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The Sensitivity of Global-mean Steric Sea Level Rise to Interior Ocean Diapycnal Diffusivity
Robert Hallberg, Bonita Samuels, and Angelique Melet
NOAA/GFDL
(Abstract received 04/30/2015 for session X)
ABSTRACT
A recent comparison between two Earth System Models that differ only in their ocean components suggested that differences in the models’ effective diapycnal diffusivities could account for much of the difference of the projected 21st century steric sea level rise between the two models. In this paper, a series of perturbation studies are carried out with a coupled climate model to document the sensitivity of steric sea level rise to changing values of diapycnal diffusivity, both during the model’s spinup to equilibrium and during the idealized climate change experiment itself. Because the thermal expansion of seawater increases with warmer temperatures, steric sea level is significantly larger for a warmer ocean state. Increasing the diffusivity either during spinup or during the climate change run itself both increase steric sea-level rise. Increasing the diffusivity during the climate change run increases the perturbation heat uptake by the model and warms the ocean’s main thermocline, both of which enhance steric sea level rise. However, increasing the diffusivity during the model’s spin-up also increases steric sea level rise during the climate change rune, primarily due to the larger thermal expansion coefficients in a warmer ocean. These results strongly suggest that a coupled climate model’s ability to reproduce the ocean’s observed thermal structure during the 20th century is a valuable metric of its utility in projecting steric sea level rise.
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2015 LOM Workshop, Copenhagen, Denmark June 2nd - 4th, 2015