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Thesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2006., Cataloged from PDF version of thesis. "February 2006.", Includes bibliographical references (page 35)., A parameter representing circulation due to wind forcing is added to the thermohaline circulation model of Marotzke (1996). The model consists of four boxes and is governed by a system of two differential equations governing the temperature and salinity differences between high latitude ocean and low latitude ocean boxes. The modified model is solved numerically for equilibrium solutions, and then solved analytically by the method of Krasovskiy and Stone (1998). At the maximum strength of wind-forced circulation studied, v = 5 x 10-¹¹ s-¹, a stable thermal mode equilibrium temperature difference of 25 K is calculated. Once v reaches a critical value, which is within the range of physically reasonable values, the stable haline mode equlibrium and unstable thermal mode equilibrium are no longer observed. It is concluded that strong wind-forced circulation suppresses the thermal mode equilibrium, but that more research is necessary to determine the degree to which this effect is present in the real world., by Elise M. Olson., S.B.

Thesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2006., Cataloged from PDF version of thesis. "February 2006.", Includes bibliographical references (page 35)., A parameter representing circulation due to wind forcing is added to the thermohaline circulation model of Marotzke (1996). The model consists of four boxes and is governed by a system of two differential equations governing the temperature and salinity differences between high latitude ocean and low latitude ocean boxes. The modified model is solved numerically for equilibrium solutions, and then solved analytically by the method of Krasovskiy and Stone (1998). At the maximum strength of wind-forced circulation studied, v = 5 x 10-¹¹ s-¹, a stable thermal mode equilibrium temperature difference of 25 K is calculated. Once v reaches a critical value, which is within the range of physically reasonable values, the stable haline mode equlibrium and unstable thermal mode equilibrium are no longer observed. It is concluded that strong wind-forced circulation suppresses the thermal mode equilibrium, but that more research is necessary to determine the degree to which this effect is present in the real world., by Elise M. Olson., S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2009., This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections., Cataloged from student submitted PDF version of thesis., Includes bibliographical references (p. 257-266)., In this thesis, the influence of boundary layer processes and seasonal forcing on baroclinic eddy equilibration is studied to understand how the baroclinic adjustment is modified when taking into account these two factors. A modified [beta] plane multilevel quasi-geostrophic (QG) model with an interactive stratification and a simplified parameterization of atmospheric boundary layer physics is used as the atmospheric model in this study. Comparisons between experiments with the modified QG model and the traditional QG model with fixed stratification show that it is necessary to include the interaction between vertical eddy heat flux and stratification to obtain a realistic equilibrated state, i.e. robust isentropic slope. A slab surface model is also coupled with the atmospheric model to provide an interactive surface temperature distribution in some experiments in this study. The effect of boundary layer processes is first studied under the situation with fixed underlying surface temperature. The boundary layer vertical thermal diffusion, along with the surface heat exchange, is found primarily responsible for the limitation of the PV homogenization in the boundary layer. The boundary layer processes can influence the eddy activity in at least two competing ways. First, in the eddy energy budget, all the boundary layer processes act to damp the eddy energy directly. On the other hand, the boundary layer processes also influence the mean flow, which can further influence the eddy behavior. For the boundary layer thermal damping, the indirect effect on the eddy activity becomes dominant., (cont.) Stronger boundary layer thermal forcing results in stronger meridional temperature gradients and eddy heat fluxes. For the frictional dissipation, the resulting changes in the zonal wind and in the location of the critical latitude lead to a meridional variation of the eddy forcing, which can further result in a non-monotonic response of the mean flow. The role of the boundary layer processes in the baroclinic eddy equilibration is further studied using a simple air-sea thermally coupled model. Although in the coupled system, each boundary layer process has more and different ways to influence the equilibrium state, their effect on the lower level PV homogenization is very robust. Surface friction and surface 3 heat flux all act to damp the lower level PV mixing and stronger surface damping prevents the PV homogenization more strongly, but the way in which each boundary layer process affects the PV homogenization is very different. Baroclinic eddy equilibration under seasonal forcing is studied in both the atmospheric model and the coupled model. In the situation with specified seasonal variation of the underlying surface temperature, a Northern-Hemisphere like seasonal variation of the surface temperature is used to act on the atmospheric flow through the boundary layer processes and the radiative-convective heating., (cont.) Under slowly varying seasonal forcing, the eddy and the mean flow behavior is characterized by four clearly divided time intervals: an eddy inactive time interval in the summer, a mainly dynamically determined eddy spinup time interval starting from mid-fall and lasting less than one month, a quasi-equilibrium time interval for the zonal mean flow available potential energy from late fall to late spring and a mainly external forcing determined eddy spindown time interval from late winter to late spring. In spite of the strong seasonality of the eddy activity, a robust PV structure is still observed through all the seasons. It is found that besides baroclinic eddies, the boundary layer thermal forcing as well as the moist convection all can help maintain the lower level PV structure. The sensitivity study of the eddy equilibration to the time scale of the external forcing also indicates that the time scale separation between the baroclinic adjustment and the external forcing in midlatitudes is only visible for external forcing cycle one year and longer. The seasonality study with the coupled model confirms the conclusions obtained in the uncoupled model., by Yang Zhang., Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1982., Microfiche copy available in Archives and Science, Bibliography: leaves 39-41., by Giovanna Salustri., M.S.