Thesis (Ph. D. in Geology and Geophysics)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2011. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from PDF version of thesis. / Includes bibliographical references. / Hurricane activity in the Northeastern Gulf of Mexico and its relationship to regional and large-scale climate variability during the Late Holocene is explored. A 4500-year record of hurricane-induced storm surges is developed from sediment cores collected from a coastal sinkhole near Apalachee Bay, Florida. Reconstructed hurricane frequency is shown to exhibit statistically significant variability with the greatest activity occurring between 2700 and 2400 years ago and the least activity between 1900 to 1600 years ago and after 600 years ago. Proxy records of storm-relevant climate variables contain similar timescales of variability and suggest both regional and large-scale mechanisms have influenced hurricane activity on centennial to millennial timescales. In particular, low-frequency migrations of the Loop Current may exercise control over regional hurricane activity by changing the thermal structure of the upper ocean and influencing the role of storm-induced upwelling on hurricane intensification. A new method for estimating the frequency of hurricane-generated storm surges is presented and applied to Apalachee Bay, Florida. Multisite paleohurricane reconstructions from this region are developed, and the effects of geographic boundary conditions and temporal resolution on estimates of paleohurricane frequency are explored. / by Daniel Philip Lane. / Ph.D.in Geology and Geophysics

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology and Physical Oceanography, 1981. / Microfiche copy available in Archives and Science / Bibliography: leaves 96-103. / by Yun-chi Tseng. / M.S.

Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2009. / 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. 207-211). / My thesis covers two general circulation problems that involve the stability of largescale oceanic flows and the importance of non-local effects. The first problem examines the stability of meridional boundary currents, which are found on both sides of most ocean basins because of the presence of continents. A linear stability analysis of a meridional boundary current on the beta-plane is performed using a quasi-geostrophic model in order to determine the existence of radiating instabilities, a type of instability that propagates energy away from its origin region by exciting Rossby waves and can thus act as a source of eddy energy for the ocean interior. It is found that radiating instabilities are commonly found in both eastern and western boundary currents. However, there are some significant dierences that make eastern boundary currents more interesting from a radiation point of view. They possess a larger number of radiating modes, characterized by horizontal wavenumbers which would make them appear like zonal jets as they propagate into the ocean interior. The second problem examines the circulation in a nonlinear thermally-forced twolayer quasi-geostrophic ocean. The only driving force for the circulation in the model is a cross-isopycnal flux parameterized as interface relaxation. This forcing is similar to the radiative damping used commonly in atmospheric models, except that it is applied to the ocean circulation in a closed basin and is meant to represent the large-scale thermal forcing acting on the oceans. / (cont.) It is found that in the strongly nonlinear regime a substantial, not directly thermally-driven barotropic circulation is generated. Its variability in the limit of weak bottom drag is dominated by highfrequency barotropic basin modes. It is demonstrated that the excitation of basin normal modes has significant consequences for the mean state of the system and its variability, conclusions that are likely to apply for any other system whose variability is dominated by basin modes, no matter the forcing. A linear stability analysis performed on a wind- and a thermally-forced double-gyre circulation reveals that under certain conditions the basin modes can arise from local instabilities of the flow. / by Hristina G. Hristova. / Ph.D.

