GFDL - Geophysical Fluid Dynamics Laboratory

Physical processes that impact the evolution of global mean sea level in ocean climate models

June 8th, 2012

S.M. Griffies and R.J. Greatbatch. Journal: Ocean Modelling. DOI: 10.1016/j.ocemod.2012.04.003

Summary

Global mean sea level reflects the combined effects of the total mass of seawater and its global mean density. Adding to the net seawater mass, as from melting land ice, raises sea level. Likewise, lowering seawater density, as when water is heated, raises sea level through so-called steric effects.

How much mass is added to or subtracted from the ocean largely involves issues related to land ice sheets and mountain glaciers. How ocean density is modified involves issues fundamental to physical oceanography, such as how heat (and to a lesser degree, salt) is transported across the ocean surface and within the ocean interior. This study focused on changes in density.

The net impact of heat on seawater density, and thus on sea level, is measured by the thermal expansion of seawater. The ability of seawater to expand is itself a function of temperature, salinity, and pressure. The thermal expansion of seawater varies by roughly a factor of ten across regions of the global ocean, thus strongly impacting the importance of ocean heating on changes to sea level. Furthermore, as heat crosses the ocean surface and moves through the ocean interior, it may move from regions where seawater more readily expands (i.e., regions of large thermal expansivity) to regions where it is less expansive.

Heat in the ocean tends to move from the tropics to the poles, in response to the net positive heating in the lower latitudes and cooling in the higher latitudes. Heat also moves from the warmer surface waters to abyss. This tendency for heat transfer corresponds to a movement of heat from regions of large thermal expansion to small thermal expansion. Consequently, surface ocean heating in the tropics, which raises sea level, is compensated by a drop in sea level as heat is transported both poleward and into the abyss.

Processes affecting this heat transport are associated with lateral eddy heat transport, ocean mixing, and cabbeling and thermobaricity. In effect, those physical processes in the ocean interior which affect heat transport create a net reduction in global mean sea level, and the net impact is to roughly balance the sea level rise from surface heating.

If the ocean were stagnant, with no turbulent mixing or eddy transport, sea level would rise much more through tropical heating than actually occurs in the real ocean, where mixing and lateral stirring processes move heat to regions of smaller thermal expansion.

This balanced situation occurs in an idealized state where all terms that may alter global mean sea level sum to zero. Global warming, in particular, changes this zero sum balance, thus causing a net sea level rise due to increases in surface warming. How the ocean interior physical processes respond to changes in surface warming remains a question for further research. The theoretical framework established in this study provides a conceptual and quantitative basis for such studies.

Figure 1: Impact on global mean sea level from surface heating is shown here based on a long time average from a global ocean model forced with an atmospheric data product. This map includes heating from shortwave, longwave, sensible, and latent heat effects. Note the large positive values in the tropics, especially in the Pacific, indicating how heating increases global mean sea level. Large negative values occur in the western boundary currents of the Atlantic and Pacific, as well as the Agulhas region of South Africa, which are regions where warm waters moving poleward are cooled by strong atmospheric forcing, thus causing heat loss from the ocean, which in turn reduces global mean sea level.
Figure 1: Impact on global mean sea level from surface heating is shown here based on a long time average from a global ocean model forced with an atmospheric data product. This map includes heating from shortwave, longwave, sensible, and latent heat effects. Note the large positive values in the tropics, especially in the Pacific, indicating how heating increases global mean sea level. Large negative values occur in the western boundary currents of the Atlantic and Pacific, as well as the Agulhas region of South Africa, which are regions where warm waters moving poleward are cooled by strong atmospheric forcing, thus causing heat loss from the ocean, which in turn reduces global mean sea level.
Figure 2: This figure shows the impact on global mean sea level from the poleward transport of heat induced by a parameterization of mesoscale eddies. Note the larger values in the strong boundary current regions as well as in the Southern Ocean. Such regions are where eddy activity is strongest. The predominance of negative values indicates that the eddies act to reduce global mean sea level, as a result of moving heat from regions of large thermal expansion to smaller.
Figure 2: This figure shows the impact on global mean sea level from the poleward transport of heat induced by a parameterization of mesoscale eddies. Note the larger values in the strong boundary current regions as well as in the Southern Ocean. Such regions are where eddy activity is strongest. The predominance of negative values indicates that the eddies act to reduce global mean sea level, as a result of moving heat from regions of large thermal expansion to smaller.