arXiv subjects

M. J. Olascoaga

Publications and source records attributed to M. J. Olascoaga.

28 records · Page 2Linked to original sources

arXiv2015

Coherent water transport across the South Atlantic

The role of mesoscale eddies in transporting Agulhas leakage is investigated using a recent technique from nonlinear dynamical systems theory applied on geostrophic currents inferred from the over two-decade-long satellite altimetry record. Eddies are found to acquire material coherence away from the Agulhas retroflection, near the Walvis Ridge in the South Atlantic. Yearly, 1 to 4 coherent material eddies are detected with diameters ranging from 40 to 280 km. A total of 23 eddy cores of about 50 km in diameter and with at least 30\pct of their contents traceable into the Indian Ocean were found to travel across the subtropical gyre with minor filamentation. Only 1 eddy core was found to pour its contents on the North Brazil Current. While ability of eddies to carry Agulhas leakage northwestward across the South Atlantic is supported by our analysis, this is more restricted than suggested by earlier ring transport assessments.

physics.ao-ph↗

arXiv2015

Dissipative inertial transport patterns near coherent Lagrangian eddies in the ocean

Recent developments in dynamical systems theory have revealed long-lived and coherent Lagrangian (i.e., material) eddies in incompressible, satellite-derived surface ocean velocity fields. Paradoxically, observed drifting buoys and floating matter tend to create dissipative-looking patterns near oceanic eddies, which appear to be inconsistent with the conservative fluid particle patterns created by coherent Lagrangian eddies. Here we show that inclusion of inertial effects (i.e., those produced by the buoyancy and size finiteness of an object) in a rotating two-dimensional incompressible flow context resolves this paradox. Specifically, we obtain that anticyclonic coherent Lagrangian eddies attract (repel) negatively (positively) buoyant finite-size particles, while cyclonic coherent Lagrangian eddies attract (repel) positively (negatively) buoyant finite-size particles. We show how these results explain dissipative-looking satellite-tracked surface drifter and subsurface float trajectories, as well as satellite-derived \emph{Sargassum} distributions.

physics.ao-ph↗

arXiv2008

Zonal Jets as Transport Barriers in Planetary Atmospheres

The connection between transport barriers and potential vorticity (PV) barriers in PV-conserving flows is investigated with a focus on zonal jets in planetary atmospheres. A perturbed PV-staircase model is used to illustrate important concepts. This flow consists of a sequence of narrow eastward and broad westward zonal jets with a staircase PV structure; the PV-steps are at the latitudes of the cores of the eastward jets. Numerically simulated solutions to the quasigeostrophic PV conservation equation in a perturbed PV-staircase flow are presented. These simulations reveal that both eastward and westward zonal jets serve as robust meridional transport barriers. The surprise is that westward jets, across which the background PV gradient vanishes, serve as robust transport barriers. A theoretical explanation of the underlying barrier mechanism is provided. It is argued that transport barriers near the cores of westward zonal jets, across which the background PV gradient is small, are found in Jupiter's midlatitude weather layer and in the Earth's summer hemisphere subtropical stratosphere.

physics.ao-ph↗

arXiv2007

Tracing the early development of harmful algal blooms with the aid of Lagrangian coherent structures

Several theories have been proposed to explain the development of harmful algal blooms (HABs) produced by the toxic dinoflagellate \emph{Karenia brevis} on the West Florida Shelf. However, because the early stages of HAB development are usually not detected, these theories have been so far very difficult to verify. In this paper we employ simulated \emph{Lagrangian coherent structures} (LCSs) to trace the early location of a HAB in late 2004 before it was transported to an area where it could be detected by satellite imagery, and then we make use of a population dynamics model to infer the factors that may have led to its development. The LCSs, which are computed based on a surface flow description provided by an ocean circulation model, delineate past and future histories of boundaries of passively advected fluid domains. The population dynamics model determines nitrogen in two components, nutrients and phytoplankton, which are assumed to be passively advected by the simulated surface currents. Two nearshore nutrient sources are identified for the HAB whose evolution is found to be strongly tied to the simulated LCSs. While one nutrient source can be associated with a coastal upwelling event, the other is seen to be produced by river runoff, which provides support to a theory of HAB development that considers nutrient loading into coastal waters produced by human activities as a critical element. Our results show that the use of simulated LCSs and a population dynamics model can greatly enhance our understanding of the early stages of the development of HABs.

physics.ao-ph↗

arXiv2006

Persistent Transport Barrier on the West Florida Shelf

Analysis of drifter trajectories in the Gulf of Mexico has revealed the existence of a region on the southern portion of the West Florida Shelf (WFS) that is not visited by drifters that are released outside of the region. This so-called ``forbidden zone'' (FZ) suggests the existence of a persistent cross-shelf transport barrier on the southern portion of the WFS. In this letter a year-long record of surface currents produced by a Hybrid-Coordinate Ocean Model simulation of the WFS is used to identify Lagrangian coherent structures (LCSs), which reveal the presence of a robust and persistent cross-shelf transport barrier in approximately the same location as the boundary of the FZ. The location of the cross-shelf transport barrier undergoes a seasonal oscillation, being closer to the coast in the summer than in the winter. A month-long record of surface currents inferred from high-frequency (HF) radar measurements in a roughly 60 km $\times$ 80 km region on the WFS off Tampa Bay is also used to identify LCSs, which reveal the presence of robust transient transport barriers. While the HF-radar-derived transport barriers cannot be unambiguously linked to the boundary of the FZ, this analysis does demonstrate the feasibility of monitoring transport barriers on the WFS using a HF-radar-based measurement system. The implications of a persistent cross-shelf transport barrier on the WFS for the development of harmful algal blooms on the shoreward side of the barrier are considered.

