The sensitivity and predictability of mesoscale eddies in an idealized model ocean

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution April, 1976Two numerical applications of two-level quasigeostrophic theory are used to investigate the interrelationships of the mean and mesoscale eddy fields in a closed-basin ocean model. The resulting techniques provide a more accurate description of the local dynamics, origins, and parametric dependences of the eddies than that available in previous modelling studies. First, we propose a novel and highly efficient quasigeostrophic closed-domain model which has among its advantages a heightened resolution in the boundary layer regions. The pseudospectral method, employing an orthogonal expansion in Fourier and Chebyshev functions, relies upon a discrete Green's function technique capable of satisfying to spectral accuracy rather arbitrary boundary conditions on the eastern and western (continental) walls. Using this formulation, a series of four primary numerical experiments tests the sensitivity of wind-driven single and double-gyred eddying circulations to a transition from free-slip to no-slip boundary conditions. These comparisons indicate that, in the absence of topography, no-slip boundaries act primarily to diffuse vorticity more efficiently. The interior transport fields are thus reduced by as much as 50%, but left qualitatively unchanged. In effect, once having separated from the western wall, the internal jet has no know1edge, apart from its characteristic flow speed, of the details of the boundary layer structure. Next, we develop a linearized stability theory to analyze the local dynamic processes responsible for the eddy fields observed in these idealized models. Given two-dimensional (x, z) velocity profiles of arbitrary horizontal orientation, the resulting eigenfunction problems are solved to predict a variety of eddy properties: growth rate, length and time scales, spatial distribution, and energy fluxes. This simple methodology accurately reproduces many of the eddy statistics of the fully nonlinear fields; for instance, growth rates of 10-100 days predicted for the growing waves by the stability analysis are consistent with observed model behavior and have been confirmed independently by a perturbation growth test. Local energetic considerations indicate that the eddy motions arise in distinct and recognizable regions of barotropic and baroclinic activity. The baroclinic instabilities deîend sensitively on the vertical shear which must exceed 0(5 cm sec-1) across the thermocline to induce eddy growth. As little as a 10% reduction in |uz|, however, severely suppresses the cascade of mean potential energy to the eddy field. In comparison, the barotropic energy conversion process scales with the horizontal velocity shear, |uy|, whose threshold values for instability, a (2 x 10-6 sec-1), is undoubtedly geophysically realizable. A simple scatter diagram of |uy| versus |uz| for all the unstable modes studied shows a clear separation between the regions of barotropic and baroclinic instability. While the existence of baroclinic modes can be deduced from either time mean or instantaneous flow profiles, barotropic modes cannot be predicted from mean circulation profiles (in which the averaging process reduces the effective horizontal shears). Finally, we conduct a separate set of stability experiments on analytically generated jet profiles. The resulting unstable modes align with the upper level velocity maxima and, although highly sensitive to local shear amplitude, depend much less strongly on jet separation and width. Thus, the spatial and temporal variability of the mesoscale statistics monitored in the nonlinear eddy simulations can be attributed almost entirely to time-dependent variations in local shear strength. While these results have been obtained in the absence of topography and in an idealized system, they yet have strong implications for the importance of the mid-ocean and boundary layer regions as possible eddy generation sites.This research has been made possible by National Science Foundation grant OCE74-03001 A03, formerly DES73-00528, and the National Science Foundation funded National Center for Atmospheric Research

Modeling Nutrient and Plankton Processes in the California Coastal Transition Zone: 2. A Three-Dimensional Physical-Bio-Optical Model

