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.

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), 2013. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 297-213). / This thesis explores the role that the circulation in the Gulf of Maine (GOM) plays in determining the distribution of dense aggregations of copepods. These aggregations are an important part of the marine ecosystem, especially for endangered North Atlantic right whales. Certain ocean processes may generate dense copepod aggregations, while others may destroy them; this thesis looks at how different characteristics of the GOM circulation fit into these two categories. The first part of the thesis investigates a hypothetical aggregation mechanism in which frontal circulation interacts with copepod behavior to generate a dense patch of copepods. The first two chapters of this thesis address this mechanism in the context of coastal river plumes and salinity fronts. One chapter describes the characteristics and variability of coastal freshwater and salinity fronts using a historical dataset and a realistic numerical model. The seasonal variability of freshwater is tied in part to seasonality in river discharge, while variability on shorter time scales in the frontal position is related to wind stress. Another chapter applies the hypothetical mechanism to idealized river plumes using a suite of numerical models. The structure of the plume and plume-relative circulation change the resulting copepod aggregation from what is expected from the hypothetical mechanism. The second part of the thesis discusses the GOM circulation and how it may eliminate copepod patches. The summertime mean surface circulation and eddy kinetic energy are computed from a Lagrangian drifter dataset and an adaptive technique that allows for higher spatial resolution while also keeping uncertainty low. Eddy diffusivity is also computed over different regions of the GOM in an attempt to quantify the spreading of a patch of copepods, and is found to be lower near the coast where right whales are often found feeding on copepod patches. In the next chapter, a numerical drifter dataset is used to understand how the results of the previous chapter depend upon the quantity of observations. It is found that the uncertainty in estimating eddy diffusivity is tightly coupled to the number of drifters in the calculation. / by Nicholas W. Woods / Ph.D.

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Thesis. 1977. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Microfiche copy available in Archives and Science. / Vita. / by Edward P. Laine. / Ph.D.

A motion compensated ADV system was evaluated to determine its ability to make measurements necessary for characterizing the variability of the ambient current in the Gulf Stream. The impact of IMU error relative to predicted turbulence spectra was quantified, as well as and the ability of the motion compensation approach to remove sensor motion from the ADV measurements. The presented data processing techniques are shown to allow the evaluated ADV to be effectively utilized for quantifying ambient current fluctuations from 0.02 to 1 Hz (50 to 1 seconds) for dissipation rates as low as 3x10-7. This measurement range is limited on the low frequency end by IMU error, primarily by the calculated transformation matrix, and on the high end by Doppler noise. Inshore testing has revealed a 0.37 Hz oscillation inherent in the towfish designed and manufactured as part of this project, which can nearly be removed using the IMU. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2014. / FAU Electronic Theses and Dissertations Collection