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1981., Microfiche copy available in Archives and Science., Vita., Bibliography: leaves 193-200., by Lee Edward Branscome., Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1981., Microfiche copy available in Archives and Science., Bibliography: leaves 63-65., by Steven John Ghan., M.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2001., Includes bibliographical references (p. 183-193)., The focus of this thesis is the sensitivity of the strength of the meridional overturning circulation to surface forcing and mixing on climatological time scales. An adjoint model is used to gain new insights into the spatial characteristics of the sensitivity patterns. Adjoint models provide the sensitivity of a diagnostic, often called cost function, to all model parameters in a single integration. In contrast, traditional sensitivity analyses are performed by repeated integrations of the so-called "forward" model, perturbing slightly the value of a single parameter at each integration. The results of the adjoint model allows us to calculate global maps of sensitivity. These maps provide a geographic picture of where on the ocean heat and freshwater flux, wind stress and diapycnal mixing perturbations in the North Atlantic are likely to have the greatest impact on the meridional overturning and its heat transport. The adjoint model provides clear identification of the physical mechanisms which can influence the meridional overturning on times scales of years to decades. Boundary and equatorial Kelvin waves and equatorially trapped Rossby waves carry information around the boundaries of the basin and across the equator in less than a decade for a basin of the size of the Atlantic. Advection of buoyancy perturbations has an important influence on the meridional overturning on the decadal time scale. Diffusion is important in determining the final equilibrated state of the meridional overturning on the centennial scale. The role of diapycnal mixing in determining the overturning's strength is confined to regions near the lateral boundaries in the Northern hemisphere and to the tropical region in both hemispheres. The important role played by the tropics in setting the overturning's strength seems to confirm the thermodynamic principles outlined by Sandstrom (1908), Jeffreys (1925) and Munk and Wunsch (1998): upward advection of heat is balanced by downward diffusion., by Véronique Bugnion., Ph.D.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1988., Includes bibliographical references., by Steven John Ghan., Sc.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1982., Microfiche copy available in Archives and Science, Bibliography: leaves 39-41., by Giovanna Salustri., M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmosphere and Planetary Sciences, 1987., Bibliography: p. 179-182., by James S. Risbey., M.S.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1982., Microfiche copy available in Archives and Science., Bibliography: leaves 236-239., by Stephen Brenner., Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1981., Microfiche copy available in Archives and Science., Vita., Bibliography: leaves 193-200., by Lee Edward Branscome., Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1987., Microfiche copy available in Archives and Science., Bibliography: leaves 101-102., by Peter John Sousounis., M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1981., Microfiche copy available in Archives and Science., Bibliography: leaves 63-65., by Steven John Ghan., M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1982., Microfiche copy available in Archives and Science, Bibliography: leaves 39-41., by Giovanna Salustri., M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1981., Microfiche copy available in Archives and Science., Bibliography: leaves 63-65., by Steven John Ghan., M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1981., Microfiche copy available in Archives and Science., Bibliography: leaves 63-65., by Steven John Ghan., M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1982., Microfiche copy available in Archives and Science, Bibliography: leaves 39-41., by Giovanna Salustri., M.S.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1981., Microfiche copy available in Archives and Science., Vita., Bibliography: leaves 193-200., by Lee Edward Branscome., Ph.D.

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Meteorology, 1979., Microfiche copy available in Archives and Science., Vita., Bibliography: leaves 242-247., by Antony Brett Mullan., Sc.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1982., Microfiche copy available in Archives and Science, Bibliography: leaves 39-41., by Giovanna Salustri., M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmosphere and Planetary Sciences, 1987., Bibliography: p. 179-182., by James S. Risbey., M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1982., Microfiche copy available in Archives and Science, Bibliography: leaves 39-41., by Giovanna Salustri., M.