http://hdl.handle.net/1721.1/7656 2013-05-21T17:30:28Z http://hdl.handle.net/1721.1/78549 Ice shelf-ocean interactions in a general circulation model : melt-rate modulation due to mean flow and tidal currents Dansereau, Véronique Interactions between the ocean circulation in sub-ice shelf cavities and the overlying ice shelf have received considerable attention in the context of observed changes in flow speeds of marine ice sheets around Antarctica. Modeling these interactions requires parameterizing the turbulent boundary layer processes to infer melt rates from the oceanic state at the ice-ocean interface. Here we explore two such parameterizations in the context of the MIT ocean general circulation model coupled to the z-coordinates ice shelf cavity model of Losch (2008). We investigate both idealized ice shelf cavity geometries as well as a realistic cavity under Pine Island Ice Shelf (PIIS), West Antarctica. Our starting point is a three-equation melt rate parameterization implemented by Losch (2008), which is based on the work of Hellmer and Olbers (1989). In this form, the transfer coefficients for calculating heat and freshwater fluxes are independent of frictional turbulence induced by the proximity of the moving ocean to the fixed ice interface. More recently, Holland and Jenkins (1999) have proposed a parameterization in which the transfer coefficients do depend on the ocean-induced turbulence and are directly coupled to the speed of currents in the ocean mixed layer underneath the ice shelf through a quadratic drag formulation and a bulk drag coefficient. The melt rate parameterization in the MITgcm is augmented to account for this velocity dependence. First, the effect of the augmented formulation is investigated in terms of its impact on melt rates as well as on its feedback on the wider sub-ice shelf circulation. We find that, over a wide range of drag coefficients, velocity-dependent melt rates are more strongly constrained by the distribution of mixed layer currents than by the temperature gradient between the shelf base and underlying ocean, as opposed to velocity-independent melt rates. This leads to large differences in melt rate patterns under PIIS when including versus not including the velocity dependence. In a second time, the modulating effects of tidal currents on melting at the base of PIIS are examined. We find that the temporal variability of velocity-dependent melt rates under tidal forcing is greater than that of velocity-independent melt rates. Our experiments suggest that because tidal currents under PIIS are weak and buoyancy fluxes are strong, tidal mixing is negligible and tidal rectification is restricted to very steep bathymetric features, such as the ice shelf front. Nonetheless, strong tidally-rectified currents at the ice shelf front significantly increase ablation rates there when the formulation of the transfer coefficients includes the velocity dependence. The enhanced melting then feedbacks positively on the rectified currents, which are susceptible to insulate the cavity interior from changes in open ocean conditions. Thesis (S.M.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2012.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 121-123). 2012-01-01T05:00:00Z http://hdl.handle.net/1721.1/78538 The effects of ocean eddies on tropical cyclones Miltenberger, Alexander Reid The purpose of this study is to understand the interactions of tropical cyclones with ocean eddies. In particular we examine the influence of a cold-core eddy on the cold wake formed during the passage of Typhoon Fanapi (2010). The three-dimensional version of the numerical Price-Weller-Pinkel (PWP) vertical mixing model has previously been used to simulate and study the cold wakes of Atlantic hurricanes. The model has not been used in comparison with observations of typhoons in the Western Pacific Ocean. In 2010 several typhoons were studied during the Impact of Typhoons on the Ocean in the Pacific (ITOP) field campaign and Fanapi was particularly well observed. We use these observations and the 3DPWP to understand the ocean cold wake generated by Fanapi. The cold wake of Fanapi was advected by a cyclonic eddy that was south of the typhoon track. The 3DPWP model outputs with and without an eddy are compared with observations made during the field campaign. These observations are compared to model outputs with eddies in a series of positions right and left of the storm track in order to study effects of mesoscale eddies on ocean vertical mixing in the cold wake of typhoons. Thesis (S.M.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2012.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 40-41). 2012-01-01T05:00:00Z http://hdl.handle.net/1721.1/78479 Assessing United States hurricane damage under different environmental conditions Maheras, Anastasia Francis Hurricane activity between 1979 and 2011 was studied to determine damage statistics under different environmental conditions. Hurricanes cause billions of dollars of damage every year in the United States, but damage locations and magnitudes vary from year to year. Seasonal hurricane forecasts predicting the strength of the upcoming hurricane season have the potential to be used by many industries and sectors to reduce and mitigate the effects of hurricanes. However, damage itself is not predicted by these forecasts. This work analyzed trends in hurricane damage due to atmospheric and oceanic conditions, and the results could be applied to and included in seasonal hurricane forecasts, thus increasing forecast applicability and value. This work used synthetic hurricane tracks generated from background climate conditions, a U.S. property portfolio, and a damage function based on wind speed to determine 1979-2011 hurricane damage. Damage was split into La Niña/El Niño and pre-/post- 1995 year sets to determine spatial and temporal trends in U.S. hurricane damage. This work concluded that different regions of the country experienced more or less hurricane damage under different environmental conditions. Knowledge of these trends can be applied to seasonal hurricane forecasts and can influence property owner, regulator, and insurer behavior across the nation. Thesis (S.M. in Atmospheric Science)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2012.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 47-53). 2012-01-01T05:00:00Z http://hdl.handle.net/1721.1/78478 Weakening of ice by magnesium perchlorate hydrate Lenferink, Hendrik J., 1985- I show that perchlorate hydrates, which have been indirectly detected at high Martian circumpolar latitudes by the Phoenix Mars Lander, have a dramatic effect upon the rheological behavior of polycrystalline water ice under conditions applicable to the north polar layered deposits (NPLD). I conducted subsolidus creep tests on mixtures of ice and magnesium perchlorate hexahydrate (MP6) of 0.02, 0.05, 0.10, and 0.47 volume fraction MP6. I found these mixtures to be increasingly weak with increasing MP6 content. For mixtures with </= 0.10 volume fraction MP6, I resolved a stress exponent of n ~~ 2 at low stresses transitioning to n ~~ 4 above 10 MPa. Scanning electron microscopy of deformed specimens revealed MP6 to be distributed as an interconnected film between ice grains. These results suggest that grain boundary sliding (GBS) may be enhanced with respect to pure ice. As the enhancement of GBS is expected in polycrystalline aggregates containing a few percent melt or otherwise weak material distributed along grain boundaries, the observed n~~ 2 is consistent with the mutual accommodation of basal slip and GBS. If ice containing trace concentrations of MP6 is also much weaker than pure ice at low stresses, flow in the NPLD could be significantly enhanced, particularly at the warmer basal temperatures associated with higher Martian obliquities. Thesis (S.M. in Geophysics)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2012.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 21-23). 2012-01-01T05:00:00Z