Dans le Pacifique sud-ouest, le cheminement des eaux depuis la gyre subtropicale vers l'équateur est réalisé via les courants de bord ouest (WBCs), associés à des changements de température ou de quantité d'eau à di?érentes échelles de temps. La mer des Salomon située au sud de la Pa- pouasie Nouvelle-Guinée est considérée comme une région clé pour étudier les WBCs juste avant qu'ils rejoignent l'équateur. Dans le cadre d'une collaboration internationale, deux campagnes en mer ont été réalisées en mer des Salomon pendant deux périodes contrastées de la saison des alizés (Juillet-Août 2012 et Mars 2014). En Juillet-Août 2012, pendant la saison de forts alizés, les WBCs transportent 36 ± 3 Sv à travers l'entrée sud de la mer des Salomon depuis la surface jusqu'à 1000 m de profondeur, mais seulement 11 ± 3 Sv en Mars 2014 (réduction de 70%) quand les alizés faiblissent. Les WBCs sont associés à des propriétés de masses d'eau distinctes qui subissent d'importantes modifications au cours de leur passage en mer des Salomon. En dessous de 2000 m de profondeur, l'Upper Circumpolar Deep Water (UCDW) est tracée depuis le bassin de Fidji et la fosse de la Nouvelle-Calédonie avant d'entrer en mer des Salomon, asso- ciée à un transport de masse de 2 à 3 ± 2 Sv. L'étude préliminaire des mouillages au niveau des détroits de sortie a permis d'identifier une modulation de l'intensité et de la structure ver- ticale des WBCs en relation avec la variabilité saisonnière des alizés. Les eaux de thermocline entre les différents mouillages varient généralement de manière cohérente et en phase à l'échelle intra-saisonnière et saisonnière. / The Low Latitude Western Boundary Currents (LLWBCs) of the Southwest Pacific establish a major connection between the subtropics and the equator. They are believed to play a key role in heat and mass budgets of the equatorial Pacific and cross, as a final passage, the Solomon Sea southeast of Papua New Guinea. As part of a France-U.S. collaboration, two oceanographic multidisciplinary cruises were conducted in the Solomon Sea during contrasting trade wind sea- sons, in July-August 2012 and March 2014 to characterize currents, water properties and their modifications. During July-August 2012, the season of strong southeasterly trade winds, an in- tense flow of 36 ± 3 Sv from the surface down to 1000 m is entering the Solomon Sea, that falls by 70% to 11 ± 3 Sv in March 2014 when the winds over the sea are weaker. The LLWBCs are associated with distinct water mass properties across the Solomon Sea that can be tracked along their pathways, and undergo significant modifications between the southern entrance and the northern exits of the sea. Below 2000 m, the Upper Circumpolar Deep Water (UCDW) has been traced from the New Caledonia Trench and the South Fiji Basin into the Solomon Sea, associ- ated with a volume transport of 2-3 ± 2 Sv. Preliminary analyses of the mooring array in each major exit channel indicate large time variability of the LLWBCs in both =intensity and vertical extent, where thermocline pathways vary in most cases, coherently and in phase at intraseasonal to seasonal time scales.

