Publication Date

2015-04-24

Availability

Embargoed

Embargo Period

2016-10-15

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Meteorology and Physical Oceanography (Marine)

Date of Defense

2015-04-03

First Committee Member

Tamay M Ozgokmen

Second Committee Member

Laurie Padman

Third Committee Member

Igor Kamenkovich

Fourth Committee Member

William Johns

Abstract

Formation of intermediate and abyssal water masses as dense water flows off continental shelves contributes to the lower limb of the meridional overturning circulation. The main goal of this dissertation is to advance the understanding of processes that determine volume flux and physical properties of oceanic outflows, focusing on outflows of dense shelf water (DSW) in Antarctica. First, the effects of grid resolution, numerical schemes, Reynolds numbers and turbulence models on mixing and stirring driven by buoyancy-driven flows are quantified. Using the lock-exchange problem, we found that mixing in an ocean general circulation model is more sensitive to the choice of grid resolution than to the other parameters tested here; however, the results do not monotonically converge towards the optimum solution as resolution is refined. We then considered the stirring of a passive tracer field by submesoscale eddies generated by surface density fronts. We found that using a second-order turbulence closure provides an accurate representation of restratification in the mixed layer. Second, idealized and realistic numerical simulations are used to investigate the excitation of topographic vorticity waves (TVWs) along the Antarctic continental slope by outflows of DSW through troughs. The modeled TVWs are sufficiently energetic to play an important role in cross-slope water mass exchanges and Antarctic Bottom Water (AABW) production. Idealized simulations show that wave frequency depends on the amount of stretching in the ambient fluid over the outflow and on the background along-slope mean flow. Frequency is higher for steeper bottom slope, larger outflow density anomaly, and stronger westward mean flow. For weak stratification and weak westward along-slope flows typical of the Antarctic slope, wave energy propagates eastward, in the opposite direction from phase velocity. In a realistic simulation of the Ross Sea, TVW properties are modulated on seasonal and shorter time scales as background ocean state varies, consistent with our idealized results. Our results are also consistent with observations of TVWs in the southern Weddell Sea. Lastly, high resolution numerical simulations are used to study the generation of a double plume pattern in oceanic outflows. Double plumes, previously observed in a laboratory study, carry water properties from the shelf into the deep ocean at two distinct depths. Our numerical model is configured to solve the nonhydrostatic Boussinesq equations in a two-dimensional configuration without rotation. A set of nine experiments were conducted by varying the ambient stratification frequency (N) and the bottom slope (α). We present the first evidence that the double plume pattern may occur in oceanic environments, such as the Antarctic outflows. The parameters needed to identify the flow regimes are α and the buoyancy number B = Q N^3/g'^2, where Q is the volume flux of the dense water flow per unit width and g' is the reduced gravity. The double plume regime occurred when B ~ 0.02, regardless of the bottom slopes tested here. When B was about one order of magnitude smaller (~0.002), a single plume was always observed. For intermediate B values (B~0.007), a double plume regime occurred for steeper slopes (α=0.1 and α=0.05) and a single plume occurred for the shallower slope (α=0.01). An important characteristic of the double plume regime is the flow transition from a supercritical condition, where the Froude number (Fr) is greater than one, to a slower and mode uniform subcritical condition (Fr<1). This transition was associated with an internal hydraulic jump and consequent mixing enhancement. We hypothesize that double plumes around Antarctica could simultaneously transport DSW offshore as AABW and as intermediate-depth water masses that cool the Circumpolar Deep Water that is the primary heat source to the Antarctic continental margins.

Keywords

Oceanic overflows; gravity current; bottom water formation; numerical simulations

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