A three-dimensional (3-D) primitive equation model, developed to simulate the circulation features (filaments) observed in the California coastal transition zone (CTZ), was coupled to a nine-component food web model and a bio-optical model. The simulated flow fields from a 3-D primitive equation model are used to advect the constituents of the food web model, which include silicate, nitrate, ammonium, two phytoplankton size fractions, copepods, doliolids, euphausiids, and a detritus pool. The bio-optical model simulates the wavelength-dependent attenuation of the subsurface irradiance field. The overall objective of this modeling study was to understand and quantify the processes that contribute to the spatial and temporal development of nutrient and plankton distributions in the CTZ. The resulting simulated 3-D nutrient, plankton and submarine light fields agree well with those observed within the CTZ. Specifically, high nutrient and plankton biomass occur onshore and within the core of the simulated filament. Variations in the depth of the 1% light level, which result from the simulated plankton distributions, shallows to less than 30 m in regions of high phytoplankton biomass, and deepens to greater than 75 m in regions of low phytoplankton biomass. The onshore and offshore surface carbon flux patterns are similar in shape due to the meander-like flow patterns of the filament; however, the net across-shore area-integrated carbon flux is predominantly offshore. The total 20-day integrated carbon transport for the model domain varies with distance from shore and is highest (35 × 109 g C) in the region where the filament circulation pattern develops into an anticyclonic and cyclonic pair of eddies. The annual integrated carbon transport by filaments along the California coast is estimated to be 1.89 × 1012 g C

Mozambique Channel Eddies in GCMs: A question of resolution and slippage

Hydrographic observations in the 21st century have shown that the flow within the MozambiqueChannel is best described by a series of large poleward-propagating anticyclonic eddies, rather than, aspreviously thought, a continuous intense western boundary current. The portrayal of this region in various runs of the NEMO 75-level model is found to vary between those two descriptions depending upon the resolution used and the implementation of the model's lateral boundary conditions. In a comparison of 1/4 ? resolution runs, the change of these conditions from free-slip to no-slip leads to the mean southward flow moving further offshore, with greater variability in the zonal and meridional velocities as the flow organises itself into eddies, and a reduction in total transport. If a realization of a model is unable to get these aspects of the physical flow correct, then this will significantly reduce its ability to show a realistic biological signal or long-term response to climate change. Further south, beyond Durban, the application of no-slip conditions similarly causes the mean Agulhas Current to lie further offshore, making it much more able to simulate Natal Pulses.<br/

Rapid Real-Time Interdisciplinary Ocean Forecasting Using Adaptive Sampling and Adaptive Modeling and Legacy Codes: Component Encapsulation Using XML

Abstract. We present the high level architecture of a real-time inter-disciplinary ocean forecasting system that employs adaptive elements in both modeling and sampling. We also discuss an important issue that arises in creating an integrated, web-accessible framework for such a system out of existing stand-alone components: transparent support for handling legacy binaries. Such binaries, that are most common in scien-tific applications, expect a standard input stream, maybe some command line options, a set of input files and generate a set of output files as well as standard output and error streams. Legacy applications of this form are encapsulated using XML. We present a method that uses XML doc-uments to describe the parameters for executing a binary.

The Afloat maintenance Command: organizational and funding issues in Navy ship maintenance, repair, and modernization

The goal of current Navy surface ship maintenance and repair strategy is to sustain readiness and to maximize both combat capability and the amount of time ships are available for employment during their lifetime. The established organizational framework of the Navy to perform this task includes a complex array of activities that are effective overall, but at the expense of efficiency. The current drawdown in budget resources requires that the goals of ship maintenance and modernization be met with improved efficiency. The Afloat Maintenance Command is a proposal to restructure the existing maintenance hierarchy. Improvements in the process of maintenance and modernization are incorporated in the Afloat Maintenance Command through the elimination of redundancies in capabilities and the improvement of funding flows to enhance organizational efficiency and effectiveness. This thesis will provide an overview of the Afloat Maintenance Command and its possible organization in consolidating assets from existing maintenance activities. Additionally, funding alternatives for the Afloat Maintenance Command will be developed and assessed.http://archive.org/details/afloatmaintenanc00haidLieutenant, United States NavyApproved for public release; distribution is unlimited

Looking Forward Transdisciplinary Modeling, Environmental Forecasting, and Management

In the 1970s, the International Decade of Ocean Exploration (IDOE) set the stage for an era of global ocean research programs (NRC, 1999). Although scientists had long explored the "seven seas," it was only in the late 1960s that observing the ocean at synoptic scales became feasible. This capability, together with the lessons learned from IDOE, allowed for the growth of major oceanographic initiatives. In particular, the late 1980s and the 1990s marked two decades of large oceanographic programs, two of which, the World Ocean Circulation Experiment (WOCE; http://www.nodc.noaa.gov/woce/wdiu/wocedocs/index.htm#design), and the Joint Global Ocean Flux Study (JGOFS; http://www1.whoi.edu), resulted in important advances and transformations in ocean research that fostered the subsequent development of the Global Ocean Ecosystem Dynamics program (GLOBEC; http://www.globec.org)