The sinking of dense shelf waters down the continental slope (or “cascading”) contributes to oceanic water mass formation and carbon cycling. Cascading is therefore of significant importance for the global overturning circulation and thus climate. The occurrence of cascades is highly intermittent in space and time and observations of the process itself (rather than its outcomes) are scarce. Global climate models do not typically resolve cascading owing to numerical challenges concerning turbulence, mixing and faithful representation of bottom boundary layer dynamics. This work was motivated by the need to improve the representation of cascading in numerical ocean circulation models. Typical 3-D hydrostatic ocean circulation models are employed in a series of numerical experiments to investigate the process of dense water cascading in both idealised and realistic model setups. Cascading on steep bottom topography is modelled using POLCOMS, a 3-D ocean circulation model using a terrain-following s-coordinate system. The model setup is based on a laboratory experiment of a continuous dense water flow from a central source on a conical slope in a rotating tank. The descent of the dense flow as characterised by the length of the plume as a function of time is studied for a range of parameters, such as density difference, speed of rotation, flow rate and (in the model) diffusivity and viscosity. Very good agreement between the model and the laboratory results is shown in dimensional and non-dimensional variables. It is confirmed that a hydrostatic model is capable of reproducing the essential physics of cascading on a very steep slope if the model correctly resolves velocity veering in the bottom boundary layer. Experiments changing the height of the bottom Ekman layer (by changing viscosity) and modifying the plume from a 2-layer system to a stratified regime (by enhancing diapycnal diffusion) confirm previous theories, demonstrate their limitations and offer new insights into the dynamics of cascading outside of the controlled laboratory conditions. In further numerical experiments, the idealised geometry of the conical slope is retained but up-scaled to oceanic dimensions. The NEMO-SHELF model is used to study the fate of a dense water plume of similar properties to the overflow of brine-enriched shelf waters from the Storfjorden in Svalbard. The overflow plume, resulting from sea ice formation in the Storfjorden polynya, cascades into the ambient stratification resembling the predominant water masses of Fram Strait. At intermediate depths between 200-500m the plume encounters a layer of warm, saline AtlanticWater. In some years the plume ‘pierces’ the Atlantic Layer and sinks into the deep Fram Strait while in other years it remains ‘arrested’ at Atlantic Layer depths. It has been unclear what parameters control whether the plume pierces the Atlantic Layer or not. In a series of experiments we vary the salinity ‘S’ and the flow rate ‘Q’ of the simulated Storfjorden overflow to investigate both strong and weak cascading conditions. Results show that the cascading regime (piercing, arrested or ‘shaving’ - an intermediate case) can be predicted from the initial values of S and Q. In those model experiments where the initial density of the overflow water is considerably greater than of the deepest ambient water mass we find that a cascade with high initial S does not necessarily reach the bottom if Q is low. Conversely, cascades with an initial density just slightly higher than the deepest ambient layer may flow to the bottom if the flow rate Q is high. A functional relationship between S/Q and the final depth level of plume waters is explained by the flux of potential energy (arising from the introduction of dense water at shallow depth) which, in our idealised setting, represents the only energy source for downslope descent and mixing. Lastly, the influence of tides on the propagation of a dense water plume is investigated using a regional NEMO-SHELF model with realistic bathymetry, atmospheric forcing, open boundary conditions and tides. The model has 3 km horizontal resolution and 50 vertical levels in the sh-coordinate system which is specially designed to resolve bottom boundary layer processes. Tidal effects are isolated by comparing results from model runs with and without tides. A hotspot of tidally-induced horizontal diffusion leading to the lateral dispersion of the plume is identified at the southernmost headland of Spitsbergen which is in close proximity to the plume path. As a result the lighter fractions in the diluted upper layer of the plume are drawn into the shallow coastal current that carries Storfjorden water onto the Western Svalbard Shelf, while the dense bottom layer continues to sink down the slope. This bifurcation of the plume into a diluted shelf branch and a dense downslope branch is enhanced by tidally-induced shear dispersion at the headland. Tidal effects at the headland are shown to cause a net reduction in the downslope flux of Storfjorden water into deep Fram Strait. This finding contrasts previous results from observations of a dense plume on a different shelf without abrupt topography. The dispersive mechanism which is induced by the tides is identified as a mechanism by which tides may cause a relative reduction in downslope transport, thus adding to existing understanding of tidal effects on dense water overflows.

Both physical circulation and biogeochemical characteristics are unique in the Southern Ocean (SO) region, and are fundamentally different from those of the northern hemisphere. Moreover, according to previous research, the oceanic response to the trend of the Southern Annual Mode (SAM) has profound impacts on the future oceanic uptake of carbon dioxide in the SO. In other words, the climate and circulation of the SO are strongly coupled to the overlying atmospheric variability. However, while we have understanding on the SO physical circulation and have the ability to predict the future changes of the SO climate and physical processes, the link between the SO physical processes, the air-sea carbon flux, and correlated climate variability remains unknown. Even though scientists have been studying the spatial and temporal variability of the SO carbon flux and the associated biogeochemical processes, the spatial patterns and the magnitudes of the air-sea carbon flux do not agree between models and observations. Therefore, in this study, we utilized a modified version of a general circulation model (GCM) to performed realistic simulations of the SO carbon on seasonal to interannual timescales, and focused on the crucial physical and biogeochemical processes that control the carbon flux. The spatial pattern and the seasonal cycle of the air-sea carbon dioxide flux is calculated, and is broadly consistent with the climatological observations. The variability of air-sea carbon flux is mainly controlled by the gas exchange rate and the partial pressure of carbon dioxide, which is in turn controlled by the compensating changes in temperature and dissolved inorganic carbon. We investigated the seasonal variability of dissolved inorganic carbon based on different regional processes. Furthermore, we also investigated the dynamical adjustment of the surface carbon flux in response to the different gas exchange parameterizations, and conclude that parameterization has little impact on spatially integrated carbon flux. Our simulation well captured the SO carbon cycle variability on seasonal to interannual timescales, and we will improve our model by employ a better scheme of nutrient cycle, and consider more nutrients as well as ecological processes in our future study.