scholarly journals Coherence Resonance in a Double-Gyre Model of the Kuroshio Extension

2010 ◽  
Vol 40 (1) ◽  
pp. 238-248 ◽  
Author(s):  
Stefano Pierini
Keyword(s):  
Time Scale ◽  
Kuroshio Extension ◽  
Relaxation Oscillation ◽  
Low Frequency ◽  
Altimeter Data ◽  
Coherence Resonance ◽  
Wind Noise ◽  
Low Frequency Variability ◽  
The Kuroshio ◽  
Double Gyre

Abstract The effect of stochastic winds on the intrinsic low-frequency variability of the Kuroshio Extension (KE) is analyzed through a double-gyre (DG) model forced by a steady climatological wind plus an idealized Ornstein–Uhlenbeck wind noise. A DG model of the KE bimodality, whose results compare well to altimeter data, is first shown to be an excitable system. In fact, the relaxation oscillation (forced by steady winds) with decadal time scale that describes the bimodality is recognized to be an internal mode of the system, which can be excited also in a dissipative parameter range (PR) in which it does not arise spontaneously, provided appropriate initial conditions are chosen. It is then shown that, if the additive wind noise is included in the forcing, the actual excitation of the relaxation oscillation in PR occurs if the noise is red with a decorrelation time greater than a minimum time scale ranging from 1 month to 1 year, depending on the dissipation. This behavior, known as “coherence resonance,” is likely to be paradigmatic of the low-frequency variability of western boundary current extensions of intrinsic origin, when it is in the form of relaxation oscillations resulting from a homoclinic bifurcation. General considerations concerning the interpretation of model results obtained within different parameter ranges are applied to this study.

scholarly journals On the Crucial Role of Basin Geometry in Double-Gyre Models of the Kuroshio Extension

2008 ◽  
Vol 38 (6) ◽  
pp. 1327-1333 ◽  
Author(s):  
Stefano Pierini
Keyword(s):  
Crucial Role ◽  
North Pacific Ocean ◽  
Kuroshio Extension ◽  
Low Frequency ◽  
Altimeter Data ◽  
Western Boundary ◽  
Low Frequency Variability ◽  
Basin Geometry ◽  
The Kuroshio ◽  
Double Gyre

Abstract The decadal chaotic relaxation oscillation obtained in a recent double-gyre model study of the Kuroshio Extension intrinsic low-frequency variability was found to compare surprisingly well with the real variability of the jet as revealed by altimeter data, despite the high degree of idealization of the model. In this note it is shown that elements of realism in the basin geometry, present in that study and absent in previous double-gyre models applied to the Kuroshio Extension, play a crucial role in shaping the low-frequency variability of the jet, and can explain the good performance of the model. A series of numerical experiments with different basin geometries of increasing degrees of simplicity are analyzed. If the schematic western boundary representing the coastline south of Japan is removed, the strong decadal variability completely disappears and only a very weak periodic oscillation about an elongated state of the jet is found. If the large zonal width of the basin (representing correctly the extension of the North Pacific Ocean) is reduced by a half, then the total meridional Sverdrup transport is reduced by the same factor, and so is the intensity of the Kuroshio and Oyashio western boundary currents: as a result, the modeled Kuroshio Extension is totally unrealistic in shape and is steady. If both simplifications are introduced the resulting jet is, again, totally unrealistic, yielding a weak periodic bimodal cycle. On the basis of these results, two main conclusions are drawn: (i) the introduction of appropriate geometrical elements of realism in double-gyre model studies of the Kuroshio Extension is essential, and (ii) the Kuroshio Extension intrinsic low-frequency variability would be dramatically different if the southwestern coastline of Japan were more meridionally oriented.

scholarly journals A Nonlinear Theory of the Kuroshio Extension Bimodality

2009 ◽  
Vol 39 (9) ◽  
pp. 2212-2229 ◽  
Author(s):  
Stefano Pierini ◽  
Henk A. Dijkstra ◽  
Angelo Riccio
Keyword(s):  
Decadal Variability ◽  
Nonlinear Dynamical Systems ◽  
North Pacific Ocean ◽  
Kuroshio Extension ◽  
Relaxation Oscillation ◽  
Low Frequency ◽  
Wind Forcing ◽  
Altimeter Data ◽  
Main Question ◽  
The Kuroshio