In this thesis, I explore the climate system's response to symmetric abrupt and transient CO₂ forcing across a range of concentrations, from ⅛ ⨉ to 8⨉CO₂, relative to pre-industrial levels. I use two CMIP6 class models: the CESM Large Ensemble (CESM-LE) model configuration and the NASA Goddard Institute for Space Studies Model E2.1-G (GISS-E2.1-G). I use a hierarchy of (1) fully coupled atmosphere-ocean-sea-ice-land, (2) slab ocean, and (3) prescribed sea surface temperature simulations to analyze and support the findings. First, I find an asymmetric response in global mean surface air temperature (𝚫𝜯_s) and effective climate sensitivity (EffCS) between colder and warmer experiments. The 𝚫𝜯_s response at 8⨉CO₂ is more than a third larger than the corresponding cooling at ⅛⨉CO₂. I attribute this assymetry primarily due to the non-logarithmic CO₂ forcing, not to changes in the radiative feedbacks. Second, I identify a non-monotonic response of EffCS in the warmer scenarios, with a minimum occurring at 4⨉CO₂ (3⨉CO₂) in CESM-LE (GISS-E2.1-G). This minimum in the warming simulations is associated with a non-monotonicity in the radiative feedback. Similar non-monotonic responses in Northern Hemisphere sea-ice, precipitation, the latitude of zero precipitation-minus-evaporation, and the strength of the Hadley cell are also identified. Comparing the climate response over the same CO₂ range between fully coupled and slab-ocean versions of the same models, I demonstrate that the climate system’s non-monotonic response is linked to changes in ocean dynamics, associated with a collapse of the Atlantic Meridional Overturning Circulation (AMOC). Third, to establish the significance of North Atlantic cooling in driving the non-monotonic changes in the radiative feedback, I conducted additional atmosphere-only (AMIP) simulations using the same models but with prescribed sea surface temperatures (SSTs) restricted to different regions. Through these simulations, I uncovered that the minimum EffCS value, characterized by notably negative radiative feedbacks, primarily originates from relative cooling of the sea surface temperature (SST) in the tropical and subtropical North Atlantic. This cooling of SSTs contributes to an increase in low-level cloud content in the eastern region of the North Atlantic, subsequently leading to a pronounced negative (stabilizing) feedback response. Furthermore, I investigated the state dependence of the effective radiative forcing (ERF) from 1/16 ⨉ to 16⨉CO₂. I found that ERF increases with CO₂ concentration due to the increase in Instantaneous Radiative Forcing (IRF). Specifically, the IRF increases at higher CO₂ values primarily due to stronger stratospheric cooling induced by CO₂ forcing. On the other hand, the radiative adjustments counteract the IRF increase, causing the ERF to rise at a slower pace compared to the corresponding increase in IRF induced by higher CO₂ concentrations. Lastly, I studied the winter storm tracks in the Southern Hemisphere, focusing on experiments up to 8⨉CO₂. Through this analysis, I identified a non-linear response in the low latitude storm tracks. It is projected that the storm tracks will experience an intensification by the end of the century. However, my findings reveal that this intensification does not scale linearly with CO₂ forcing. In fact, the storm tracks shift poleward, including a reduction of the storm tracks at low-mid latitudes and intensification at mid-high latitudes.

The Atlantic Meridional Overturning Circulation (AMOC) is thought to exert considerable influence over the climate via heat redistribution and carbon storage. Its repeated variations along with the regional and global climate during the last glacial cycle suggest that the state of the AMOC may be roughly divided into “warm,” “cold,” and “off” modes. The three modes correspond to the vigorous deepwater formation in the subpolar North Atlantic, a reduced deepwater formation, and the widespread disruption of the AMOC, respectively. Questions remain about the cause and response of AMOC perturbations in each of the three modes.Reconstruction of the burial flux of ice-rafted debris can resolve questions about the timing and rates of ice sheet calving, which may have been responsible for the “off” mode of the AMOC, given the association of freshwater forcing with AMOC strength. The first chapter quantified the flux of ice-rafted debris in a pair of cores collected from sites in the western North Atlantic. The results show higher ice-rafted debris flux during all Heinrich events and that the western North Atlantic fluxes were higher than the east. The data demonstrate that the Laurentide Ice Sheet played a role in all Heinrich events. A catastrophic last interglacial Laurentide outburst (LILO) event some 125,000 years ago (125 ka) may have contributed to abrupt climate change during the Eemian, when the AMOC was in the “warm” mode. The LILO event was previously proposed to be an analog of the Holocene 8.2 ka event. The second chapter investigated the age and chemical compositions of a layer of red sediments deposited across much of the Northwest Atlantic at 125 ka. The results provide strong support for the occurrence of the LILO event that was analogous to the 8.2 ka event in provenance, timing, and delivery. Little is known about the zonal (east/west) characteristics of the AMOC when in the “cold” mode during the Last Glacial Maximum. Authigenic uranium preserved in sediments is a sensitive redox tracer and can shed light on bottom water oxygen, carbon storage, and water mass distributions. In the third chapter, new and published authigenic uranium data were used to reconstruct deep ocean oxygenation. The compilation shows that lower-than-Holocene oxygen and correspondingly greater respired carbon storage were persistent features of the LGM in the deep North Atlantic. The eastern basin was substantially less well oxygenated than the west. A farther advance and greater infilling in the east of deep waters originating from the Southern Ocean may have caused the zonal difference. Alternatively, deep waters originating from the subpolar North Atlantic may have increased in their residence time in the eastern transect. Questions remain about the flux of freshwater necessary to induce the AMOC to enter the “off” mode. Existing estimates do not agree on the freshwater fluxes associated with Heinrich events. The fourth chapter uses compiled 230Thxs-based mass fluxes in the North Atlantic during the last glacial period to calculate the surge mass fluxes as a measure of the rate of ice-rafted debris deposition. The surge mass fluxes were then converted into freshwater fluxes. Freshwater fluxes for an arbitrarily defined 2000-year period and total freshwater volumes between 20° and 70° N were as high as 0.11 Sv and 6.9 × 1015 m³ during Heinrich event 4 and as low as 0.0012 Sv and 7.6 × 1013 m³ during Heinrich event 3. The relatively low freshwater fluxes we reconstructed for Heinrich events might suggest potentially a high sensitivity of the Atlantic Meridional Overturning Circulation to freshwater perturbations, although the freshwater volumes are in line with previous reconstructions. Our project represents the first time an attempt made to reconstruct the freshwater fluxes and volumes during all Heinrich events of the last glacial period.

