Thesis (Ph. D.)--Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1982. / Vita. Includes bibliographical references (p. 221-225).

Thesis (Ph. D.)--Massachusetts Institute of Technology, 1988. / "June 1988." Funding provided by the National Science Foundation under grant Numbers OCE 82-13967, and OCE 85-15642, and by the WHO/MIT Joint Program Ventures Fund. Includes bibliographical references (p. 142-146).

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1980. / Supervised by Charles D. Hollister. Includes bibliographical references (leaves 206-211).

Thesis (Ph. D.)--Oregon State University, 2007. / Printout. Includes bibliographical references (leaves 169-179). Also available on the World Wide Web.

An idealized atmospheric model consisting of energy and moisture conservation equations is developed for studies of the ocean's role in climate. Testing under fixed oceanic conditions yields a climatology comparable with direct observations, as does the case when the interpentadal (1955–59; 1970–74) sea surface temperature fields are applied. The atmospheric model is then coupled to an ocean general circulation model as well as a thermodynamic ice model without the use of flux adjustments. When configured for a global realistic geometry, the model faithfully represents deep water formation in the Atlantic and Southern Oceans with upwelling throughout the Pacific and Indian Oceans. The model is then utilized to investigate the influence of meltwater discharge on the stability of North Atlantic Deep Water (NADW) production and the Younger Dryas (YD ∼ 14ka). Results suggest pre-YD meltwater is capable of diminishing NADW to the point where diversion of meltwater from the Gulf of Mexico to the St. Lawrence completely inhibits its production. The coupled model appears to be stable in this state, equivalent to the “Southern Sinking” equilibrium identified in previous models. Inclusion of the wind stress/speed feedback, however, has a dramatic effect causing a reestablishment of NADW production. The model is then configured in a four basin-two hemisphere sector geometry, crudely representative of the global oceans. Two identically formulated models (one of which employs flux adjustments) are then perturbed to assess the role of flux adjustments on the ocean's response to a “global warming-like” scenario. Significant global and basin-scale differences exist between the cases which is linked to the influence of the salt-flux adjustment on the overturning cells within the model Atlantic and Southern Oceans. Results further suggest that minimizing the coupling shock prior to applying the perturbation leads to results slightly closer between the models, although large differences still persist. The model is then configured for a highly idealized 60° sector geometry to study the influence of horizontal resolution and parameterized eddy processes on poleward heat transport. As resolution increases, the total oceanic heat transport steadily increases. This result is also evidenced in a parallel series of ocean-only model studies driven by restoring boundary conditions. In each case the increase in heat transport is associated with the steady currents. In particular the baroclinic gyre transport (our model analog of the transport associated with the “Warm Core” jet region of the Gulf Stream) increases by a factor of 5 between coarsest and finest resolution. Spontaneous decadal-scale variability is also found to exist in the higher resolution experiments (with the exception of one of the restoring cases). The oscillation is thermally driven by an advective-convective mechanism and linked to the value of the horizontal diffusivity employed. Increasing the diffusivity in the high resolution cases is enough to destroy the variability, while decreasing the diffusivity in the moderately coarse resolution case is enough to induce the variability. These results point to the importance of higher resolution in the oceanic component of current climate models, yielding enhanced poleward heat transports and revealing the existence of richer decadal-scale variability in models which require less parameterized viscosity and diffusion. / Graduate

Parameterization of diapycnal mixing is required for accurate modeling of ocean circulation. Almost 50% of all warm Atlantic water flowing towards the Arctic and more than 20% of the returning cold water passes through the Faroese Channels. Diapycnal mixing in the Wyville Thomson Basin (WTB), at the centre of the Faroese Channels, removes heat from Atlantic water flowing northwards and preconditions the cooler waters flowing south over the Wyville Thomson Ridge (WTR). Direct measurements of the dissipation rate of turbulent kinetic energy (epsilon) were obtained from the WTB in September 2007 along with CTD, LADCP and mooring data. Microstructure profiles to a depth of 800m were used to estimate diapycnal diffusivity from epsilon. Large values of epsilon (10-5 Wkg-1) were observed and diffusivity (10-2m2s-1) calculated at approximately 500m depth in a water column depth of 900m. Elevated values coincided with a very strong thermocline between surface Atlantic and cold deep waters in the WTB. Oscillations within mooring data show large O (100m) vertical displacements of the thermocline with a semidiurnal period, hence generation and breaking of tidally forced internal waves by interaction with the ridge topography is proposed as an explanation for observed high diffusivity. Non-hydrostatic 2D model data are used to explore generation and breaking mechanisms of such waves and it is shown that the WTR is an effective generator of internal tides and furthermore that baroclinic energy is trapped and dissipated within the WTB. Conservative calculation of a basin-wide diffusivity (2.51 x 10-4 m2s-1) exceeds that required to maintain global ocean circulation, but heat flux into the dense layer (~14 Wm-2) is found to influence the Faroe-Bank Channel outflow volume by <2%.