Numerical modelling in a multiscale ocean

Systematic improvement in ocean modelling and prediction systems over the past several decades has resulted from several concurrent factors. The first of these has been a sustained increase in computational power, as summarized in Moore\u27s Law, without which much of this recent progress would not have been possible. Despite the limits imposed by existing computer hardware, however, significant accruals in system performance over the years have been achieved through novel innovations in system software, specifically the equations used to represent the temporal evolution of the oceanic state as well as the numerical solution procedures employed to solve them. Here, we review several recent approaches to system design that extend our capability to deal accurately with the multiple time and space scales characteristic of oceanic motion. The first two are methods designed to allow flexible and affordable enhancement in spatial resolution within targeted regions, relying on either a set of nested structured grids or, alternatively, a single unstructured grid. Finally, spatial discretization of the continuous equations necessarily omits finer, subgrid-scale processes whose effects on the resolved scales of motion cannot be neglected. We conclude with a discussion of the possibility of introducing subgrid-scale parameterizations to reflect the influences of unresolved processes

John H. Steele, 1926–2013

This special issue of Oceanography is dedicated to the memory of John H. Steele, who passed away on November 4, 2013, at the age of 86. John was a seminal figure in the creation of the US Global Ocean Ecosystem Dynamics (GLOBEC) and International GLOBEC programs, and remained involved in them through his scientific endeavors and program leadership. We benefitted from his keen intellect, wide-ranging knowledge, deep insight, and unfailing good humor and gentlemanly manner

FVCOM validation experiments : comparisons with ROMS for three idealized barotropic test problems

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 113 (2008): C07042, doi:10.1029/2007JC004557.The unstructured-grid Finite-Volume Coastal Ocean Model (FVCOM) is evaluated using three idealized benchmark test problems: the Rossby equatorial soliton, the hydraulic jump, and the three-dimensional barotropic wind-driven basin. These test cases examine the properties of numerical dispersion and damping, the performance of the nonlinear advection scheme for supercritical flow conditions, and the accuracy of the implicit vertical viscosity scheme in barotropic settings, respectively. It is demonstrated that FVCOM provides overall a second-order spatial accuracy for the vertically averaged equations (i.e., external mode), and with increasing grid resolution the model-computed solutions show a fast convergence toward the analytic solutions regardless of the particular triangulation method. Examples are provided to illustrate the ability of FVCOM to facilitate local grid refinement and speed up computation. Comparisons are also made between FVCOM and the structured-grid Regional Ocean Modeling System (ROMS) for these test cases. For the linear problem in a simple rectangular domain, i.e., the wind-driven basin case, the performance of the two models is quite similar. For the nonlinear case, such as the Rossby equatorial soliton, the second-order advection scheme used in FVCOM is almost as accurate as the fourth-order advection scheme implemented in ROMS if the horizontal resolution is relatively high. FVCOM has taken advantage of the new development in computational fluid dynamics in resolving flow problems containing discontinuities. One salient feature illustrated by the three-dimensional barotropic wind-driven basin case is that FVCOM and ROMS simulations show different responses to the refinement of grid size in the horizontal and in the vertical.For this work, H. Huang and G. Cowles were supported by the Massachusetts Marine Fisheries Institute (MFI) through NOAA grants DOC/NOAA/NA04NMF4720332 and DOC/ NOAA/NA05NMF472113; C. Chen was supported by NSF grants (OCE0234545, OCE0606928, OCE0712903, OCE0732084, and OCE0726851), NOAA grants (NA160P2323, NA06RG0029, and NA960P0113), MIT Sea grant (2006-RC-103), and Georgia Sea grant (NA26RG0373 and NA66RG0282); C. Winant was supported through NSF grant OCE-0726673; R. Beardsley was supported through NSF OCE—0227679 and the WHOI Smith Chair; K. Hedstrom was supported through NASA grant NAG13– 03021 and the Arctic Region Supercomputing Center; and D. Haidvogel was supported through grants ONR N00014- 03-1-0683 and NSF OCE 043557