Ocean Sciences [OS]

OS33C  MW:3001   Wednesday
Global and Basin Circulation Studies: Tracers and Models
Presiding: W Han, University of Colorado, Boulder; P Schlosser, Lamont-Doherty Earth Observatory, Columbia University; W J Jenkins, Woods Hole Oceanographic Institution

OS33C-01 

An investigation of the Indian Ocean thermocline cooling during recent decades using an ocean general circulation model

* Han, W (whan@enso.colorado.edu), Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, Co 80309, United States Trenary, L L (laurie.trenary@colorado.edu), Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, Co 80309, United States

An ocean general circulation model, the HYbrid Coordinate Ocean Model (HYCOM), is used to understand the causes for the observed cooling trend of the Indian Ocean thermocline in the past few decades. The model basin covers the Indian and Pacific Oceans of 40oS-55oN, and is forced by 3-day mean ERA40 reanalysis products for the period of 1958-2000. HYCOM reasonably reproduced the thermocline cooling, which agrees well with the cooling trends shown in the Levitus data and SODA-POP data. Results from HYCOM suggest that the observed strongest cooling in the upper thermocline within 15oS-2oS is associated with enhanced upwelling caused by the enhanced upward Ekman pumping velocity ( ). This increased upwelling is consistent with the significantly strengthened Southern Subtropical Cell (STC), when the Cross-Equatorial Cell weakens. Remote influence from the Pacific shallow thermocline through wave transmission via the Indonesian Throughflow does not seem to have a significant contribution in this area, because the maximum shoaling of the thermocline occurs in the central-western basin and is weak near the ITF region. At deeper layers (below ~250m) and south of 15oS, however, remote forcing from the Pacific appears to be more important.

OS33C-02 

A high-resolution global ocean model of ROMS and its application to GRACE

* Colberg, F (fcolberg@pacific.jpl.nasa.gov), JPL/ CalTech, 4800 Oak Grove Dr., Pasadena, CA 91109, United States Song, T Y (song@pacific.jpl.nasa.gov), JPL/ CalTech, 4800 Oak Grove Dr., Pasadena, CA 91109, United States

We report on a high-resolution global ocean modeling effort using the regional ocean modeling system (ROMS- Ruttgers version). This study is motivated by the availability of increasingly accurate measurements of the earth gravitational field (e. g. GRACE), which allow for the deduction of ocean bottom pressure and hence an estimation of the total oceanic mass flux. Since ocean models using Boussinesq approximation a priori inhibit the calculation of ocean bottom pressure signals our effort is directed towards developing a global ocean model with non-Boussinesq formulation. At the current developing stage we are able to present preliminary results from the initial spin-up phase of the hydrostatic model version. The model resolution is on average 1/4 degree horizontally, with highest resolution over the equatorial belt decreasing towards the subtropics to midlatitudes. To overcome the north-pole singularity the grid has been rotated following Madec and Imbard (1996). The ocean model is then coupled to a Sea-Ice model at the poles, hence allowing for a more realistic representation of fresh water fluxes there and for the investigation of possible Ocean-Ice interaction. The model is spun-up with monthly climatologies of the NCEP/ NCAR Reanalysis and then run with monthly means of NCEP/ NCAR windstress and fluxes from 1948 to present.

OS33C-03 

Meridional coherence of the North Atlantic meridional overturning circulation.

* Bingham, R J (rjbi@pol.ac.uk), Proudman Oceanographic Laboratory, 6 Brownlow Street, Liverpool, L3 5DA, United Kingdom Hughes, C W (cwh@pol.ac.uk), Proudman Oceanographic Laboratory, 6 Brownlow Street, Liverpool, L3 5DA, United Kingdom Roussenov, V (enov@liv.ac.uk), Department of Earth and Ocean Sciences, University of Liverpool, Liverpool, L69 3GP, United Kingdom Williams, R G (ric@liv.ac.uk), Department of Earth and Ocean Sciences, University of Liverpool, Liverpool, L69 3GP, United Kingdom

The North Atlantic Meridional Overturning Circulation (MOC) is associated with deep water formation at high latitudes, and climatically-important ocean-atmosphere heat fluxes, hence the current substantial effort to monitor the MOC. While it is expected that, on sufficiently long time scales, variations in the MOC would be coherent across latitudes south of the deep water formation region, it is not clear whether coherence should be expected at shorter timescales. In this paper, we investigate the coherence of MOC variations in a range of ocean models. We find that, across a range of model physics, resolution, and forcing scenarios, there is a change in the character of the overturning north and south of about 40° N. To the north the variability has a strong decadal component, while to the south higher frequencies dominate. This acts to significantly reduce the meridional coherence of the MOC, even on interannual timescales. A physical interpretation in terms of an underlying meridionally coherent mode, strongest at high latitudes, but swamped by higher frequency, more localised processes south of 40N is provided.