Thesis. 1977. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Microfiche copy available in Archives and Science. / Vita. / Bibliography : leaves 162-165. / Ph.D.

Thesis submitted in fulfilment of the requirements for the degree Master of Technology: Oceanography in the Faculty of Applied Sciences at the Cape Peninsula University of Technology / In order to validate remote sensing products and to provide data for model assimilation, a real-time monitoring line consisting of three moorings was deployed across the Agulhas Current off Port Edward, South Africa. This deployment formed part of a Technology and Human Resource for Industry Programme (THRIP) funded initiative to develop a real-time mooring system capable of measuring ocean parameters in the Agulhas Current during 2011. The slope and offshore moorings displayed a distinct stratified regime within the Agulhas Current, a northeastward flowing Agulhas Undercurrent and the southwestward flowing Agulhas Current. Three major reversal events, with northeastward currents occurred on 23 July, 02 September and on 11 October 2011. All current reversals caused a decrease in current velocity. The Agulhas Undercurrent was a persistent feature and average velocities between the line of moorings ranged between 13.38 cm/s and 15.52 cm/s. The results obtained from the mooring systems were consistent in terms of velocity, direction and hydrographic properties of the Agulhas Current as described in previous literature. The low directional variability in the surface layers at the offshore mooring and dominant southwestward flow, except during reversal events indicate the strong influence of the Agulhas Current in this region. The inshore mooring showed less occurrences of the Agulhas Undercurrent if northward flow in the bottom layers was to be considered as signs of the Agulhas Undercurrent. General current characteristics as well as the characterisation of the mesoscale features affecting the coast off Port Edward was accomplished through the use of the in situ moorings. All current reversals encountered were associated with the process of vortex shedding from the Natal Bight. These events may be related to the shedding of the Durban Cyclonic Eddy from its origin in the Natal Bight. Data from the offshore mooring suggested that for monitoring Agulhas Current core dynamics, it was ideally placed as highest surface velocities were measured by this mooring system. The slope mooring recorded highest velocities within the Agulhas Undercurrent and was thus ideally placed to measure the Agulhas Undercurrent’s core. Shelf dynamics were under the influence of the Agulhas Current and northerly current reversals and were aptly recorded by the inshore mooring which was placed on the continental shelf, close to the shelf break.

