distributions, particularly in regions influenced by strong currents or upwelling. Despite their limitations, slab-ocean models are valuable for assessing global climate sensitivity and understanding the interaction between ocean temperatures and atmospheric changes, especially when coupled with atmospheric models and other environmental factors.
Slab-ocean model
Definition
A slab-ocean model is a simple, nondynamic, computer-generated ocean model used in coupled simulations between ocean and atmosphere where the ocean is represented as a motionless layer of water with a depth of 50-100 meters. “Slab” refers to the layer of nominal thickness of water chosen to represent simple upper-ocean seasonal heat capacity. In slab models, sea surface temperatures (SSTs) are computed from surface and heat storage, appearing as a fixed-depth, homogeneous, mixed layer of water with no ocean currents. Because of their general nature, slab-ocean models are also referred to as “simple mixed-layer models” or “bulk models.” Standardized equations are used to define the momentum, temperature, and of the model’s mixed layer, implying that these parameters are constant over the depth of the model’s mixed layer.
When run coupled with atmospheric data, a slab-ocean model can simulate seasonal cycles of upper-ocean heat storage, permitting a simplified understanding of the seasonal cycle and its effects on climate sensitivity: Shifts in climate are reflected as changes to the seasonal heat capacity of the upper ocean. Changing input values can cause the defined slab-ocean layer to act as a heat source or sink for a larger model, and to some extent it can simulate the effect of moving currents. However, there is no true current or upwelling (vertical motion) in a slab-ocean model, so it can only simulate the climate effects of a steady ocean current. As a result, a slab-ocean model cannot simulate a dynamic change in an ocean current, upwelling, or overturning of circulation due to atmospheric climatic changes. Since ocean currents are a major portion of real ocean morphology, and they are not represented in slab-ocean models, the models cannot simulate all aspects of SST distributions.
Ocean currents work to move heat horizontally from the tropics poleward, and excluding currents from slab models tends to produce simulations in which SSTs are higher in modeled tropical environments and colder in modeled high latitudes than they are in actuality. Slab models also exclude the contributions of upwelling, which produces temperature changes in upper ocean layers. As a result, areas rich in upwelling regimes require input corrections.
In an effort to correct such temperature errors in slab-ocean models, simulations often include a heat value called the Q-flux. The heat flux correction simulates a source of heat transport producing effects similar to those generated by real ocean currents, but, when running slab-ocean models, the same Q-flux corrections must be run in both the control and forced simulations to maintain experimental integrity. Changes to upper-ocean temperatures due to increased atmospheric temperatures, melting ice, or increased radiation can be simulated simplistically and run to with Q-flux corrections.
Equilibrium for a slab-ocean model is the point at which the atmosphere and the ocean reach a balanced state, such that the climate variables being measured do not drift toward another state. Taken as a representation of thermodynamic equilibrium, slab models may reflect changes to the climate system, assuming the observed ocean heat transport is the same in both controlled and forced experiments. Forcing parameters in slab-ocean models include energetic effects of surface fluxes, such as solar radiation; work from waves; buoyancy dissipation; and frictional effects of wind.
Significance for Climate Change
In climate studies, slab-ocean models are used to estimate the equilibrium response of climate to a given forcing, not the transient evolution of climate. Also, as the model’s layer is considered motionless, the model cannot reflect ocean circulation changes resulting from environmental changes. In spite of their simplistic nature and problems with SST errors due to known ocean currents and upwelling, slab-ocean models are useful in researching global climate sensitivity and have helped establish data for the assessment of global response to such forcing mechanisms as increased atmospheric carbon dioxide and melting ice. In practice, their sensitivity to upper-ocean-layer temperature changes causes slab-ocean models to be affected by data reflecting changes in and glacial melt waters.
The Earth’s oceans and atmosphere have different response times to changes: ocean depths may take centuries to react to a change in forcing, whereas the atmosphere may respond in days. Because of their limited thickness parameters, slab models do not take a long time to adjust to forcings applied to the model. Where complex models reflecting the slow response time of deep-ocean circulation may require changes to develop over hundreds or thousands of model years to reach equilibrium, slab models reach equilibrium in twenty to thirty model years for a 50-meter-thick slab. As a result of a slab model’s simplified parameters, researchers can get reasonable results for the overall effect of a forcing as a percent increase or decrease using less computing time than would be consumed by a more complex model.
Coupling slab-ocean models with atmospheric models allows for a quick comparison of differences in the timescales over which ocean and atmospheric phenomena form and react to changes. When coupled ocean and atmospheric models are further coupled with additional models, such as carbon or nitrogen models, they produce increasingly accurate simulations of the Earth’s climate. These coupled models, however, retain all errors introduced in each individual simulation, and this fact imposes an important constraint on the models’ collective findings. Continued advancements in slab ocean models allowed scientists to accurately study the impact of ocean heat transplant.
Bibliography
Griffies, S. Fundamentals of Ocean Climate Models. Princeton, N.J.: Princeton University Press, 2004.
Kiehl, J. T., and V. Ramanathan, eds. Frontiers of Climate Modeling. New York: Cambridge University Press, 2006.
"Slab Ocean Model Shows Benefits Assessing Ocean Temperature Changes." Pacific Northwest National Laboratory, 30 July 2024, www.pnnl.gov/publications/slab-ocean-model-shows-benefits-assessing-ocean-temperature-changes. Accessed 21 Dec. 2024.
Siedler, G., J. Church, and J. Gould, eds. Ocean Circulation and Climate: Observing and Modeling the Global Ocean. New York: Academic Press, 2001.
Toba, Y., ed. Ocean-Atmosphere Interactions. Tokyo: Terra Scientific, 2003.