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  • Author or Editor: Scott C. Doney x
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Scott C. Doney
and
William J. Jenkins

Abstract

The tritium and excess 3He data from the 1981 TTO/NAS program are used to study the time scales for the ventilation of the deep western basin by recently formed North Atlantic Deep Water (NADW). The large-scale distributions of tritium and 3He in the deep North Atlantic are presented, and tracer inventories are computed for individual deep water basins. The bulk of the bomb tritium (and thus new NADW) resided in 1981 in the deep Labrador Sea and western subpolar gyre, with a slightly smaller amount in the deep western subtropical gyre. The maximum excess 3He values were located south of the overflows in the Labrador Sea the result of competition between ventilation and in situ tritium decay. The subpolar gyre was also the site of the strongest increase in decay-corrected tritium (∼120%) and excess 3He (∼100%) between the 1972 GEOSECS survey and the 1981 TTO/NAS program. The observed deep water tritium inventory is in reasonable agreement with model tracer inputs computed for the combined overflows from the Greenland/Norwegian Seas.

Elevated tritium and anomalous 3He values are found in the deep western boundary current (DWBC) along the entire North American coast. The cross-stream and alongstream structure of the transient tracer distributions in the DWBC is examined using high-resolution, midlatitude sections and a composite boundary current section from the overflows to the tropics. The observed evolution of tritium and excess 3He along the DWBC are used, along with the large-scale tracer distributions, to constrain a conceptual ventilation model for the deep western basin. The model results highlight the important role of turbulent mixing and/or recirculation between the DWBC and the interior and suggest that on average the water in the boundary current is exchanged with the interior every 2500–3500 km. The net effect of the large recirculation between the boundary current and the interior is twofold: rapid O(10–15 years) ventilation of the deep Labrador Sea and western subpolar gyre by newly formed NADW and reduction in the southward spreading rate of NADW to about 0.75–1.5 cm s−1, a factor of 5–10 smaller than observed DWBC velocities. The results have important implications for understanding the response of the deep North Atlantic to climatic variability on decadal time scales and for the invasion of anthropogenic pollutants (e.g., CO2) into the deep ocean.

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Scott C. Doney
and
Matthew W. Hecht

Abstract

The ocean distributions of chlorofluorocarbons (CFCs) have been measured extensively in order to determine the mechanisms, rates, and pathways associated with thermohaline deep-water formation. Model temperature, salinity, and CFC-11 fields from the National Center for Atmospheric Research (NCAR) global ocean climate model are compared against observations with emphasis on the patterns of Antarctic Bottom Water (AABW) production, properties, and circulation in the Southern Ocean. The model control simulation forms deep water as observed in both the Weddell and Ross Seas, though not along other sectors of the Antarctic coast. Examination of the deep water CFC-11 distribution, total inventory, and profiles along individual observational sections demonstrates that the decadal-scale deep-water ventilation in the model Southern Ocean is both too weak and too restricted to the Ross and Weddell Sea source regions. A series of sensitivity experiments is conducted to determine the factors contributing to these deficiencies. The incorporation of a simple bottom boundary layer (BBL) scheme leads to only minor reductions in overall model–data error. The limited impact of the BBL may reflect in part other model large-scale circulation problems, for example, the lack of saline Circumpolar Deep Water along the Antarctic slope, and the coarse vertical resolution of the model. The surface boundary conditions in the permanent sea-ice-covered regions are a more major factor, leading to inadequate formation of dense, cold, and relatively saline shelf water, the precursors of AABW. Improved model–data agreement is found by combining the BBL parameterization with reasonably small adjustments in the surface restoring salinities on the Weddell and Ross Shelfs, justified by undersampling of winter conditions in standard climatologies. The modified salinities result in increased AABW production and enhanced signature of shelf water properties in the deep Southern Ocean similar in character to the effect of coupling with an active sea ice model.

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Scott C. Doney
,
Steve Yeager
,
Gokhan Danabasoglu
,
William G. Large
, and
James C. McWilliams

Abstract

The interannual variability in upper-ocean (0–400 m) temperature and governing mechanisms for the period 1968–97 are quantified from a global ocean hindcast simulation driven by atmospheric reanalysis and satellite data products. The unconstrained simulation exhibits considerable skill in replicating the observed interannual variability in vertically integrated heat content estimated from hydrographic data and monthly satellite sea surface temperature and sea surface height data. Globally, the most significant interannual variability modes arise from El Niño–Southern Oscillation and the Indian Ocean zonal mode, with substantial extension beyond the Tropics into the midlatitudes. In the well-stratified Tropics and subtropics, net annual heat storage variability is driven predominately by the convergence of the advective heat transport, mostly reflecting velocity anomalies times the mean temperature field. Vertical velocity variability is caused by remote wind forcing, and subsurface temperature anomalies are governed mostly by isopycnal displacements (heave). The dynamics at mid- to high latitudes are qualitatively different and vary regionally. Interannual temperature variability is more coherent with depth because of deep winter mixing and variations in western boundary currents and the Antarctic Circumpolar Current that span the upper thermocline. Net annual heat storage variability is forced by a mixture of local air–sea heat fluxes and the convergence of the advective heat transport, the latter resulting from both velocity and temperature anomalies. Also, density-compensated temperature changes on isopycnal surfaces (spice) are quantitatively significant.

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William G. Large
,
Gokhan Danabasoglu
,
Scott C. Doney
, and
James C. McWilliams

Abstract

The effects of more realistic bulk forcing boundary conditions, a more physical subgrid-scale vertical mixing parameterization, and more accurate bottom topography are investigated in a coarse-resolution, global oceanic general circulation model. In contrast to forcing with prescribed fluxes, the bulk forcing utilizes the evolving model sea surface temperatures and monthly atmospheric fields based on reanalyses by the National Centers for Environmental Prediction and on satellite data products. The vertical mixing in the oceanic boundary layer is governed by a nonlocal K-profile parameterization (KPP) and is matched to parameterizations of mixing in the interior. The KPP scheme is designed to represent well both convective and wind-driven entrainment. The near- equilibrium solutions are compared to a baseline experiment in which the surface tracers are strongly restored everywhere to climatology and the vertical mixing is conventional with constant coefficients, except where there is either convective or near-surface enhancement.

The most profound effects are due to the bulk forcing boundary conditions, while KPP mixing has little effect on the annual-mean state of the ocean model below the upper few hundred meters. Compared to restoring boundary conditions, bulk forcing produces poleward heat and salt transports in better agreement with most oceanographic estimates and maintains the abyssal salinity and temperature closer to observations. The KPP scheme produces mixed layers and boundary layers with realistically large temporal and spatial variability. In addition, it allows for more near-surface vertical shear, particularly in the equatorial regions, and results in enhanced large-scale surface divergence and convergence. Generally, topographic effects are confined locally, with some important consequences. For example, realistic ocean bottom topography between Greenland and Europe locks the position of the sinking branch of the Atlantic thermohaline circulation to the Icelandic Ridge. The model solutions are especially sensitive to the under-ice boundary conditions where model tracers are strongly restored to climatology in all cases. In particular, a factor of 4 reduction in the strength of under-ice restoring diminishes the abyssal salinity improvements by about 30%.

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