Publication Date

2015-04-30

Availability

Embargoed

Embargo Period

2017-04-29

Degree Type

Doctoral Essay

Degree Name

Doctor of Philosophy (PHD)

Department

Meteorology and Physical Oceanography (Marine)

Date of Defense

2015-04-01

First Committee Member

Tamay M. Ozgokmen

Second Committee Member

Annalisa Griffa

Third Committee Member

William Johns

Fourth Committee Member

Igor Kamenkovich

Fifth Committee Member

Zulema Garraffo

Sixth Committee Member

Milena Veneziani

Abstract

The goal of this work is to study the Lagrangian and Eulerian properties of the scales below the mesoscale. In the last few years, developments in the observations and numerical modeling capabilities allowed to venture in the domain of the submesoscale (SM) opening a broad range of questions on the dynamics of these scales and their role in the ocean circulation. In the first part of this dissertation we investigate some of the properties of these scales while in the last part we enter the domain of the fully 3D dynamics with a study of dynamics of the convective mixed layer. The formation mechanism of SM is investigated in Chapter 2. In particular, we focus on the seasonality of SM. In order to approach this problem, a realistic simulation of the Gulf Stream (GS) region with the Hybrid Coordinate Ocean Model is integrated for 18 months at two horizontal resolutions: a high-resolution (1/48th) simulation able to resolve part of the submesoscale regime and the full range of mesoscale dynamics, and a coarser resolution (1/12th) case, in which submesoscales are not resolved. Results provide an insight into submesoscale dynamics highlighting a clear seasonal cycle, with submesoscale features mostly present during winter. The limiting and controlling factor in the occurrence of submesoscales appears to be the depth of the mixed layer, which controls the reservoir of available potential energy available at the mesoscale fronts that are present most of the year. Atmospheric forcings are the main energy source behind submesoscale formation, but mostly indirectly through mixed layer deepening. This result represented the first evidence of seasonality of SM features and was successively confirmed by Shcherbina et al. (2013) via direct observations. SM features have been found to play an important role on ocean material transport, governing horizontal dispersion at the small scales Poje et al. (2014). Similarly, it is thought that SM features, given their large vertical velocities, might play an important role in the transport of biogeochemical nutrients in the upper ocean. In particular, vertical transport of nutrients is thought to be located mostly in ocean eddies. Motivated by the lack of a clear understanding of these processes in Chapter 3 we perform a set of numerical simulations in order to study some of the proposed vertical transport mechanisms. The MITgcm is integrated in four different configurations: two summer configurations with shallow mixed layer, one with wind and one without, and two winter simulations with deep mixed layer, one with wind and one without. The goal is to simulate the effect of Eddy- Ekman pumping and SM pumping. Results show that wind forced simulations present strong internal wave activity in the near-inertial band. Also, large vertical velocities are found in the mixed layer of the winter simulations. No clear sign of Eddy-Ekman pumping is observed in the vertical velocity field. In order to investigate the associated vertical transport, synthetic particles are released in all simulations. Results show that wind forced summer simulation can provide only weak vertical transport and that including a mixed layer and SM features, strong vertical transport is observed within the mixed layer and across the mixed layer and the water column. Motivated by the importance that SM plays on the dynamics of the upper ocean in Chapter 4 we venture into smaller scales trying to bridge the gap between the SM and the fully 3D dynamics of the upper ocean. Here we present results from two non-hydrostatic simulations of a weakly wind and buoyancy forced mixed layer with semi-realistic diurnal cycling. Both purely buoyancy-forced and wind- and buoyancy-forced flows are sampled using passive tracers, as well as 2D and 3D particles to investigate characteristics of horizontal and vertical dispersion. It is found through tracer releases that the surface patterns of the tracer were determined by the convergence zones created by buoyancy-driven convection within a time scale of a few hours. For pure convections the results display the classic signature of Rayleigh-Benard cells. When combined with a wind stress the convective cells become organized such that the along-wind length scale becomes much larger than the cross-wind scale of the convective cells. Relative dispersion computed by sampling the flow fields using both 2D and 3D particles shows Richardson regimes meaning that particle separation is driven by processes at the scale of the particle separation. Relative dispersion is found to be much higher in wind driven mixed layer and 2D surface-releases transitioned to Richardson regime faster in the wind forced simulation. We also show that the buoyancy-forced case results in significantly lower amplitudes of scale-dependent relative diffusivity, k_D (l), than those reported by Okubo (1970), but the wind- and buoyancy-forced case was in good agreement with Okubo’s diffusivity amplitude, and scaling was consistent with the Richardson law, k_D~ l^(4/3). These results represent a first investigation of the Lagrangian properties of the ∼3D flows of the upper ocean and suggest that transport is governed by local processes. The ultimate goal of this dissertation is to shed some light on the properties of the fine structure of the oceans in the belief that a complete understanding of the ocean dynamics cannot prescind from the understanding of its smaller scales.

Keywords

submesoscale; material transport; mixed layer; lagrangian dispersion

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