The location of diapycnal mixing and the meridional overturning circulation

The large-scale consequences of diapycnal mixing location are explored using an idealized threedimensional model of buoyancy-forced flow in a single hemisphere. Diapycnal mixing is most effective in supporting a strong meridional overturning circulation (MOC) if mixing occurs in regions of strong stratification, that is, in the low-latitude thermocline where diffusion causes strong vertical buoyancy fluxes. Where stratification is weak, such as at high latitudes, diapycnal mixing plays little role in determining MOC strength, consistent with weak diffusive buoyancy fluxes at these latitudes. Boundary mixing is more efficient than interior mixing at driving the MOC; with interior mixing the planetary vorticity constraint inhibits the communication of interior water mass properties and the eastern boundary. Mixing below the thermocline affects the abyssal stratification and upwelling profile, but does not contribute significantly to the MOC through the thermocline or the ocean’s meridional heat transport. The abyssal heat budget is dominated by the downward mass transport of buoyant water versus the spread of denser water tied to the properties of deep convection, with mixing of minor importance. These results are in contrast to the widespread expectation that the observed enhanced abyssal mixing can maintain the MOC; rather, they suggest that enhanced boundary mixing in the thermocline needs to be identified in observations

Impact of geothermal heating on the global ocean circulation

The response of a global circulation model to a uniform geothermal heat flux of 50 mW m-2 through the sea floor is examined. If the geothermal heat input were transported upward purely by diffusion, the deep ocean would warm by 1.2°C. However, geothermal heating induces a substantial change in the deep circulation which is larger than previously assumed and subsequently the warming of the deep ocean is only a quarter of that suggested by the diffusive limit. The numerical ocean model responds most strongly in the Indo-Pacific with an increase in meridional overturning of 1.8 Sv, enhancing the existing overturning by approximately 25%

Space station thermal control surfaces

Mission planning documents were used to analyze the radiator design and thermal control surface requirements for both space station and 25-kW power module, to analyze the missions, and to determine the thermal control technology needed to satisfy both sets of requirements. Parameters such as thermal control coating degradation, vehicle attitude, self eclipsing, variation in solar constant, albedo, and Earth emission are considered. Four computer programs were developed which provide a preliminary design and evaluation tool for active radiator systems in LEO and GEO. Two programs were developed as general programs for space station analysis. Both types of programs find the radiator-flow solution and evaluate external heat loads in the same way. Fortran listings are included

Paper Session II-B - Solid State Oxygen Sensor Development

To anticipate future long-duration mission needs for life support sensors, we explored the feasibility of using thin-film metal-oxide semiconductors. The objective of this task was to develop gas sensors for life support applications which would be suitable for long-duration missions. Metal oxides, such as ZnO, SnO2, and TiO2 have been shown to react with oxygen molecules. Oxygen lowers the metal oxide\u27s electrical resistance. Critical to the performance is the application of the oxide in a thin film on an inert substrate: the thinner the film, the more readily the oxygen penetration and hence the more rapid and sensitive the sensor. Metal oxides are not limited to oxygen detection, rather, oxides offer detection and quantification applications to the complete range of gases of interest, not only for life support systems, but for propellants as well

'Climate Response Functions' for the Arctic Ocean: a proposedcoordinated modeling experiment

A coordinated set of Arctic modelling experiments, which explore how the Arctic responds to changes in external forcing, is proposed. Our goal is to compute and compare “climate response functions” (CRFs) – the transient response of key observable indicators such as sea-ice extent, freshwater content of the Beaufort Gyre, etc. – to abrupt “step” changes in forcing fields across a number of Arctic models. Changes in wind, freshwater sources, and inflows to the Arctic basin are considered. Convolutions of known or postulated time series of these forcing fields with their respective CRFs then yield the (linear) response of these observables. This allows the project to inform, and interface directly with, Arctic observations and observers and the climate change community. Here we outline the rationale behind such experiments and illustrate our approach in the context of a coarse-resolution model of the Arctic based on the MITgcm. We conclude by summarizing the expected benefits of such an activity and encourage other modelling groups to compute CRFs with their own models so that we might begin to document their robustness to model formulation, resolution, and parameterization.National Science Foundation (U.S.) (Award 1603557