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2009., This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections., Cataloged from student submitted PDF version of thesis., Includes bibliographical references (p. 257-266)., In this thesis, the influence of boundary layer processes and seasonal forcing on baroclinic eddy equilibration is studied to understand how the baroclinic adjustment is modified when taking into account these two factors. A modified [beta] plane multilevel quasi-geostrophic (QG) model with an interactive stratification and a simplified parameterization of atmospheric boundary layer physics is used as the atmospheric model in this study. Comparisons between experiments with the modified QG model and the traditional QG model with fixed stratification show that it is necessary to include the interaction between vertical eddy heat flux and stratification to obtain a realistic equilibrated state, i.e. robust isentropic slope. A slab surface model is also coupled with the atmospheric model to provide an interactive surface temperature distribution in some experiments in this study. The effect of boundary layer processes is first studied under the situation with fixed underlying surface temperature. The boundary layer vertical thermal diffusion, along with the surface heat exchange, is found primarily responsible for the limitation of the PV homogenization in the boundary layer. The boundary layer processes can influence the eddy activity in at least two competing ways. First, in the eddy energy budget, all the boundary layer processes act to damp the eddy energy directly. On the other hand, the boundary layer processes also influence the mean flow, which can further influence the eddy behavior. For the boundary layer thermal damping, the indirect effect on the eddy activity becomes dominant., (cont.) Stronger boundary layer thermal forcing results in stronger meridional temperature gradients and eddy heat fluxes. For the frictional dissipation, the resulting changes in the zonal wind and in the location of the critical latitude lead to a meridional variation of the eddy forcing, which can further result in a non-monotonic response of the mean flow. The role of the boundary layer processes in the baroclinic eddy equilibration is further studied using a simple air-sea thermally coupled model. Although in the coupled system, each boundary layer process has more and different ways to influence the equilibrium state, their effect on the lower level PV homogenization is very robust. Surface friction and surface 3 heat flux all act to damp the lower level PV mixing and stronger surface damping prevents the PV homogenization more strongly, but the way in which each boundary layer process affects the PV homogenization is very different. Baroclinic eddy equilibration under seasonal forcing is studied in both the atmospheric model and the coupled model. In the situation with specified seasonal variation of the underlying surface temperature, a Northern-Hemisphere like seasonal variation of the surface temperature is used to act on the atmospheric flow through the boundary layer processes and the radiative-convective heating., (cont.) Under slowly varying seasonal forcing, the eddy and the mean flow behavior is characterized by four clearly divided time intervals: an eddy inactive time interval in the summer, a mainly dynamically determined eddy spinup time interval starting from mid-fall and lasting less than one month, a quasi-equilibrium time interval for the zonal mean flow available potential energy from late fall to late spring and a mainly external forcing determined eddy spindown time interval from late winter to late spring. In spite of the strong seasonality of the eddy activity, a robust PV structure is still observed through all the seasons. It is found that besides baroclinic eddies, the boundary layer thermal forcing as well as the moist convection all can help maintain the lower level PV structure. The sensitivity study of the eddy equilibration to the time scale of the external forcing also indicates that the time scale separation between the baroclinic adjustment and the external forcing in midlatitudes is only visible for external forcing cycle one year and longer. The seasonality study with the coupled model confirms the conclusions obtained in the uncoupled model., by Yang Zhang., Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1981., Microfiche copy available in Archives and Science., Vita., Bibliography: leaves 193-200., by Lee Edward Branscome., Ph.D.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1988., Includes bibliographical references., by Steven John Ghan., Sc.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric and Planetary Sciences, 1987., Vita., Bibliography: leaves 131-133., by Yung-An Lee., Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1982., Microfiche copy available in Archives and Science., Bibliography: leaves 236-239., by Stephen Brenner., Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1982., Microfiche copy available in Archives and Science, Bibliography: leaves 39-41., by Giovanna Salustri., M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1981., Microfiche copy available in Archives and Science., Bibliography: leaves 63-65., by Steven John Ghan., M.S.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmosphere and Planetary Sciences, 1987., Bibliography: p. 179-182., by James S. Risbey., M.S.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1982., Microfiche copy available in Archives and Science., Bibliography: leaves 236-239., by Stephen Brenner., Ph.D.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1988., Includes bibliographical references., by Steven John Ghan., Sc.D.

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Meteorology, 1979., Microfiche copy available in Archives and Science., Vita., Bibliography: leaves 242-247., by Antony Brett Mullan., Sc.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2001., Includes bibliographical references (p. 183-193)., The focus of this thesis is the sensitivity of the strength of the meridional overturning circulation to surface forcing and mixing on climatological time scales. An adjoint model is used to gain new insights into the spatial characteristics of the sensitivity patterns. Adjoint models provide the sensitivity of a diagnostic, often called cost function, to all model parameters in a single integration. In contrast, traditional sensitivity analyses are performed by repeated integrations of the so-called "forward" model, perturbing slightly the value of a single parameter at each integration. The results of the adjoint model allows us to calculate global maps of sensitivity. These maps provide a geographic picture of where on the ocean heat and freshwater flux, wind stress and diapycnal mixing perturbations in the North Atlantic are likely to have the greatest impact on the meridional overturning and its heat transport. The adjoint model provides clear identification