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), 2011. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 63-65). / Remote sensing and in situ observations are used to investigate the ocean response to the Tokar Wind Jet in the Red Sea. The wind jet blows down the pressure gradient through the Tokar Gap on the Sudanese coast, at about 18°N, during the summer monsoon season. It disturbs the prevailing along-sea (southeastward) winds with strong cross-sea (northeastward) winds that can last from days to weeks and reach amplitudes of 20-25 m/s. By comparing scatterometer winds with along-track and gridded sea level anomaly observations, it is shown that an intense dipolar eddy spins up in less than seven days in response to the wind jet. The eddy pair has a horizontal scale of 140 km. Maximum ocean surface velocities can reach 1 m/s and eddy currents extend at least 200 m into the water column. The eddy currents appear to cover the width of the sea, providing a pathway for rapid transport of marine organisms and other drifting material from one coast to the other. Interannual variability in the strength of the dipole is closely matched with variability in the strength of the wind jet. The dipole is observed to be quasi-stationary, although there is some evidence for slow eastward propagation-simulation of the dipole in an idealized high-resolution numerical model suggests that this is the result of self-advection. These and other recent in situ observations in the Red Sea show that the upper ocean currents are dominated by mesoscale eddies rather than by a slow overturning circulation. / by Ping Zhai. / S.M.

Thesis (S.M.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Mechanical Engineering; and the Woods Hole Oceanographic Institution), June 2011. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 55-56). / Throughout this thesis we will discuss the theoretical background and empirical observation of a swell band shore normal flux divergence reversal. Specifically, we will demonstrate the existence and persistence of the energy flux divergence reversal in the nearshore region of Atchafalaya Bay, Gulf of Mexico, across storms during the March through April 2010 deployment. We will show that the swell band offshore component of energy flux is rather insignificant during the periods of interest, and as such we will neglect it during the ensuing analysis. The data presented will verify that the greatest flux divergence reversal is seen with winds from the East to Southeast, which is consistent with theories which suggest shoreward energy flux as well as estuarine sediment transport and resuspension prior to passage of a cold front. Employing the results of theoretical calculations and numerical modeling we will confirm that a plausible explanation for this phenomena can be found in situations where temporally varying wind input may locally balance or overpower bottom induced dissipation, which may also contravene the hypothesis that dissipation need increase shoreward due to nonlinear wave-wave interactions and maturation of the spectrum. Lastly, we will verify that the data presented is consistent with other measures collected during the same deployment in the Atchafalaya Bay during March - April 2010. / by Shmuel G. Link. / S.M.

Thesis: Ph. D., Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2016. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages [191]-201). / Submesoscale flows, current systems 1-100 km in horizontal extent, are increasingly coming into focus as an important component of upper-ocean dynamics. A range of processes have been proposed to energize submesoscale flows, but which process dominates in reality must be determined observationally. We diagnose from observed flow statistics that in the thermocline the dynamics in the submesoscale range transition from geostrophic turbulence at large scales to inertia-gravity waves at small scales, with the transition scale depending dramatically on geographic location. A similar transition is shown to occur in the atmosphere, suggesting intriguing similarities between atmospheric and oceanic dynamics. We furthermore diagnose from upper-ocean observations a seasonal cycle in submesoscale turbulence: fronts and currents are more energetic in the deep wintertime mixed layer than in the summertime seasonal thermocline. This seasonal cycle hints at the importance of baroclinic mixed layer instabilities in energizing submesoscale turbulence in winter. To better understand this energization, three aspects of the dynamics of baroclinic mixed layer instabilities are investigated. First, we formulate a quasigeostrophic model that describes the linear and nonlinear evolution of these instabilities. The simple model reproduces the observed wintertime distribution of energy across scales and depth, suggesting it captures the essence of how the submesoscale range is energized in winter. Second, we investigate how baroclinic instabilities are affected by convection, which is generated by atmospheric forcing and dominates the mixed layer dynamics at small scales. It is found that baroclinic instabilities are remarkably resilient to the presence of convection and develop even when rapid overturns keep the mixed layer unstratified. Third, we discuss the restratification induced by baroclinic mixed layer instabilities. We show that the rate of restratification depends on characteristics of the baroclinic eddies themselves, a dependence not captured by a previously proposed parameterization. These insights sharpen our understanding of submesoscale dynamics and can help focus future inquiry into whether and how submesoscale flows influence the ocean's role in climate. / by Jörn Callies. / Ph. D.