The roles of mixing, geothermal heating, and surface buoyancy forcing in ocean meridional overturning dynamics

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2000.Includes bibliographical references (p. 121-128).The dynamics of the oceanic large-scale meridional overturning circulation are investigated through a series of numerical experiments using an idealized single-hemisphere general circulation model. In addition to the system's scaling behavior, the consequences of diapycnal mixing location, the impact of deep buoyancy fluxes, and the importance of the surface restoring timescale are considered. As required by advective-diffusive balance, upwelling across isopycnals in low latitudes occurs where diapycnal mixing is specified. Downward mass transport into the abyss is relatively buoyant; the abyssal heat budget is such that this flow is subsequently cooled through deep convective mixing and re-warmed by diapycnal heat fluxes. Thus, mixing below the thermocline affects the abyssal stratification and upwelling profile, but does not contribute significantly to the zonally averaged circulation through the thermocline or the meridional oceanic heat transport. Boundary mixing is more efficient than interior mixing at driving the meridional overturning circulation; with interior mixing, the planetary vorticity constraint interferes with the communication of interior water mass properties and the eastern boundary. The results are consistent with thermodynamic considerations that suggest the strength of the overturning is a function of the vertical heat fluxes through the thermocline. Accordingly, diapycnal mixing must result in surface heat input to influence the portion of large scale overturning that effects the meridional heat transport. When a buoyancy flux (e.g., geothermal heating) is applied to the ocean floor, a perturbation deep meridional overturning cell on the order of several Sv is produced. The surface flow is also perturbed at high latitudes, allowing the additional heat to be released to the atmosphere. Rising motion is concentrated near the equator. The upward penetration of the deep cell is limited by the thermocline, analogous to the role of the stratosphere in limiting the upward penetration of convective plumes in the atmosphere. The magnitude of the advective response is inversely proportional to the deep stratification; with a weaker meridional overturning circulation and hence a less stratified abyss, the overturning maximum of the deep cell is increased. These results suggest that geothermal heat fluxes, typically ignored in general circulation models, might play a more significant role than thought in the determining the abyssal circulation. For the lowest two decades of changes to diapycnal mixing diffusivity (K), the system's response is largely "self-similar", but experiences a transition to a different regime at very high values of diffusivity. The maximum in overturning circulation obeys an approximate 2/3 power scaling law over both regimes. In contrast, given changes in the imposed equator-to-pole temperature difference AT, the behavior is not self-similar except in the meridional pro.- file of surface heat exchange. Moreover, the power law scaling of overturning with AT is similar to that of K, in contradiction with the 1/3 law predicted by scaling arguments and the Marotzke (1997) theory. The ocean's dynamical behavior is also strongly influenced by the restoring timescale at which the surface temperature is restored; with weaker restoring, the deep sinking region of the ocean becomes more narrow in the zonal mean, and the maximum in meridional heat flux declines even though the maximum in overturning remains nearly constant. These results are interpreted by considering the fundamental thermodynamics of the system.by Jeffery R. Scott.Ph.D

Winners and losers: Ecological and biogeochemical changes in a warming ocean

We employ a marine ecosystem model, with diverse and flexible phytoplankton communities, coupled to an Earth system model of intermediate complexity to explore mechanisms that will alter the biogeography and productivity of phytoplankton populations in a warming world. Simple theoretical frameworks and sensitivity experiments reveal that ecological and biogeochemical changes are driven by a balance between two impacts of a warming climate: higher metabolic rates (the “direct” effect), and changes in the supply of limiting nutrients and altered light environments (the “indirect” effect). On globally integrated productivity, the two effects compensate to a large degree. Regionally, the competition between effects is more complicated; patterns of productivity changes are different between high and low latitudes and are also regulated by how the supply of the limiting nutrient changes. These complex regional patterns are also found in the changes to broad phytoplankton functional groups. On the finer ecological scale of diversity within functional groups, we find that ranges of some phytoplankton types are reduced, while those of others (potentially minor players in the present ocean) expand. Combined change in areal extent of range and in regionally available nutrients leads to global “winners and losers.” The model suggests that the strongest and most robust signal of the warming ocean is likely to be the large turnover in local phytoplankton community composition.United States. Dept. of Energy. Office of Science (Grant DE-FG02-94ER61937)United States. National Oceanic and Atmospheric AdministrationGordon and Betty Moore Foundatio