of the physical mechanisms which can influence the meridional overturning on times scales of years to decades. Boundary and equatorial Kelvin waves and equatorially trapped Rossby waves carry information around the boundaries of the basin and across the equator in less than a decade for a basin of the size of the Atlantic. Advection of buoyancy perturbations has an important influence on the meridional overturning on the decadal time scale. Diffusion is important in determining the final equilibrated state of the meridional overturning on the centennial scale. The role of diapycnal mixing in determining the overturning's strength is confined to regions near the lateral boundaries in the Northern hemisphere and to the tropical region in both hemispheres. The important role played by the tropics in setting the overturning's strength seems to confirm the thermodynamic principles outlined by Sandstrom (1908), Jeffreys (1925) and Munk and Wunsch (1998): upward advection of heat is balanced by downward diffusion., by Véronique Bugnion., Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1982., Microfiche copy available in Archives and Science, Bibliography: leaves 39-41., by Giovanna Salustri., M.S.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric and Planetary Sciences, 1984., Microfiche copy available in Archives and Science., Vita., Bibliography: leaves 287-293., by William Joseph Gutowski, Jr., Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2009., This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections., Cataloged from student submitted PDF version of thesis., Includes bibliographical references (p. 257-266)., In this thesis, the influence of boundary layer processes and seasonal forcing on baroclinic eddy equilibration is studied to understand how the baroclinic adjustment is modified when taking into account these two factors. A modified [beta] plane multilevel quasi-geostrophic (QG) model with an interactive stratification and a simplified parameterization of atmospheric boundary layer physics is used as the atmospheric model in this study. Comparisons between experiments with the modified QG model and the traditional QG model with fixed stratification show that it is necessary to include the interaction between vertical eddy heat flux and stratification to obtain a realistic equilibrated state, i.e. robust isentropic slope. A slab surface model is also coupled with the atmospheric model to provide an interactive surface temperature distribution in some experiments in this study. The effect of boundary layer processes is first studied under the situation with fixed underlying surface temperature. The boundary layer vertical thermal diffusion, along with the surface heat exchange, is found primarily responsible for the limitation of the PV homogenization in the boundary layer. The boundary layer processes can influence the eddy activity in at least two competing ways. First, in the eddy energy budget, all the boundary layer processes act to damp the eddy energy directly. On the other hand, the boundary layer processes also influence the mean flow, which can further influence the eddy behavior. For the boundary layer thermal damping, the indirect effect on the eddy activity becomes dominant., (cont.) Stronger boundary layer thermal forcing results in stronger meridional temperature gradients and eddy heat fluxes. For the frictional dissipation, the resulting changes in the zonal wind and in the location of the critical latitude lead to a meridional variation of the eddy forcing, which can further result in a non-monotonic response of the mean flow. The role of the boundary layer processes in the baroclinic eddy equilibration is further studied using a simple air-sea thermally coupled model. Although in the coupled system, each boundary layer process has more and different ways to influence the equilibrium state, their effect on the lower level PV homogenization is very robust. Surface friction and surface 3 heat flux all act to damp the lower level PV mixing and stronger surface damping prevents the PV homogenization more strongly, but the way in which each boundary layer process affects the PV homogenization is very different. Baroclinic eddy equilibration under seasonal forcing is studied in both the atmospheric model and the coupled model. In the situation with specified seasonal variation of the underlying surface temperature, a Northern-Hemisphere like seasonal variation of the surface temperature is used to act on the atmospheric flow through the boundary layer processes and the radiative-convective heating., (cont.) Under slowly varying seasonal forcing, the eddy and the mean flow behavior is characterized by four clearly divided time intervals: an eddy inactive time interval in the summer, a mainly dynamically determined eddy spinup time interval starting from mid-fall and lasting less than one month, a quasi-equilibrium time interval for the zonal mean flow available potential energy from late fall to late spring and a mainly external forcing determined eddy spindown time interval from late winter to late spring. In spite of the strong seasonality of the eddy activity, a robust PV structure is still observed through all the seasons. It is found that besides baroclinic eddies, the boundary layer thermal forcing as well as the moist convection all can help maintain the lower level PV structure. The sensitivity study of the eddy equilibration to the time scale of the external forcing also indicates that the time scale separation between the baroclinic adjustment and the external forcing in midlatitudes is only visible for external forcing cycle one year and longer. The seasonality study with the coupled model confirms the conclusions obtained in the uncoupled model., by Yang Zhang., Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1981., Microfiche copy available in Archives and Science., Bibliography: leaves 63-65., by Steven John Ghan., M.