physics.ao-ph↗

arXiv2006

The Effects of Lagrangian vs. Population Dynamics on Plankton Patchiness Generation

The relative importance of Lagrangian and population dynamics on spatial pattern formation in the distribution of plankton near the ocean's surface is investigated. Phytoplankton and zooplankton are treated as biologically interacting tracers that are passively advected by a divergence-free two-dimensional nonsteady velocity field. The time-dependence of this field is assumed to be quasiperiodic, which extends the analysis presented beyond standard chaotic advection. Various forms of predator--prey interactions, including complex interactions, are considered. Numerical simulations illustrate how Lagrangian dynamics, which is characterized by a mixed phase space with coexisting regular and chaotic motion regions, influences plankton patchiness generation depending on the nature of the biological interactions.

nlin.CD↗

arXiv2006

On the Lagrangian Dynamics of Atmospheric Zonal Jets and the Permeability of the Stratospheric Polar Vortex

The Lagrangian dynamics of zonal jets in the atmosphere are considered, with particular attention paid to explaining why, under commonly encountered conditions, zonal jets serve as barriers to meridional transport. The velocity field is assumed to be two-dimensional and incompressible, and composed of a steady zonal flow with an isolated maximum (a zonal jet) on which two or more travelling Rossby waves are superimposed. The associated Lagrangian motion is studied with the aid of KAM (Kolmogorov--Arnold--Moser) theory, including nontrivial extensions of well-known results. These extensions include applicability of the theory when the usual statements of nondegeneracy are violated, and applicability of the theory to multiply periodic systems, including the absence of Arnold diffusion in such systems. These results, together with numerical simulations based on a model system, provide an explanation of the mechanism by which zonal jets serve as barriers to meridional transport of passive tracers under commonly encountered conditions. Causes for the breakdown of such a barrier are discussed. It is argued that a barrier of this type accounts for the sharp boundary of the Antarctic ozone hole at the perimeter of the stratospheric polar vortex in the austral spring.

physics.ao-ph↗

arXiv2005

Phase space structure of nonautonomous one-degree-of-freedom Hamiltonian systems with nontrivial time dependence

It has been recently argued that near-integrable nonautonomous one-degree-of-freedom Hamiltonian systems are constrained by KAM theory even when the time-dependent (nonintegrable) part of the Hamiltonian is given in the form of a superposition of time-periodic functions with incommensurate frequencies. Furthermore, such systems are constrained by one fewer integral of motion than is required to render the system completely integrable. As a consequence, the phase space in systems of this type is expected to be partitioned into nonintersecting regular and chaotic regions. In this note we provide numerical evidence of the existence of such a characteristic mixed phase space structure. This is done by considering the problem of acoustic ray dynamics in deep ocean environments, which is naturally described as a nonautonomous one-degree-of-freedom Hamiltonian system with a multiply periodic Hamiltonian in the independent (time-like) variable. Also, we discuss the implications of a mixed phase space for the dynamics of that geophysical system and another one which describes Lagrangian motion in the ocean. The latter is also naturally described as a nonautonomous one-degree-of-freedom Hamiltonian system with a multiply time-periodic Hamiltonian.

nlin.CD↗

arXiv2004

Linear waves and baroclinic instability in an inhomogeneous-density layered primitive-equation ocean model

We consider a multilayer generalization of Ripa's inhomogeneous-density single-layer primitive-equation model. In addition to vary arbitrarily in horizontal position and time, the horizontal velocity and buoyancy fields are allowed to vary linearly with depth within each layer of the model. Preliminary results on linear waves and baroclinic instability suggest that a configuration involving a few layers may set the basis for a quite accurate and numerically efficient ocean model.

physics.ao-ph↗

arXiv2003

Deep ocean influence on upper ocean baroclinic instability saturation

In this paper we extend earlier results regarding the effects of the lower layer of the ocean (below the thermocline) on the baroclinic instability within the upper layer (above the thermocline). We confront quasigeostrophic baroclinic instability properties of a 2.5-layer model with those of a 3-layer model with a very thick deep layer, which has been shown to predict spectral instability for basic state parameters for which the 2.5-layer model predicts nonlinear stability. We compute and compare maximum normal-mode perturbation growth rates, as well as rigorous upper bounds on the nonlinear growth of perturbations to unstable basic states, paying particular attention to the region of basic state parameters where the stability properties of the 2.5- and 3-layer model differ substantially. We found that normal-mode perturbation growth rates in the 3-layer model tend to maximize in this region. We also found that the size of state space available for eddy-amplitude growth tends to minimize in this same region. Moreover, we found that for a large spread of parameter values in this region the latter size reduces to only a small fraction of the total enstrophy of the system, thereby allowing us to make assessments of the significance of the instabilities.

physics.ao-ph↗