The deep ocean buoyancy budget and its temporal variability
Palter, JB and Griffies, SM and Samuels, BL and Galbraith, ED and Gnanadesikan, A and Klocker, A, The deep ocean buoyancy budget and its temporal variability, Journal of Climate, 27, (2) pp. 551-573. ISSN 0894-8755 (2014) [Refereed Article]
Despite slow rates of ocean mixing, observational and modeling studies suggest that buoyancy is redistributed to
all depths of the ocean on surprisingly short interannual to decadal time scales. The mechanisms responsible for this
redistribution remain poorly understood. This work uses an Earth system model to evaluate the global steady-state
ocean buoyancy (and related steric sea level) budget, its interannual variability, and its transient response to
a doubling of CO2 over 70 years, with a focus on the deep ocean. At steady state, the simple view of vertical
advective–diffusive balance for the deep ocean holds at low tomidlatitudes. At higher latitudes, the balance depends
on a myriad of additional terms, namely mesoscale and submesoscale advection, convection and overflows from
marginal seas, and terms related to the nonlinear equation of state. These high-latitude processes rapidly communicate
anomalies in surface buoyancy forcing to the deep ocean locally; the deep, high-latitude changes then influence
the large-scale advection of buoyancy to create transient deep buoyancy anomalies at lower latitudes.
Following a doubling of atmospheric carbon dioxide concentrations, the high-latitude buoyancy sinks are suppressed
by a slowdown in convection and reduced dense water formation. This change is accompanied by a slowing of both
upper and lower cells of the global meridional overturning circulation, reducing the supply of dense water to low
latitudes beneath the pycnocline and the commensurate flow of light waters to high latitudes above the pycnocline.
By this mechanism, changes in high-latitude buoyancy are communicated to the global deep ocean on relatively fast
advective time scales.