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2008., This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections., Includes bibliographical references (p. 231-245)., The Madden-Julian oscillation (MJO), the dominant tropical intraseasonal variability with widespread meteorological impacts, continues to puzzle the climate research community on both theoretical and modeling fronts. Motivated by a recent interest in the role of humidity in tropical dynamics, this thesis hypothesizes that the MJO is a nonlinear moisture mode whose existence depends on moisture-convection feedback (the feedback between deep convection and environmental free-tropospheric humidity), and that weak moisture convection feedback in general circulation models accounts for their deficiencies with the MJO simulations. Moisture modes are found to exist in a large class of linear primitive equation models on the equatorial beta-plane. For models with standard quasi-equilibrium parameterizations,perturbation expansion analyses demonstrate that the weak temperature gradient (WTG) approximation of Sobel et al. describes the small-scale limit of the moisture mode accurately,with the small expansion parameter being the ratio between temperature tendency and adiabatic cooling. Under the WTG balance, the only leading order variables are humidity and vertical motion. Analyses of three models in the literature show that a moisture mode is unstable if moist static energy (MSE) sources such as cloud radiative forcing or gust-enhanced surface heat flux exceed the MSE export. Numerical calculations of a single-column model under the WTG configuration show that a realistic convective scheme can reproduce moisture mode instability. Sensitivity tests on the strength of moisture-convection feedback in the Emanuel scheme indicate that such a feedback is essential for moisture mode instability, confirming the prediction from simple models. To explore the nonlinear regime of a moisture mode, numerical calculations of a simplified Quasi-equilibrium Tropical Circulation Model on the equatorial beta-plane have been performed., A classical Gill model augmented with a prognostic humidity variable can capture nonlinear dynamics of the moisture mode. In particular, nonlinear advection of dry air by Rossby gyres is found to move the moisture mode eastward. Notwithstanding progress made in the present thesis, numerous difficulties abound. The most significant issue is that moisture mode instability favors the smallest scale in the linear regime. The author suggests that scale interaction and its effect on the humidity budget might be an important research topic., by Masahiro Sugiyama., Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2008., This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections., Includes bibliographical references (p. 231-245)., The Madden-Julian oscillation (MJO), the dominant tropical intraseasonal variability with widespread meteorological impacts, continues to puzzle the climate research community on both theoretical and modeling fronts. Motivated by a recent interest in the role of humidity in tropical dynamics, this thesis hypothesizes that the MJO is a nonlinear moisture mode whose existence depends on moisture-convection feedback (the feedback between deep convection and environmental free-tropospheric humidity), and that weak moisture convection feedback in general circulation models accounts for their deficiencies with the MJO simulations. Moisture modes are found to exist in a large class of linear primitive equation models on the equatorial beta-plane. For models with standard quasi-equilibrium parameterizations,perturbation expansion analyses demonstrate that the weak temperature gradient (WTG) approximation of Sobel et al. describes the small-scale limit of the moisture mode accurately,with the small expansion parameter being the ratio between temperature tendency and adiabatic cooling. Under the WTG balance, the only leading order variables are humidity and vertical motion. Analyses of three models in the literature show that a moisture mode is unstable if moist static energy (MSE) sources such as cloud radiative forcing or gust-enhanced surface heat flux exceed the MSE export. Numerical calculations of a single-column model under the WTG configuration show that a realistic convective scheme can reproduce moisture mode instability. Sensitivity tests on the strength of moisture-convection feedback in the Emanuel scheme indicate that such a feedback is essential for moisture mode instability, confirming the prediction from simple models. To explore the nonlinear regime of a moisture mode, numerical calculations of a simplified Quasi-equilibrium Tropical Circulation Model on the equatorial beta-plane have been performed., A classical Gill model augmented with a prognostic humidity variable can capture nonlinear dynamics of the moisture mode. In particular, nonlinear advection of dry air by Rossby gyres is found to move the moisture mode eastward. Notwithstanding progress made in the present thesis, numerous difficulties abound. The most significant issue is that moisture mode instability favors the smallest scale in the linear regime. The author suggests that scale interaction and its effect on the humidity budget might be an important research topic., by Masahiro Sugiyama., Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2003., Includes bibliographical references (leaves 61-67)., The diapycnal diffusivity of the ocean is one of the least known parameters in cur- rent climate models. Measurements of this diffusivity are sparse and insufficient for compiling a global map. Inferences from inverse methods and energy budget calculations suggests as much as a factor of 5 difference in the global mean value of the diapycnal diffusivity. Yet, the climate is extremely sensitive to the diapycnal diffusivity, as shown by studies using single-hemispheric ocean General Circulation Models (GCMs) and 2-dimensional coupled models. In this thesis we study the sensitivity of both the current climate and the climate change to the diapycnal diffusivity - using, for the first time, a coupled model with a 3-dimensional global ocean component and idealized geometry. Our results show that, at equilibrium, the strength of the thermohaline circulation in the North Atlantic scales with the 0.44 power of the diapycnal diffusivity, in contrast to the theoretical value of 2/3. On the other hand, the strength of the circulation in the South Pacific scales with the 0.63 power of the diapycnal diffusivity. The implication is that the amount of water upwelling from the deep ocean may be regulated by the diapycnal diffusion in the Indo-Pacific ocean. The vertical heat balance in the ocean is controlled by: in the downward direction, (i) advection and (ii) diapycnal diffusion; in the upward direction, (iii) isopycnal diffusion and (iv) bolus velocity (GM) advection. The size of the latter three fluxes increases with diapycnal diffusivity., (cont.) The thickness of the thermocline also increases with diapycnal diffusivity leading to greater isopycnal slopes at high latitudes, and hence enhanced isopycnal diffusion and GM advection. Larger diapycnal diffusion compensates for changes in isopycnal diffusion and GM advection. Little changes are found for the advective flux because of compensation between changes in downward and upward advective fluxes. We present sensitivity results for the hysteresis curve of the thermohaline circulation. The stability of the climate system to slow freshwater perturbations is reduced as a consequence of a smaller diapycnal diffusivity. This result confirms the findings of 2-dimensional climate models. However, contrary to the results of these studies, a common threshold for the shutdown of the thermohaline circulation is not found in our model. In our global warming experiments, the thermohaline circulation slows down for about 100 years and recovers afterward, for any value of the diapycnal diffusivity. The rates of slowdown and of recovery, as well as the percentage recovery of the circulation at the end of 1000-year integration, is variable but a direct relation with the diapycnal diffusivity cannot be found. The steric height gradient is divided into a temperature component and a salinity component. It appears that, in the first 70 years of simulated global warming, temperature variations dominate the salinity ones in weakly diffusive models, whereas the opposite occurs in strongly diffusive models. The analysis of the vertical heat balance reveals that, in global warming experiments, deep ocean heat uptake is due to reduced upward isopycnal diffusive flux and GM advective flux ..., by Fabio Dalan., S.M.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2003., Includes bibliographical references (leaves 61-67)., The diapycnal diffusivity of the ocean is one of the least known parameters in cur- rent climate models. Measurements of this diffusivity are sparse and insufficient for compiling a global map. Inferences from inverse methods and energy budget calculations suggests as much as a factor of 5 difference in the global mean value of the diapycnal diffusivity. Yet, the climate is extremely sensitive to the diapycnal diffusivity, as shown by studies using single-hemispheric ocean General Circulation Models (GCMs) and 2-dimensional coupled models. In this thesis we study the sensitivity of both the current climate and the climate change to the diapycnal diffusivity - using, for the first time, a coupled model with a 3-dimensional global ocean component and idealized geometry. Our results show that, at equilibrium, the strength of the thermohaline circulation in the North Atlantic scales with the 0.44 power of the diapycnal diffusivity, in contrast to the theoretical value of 2/3. On the other hand, the strength of the circulation in the South Pacific scales with the 0.63 power of the diapycnal diffusivity. The implication is that the amount of water upwelling from the deep ocean may be regulated by the diapycnal diffusion in the Indo-Pacific ocean. The vertical heat balance in the ocean is controlled by: in the downward direction, (i) advection and (ii) diapycnal diffusion; in the upward direction, (iii) isopycnal diffusion and (iv) bolus velocity (GM) advection. The size of the latter three fluxes increases with diapycnal diffusivity., (cont.) The thickness of the thermocline also increases with diapycnal diffusivity leading to greater isopycnal slopes at high latitudes, and hence enhanced isopycnal diffusion and GM advection. Larger diapycnal diffusion compensates for changes in isopycnal diffusion and GM advection. Little changes are found for the advective flux because of compensation between changes in downward and upward advective fluxes. We present sensitivity results for the hysteresis curve of the thermohaline circulation. The stability of the climate system to slow freshwater perturbations is reduced as a consequence of a smaller diapycnal diffusivity. This result confirms the findings of 2-dimensional climate models. However, contrary to the results of these studies, a common threshold for the shutdown of the thermohaline circulation is not found in our model. In our global warming experiments, the thermohaline circulation slows down for about 100 years and recovers afterward, for any value of the diapycnal diffusivity. The rates of slowdown and of recovery, as well as the percentage recovery of the circulation at the end of 1000-year integration, is variable but a direct relation with the diapycnal diffusivity cannot be found. The steric height gradient is divided into a temperature component and a salinity component. It appears that, in the first 70 years of simulated global warming, temperature variations dominate the salinity ones in weakly diffusive models, whereas the opposite occurs in strongly diffusive models. The analysis of the vertical heat balance reveals that, in global warming experiments, deep ocean heat uptake is due to reduced upward isopycnal diffusive flux and GM advective flux ..., by Fabio Dalan., S.M.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2003., Includes bibliographical references (leaves 87-97)., A thorough analysis of the stability of uncoupled and coupled versions of an inter-hemispheric 3-box model of Thermohaline Circulation (THC) is presented. The model consists of a northern high latitudes box, a tropical box, and a southern high latitudes box, which respectively can be thought as corresponding to the northern, tropical and southern Atlantic ocean. We study how the strength of THC changes when the system undergoes forcings that are analogous to those of global warming conditions. In each class of experiments, we determine, using suitably defined metrics, the boundary dividing the set of forcing scenarios that lead the system to equilibria characterized by a THC pattern similar to the present one, from those that drive the system to equilibria where the THC is reversed. In the case of the uncoupled model, we apply to the equilibrium state perturbations to the moisture and heat fluxes into