The life of a thermohaline intrusion

The effects of salt finger and diffusive interface transports on the thermohaline properties of an oceanic intrusion are modelled by considering the time evolution of a well-mixed layer with a single diffusive interface below it and a single finger interface above. Both cause the T-S anomaly to decrease. At short times, the diffusive interface buoyancy flux dominates, and can cause the central layer to overturn the finger interface. Initial conditions are deduced for which the central layer must lose its S contrast while remaining intermediate in density between the top and bottom layers. Under these conditions, the diffusive interface buoyancy flux is large initially, but at long time, the finger fluxes prevail. The balance between the two transports provides a \u27focussing\u27 such that the final density of the central layer is approximately independent of its initial density. Thus the time scale of run-down and the final state are predictable.The conditions sufficient to guarantee stable run-down are plotted for the analogue sugar-salt system. Too little is known about the relative buoyancy fluxes of the heat-salt finger and diffusive interfaces at low stability to permit plotting of the stability boundaries of the heat-salt system. However, the general features of the theory are expected to hold.Two laboratory experiments were performed demonstrating stable and unstable evolution in the sugar-salt system. The measured time variations in layer properties agreed well with those expected from numerical integration of the governing equations

The mythical thermohaline oscillator?

The system discussed by Stommel (1987) and Welander (1982), in which heating and evaporation at the surface of the ocean are balanced by vertical turbulent mixing, is studied analytically and numerically for mixing laws appropriate to salt fingers, rather than mechanical turbulence. Stommel and Welander found for mechanically-driven turbulent mixing that a limit cycle of T and S exists (that is, T and S oscillate) in the presence of steady forcing. We find that the usual salt finger parameterizations, in which salinity flux coefficient and buoyancy flux ratio decrease with increasing density ratio, do not allow a limit cycle. This result holds whether the flux parameterization is for an interface using the “4/3 power law” laboratory relationships or in terms of vertical gradients. Rather, all initial conditions either evolve to a steady balance or lead to the upper layer becoming denser than the lower layer and overturning. In addition, we find that commonly used mechanical turbulence parameterizations for eddy diffusivity vs. Richardson number do not vary rapidly enough to allow a limit cycle in the Stommel/Welander model, although recent observations of equatorial turbulence do. Hence the possible existence of a limit oscillation in evaporatively-driven areas of the ocean depends critically on the type of vertical mixing which occurs, and on the precise form of its parameterization

Observations of interaction between the internal wavefield and low-frequency flows in the North Atlantic

Also published as: Journal of Physical Oceanography 9 (1979): 489-517A total of four moorings from POLYMODE array I and II were analyzed in an investigation of the interaction of wavefields and mean flow. In particular, evidence for internal wave-mean flow interaction was sought by searching for time correlations between the vertically acting Reynolds stress of the wavefield (estimated using the temperature and velocity records), and the mean shear. No significant stress-shear correlations were found at the less energetic moorings (u¯≲10 cm s−1), indicating that the magnitude of the eddy viscosity was under 200 cm2 s−1, with the sign of the energy transfer uncertain. This is considerably below the O(4500 cm2 s−1) predicted by Müller (1976). An extensive error analysis indicates that the large wave stress predicted by the theory should have been observable clearly under the conditions of measurement. At moorings typified by a higher mean velocity (u¯≈25 cm s−1), statistically significant stress-shear correlations were found, and the wavefield energy level was observed to modulate with the strength of the mean shear. The observations were consistent with generation of short (∼1 km horizontal wavelength) internal waves by the mean shear near the thermocline, resulting in an effective eddy viscosity of ∼100 cm2 s−1. Theoretical computations indicate that the wavefield “basic state” may not be independent of the mean flow as assumed by Müller (1976) but can actually be modified by large-scale vertical shear and still remain in equilibrium. In that case, the wavefield does not exchange momentum with a large-scale vertical shear flow and, excepting critical-layer effects, a small vertical eddy viscosity is to be expected. Using the Garrett-Munk (1975) model internal wave spectrum, estimates were made of the maximum momentum flux (stress) expected to be lost to critical-layer absorption. This stress was found to increase almost linearly with the velocity difference across the shear zone, corresponding to a vertical eddy viscosity of −100 cm2 s−1. Stresses indicative of this effect were not observed in the data.Prepared for the Office of Naval Research under Contract N00014-76-C-0197; NR 083-400

Observations of interaction between the internal wavefield and low frequency flows in the North Atlantic.

