OS43C-01 13:40h
Determination Of The Diapycnal Diffusion Rates In The Upper Thermocline In The North Atlantic Ocean Using Sulfur Hexafluoride
The apparent diapycnal diffusivity below the wind-driven surface mixed layer of the ocean was determined in a cyclonic (warm core) eddy in the eastern North Atlantic using sulfur hexafluoride (SF6) tracer data collected during the Gas Ex-98 cruise in June 1998. In this tracer experiment, the downward penetration of SF6 was measured for 3 weeks following the deliberate injection of SF6 at the base of the surface mixed layer. The resulting data were used to constrain the Price, Weller, and Pinkel (1986) mixed layer model to estimate the diapycnal diffusivity. The model includes the lateral diffusion component so that it can more accurately represent the time evolution of the [SF6] along the isopycnal surface. The lateral spreading affects the estimation of the diapycnal diffusivity. For the upper thermocline immediately below the surface mixed layer, we estimated the diapycnal diffusivity for the 3-week period as 0.4 $\pm$ 0.2 cm$^{2}$ s$^{-1}$ at a buoyancy frequency of 8.2 cph. This result is about two times lower than the estimate based on an analytical solutions using the second moment of the profiles over time.
OS43C-02 13:55h
Field and numerical study of entrainment laws for surface mixed layer
Entrainment at the base of mixed layer is a key process that controls climate changes and marine ecosystem. Though many mixed layer models have been developed based on the laboratory results of entrainment and have been evaluated by comparisons with field observation of mixed layer depth and sea surface temperature, there are only few studies to test the empirical entrainment laws found at laboratory. In present study, a steady deepening of the diurnal mixed layer was observed during nocturnal cooling in a large lake. The observed dissipation rates normalized with surface buoyancy flux and wind stress were in good agreement with a previously reported value. We simulated the observed time series of the water column using the Mellor-Yamada second order turbulence closure scheme. The time series of the observed diurnal mixing layer depth agreed well with the simulated time series. The details of entrainment were investigated using both data sets. Because the deepening was due to both cooling and wind stress, the entrainment was separated into two parts. Each part followed the empirical relationships reported from laboratory studies. In order to use both forcings, namely convection and wind stress, we propose a new entrainment diagram taking both factors into consideration.
OS43C-03 14:10h
Mixing Associated with Sills in a Canyon on the Mid-Ocean Ridge Flank
In order to close the global overturning circulation, the production and sinking of dense water at high latitudes must be balanced elsewhere by buoyancy gain and upward vertical motion. Microstructure observations from the western basin of the South Atlantic indicate that most of the abyssal mixing there takes place over the topographically rough flank of the mid-ocean ridge. In previous studies it has been suggested that the enhanced mixing is primarily caused by breaking internal waves forced by tidal flows. Here, the results from a detailed analysis of hydrographic data from a ridge-flank canyon, augmented by microstructure profiles, current-meter records and high-resolution bathymetry, are presented. Most of the strong dissipation is observed within the canyon, rather than above the ridge-flank topography. The largest dissipation values were recorded in the lee of a narrow sill extending across the full width of the canyon. Along the entire canyon, there is a strong correlation between the presence of sills and along-axial density gradients, while there is no similar correlation between the presence of depressions and the horizontal density gradients. Together, these observations suggest that sill-related mixing contributes at least as much to the diapycnal buoyancy flux in the canyon as tidally forced internal-wave breaking, which is not expected to be associated preferentially with sills. The current-meter records furthermore indicate that, within the canyon, the kinetic energy of low-frequency flows is larger than that of the tides, consistent with a significant low-frequency energy source for the enhanced mixing. The available data indicate that while only $\approx$15% of the interfacial area between Antarctic Bottom Water and North Atlantic Deep Water in the western subtropical basin of the South Atlantic lies inside canyons, approximately half of the total energy dissipation takes place there. In contrast, only about a third takes place above the ridge-flank topography. The apparent importance of sill-related processes for mixing in the canyons is therefore of global significance, especially considering that a large portion of the global mid-ocean ridge is associated with deep cross-flank canyons.
OS43C-04 14:25h
Variations of Temperature-Salinity Relationship in the Pycnocline of the Eastern Equatorial Pacific Ocean Associated With El Ni\~{n}o
Temperature-Salinity (T-S) relationship variability in the pycnocline of the eastern equatorial Pacific Ocean (NINO3 region, $5\deg$S-$5\deg$N, $150\deg$W-$90\deg$W) over the last two decades is investigated using observational data and model simulation. A numerical model simulation using the MITgcm (Massachusetts Institute of Technology General Circulation Model) suggests that, during El Ni\~{n}o years, the water in the eastern equatorial Pacific Ocean becomes saltier (by 0.1 to 0.2) and warmer (by 0.5 to $1\deg$C) on density surfaces within the pycnocline. This simulation is consistent with Conductivity-Temperature-Depth (CTD) data collected mostly during Tropical Atmosphere Ocean (TAO) mooring maintenance cruises. The cause of observed variations in T-S relationship between El Ni\~{n}o and non-El Ni\~{n}o years is numerically investigated. The origin of the subject water mass is identified using the adjoint of a simulated passive tracer. The higher salinity during El Ni\~{n}o is attributed to larger convergence of saltier water from the Southern Hemisphere and smaller convergence of fresher water from the Northern Hemisphere.