the three boxes. High rates of increase in the moisture flux into the northern high-latitude box lead to a THC breakdown at smaller total final increases in the moisture flux than low rates, while the presence of moisture flux increases into the southern high-latitude box strongly inhibit the breakdown and can prevent it, in the case of slow rates in the Northern Hemisphere. Similarly, a fast heat flux increases in the North Hemisphere destabilize the system more effectively than slow ones, and again the enhancement of the heat fluxes in the Southern Hemisphere tend to drive the system towards stability. In all cases analyzed slow forcings, if sufficiently weak in the Southern Hemisphere, lead to the reversal of the THC., (cont.) In the coupled model a direct representation of the radiative forcing is possible, since the main atmospheric physical processes responsible for freshwater and heat fluxes are formulated separately. Although only weakly asymmetric or symmetric radiative forcings are representative of physically reasonable conditions, we consider general asymmetric forcings, in order to get a more complete picture of the mathematical properties of the system. We also consider different choices for the atmospheric transport parametrizations and for the ratio between the high latitude to tropical radiative forcing, and analyze the sensitivity of our results to changes in these parameters. We generally find that fast forcings are more effective than slow forcings in disrupting the present THC patterns, forcings that are stronger in the northern box are also more effective in destabilizing the system, and that very slow forcings do not destabilize the system whatever their asymmetry, unless the radiative forcings are very asymmetric and the atmospheric transport is a relatively weak function of the meridional temperature gradient. The changes in the strength of the THC are primarily forced by changes in the latent heat transports, because of their sensitivity to temperature that arises from the Clausius-Clapeyron relation., by Valerio Lucarini., S.M.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2003., Includes bibliographical references (leaves 87-97)., A thorough analysis of the stability of uncoupled and coupled versions of an inter-hemispheric 3-box model of Thermohaline Circulation (THC) is presented. The model consists of a northern high latitudes box, a tropical box, and a southern high latitudes box, which respectively can be thought as corresponding to the northern, tropical and southern Atlantic ocean. We study how the strength of THC changes when the system undergoes forcings that are analogous to those of global warming conditions. In each class of experiments, we determine, using suitably defined metrics, the boundary dividing the set of forcing scenarios that lead the system to equilibria characterized by a THC pattern similar to the present one, from those that drive the system to equilibria where the THC is reversed. In the case of the uncoupled model, we apply to the equilibrium state perturbations to the moisture and heat fluxes into the three boxes. High rates of increase in the moisture flux into the northern high-latitude box lead to a THC breakdown at smaller total final increases in the moisture flux than low rates, while the presence of moisture flux increases into the southern high-latitude box strongly inhibit the breakdown and can prevent it, in the case of slow rates in the Northern Hemisphere. Similarly, a fast heat flux increases in the North Hemisphere destabilize the system more effectively than slow ones, and again the enhancement of the heat fluxes in the Southern Hemisphere tend to drive the system towards stability. In all cases analyzed slow forcings, if sufficiently weak in the Southern Hemisphere, lead to the reversal of the THC., (cont.) In the coupled model a direct representation of the radiative forcing is possible, since the main atmospheric physical processes responsible for freshwater and heat fluxes are formulated separately. Although only weakly asymmetric or symmetric radiative forcings are representative of physically reasonable conditions, we consider general asymmetric forcings, in order to get a more complete picture of the mathematical properties of the system. We also consider different choices for the atmospheric transport parametrizations and for the ratio between the high latitude to tropical radiative forcing, and analyze the sensitivity of our results to changes in these parameters. We generally find that fast forcings are more effective than slow forcings in disrupting the present THC patterns, forcings that are stronger in the northern box are also more effective in destabilizing the system, and that very slow forcings do not destabilize the system whatever their asymmetry, unless the radiative forcings are very asymmetric and the atmospheric transport is a relatively weak function of the meridional temperature gradient. The changes in the strength of the THC are primarily forced by changes in the latent heat transports, because of their sensitivity to temperature that arises from the Clausius-Clapeyron relation., by Valerio Lucarini., S.M.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2003., Includes bibliographical references (leaves 61-67)., The diapycnal diffusivity of the ocean is one of the least known parameters in cur- rent climate models. Measurements of this diffusivity are sparse and insufficient for compiling a global map. Inferences from inverse methods and energy budget calculations suggests as much as a factor of 5 difference in the global mean value of the diapycnal diffusivity. Yet, the climate is extremely sensitive to the diapycnal diffusivity, as shown by studies using single-hemispheric ocean General Circulation Models (GCMs) and 2-dimensional coupled models. In this thesis we study the sensitivity of both the current climate and the climate change to the diapycnal diffusivity - using, for the first time, a coupled model with a 3-dimensional global ocean component and idealized geometry. Our results show that, at equilibrium, the strength of the thermohaline circulation in the North Atlantic scales with the 0.44 power of the diapycnal diffusivity, in contrast to the theoretical value of 2/3. On the other hand, the strength of the circulation in the South Pacific scales with