Thesis. 1977. Ph.D.--Massachusetts Institute of Technology. Dept. of Meteorology.Microfiche copy available in Archives and Science.Vita.Bibliography : p. [323-328].Ph.D

Measuring lateral heat flux across a thermohaline front: A model and observational test

We develop and test a method to observationally estimate lateral intrusive heat flux across a front. The model combines that of Joyce (1977), in which lateral cross-frontal advection by intrusions creates vertical temperature gradients, and Osborn and Cox (1972) in which vertical mixing of those gradients creates thermal microstructure that is dissipated by molecular conduction of heat. Observations of thermal microstructure dissipation χT are then used to estimate the production by intrusions, and hence the lateral heat flux and diffusivity. This method does not depend on the precise mechanism(s) of mixing, or on the dynamical mechanisms driving the frontal intrusions. It relies on several assumptions: (1) lateral cross-frontal advection produces diapycnal temperature gradients that are mixed locally, (2) thermal variance is dissipated locally and not exported, (3) intrusion scales are larger than turbulence scales, and (4) isotropy of temperature microstructure is assumed in order to estimate χT.The method is tested using microstructure observations in Meddy Sharon, where the erosion rate and associated lateral heat flux are known from successive mesoscale hydrographic observations (Hebert et al., 1990). An expression is developed for the production (lateral heat flux times lateral temperature gradient, expected to equal χT) in a front of steady shape that is eroding (detraining) at a steady rate; the production is proportional to the erosion speed and the square of the cross-frontal temperature contrast, both of which are well-known from observations. The qualitative structure and integrated value of the dissipation agree well with model assumptions and predictions: thermal variance produced by lateral intrusive heat flux is dissipated locally, dissipation in intrusive regions dominates total dissipation, and the total dissipation agrees with the observed erosion rate, all of which suggests that microstructure observations can be used to estimate intrusive heat flux. A direct comparison was made between lateral heat flux estimated from mesoscale Meddy structure plus the known rate of erosion, and lateral flux based on microscale temperature dissipation, with excellent agreement in the frontal zone and poorer agreement where lateral temperature gradient is too small to accurately measure

Effects of Ambient Turbulence on Interleaving at a Baroclinic Front

In this paper the authors investigate the action of ambient turbulence on thermohaline interleaving using both theory and numerical calculations in combination with observations from Meddy Sharon and the Faroe Front. The highly simplified models of ambient turbulence used previously are improved upon by allowing turbulent diffusivities of momentum, heat, and salt to depend on background gradients and to evolve as the instability grows. Previous studies have shown that ambient turbulence, at typical ocean levels, can quench the thermohaline interleaving instability on baroclinic fronts. These findings conflict with the observation that interleaving is common in baroclinic frontal zones despite ambient turbulence. Another challenge to the existing theory comes from numerical experiments showing that the Schmidt number for sheared salt fingers is much smaller than previously assumed. Use of the revised value in an interleaving calculation results in interleaving layers that are both weaker and thinner than those observed. This study aims to resolve those paradoxes. The authors show that, when turbulence has a Prandtl number greater than unity, turbulent momentum fluxes can compensate for the reduced Schmidt number of salt fingering. Thus, ambient turbulence determines the vertical scale of interleaving. In typical oceanic interleaving structures, the observed property gradients are insufficient to predict interleaving growth at an observable level, even when improved turbulence models are used. The deficiency is small, though: gradients sharper by a few tens of percent are sufficient to support instability. The authors suggest that this is due to the efficiency of interleaving in erasing those property gradients. A new class of mechanisms for interleaving, driven by flow-dependent fluctuations in turbulent diffusivities, is identified. The underlying mechanism is similar to the well-known Phillips layering instability; however, because of Coriolis effects, it has a well-defined vertical scale and also a tilt angle opposite to that of finger-driven interleaving

Temperature and salinity observations with high lateral resolution using acoustic data in the Gulf of Cadiz, NE Atlantic Ocean

