Assimilation of Radiocarbon and Chlorofluorocarbon Data to Constrain Deep and Bottom Water Transports in the World Ocean

Abstract

A coarse-resolution global model with time-invariant circulation is fitted to hydrographic and tracer data by means of the adjoint method. Radiocarbon and chlorofluorocarbon data are included to constrain deep and bottom water transport rates and spreading pathways as well as the strength of the global overturning circulation. It is shown that realistic global ocean distributions of hydrographic parameters and tracers can be obtained simultaneously. The model correctly reproduces the deep ocean radiocarbon field and the concentrations gradients between different basins. The spreading of CFC plumes in the deep and bottom waters is simulated in a realistic way, and the spatial extent as well as the temporal evolution of these plumes agrees well with observations. Radiocarbon and CFC observations place upper bounds on the northward transports of Antarctic Bottom Water (AABW) into the Pacific, Atlantic, and Indian Oceans. Long-term mean AABW transports larger than 5 Sv (Sv ϵ 10 6 m 3 s Ϫ1) through the Vema and Hunter Channels in the South Atlantic and net AABW transports across 30°S into the Indian Ocean larger than 10 Sv are found to be incompatible with CFC data. The rates of equatorward deep and bottom water transports from the North Atlantic and Southern Ocean are of similar magnitude (15.7 Sv at 50°N and 17.9 Sv at 50°S). Deep and bottom water formation in the Southern Ocean occurs at multiple sites around the Antarctic continent and is not confined to the Weddell Sea. A CFC forecast based on the assumption of unchanged abyssal transports shows that by 2030 the entire deep west Atlantic exhibits CFC-11 concentrations larger than 0.1 pmol kg Ϫ1, while most of the deep Indian and Pacific Oceans remain CFC free. By 2020 the predicted CFC concentrations in the deep western boundary current (DWBC) in the North Atlantic exceed surface water concentrations and the vertical CFC gradients start to reverse.

Molecular transformation and degradation of refractory dissolved organic matter in the Atlantic and Southern Ocean

“…The concepts of the degradation continuum (Flerus et al, 2012) and the microbial carbon pump (as inferred from the successive and repetitive processing of DOM by microbes; Jiao et al, 2010) imply that degradation of DOM can be described by a single function. Ocean models describing DOC production and decay also consider an exponential decay/removal of individual DOC fractions (Schlitzer, 2007;Hansell et al, 2012). Hence, we apply Eq.…”

The deep distributions of helium isotopes, radiocarbon, and noble gases along the U.S. GEOTRACES East Pacific Zonal Transect (GP16)

“…As was known from the earliest observations (Bien et al, 1960;Bien et al, 1965), the deep distribution of ∆ 14 C in the Pacific is dominated by a mid-depth (2000-3000 m) minimum. This minimum results from the incursion below 4000 m in the west of younger, higher ∆ 14 C waters ultimately derived from Antarctic Bottom and North Atlantic Deep Waters (Ostlund and Stuiver, 1980;Schlitzer, 2007). The intensity of this minimum is governed by a balance between cumulative decay along the global thermohaline overturning circulation (Kuhlbrodt et al, 2007;Schlitzer, 2007), distributed upwelling to intermediate depths (Munk,pg.…”

“…This minimum results from the incursion below 4000 m in the west of younger, higher ∆ 14 C waters ultimately derived from Antarctic Bottom and North Atlantic Deep Waters (Ostlund and Stuiver, 1980;Schlitzer, 2007). The intensity of this minimum is governed by a balance between cumulative decay along the global thermohaline overturning circulation (Kuhlbrodt et al, 2007;Schlitzer, 2007), distributed upwelling to intermediate depths (Munk,pg. 10 1966), in situ remineralization of relatively modern carbon from the rain of particulate material from above (Craig, 1969), and mediated by vertical mixing (Munk, 1966;Craig, 1969).…”

Distribution of helium-3 plumes and deep-sea circulation in the central Indian Ocean

“…Reid (2003) estimated an anti-clockwise flow in deep layers from physical water properties such as temperature, salinity and dissolved oxygen concentration. Schlitzer (2007) estimated a clockwise flow from hydrographic and tracer data such as radiocarbon and chlorofluorocarbons. Our result inferred from the δ 3 He distribution supports the clockwise pattern estimated by Schlitzer (2007).…”

“…Schlitzer (2007) estimated a clockwise flow from hydrographic and tracer data such as radiocarbon and chlorofluorocarbons. Our result inferred from the δ 3 He distribution supports the clockwise pattern estimated by Schlitzer (2007).…”