the 0.63 power of the diapycnal diffusivity. The implication is that the amount of water upwelling from the deep ocean may be regulated by the diapycnal diffusion in the Indo-Pacific ocean. The vertical heat balance in the ocean is controlled by: in the downward direction, (i) advection and (ii) diapycnal diffusion; in the upward direction, (iii) isopycnal diffusion and (iv) bolus velocity (GM) advection. The size of the latter three fluxes increases with diapycnal diffusivity., (cont.) The thickness of the thermocline also increases with diapycnal diffusivity leading to greater isopycnal slopes at high latitudes, and hence enhanced isopycnal diffusion and GM advection. Larger diapycnal diffusion compensates for changes in isopycnal diffusion and GM advection. Little changes are found for the advective flux because of compensation between changes in downward and upward advective fluxes. We present sensitivity results for the hysteresis curve of the thermohaline circulation. The stability of the climate system to slow freshwater perturbations is reduced as a consequence of a smaller diapycnal diffusivity. This result confirms the findings of 2-dimensional climate models. However, contrary to the results of these studies, a common threshold for the shutdown of the thermohaline circulation is not found in our model. In our global warming experiments, the thermohaline circulation slows down for about 100 years and recovers afterward, for any value of the diapycnal diffusivity. The rates of slowdown and of recovery, as well as the percentage recovery of the circulation at the end of 1000-year integration, is variable but a direct relation with the diapycnal diffusivity cannot be found. The steric height gradient is divided into a temperature component and a salinity component. It appears that, in the first 70 years of simulated global warming, temperature variations dominate the salinity ones in weakly diffusive models, whereas the opposite occurs in strongly diffusive models. The analysis of the vertical heat balance reveals that, in global warming experiments, deep ocean heat uptake is due to reduced upward isopycnal diffusive flux and GM advective flux ..., by Fabio Dalan., S.M.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1991., Includes bibliographical references (p. 207-215)., by Shuntai Zhou., Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2004., Includes bibliographical references (leaves 137-154)., This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections., Following Milankovitch's original hypothesis most model studies of changes in global ice volume on orbital time scales have focused on the impact of ablation on ice sheet mass balance. In most cases, poleward moisture flux is fixed and accumulation of snow only depends on local temperature. In this study, a simple coupled atmosphere-ice process model is introduced. An improved representation of the atmospheric hydrological cycle is included, and accumulation is related to the meridional flux of moisture by large scale baroclinic eddies. The ice sheets in the Northern Hemisphere respond to both precession and obliquity frequencies when the model is forced with seasonal insolation. Obliquity variations are introduced by the impact of earth's tilt on the meridional temperature gradient and the poleward flux of moisture, whereas precession governs surface melting by regulating summer temperatures. The response of the ice sheet to obliquity and precession is comparable, and significantly smaller than what is observed in the oxygen isotope record of the late Pliocene and early Pleistocene (2.7 - 0.8 Ma BP). This suggests that in order to successfully reproduce the strong 41 Ka periodicity observed in the record, other mechanisms must be involved such as nonlinear self-sustained, or stochastic processes, or alternatively the obliquity dominated signal originates from Antarctica. In Antarctica the seasonal cycle is damped due to the large thermal mass of the southern ocean, and surface melt is insignificant. Both of these factors reduce the influence of precession in regulating ice volume. Instead, the mass balance is dominated by accumulation and calving, thereby enhancing the role of obliquity in controlling ice volume., by Kerim Hestnes Nisancioglu., Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1991., Includes bibliographical references (p. 207-215)., by Shuntai Zhou., Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2004., Includes bibliographical references (leaves 137-154)., This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections., Following Milankovitch's original hypothesis most model studies of changes in global ice volume on orbital time scales have focused on the impact of ablation on ice sheet mass balance. In most cases, poleward moisture flux is fixed and accumulation of snow only depends on local temperature. In this study, a simple coupled atmosphere-ice process model is introduced. An improved representation of the atmospheric hydrological cycle is included, and accumulation is related to the meridional flux of moisture by large scale baroclinic eddies. The ice sheets in the Northern Hemisphere respond to both precession and obliquity frequencies when the model is forced with seasonal insolation. Obliquity variations are introduced by the impact of earth's tilt on the meridional temperature gradient and the poleward flux of moisture, whereas precession governs surface melting by regulating summer temperatures. The response of the ice sheet to obliquity and precession is comparable, and significantly smaller than what is observed in the oxygen isotope record of the late Pliocene and early Pleistocene (2.7 - 0.8 Ma BP). This suggests that in order to successfully reproduce the strong 41 Ka periodicity observed in the record, other mechanisms must be involved such as nonlinear self-sustained, or stochastic processes, or alternatively the obliquity dominated signal originates from Antarctica. In Antarctica the seasonal cycle is damped due to the large thermal mass of the southern ocean, and surface melt is insignificant. Both of these factors reduce the influence of precession in regulating ice volume. Instead, the mass balance is dominated by accumulation and calving, thereby enhancing the role of obliquity in controlling ice volume., by Kerim Hestnes Nisancioglu., Ph.D.