The oceanic surface boundary layer is of great importance and interest as its dynamics provides for the exchange of energy, momentum, heat and matter between the atmosphere and the ocean. It is crucial to have a thorough understanding of physical processes that might have a significant influence on its properties and variability. In this study I consider several different facets of mixed layer/boundary layer dynamics. One aspects concerns the consequences of the nonlinearity of the equation of state in mixed layer models. The nonlinearity of the equation of state gives rise to a term in the averaged surface buoyancy flux which can be comparable in magnitude to other terms. Its magnitude is shown to be proportional to the area enclosed by the seasonal cycle of sea-surface temperature T₅ versus the oceanic heat content H. The term always represents a buoyancy input into the ocean and is compensated exactly by the buoyancy loss via cabbeling (densification on mixing) whenever the mixed layer entrains water with different properties from below. Another problem of interest is the role of the coherent wind-induced vortices, commonly known as Langmuir circulation, in generating the surface mixed layer. A simple parameterization of the mixing due to Langmuir circulation is examined in the light of an oceanic dataset. Some evidence for the validity of the parameterization is found, thus drawing attention to Langmuir's assertion that Langmuir circulation is one of the key physical processes in the oceanic boundary layer. The third aspect of surface boundary layer dynamics explored is the mean effect on mixed layer entrainment of periodic vertical movement of isopycnals in the thermocline due to non-breaking internal waves (referred to as heaving). Seasonal model runs incorporating idealized heaving show that heaving can produce significant seasonal differences in sea-surface temperature compared to a reference case without heaving. It is inferred that by periodically stretching and compressing the mixed layer, heaving causes instabilities that result in additional entrainment of colder water from below. A heaving number RH is proposed, and two parameterizations of heaving for use in mixed layer models are suggested. / Graduate

Graduation date: 2000

Ocean circulation changes in the tropical Pacific strongly influence global climate, as demonstrated during El Niño-Southern Oscillation (ENSO) extremes. Understanding the causes of past variability in tropical Pacific circulation and their relationship to climate change will help to predict how future climate may evolve under anthropogenic radiative forcing. I measure fossil coral radiocarbon (Δ¹⁴C) from Palmyra (6°N, 162°W) and Christmas (2°N, 157°W) Islands in the central tropical Pacific to reconstruct high-resolution records of tropical Pacific ocean circulation variability over the last millennium. Variations in coral Δ¹⁴C from Palmyra and Christmas reflect a combination of the atmospheric concentration of ¹⁴C at the time of growth, Δ¹⁴C-depleted waters associated with equatorial upwelling, and Δ¹⁴C -enriched waters advected from the western tropical Pacific. Existing oxygen isotopic (δ ¹⁸O) records of the Palmyra and Christmas fossil corals reveal a rich history of interannual to centennial variability in sea-surface temperature (SST) and salinity over the last millennium [Cobb et al., 2003b]. My approach targets specific time intervals associated with strong interannual to centennial-scale coral δ ¹⁸O anomalies for high-resolution Δ¹⁴C analysis. Seasonally-resolved Δ¹⁴C measurements are used to compare interannual Δ¹⁴C variability across the 10th, 13th, 15th, 17th, and 20th centuries. Annually-resolved Δ¹⁴C measurements are used to compare decadal to centennial-scale Δ¹⁴C variations from the 10th, 12th - 15th and 17th centuries. SEM photos are used to assess the fidelity of the coral Δ¹⁴C records with respect to post-depositional alteration of the coral skeleton. I find evidence for minor dissolution and addition of secondary aragonite, but my results indicate that coral Δ¹⁴C is only compromised after moderate to severe diagenesis. Despite strong ENSO signals in modern and fossil coral δ ¹⁸O, our data show no statistically significant interannual variability in coral ¹⁴C. There is a centennial-scale increase in coral radiocarbon from the Medieval Climate Anomaly (MCA, ~900-1200AD) to the Little Ice Age (LIA, ~1500-1800). I use a box model of central tropical Pacific Δ¹⁴C contributions to show that this centennial-scale trend over the last millennium is largely explained by centennial-scale changes in atmospheric ¹⁴C. However, large 12th century depletions in Palmyra coral ∆¹⁴C data cannot be explained by atmospheric ¹⁴C variability and likely reflect a roughly two-fold increase in upwelling and/or a significant change in the ¹⁴C of higher-latitude source waters reaching the equatorial Pacific during this time. Conversely, significantly enriched Christmas coral ∆¹⁴C values during the 16th century are consistent with a two-fold reduction in upwelling strength and/or the advection of high-¹⁴C waters to the equatorial thermocline from higher latitudes.

Shark Bay is a large inverse estuary, located in Western Australia. It has a number of unique habitats that support important species. The dynamics of circulation in Shark Bay have an influence on the species that inhabit the region, on small, local scales as well as on large Bay-wide scales. Numerical modeling and field data were used to examine small-scale dynamics in relation to an important recreational fish, pink snapper (Pagrus auratus). Icthyoplankton surveys collected and recorded egg density in regions where snapper are found. A barotropic three-dimensional hydrodynamic model was coupled with a two-dimensional Lagrangian particle-tracking program to simulate the passive transport of eggs through regions where spawning is known to occur. Circulation modeling results indicated residual flows on small scales that served to retain the eggs in the region where they were originally spawned. Results corroborate genetic work on adult snapper, which found no evidence intermixing of populations in Shark Bay. The numerical model was then further refined to run in a baroclinic mode. Simulations of salinity and temperature gradients were used to recreate frontal systems in Shark Bay. Frontal regions divide the Bay into a northern and a southern section as well as separate it from the ocean. Application of an analytical method for calculating front locations was consistent with the observed results and indicated that the primary forces determining frontal locations in the Bay are tides and gravitational circulation. Winds are a secondary influence, and solar heating is minimal in influence