Improved estimates of global ocean circulation, heat transport and mixing from hydrographic data

Abstract

Through its ability to transport large amounts of heat, fresh water and nutrients, the ocean is an essential regulator of climate. The pathways and mechanisms of this transport and its stability are critical issues in understanding the present state of climate and the possibilities of future changes. Recently, global high-quality hydrographic data have been gathered in the World Ocean Circulation Experiment (WOCE), to obtain an accurate picture of the present circulation. Here we combine the new data from high-resolution trans-oceanic sections and current meters with climatological wind fields, biogeochemical balances and improved a priori error estimates in an inverse model, to improve estimates of the global circulation and heat fluxes. Our solution resolves globally vertical mixing across surfaces of equal density, with coefficients in the range (3-12) x 10(-4) m2 s(-1). Net deep-water production rates amount to (15 +/- 12) x 10(6) m3 s(-1) in the North Atlantic Ocean and (21 +/- 6) x 10(6) m3 s(-1) in the Southern Ocean. Our estimates provide a new reference state for future climate studies with rigorous estimates of the uncertainties.

High mixing rates in the abyssal Southern Ocean

“…An order of magnitude of 10 -4 m 2 s -1 for the globally averaged value of K has been obtained by estimating that approximately 25 Sv (1 Sv 10 6 m 3 s 21 ) of cold, dense abyssal water forms in polar regions, and by assuming for simplicity that it upwells uniformly elsewhere into the thermocline 1,2 . More sophisticated versions of this calculation using inverse techniques and continent-to-continent hydrographic sections reveal basin-averaged diffusivities of similar magnitude, ranging from 3±4 3 10 24 m 2 s 21 for mid-depths to 9±12 3 10 24 m 2 s 21 for bottom layers 19 . Yet direct measurements suggest that K in most regions of the world ocean is an order of magnitude less than this 4 .…”

Measuring surface currents with Surface Velocity Program drifters: the instrument, its data, and some recent results

“…This decomposition was performed using a GaussMarkov technique familiar in the oceanographic literature for its use in box inverse models (e.g.. Ganachaud and Wunsch, 2000). This approach can resolve the amplitudes and phases of seasonal fluctuations throughout most of the tropical Atlantic basin with the present density of SVP drifter observations (Lumpkin and Garraffo, 2005).…”

Using a CFC effective age to estimate propagation and storage of climate anomalies in the deep western North Atlantic Ocean

“…[4] About 17 Sv of cold, dense North Atlantic Deep Water (NADW) (including ULSW) has been transported in the lower limb of the thermohaline circulation (THC) during the last several decades [e.g., Ganachaud and Wunsch, 2000;Smethie and Fine, 2001]. Included in the THC transport is LSW formation, which is highly variable [e.g., Marsh, 2000;Rhein et al, 2002].…”