OS43C-05 14:40h
Quantifying Eddy Mixing in the Southern Ocean
An outstanding problem in large-scale ocean dynamics is the understanding, characterisation and representation of tracer transport by geostrophic eddies. Eddy transport in the coarse resolution ocean models used in climate research is parameterised by assuming that eddy tracer flux is related to mean tracer gradient through an eddy diffusivity. The very assumption of a flux gradient relationship is open to question. Arguably the key uncertainty in these models is lack of knowledge of the magnitude of the diffusivities and their variation in space and time. These matters are further complicated by the fact that eddy transfer has not been well characterised by in-situ measurements. An attempt to arrive at a more definitive estimate of ocean near-surface eddy diffusivities using a method pioneered to diagnose tracer transport in the stratosphere will be described. The transport properties of the eddy field are diagnosed by calculating an `effective diffusivity' diagnostic from a tracer. The technique uses Topex/Poseidon sea-surface altimeter data to derive a geostrophic surface velocity field, which is then used in a numerical model to transport a passive tracer around the Antarctic Circumpolar Current. Using the effective diffusivity diagnsotic, quantify the eddy diffusivity from this tracer field. The results from the application of our technique show high spatial variations in effective diffusivity accross the ACC, with values ranging from a few hundred ($m^2/s$) to several thousand. The implications of these results for the understanding of the residual circulation in the Southern Ocean and eddy parameterisation in models will be discussed.
OS43C-06 14:55h
Mixing and Dissipation Caused by Baroclinic Internal Tides: Simulations and Analysis
Observations from the Hawaiian Ocean Mixing Experiment (HOME) survey component suggest an increase in diapycnal mixing in a region above a steep slope. Using Large Eddy Simulations (LES), numerical experiments of the frictional benthic boundary layer subjected to baroclinic tidal forcing on a slope are investigated. The baroclinicity includes both depth dependency and phase variation based on HOME data. Previous results have shown the possibility of "off-slope mixing" occurring; potential energy can be released to mixing during phases of down-slope flow in an outer region of the boundary layer, an effect not seen in barotropic flows. Requirements for such mixing are provided. Additionally, parameterizations of both dissipation and mixing based on slope angle, Reynolds number, tidal excursion length, ambient stratification and mean shear are developed for both barotropic and baroclinic flows. Results are compared to observations from HOME and previous parameterizations.
OS43C-07 15:10h
Parameterizing Ocean Eddy Transports From Surface to Bottom
To improve subgrid-scale physics of climate ocean models, in particular near the top and bottom boundaries, we consider new parameterization schemes for the extra transport velocity by waves and eddies in baroclinic instability. These come in the form of elliptic equations, previously unmentioned, which we derive for the eddy-induced overturning stream function. They guarantee decrease of the mean field potential energy. Our principal example gives a relationship between the vertical shear of the overturning velocity and the buoyancy torque of the main geostrophic current. Interestingly the parameterized velocity is nonsingular at the bottom and the sea surface, contrasting with the constant-coefficient Gent and McWilliams (1990)scheme. Idealized two-dimensional numerical experiments uccessfully reproduce meridional overturning circulation even when the background density gradient is uniform everywhere (the Eady problem) or when the bottom is steeply sloped. We further demonstrate that adding an eddy form drag (wave tress) term in the TRM momentum equations yields overturning of the velocity field.
OS43C-08 15:25h
Upper Thermocline Salinity Anomalies in the Indian Ocean
An analysis of salinity data from Argo floats operating in the Indonesian-Australian Basin during 1999-2004 revealed an intense, wide spread fresh anomaly penetrating to 200 metres depth during 1999-2002. Shipboard CTD and thermosalinograph data collected during the same period supported the Argo findings. Historical CTDs document strong interannual variability in the freshwater content of the region. To first order the observed upper ocean freshwater anomalies are the result of strong interannual variability in local surface freshwater fluxes. We now extend the analysis to more recently deployed Argo floats across the entire Indian Ocean, using seasonal maps of large-scale salinity anomalies at various density levels to track the Indonesian-Australian fresh event and identify other regions of high variability. We also employ a hindcast simulation from a high-resolution z-coordinate model to explore the evolution of the salinity anomalies, and to determine the relative roles of advection and diffusion in dispersing the anomalies.