http://hdl.handle.net/1721.1/7803 http://hdl.handle.net/1721.1/33723 On the maintenance of weak meridional temperature gradients during warm climates Korty, Robert Lindsay This thesis examines the dynamics of equable climates. The underlying physics of two mechanisms by which weak meridional temperature gradients might be maintained are studied. First, I examine the evolution of stratospheric dynamics and winter temperatures when the surface temperature gradient and tropospheric eddy energy decrease in order to assess whether large-scale conditions are more favorable for polar stratospheric cloud formation. Second, I examine whether the combination of high carbon dioxide and interactive, tropical cyclone dependent ocean mixing is sufficient to maintain a weak temperature gradient. I examine planetary wave generation, the energetics of the general circulation, and vertical wave propagation in a general circulation model with a resolved stratosphere forced with a weak surface temperature gradient. Compared to the present climate, transient eddy energy decreases, but stationary eddy energy does not. The polar tropopause rises, which supports a weaker temperature gradient in the lower stratosphere, a weaker stratospheric jet, and an increase in the wave activity vertically propagating into the stratosphere. (cont.) As a result, the residual mean circulation strengthens and temperatures in the polar stratosphere change little even when the surface temperature gradient is quite weak. Temperatures in the Arctic polar vortex remain much warmer than radiative equilibrium, inhibiting large-scale polar stratospheric cloud formation. The height of the extratropical tropopause rises and the tropospheric lapse rate follows a moist adiabat when surface temperatures are warm, suggesting convection plays a significant role in setting extratropical tropospheric stratification during warm climates. The second part of the thesis addresses the role of tropical cyclone induced mixing in the oceans' general circulation. I examine the sensitivity of the oceans' meridional overturning circulation and heat flux to the locations at which mixing occurs. When confined to the tropical Atlantic, a robust single-basin circulation can be maintained, but the Indian and Pacific become quiescent, cut off from the dynamics occurring in the Atlantic. Mixing isolated in the tropical Pacific, however, can support a global circulation by mechanically lifting deep fluid parcels formed in the Atlantic, raising their potential energy. (cont.) The oceans' total heat flux is found to be sensitive to mixing in the tropics, in both the Atlantic and the Pacific, and in the upper 400 meters of the ocean. Coupling mixing with a measure of tropical cyclone intensity and frequency creates a positive feedback between climate and the poleward energy flux. When combined with a parameterization of the background mixing that evolves with stratification, a warmer, less stratified ocean can support a stronger diapycnal mixing during warm climates with high loads of carbon dioxide. In these simulations, tropical cyclones are stronger and more frequent, providing an increased energy source for more vigorous mixing in the tropical oceans. Combined with a stratification-dependent mixing scheme, such mixing provides a sufficiently strong heat flux that is able to maintain weak sea surface temperature gradients. Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2005. Includes bibliographical references (p. 238-248). Fri, 29 Oct 2004 22:58:59 GMT http://hdl.handle.net/1721.1/42833 The most conserved genome segments for life detection on Earth and other planets (Supplemental material) Ruvkun, Gary Zuber, Maria T. Gilbert, Walter Church, George M. Finney, Michael Johnson, Sarah Stewart Carr, Christopher E. Isenbarger, Thomas A. Mon, 22 Sep 2008 01:58:39 GMT http://hdl.handle.net/1721.1/42283 Oceanic lithosphere magnetization : marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments Williams, Clare Margaret The origin of symmetric alternating magnetic polarity stripes on the seafloor is investigated in two marine environments; along the ridge axis of the fast spreading East Pacific Rise (EPR) (90 25'-90 55'N) and at Kane Megamullion (KMM) (230 40'N), near the intersection of the slow-spreading Mid Atlantic Ridge with Kane Transform Fault. Marine magnetic anomalies and magnetic properties of seafloor samples are combined to characterize the magnetic source layer in both locations. The EPR study suggests that along-axis variations in the observed axial magnetic anomaly result from changing source layer thickness alone, consistent with observed changes in seismic Layer 2a. The extrusive basalts of the upper crust therefore constitute the magnetic source layer along the ridge axis and long term crustal accretion patterns are reflected in the appearance of the axial anomaly. At KMM the C2r.2r/C2An. In (- 2.581 Ma) polarity reversal boundary cuts through lower crust (gabbro) and upper mantle (serpentinized peridotites) rocks exposed by a detachment fault on the seafloor, indicating that these lithologies can systematically record a magnetic signal. Both lithologies have stable remanent magnetization, capable of contributing to the magnetic source layer. The geometry of the polarity boundary changes from the northern to the central regions of KMM and is believed to be related to changing lithology. In the northern region, interpreted to be a gabbro pluton, the boundary dips away from the ridge axis and is consistent with a rotated conductively cooled isotherm. In the central region the gabbros have been removed and the polarity boundary, which resides in serpentinized peridotite, dips towards the ridge axis and is thought to represent an alteration front. The linear appearance of the polarity boundary across both regions indicates that the two lithologies acquired their magnetic remanence during approximately the same time interval. Seismic events caused by detachment faulting at Kane and Atlantis Transform Faults are investigated using hydroacoustic waves (T-phases) recorded by a hydrophone array. Observations and ray trace models of event propagation show bathymetric blockage along propagation paths, but suggest current models of T-phase excitation and propagation need to be improved to explain observed characteristics of T-phase data. Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2007. Includes bibliographical references. Sun, 29 Oct 2006 22:58:59 GMT http://hdl.handle.net/1721.1/42282 A determination of air-sea gas exchange and upper ocean biological production from five noble gasses and tritiugenic helium-3 Stanley, Rachel H. R The five noble gases (helium, neon, argon, krypton, and xenon) are biologically and chemically inert, making them ideal oceanographic tracers. Additionally, the noble gases have a wide range of solubilities and molecular diffusivities, and thus respond differently to physical forcing. Tritium, an isotope of hydrogen, is useful in tandem with its daughter helium-3 as a tracer for water mass ages. In this thesis, a fourteen month time-series of the five noble gases, helium-3 and tritium was measured at the Bermuda Atlantic Time-series Study (BATS) site. The time-series of five noble gases was used to develop a parameterization of air-sea gas exchange for oligotrophic waters and wind speeds between 0 and 13 m s-1 that explicitly includes bubble processes and that constrains diffusive gas exchange to ± 6% and complete and partial air injection processes to ± 15%. Additionally, the parameterization is based on weeks to seasonal time scales, matching the time scales of many relevant biogeochemical cycles. The time-series of helium isotopes, tritium, argon, and oxygen was used to constrain upper ocean biological production. Specifically, the helium flux gauge technique was used to estimate new production, apparent oxygen utilization rates were used to quantify export production, and euphotic zone seasonal cycles of oxygen and argon were used to determine net community production. The concurrent use of these three methods allows examination of the relationship between the types of production and begins to address a number of apparent inconsistencies in the elemental budgets of carbon, oxygen, and nitrogen. Thesis (Ph. D.)--Joint Program in Chemical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2007. Includes bibliographical references (p. 215-225). Sun, 29 Oct 2006 22:58:59 GMT