Simulations of radiocarbon in a coarse‐resolution world ocean model: 1. Steady state prebomb distributions

Abstract

This paper presents the results of five numerical simulations of the radiocarbon distribution in the ocean using the Geophysical Fluid Dynamics Laboratory primitive equation world ocean general circulation model. The model has a 4.5° latitude by 3.75° longitude grid, 12 vertical levels, and realistic continental boundaries and bottom topography. The model is forced at the surface by observed, annually averaged temperatures, salinities, and wind stresses. There are no chemical transformations or transport of 14C by biological processes in the model. Each simulation in this paper has been run out the equivalent of several thousand years to simulate the natural, steady state distribution of 14C in the ocean. In a companion paper the final state of these simulations is used as the starting point for simulations of the ocean's transient uptake of bomb‐produced 14C. The model reproduces the mid‐depth 14C minimum observed in the North Pacific and the strong front near 45°S between old, deep Pacific waters and younger circumpolar waters. In the Atlantic, the model's deep 14C distribution is much too strongly layered with relatively old water from the Antarctic penetrating into the northern reaches of the North Atlantic basin. Two thirds of the decay of 14C between 35°S and 35°N is balanced by local 14C input from the atmosphere and downward transport by vertical mixing (both diffusion and advective stirring). Only one third is balanced by transport of 14C from high latitudes. A moderately small mixing coefficient of 0.3 cm2 s−1 adequately parameterizes vertical diffusion in the upper kilometer. Spatial variation in gas exchange rates is found to have a negligible effect on deepwater radiocarbon values. Ventilation of the circumpolar region is organized in the model as a deep overturning cell which penetrates as much as 3500 m below the surface. While allowing the circumpolar deep water to be relatively well ventilated, the overturning cell restricts the ventilation of the deep Pacific and Indian basins to the north. This study utilizes three different realizations of the ocean circulation. One is generated by a purely prognostic model, in which only surface temperatures and salinities are restored to observed values. Two are generated by a semidiagnostic model, in which interior temperatures and salinities are restored toward observed values with a 1/50 year−1 time constant. The prognostic version is found to produce a clearly superior deep circulation in spite of producing interior temperatures and salinities which deviate very noticeably from observed values. The weak restoring terms in the diagnostic model suppress convection and other vertical motions, causing major disruptions in the diagnostic model's deep sea ventilation.

Ocean general circulation from a global eddy‐resolving model

“…In such circumstances, the models are "eddy-resolving," and they should begin to reproduce the state of the ocean from first principles. An additional desirable feature in such models is the capability to perform extended simulations on climatic time scales, as has been done by Toggweiler et al [1989]. At present, the latter capability can only be attained by sacrificing grid resolution.…”

Anthropogenic CO2 distribution in the North Pacific Ocean

“…The agreement with the excess CO 2 results is rather good between Equator and 30°N. At higher latitudes the agreement is poor because the subsurface waters in the Subarctic and Antarctic regions have a pre-industrial ∆C-14 concentration below -110‰ (Toggweiler et al, 1989;Chen and Rodman, 1990). Figure 10 shows an isogram map of the depth at which ∆C-14 equals -110‰.…”

“…The preindustrial ∆C-14 concentration for low and mid-latitude waters in the Pacific Ocean is probably about -50‰, similar to the value in the Atlantic Ocean (Toggweiler et al, 1989). Figure 9 shows an isogram map of the depth where the ∆C-14 concentration equals to this value.…”

Evaluation and intercomparison of three-dimensional global marine carbon cycle models

“…Atmospheric A14C (the fractionation-corrected permil departure from a standard 14C/t2C ratio) is set to 0 permil and gas exchange is parameterized usin If the same formulation as for CO, In th model, a simplification is made by assuming that the C/12C ratio is conservative with respec ,mixmg b (Fiadiero, 1982, Toggweiler et al, 1989a) These simulations will be run until the deep ocean comes into equilibrium, which typically takes several thousand years Natural radiocarbon is used most effectively as a diagnostic of deep ocean circulation because of the limited number of observations made before the influence of nuclear bomb tests on the radiocarbon distribution 3 3.2 Bomb radiocarbon Beginning with the atmospheric nuclear weapons tests during the 195Os, the 14C content of the atmosphere and upper ocean increased sharply As shown in Figure 8, oceanographers have been able to use the bomb 14C signal in the ocean as a primary data set with which to evaluate ocean circulation models on decadal time scales. Here the observed surface A14C values from the eastern Pacific GEOSECS stations and the WOCE P17 line are compared with the global ocean circulation model of Toggweiler and Samuels (1995a,b) Interpretation of the bomb signal in surface waters for one time period is difficult if the natural signal is not removed (Broecker er al, 1985, Broecker et al, 1995 But taking the difference between two time periods easily isolates the bomb signal In doing so, we see that the model difference between 1973 and 1993 is much larger than the GEOSECS to WOCE difference, implying that the model is removing t4C from c-17 the surface ocean (presumably downward) much faster than is actually occurring This might suggest, for example, that this model would overestimate the uptake of anthropogenic CO, and the delivery of nutrients to the euphotic zone, particularly in the high latitudes.…”

“…We will simulate the natural radiocarbon distribution in our four models following OCMIP guidelines, which generally adopt the approach of Toggweiler et al (1989a). Atmospheric A14C (the fractionation-corrected permil departure from a standard 14C/t2C ratio) is set to 0 permil and gas exchange is parameterized usin If the same formulation as for CO, In th model, a simplification is made by assuming that the C/12C ratio is conservative with respec ,mixmg b (Fiadiero, 1982, Toggweiler et al, 1989a) These simulations will be run until the deep ocean comes into equilibrium, which typically takes several thousand years Natural radiocarbon is used most effectively as a diagnostic of deep ocean circulation because of the limited number of observations made before the influence of nuclear bomb tests on the radiocarbon distribution 3 3.2 Bomb radiocarbon Beginning with the atmospheric nuclear weapons tests during the 195Os, the 14C content of the atmosphere and upper ocean increased sharply As shown in Figure 8, oceanographers have been able to use the bomb 14C signal in the ocean as a primary data set with which to evaluate ocean circulation models on decadal time scales.…”