Abstract The Kuroshio Extension (KE) flow in the North Pacific Ocean displays a very distinctive decadal variability of bimodal character involving two completely different states (a large-meander “elongated” state and a small-meander “contracted” state) connected by very asymmetric temporal transitions. Although such a flow has been widely studied by means of a suite of mathematical models and by using several observational platforms, a satisfactory theoretical framework answering quite elementary questions is still lacking, the main question being whether such variability is induced by a time-varying wind forcing or, rather, by intrinsic oceanic mechanisms. In this context, the chaotic relaxation oscillation produced by a process-oriented model of the KE low-frequency variability, with steady climatological wind forcing, was recently recognized to be in substantial agreement with altimeter data. Here those model results are further compared with a comprehensive altimeter dataset. The positive result of such a comparison allows the conclusion that a minimal model for the KE bimodality has been identified and that, consequently, nonlinear intrinsic oceanic mechanisms are likely to be the main cause of the observed variability. By applying the methods of nonlinear dynamical systems theory, relevant dynamical features of the modeled flow are then explained, such as the origin of the relaxation oscillation as a consequence of a homoclinic bifurcation, the spatiotemporal character of the bimodal behavior, and the degree of predictability of the flow in the different stages of the oscillation (evaluated through a field of finite-time Lyapunov exponents and the corresponding Lagrangian time series).

scholarly journals Low-Frequency Variability, Coherence Resonance, and Phase Selection in a Low-Order Model of the Wind-Driven Ocean Circulation

2011 ◽  
Vol 41 (9) ◽  
pp. 1585-1604 ◽  
Author(s):  
Stefano Pierini
Keyword(s):  
Ocean Circulation ◽  
Kuroshio Extension ◽  
Global Bifurcation ◽  
Low Frequency ◽  
Galerkin Projection ◽  
Coherence Resonance ◽  
Primitive Equation ◽  
Phase Selection ◽  
Low Frequency Variability ◽  
Low Order

Abstract In this paper, a low-order spectral quasigeostrophic (QG) model of the wind-driven ocean circulation is derived and used to analyze the low-order character of the intrinsic low-frequency variability of the midlatitude double-gyre ocean circulation and of the related coherence resonance and phase selection phenomena. The model includes an exponential in the basis functions that allows for westward intensification, retains only four modes in the Galerkin projection, is defined in a rectangular domain, and is forced by deterministic and stochastic winds, thus extending previous low-order QG ocean models. The solution under steady forcing is first obtained, and the results are also analyzed in terms of dynamical systems theory. A homoclinic bifurcation (with the wind amplitude chosen as the control parameter) leads to intrinsic decadal relaxation oscillations (ROs) similar in several respects to those obtained with primitive equation models. The system is then forced with an additional red noise wind, and, in a parameter range preceding the global bifurcation, a coherence resonance scenario very similar to the one found with a primitive equation model of the Kuroshio Extension is obtained: this suggests that such a phenomenon is of low-order character. To study the RO excitation mechanism, a method denoted as phase selection is proposed. The system is forced with additional fictitious periodic winds that produce an emergence of ROs yielding strong phase dependence with the periodic forcing. The subsequent analysis reveals the character of the wind forcing that is most likely to excite a RO. All the results are discussed within the general framework of climate dynamics.

scholarly journals Intrinsic low-frequency variability and predictability of the Kuroshio Current and of its extension

2014 ◽  
Vol 5 (2) ◽  
pp. 79 ◽  
Author(s):  
Stefano Pierini ◽  
Henk A. Dijkstra ◽  
Mu Mu
Keyword(s):  
Particle Filtering ◽  
Current System ◽  
Kuroshio Extension ◽  
Low Frequency ◽  
Kuroshio Current ◽  
Western Boundary ◽  
Boundary Current ◽  
Model Studies ◽  
Low Frequency Variability ◽  
The Kuroshio