OS33C-04 

Meridional Overturning Circulation of the Deep Pacific Estimated Assuming the Vertical Advective-diffusive Balance

* Endoh, T (endo@riam.kyushu-u.ac.jp), Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga-kouen, Kasuga, Fukuoka, 816-8580, Japan Hibiya, T (hibiya@eps.s.u-tokyo.ac.jp), Department of Earth and Planetary Science, Graduate School of Science, the University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan

\hspace*{3em}Assuming the vertical advective-diffusive balance, we calculate the meridional overturning circulation of the deep Pacific based on the observed density field. In addition to the global distribution of diapycnal diffusivity in the pycnocline [ Hibiya et al., 2006], bottom-intensified mixing that follows a Gaussian distribution with depth is taken into account. When diapycnal diffusivity in the pycnocline is assumed to extend uniformly to the bottom, the northward transport of the Lower Circumpolar Deep Water (LCDW) across the equator is only 1 Sv. When the bottom-intensified mixing is incorporated, the meridional overturning circulation increases, decreases, or even reverses depending on the vertical gradient of diapycnal diffusivity. The northward transport of the LCDW crossing the equator becomes comparable to the estimates from previous current meter moorings as well as hydrographic surveys only when the mixing extends well above the rough ocean bottom and hence the vertical gradient of diapycnal diffusivity is small.

OS33C-05 

Ocean Forecasting and Monitoring Products for Scientific Community: the Mercator Ocean portfolio of Services and the future European Marine Core Service

* Toumazou, V (vincent.toumazou@mercator-ocean.fr), Mercator Ocean, 8-10 rue Hermes, Parc Technologique du Canal, Ramonville, 31520, France Vinay, G (gvinay@mercator-ocean.fr), MGC, 2bis rue Marcel Doret, Blagnac, 31700, France Baudel, S (sbaudel@cls.Fr), CLS, 8-10 rue Hermes, Parc Technologique du Canal, Ramonville, 31520, France Palin, D (dpalin@mercator-ocean.fr), Mercator Ocean, 8-10 rue Hermes, Parc Technologique du Canal, Ramonville, 31520, France Kempa, D (dkempa@mercator-ocean.fr), Mercator Ocean, 8-10 rue Hermes, Parc Technologique du Canal, Ramonville, 31520, France Bahurel, P (pbahurel@mercator-ocean.fr

Mercator Ocean is a French public consortium formed in Toulouse in early 2002 by the six major players in the French oceanography community: the space agency CNES, the scientific research centre CNRS, IFREMER (the institute of marine research and exploration), the development research institute IRD, the Météo France weather service, and SHOM (the French Navy's hydrography & oceanography department). The services offered by Mercator-Ocean consist in a real-time general description of the physical state of the ocean (3D currents, salinity, temperature…) in order to provide the users' community with a wide range of ocean products. Many downstream activities are served in that framework covering applications like scientific research, off-shore, ship routing, environment and security (within the GMES European program). Within the future European Marine Core Service, Mercator Ocean as project leader and its partners will propose an extended portfolio of products and services covering a large part of the scientists' needs (coastal modelling, biology, climate…). In this talk, a review of the systems operated by Mercator Ocean will be proposed. In addition, a description of the products and services made available by both Mercator Ocean and the Marine Core Service will be given with a special focus on the part dedicated to the scientific community. http://www.mercator-ocean.fr/

OS33C-06 

Global ocean heat content 1955-2006 in light of recently revealed instrumentation problems

Levitus, S (sydney.levitus@noaa.gov), Ocean Climate Laboratory NODC/NESDIS/NOAA, 1315 East-West Highway, Silver Spring, MD 20910, United States Antonov, J (john.antonov@noaa.gov), Ocean Climate Laboratory NODC/NESDIS/NOAA, 1315 East-West Highway, Silver Spring, MD 20910, United States * Boyer, T (boyer@nodc.noaa.gov), Ocean Climate Laboratory NODC/NESDIS/NOAA, 1315 East-West Highway, Silver Spring, MD 20910, United States

