Steric Sea Level in the Los Alamos POP Code—Non-Boussinesq Effects

Abstract

ABSTRACT The recent availability of accurate global ocean surface elevation data from satellites permits intercomparison with model results, and therefore raises the question of the accuracy of the computed sea level in ocean models. Most models, even those that incorporate a free surface, make use of the Boussinesq approximation—that is, they assume an incompressible fluid and conserve volume rather than mass. This results in an error in predicting steric sea level changes. The Los Alamos POP model, a free-surface Bryan-Cox type of ocean model developed for parallel computers, is in this category. We generalize the POP (Parallel Ocean Program) model to account for non-zero dilatation (non-zero velocity divergence—the primary non-Boussinesq effect) due to a variety of causes, such as expansion due to thermal and salinity mixing, as well as compressibility. We then compute both local and global changes of the sea level due to these effects and conclude that overall global dynamic effects are entirely negligible. There are some small local effects such as a very small increase in the Antarctic Circumpolar Current transport, for example. We also observe a global mean sea level decrease at a rate on the order of a half-metre per Century. While in this case this is most likely due to the ocean relaxing toward equilibrium (the computation is not initiated from a state of equilibrium), this does indicate that mean ocean expansion/contraction may be Significant from the point of view of climate and ought to be included in models.