AN ECONOMIC AND RISK ANALYSIS OF THE EFFECTS OF TILLAGE AND NITROGEN SOURCE ON SOIL CARBON SEQUESTRATION IN CORN PRODUCTION

The economic potential of no-tillage versus conventional tillage to sequester soil carbon using either commercial nitrogen or manure for continuous corn production is evaluated. Results indicate which system provides the highest net returns, which system is preferred by risk averse decision makers, and the price of carbon credits under alternative risk aversion preferences.Risk and Uncertainty,

Relative Roles of Climate Sensitivity and Forcing in Defining the Ocean Circulation Response to Climate Change

Abstract in HTML and technical report in PDF available on the Massachusetts Institute of Technology Joint Program on the Science and Policy of Global Change website (http://mit.edu/globalchange/www/).The response of the ocean’s meridional overturning circulation (MOC) to increased greenhouse gas forcing is examined using a coupled model of intermediate complexity, including a dynamic 3D ocean subcomponent. Parameters are the increase in CO2 forcing (with stabilization after a specified time interval) and the model’s climate sensitivity. In this model, the cessation of deep sinking in the north “Atlantic” (hereinafter, a “collapse”), as indicated by changes in the MOC, behaves like a simple bifurcation. The final surface air temperature (SAT) change, which is closely predicted by the product of the radiative forcing and the climate sensitivity, determines whether a collapse occurs. The initial transient response in SAT is largely a function of the forcing increase, with higher sensitivity runs exhibiting delayed behavior; accordingly, high CO2-low sensitivity scenarios can be assessed as a recovering or collapsing circulation shortly after stabilization, whereas low CO2-high sensitivity scenarios require several hundred additional years to make such a determination. We also systemically examine how the rate of forcing, for a given CO2 stabilization, affects the ocean response. In contrast with previous studies based on results using simpler ocean models, we find that except for a narrow range of marginally stable to marginally unstable scenarios, the forcing rate has little impact on whether the run collapses or recovers. In this narrow range, however, forcing increases on a time scale of slow ocean advective processes results in weaker declines in overturning strength and can permit a run to recover that would otherwise collapse.This research was supported in part by the Methods and Models for Integrated Assessments Program of the National Science Foundation, Grant ATM-9909139, by the Office of Science (BER), U.S. Department of Energy, Grant No. DE-FG02-93ER61677, and by the MIT Joint Program on the Science and Policy of Global Change (JPSPGC)

Integrated economic and climate projections for impact assessment

We designed scenarios for impact assessment that explicitly address policy choices and uncertainty in climate response. Economic projections and the resulting greenhouse gas emissions for the “no climate policy” scenario and two stabilization scenarios: at 4.5 W/m2 and 3.7 W/m2 by 2100 are provided. They can be used for a broader climate impact assessment for the US and other regions, with the goal of making it possible to provide a more consistent picture of climate impacts, and how those impacts depend on uncertainty in climate system response and policy choices. The long-term risks, beyond 2050, of climate change can be strongly influenced by policy choices. In the nearer term, the climate we will observe is hard to influence with policy, and what we actually see will be strongly influenced by natural variability and the earth system response to existing greenhouse gases. In the end, the nature of the system is that a strong effect of policy, especially directed toward long-lived GHGs, will lag by 30 to 40 years its implementation.United States. Environmental Protection Agency (Cooperative Agreement #XA-83600001