Recent focus on instrumental biases and data recording problems have prompted the reexamination of ocean heat content calculated from in situ observations. Warm biases have been shown in XBT (and MBT) temperatures in comparison with bottle and CTD observations. But for most of the 1970s through the 1990s, data distribution of temperature observations without inclusion of XBT data is too sparse for global heat content calculations. Time varying XBT and MBT biases are estimated by statistical checks against nearby CTDs and bottle temperature measurements. Cool biases due to pressure offsets during data recording in a significant subset of profiling floats have also recently been discovered. It is anticipated that these pressure offsets can be corrected, but these corrections are not yet available. Here we present yearly heat content calculated using XBT and MBT profiles corrected for the instrumental biases and excluding floats with a cool bias. Comparisons are made with heat content without bias corrections. Other possible methods for dealing with the XBT warm bias are discussed.

OS33C-07 

The penetration of tritium and generation of 3He in the North Atlantic

Lott, D E (dlott@whoi.edu), Woods Hole Oceanographic Institution, 360 Woods Hole Road, Woods Hole, MA 02543, United States * Jenkins, W J (wjenkins@whoi.edu), Woods Hole Oceanographic Institution, 360 Woods Hole Road, Woods Hole, MA 02543, United States

Based on large scale surveys such as GEOSECS, TTO, WOCE and CLIVAR, as well as smaller cruises, we now have observations that cover more nearly 35 years of the penetration of bomb-produced tritium and its daughter 3He in the North Atlantic Ocean. This data set offers us the opportunity to characterize the decade time-scale ventilation and circulation of the North Atlantic basin, and some insights into climate change and variability. Perhaps the most important aspect of this tracer pair is that the tritiugenic 3He is a unique transient tracer in that it highlights the return pathways of the ventilation process. This permits us to use it to constrain large scale fluxes of remineralized thermocline nutrients to the surface ocean, thus constraining basin scale new production. We describe the patterns of evolving tritium and 3He distributions within the subtropical North Atlantic and relate these to large scale circulation and ventilation. In addition, the evolving inventories of these tracers provide useful insights into the character of the meridional overturning circulation.

OS33C-08 

Basin-Scale Vertical Mixing Coefficients in the Deep Southern Pacific Derived From the Distribution of Mantle He-3

* Schlosser, P (schlosser@ldeo.columbia.edu), Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, NY 10964, United States * Schlosser, P (schlosser@ldeo.columbia.edu), Department of Earth and Environmental Sciences, Columbia University, 2960 Broadway, New York, NY 10027, United States * Schlosser, P (schlosser@ldeo.columbia.edu), Department of Earth and Environmental Engineering, Columbia University, 2960 Broadway, New York, NY 10027, Newton, R (bnewton@ldeo.columbia.edu), Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, NY 10964, United States Winckler, G (winckler@ldeo.columbia.edu), Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, NY 10964, United States

Most of the oceanic excess 3He above solubility equilibrium with the atmosphere is due to injection of mantle helium with high 3He/4He ratios at the crests of the global mid-ocean ridge system with the strongest signal of mid-ocean ridge 3He being found in the South Pacific. The mantle 3He is injected over a fairly narrow depth interval and is re-distributed vertically on its pathway from the mid-ocean ridges to the boundaries of the deep ocean basins. Here, we use a comprehensive 3He data set for the South Pacific and document the vertical spreading of the plume injected at the crest of the Mid-Ocean Ridge System. The diapycnal spreading of the 3He plume is interpreted as the combined effect of vertical turbulent exchange and isopycnal transport to and from regions with rough topography and the related enhanced vertical mixing at these sites. Using a simple 1-D advection/diffusion 'Munk' model, we place constraints on the average vertical exchange that can be interpreted as an apparent vertical mixing coefficient (kz). We performed sensitivity tests of this simple model to the width over which 3He injection occurs, as well as to kz. Apparent kz values range from ca. 0.5*10-4 m2 sec-1 to 0.9*10-4 m2 sec-1, i.e., they are close to the average values obtained for the deep ocean by Munk (1966). We discuss the implications of these values for large scale distributions of properties in the deep ocean.