European Geosciences Union General Assembly 2015 (EGU2015), 12-17 April 2015, Vienna, Austria.-- 1 pageWe present a methodology for inverting temperature and salinity from time and space-coincident acoustic reflectivity and XBT data. This method recovers low frequency content ( 10 Hz) from acoustic reflectivity. Afterwards, maps of temperature and salinity are calculated from impedance using the GSW equations of state and an empirical T-S relation. Acoustic data allows to recover the main physical parameters of the ocean along lateral sections of hundreds of km, covering all the full-depth water column and with vertical and lateral resolutions of 10 m and 100 m, respectively. This method was applied in the Gulf of Cadiz, NE Atlantic Ocean to recover the main physical oceanographic parameters in the ocean with accuracies of δTsd = 0.1 C, δSsd = 0.09 and δsd = 0.02kg/m3 for temperature, salinity and potential density. Inverted temperature anomalies reveal baroclinic thermohaline fronts with intrusions.The observations support a mix of thermohaline features created by both double-diffusive and isopycnal stirring mechanismsPeer Reviewe

Synthetic Modeling for an Acoustic Exploration System for Physical Oceanography

10 pages, 6 figures, 1 tableMarine multichannel seismic (MCS) data, used to obtain structural reflection images of the earth¿s subsurface, can also be used in physical oceanography exploration. This method provides vertical and lateral resolutions of O(10¿100) m, covering the existing observational gap in oceanic exploration. All MCS data used so far in physical oceanography studies have been acquired using conventional seismic instrumentation originally designed for geological exploration. This work presents the proof of concept of an alternative MCS system that is better adapted to physical oceanography and has two goals: 1) to have an environmentally low-impact acoustic source to minimize any potential disturbance to marine life and 2) to be light and portable, thus being installed on midsize oceanographic vessels. The synthetic experiments simulate the main variables of the source, shooting, and streamer involved in the MCS technique. The proposed system utilizes a 5-s-long exponential chirp source of 208 dB relative to 1 ¿Pa at 1 m with a frequency content of 20¿100 Hz and a relatively short 500-m-long streamer with 100 channels. This study exemplifies through numerical simulations that the 5-s-long chirp source can reduce the peak of the pressure signal by 26 dB with respect to equivalent air gun¿based sources by spreading the energy in time, greatly reducing the impact to marine life. Additionally, the proposed system could be transported and installed in midsize oceanographic vessels, opening new horizons in acoustic oceanography researchThe first author’s work has been supported by the European Commission through Marie Curie Actions FP7-PEOPLE-2010-IOF-271936 and FP7-PEOPLE-2012-COFUND-600407. This work has been done in the framework of the Spanish project POSEIDON (CTM2010-25169) and the Italian National Flagship Programme RITMARE (Programma Nazionale della Ricerca 2011-2013 MIUR). We want to acknowledge the team of GO project funded by the EU (015603-GO-STREP)Peer Reviewe

Toward quantifying the increasing role oceanic heat in sea ice loss in the new Arctic

Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Bulletin of the American Meteorological Society 96 (2015): 2079–2105, doi:10.1175/BAMS-D-13-00177.1.The loss of Arctic sea ice has emerged as a leading signal of global warming. This, together with acknowledged impacts on other components of the Earth system, has led to the term “the new Arctic.” Global coupled climate models predict that ice loss will continue through the twenty-first century, with implications for governance, economics, security, and global weather. A wide range in model projections reflects the complex, highly coupled interactions between the polar atmosphere, ocean, and cryosphere, including teleconnections to lower latitudes. This paper summarizes our present understanding of how heat reaches the ice base from the original sources—inflows of Atlantic and Pacific Water, river discharge, and summer sensible heat and shortwave radiative fluxes at the ocean/ice surface—and speculates on how such processes may change in the new Arctic. The complexity of the coupled Arctic system, and the logistic and technological challenges of working in the Arctic Ocean, require a coordinated interdisciplinary and international program that will not only improve understanding of this critical component of global climate but will also provide opportunities to develop human resources with the skills required to tackle related problems in complex climate systems. We propose a research strategy with components that include 1) improved mapping of the upper- and middepth Arctic Ocean, 2) enhanced quantification of important process, 3) expanded long-term monitoring at key heat-flux locations, and 4) development of numerical capabilities that focus on parameterization of heat-flux mechanisms and their interactions.2016-06-0