Investigations of the intrinsic low-frequency variability and predictability of the Kuroshio Current and of its extension jet (the Kuroshio Extension, KE) are reviewed. The Kuroshio and KE in the North Pacific constitute a western boundary current system of great relevance from climatological and ecological viewpoints. Both the Kuroshio south of Japan and the KE display remarkable changes of bimodal character on interannual time scales that are believed to be intrinsic, i.e., basically generated by nonlinear oceanic mechanisms rather than by direct atmospheric forcing. Model studies of the Kuroshio and KE with climatological forcing are thus reviewed. Moreover, as these changes are chaotic, their predictability requires peculiar mathematical approaches: theoretical results concerning this important issue are therefore reviewed as well. Model studies aimed at determining the optimal precursors and optimally growing initial errors for the Kuroshio are described. Techniques based on Lyapunov exponents (including their Lagrangian extension) and on data assimilation techniques (namely, sequential importance sampling using a particle-filtering approach) are reviewed for the KE. The key problem of how to identify the areas where targeted observations can improve the forecast is also addressed. The role of wind forcing in triggering the KE oscillations is finally considered.

scholarly journals Low-frequency variability of the Kuroshio Extension

2009 ◽  
Vol 16 (6) ◽  
pp. 665-675 ◽  
Author(s):  
S. Pierini ◽  
H. A. Dijkstra
Keyword(s):  
Nonlinear Dynamics ◽  
Large Scale ◽  
Rossby Waves ◽  
Kuroshio Extension ◽  
Baroclinic Instability ◽  
Low Frequency ◽  
Wave Dynamics ◽  
Low Frequency Variability ◽  
Low Dimensional ◽  
The Kuroshio

Abstract. In this paper, we provide a review of recent results targeted at the understanding of the low-frequency variability of the Kuroshio Extension. We provide the background and main arguments of two views which have recently been proposed to explain this variability. In the first view, wind-induced Rossby waves and the effects of mesocale eddies are crucial. The second view is based on low-dimensional equivalent-barotropic large-scale nonlinear dynamics, with neither Rossby wave dynamics nor baroclinic instability being important. Results from models supporting each view are discussed and confronted with results from available observations.

scholarly journals A Kuroshio Extension System Model Study: Decadal Chaotic Self-Sustained Oscillations

2006 ◽  
Vol 36 (8) ◽  
pp. 1605-1625 ◽  
Author(s):  
Stefano Pierini
Keyword(s):  
Decadal Variability ◽  
Kuroshio Extension ◽  
Low Frequency ◽  
Internal Oscillation ◽  
Extension System ◽  
Model Study ◽  
Low Frequency Variability ◽  
South Of Japan ◽  
Altimetric Measurements ◽  
The Kuroshio

Abstract A model study of the Kuroshio Extension system, in which forcing is provided by a time-independent climatological wind, yields a mean meandering path and a decadal variability of the jet in significant agreement with in situ and altimetric measurements. A reduced-gravity primitive equation ocean model is implemented in a box spanning the whole North Pacific, including a schematic coastline at the western side, and an analytical wind forcing is determined according to the ECMWF and Comprehensive Ocean–Atmosphere Data Set (COADS) climatologies. The modeled time-averaged Kuroshio Extension shows meanders, northern and southern recirculation regions, and a jet penetration that are in good agreement with the corresponding observed climatological features. This result suggests that intrinsic nonlinear mechanisms are likely to play a major role in determining the meander pattern of the mean flow. The internal low-frequency variability is found to be a chaotic bimodal self-sustained oscillation between an energetic meandering state and a much weaker state with a reduced zonal penetration of the jet. These high and low energy states are found to be very similar to the “elongated” and “contracted” modes of the Kuroshio Extension detected through altimetric measurements; moreover, the characteristic period (of around 10 yr), flow patterns, and transition details of a typical bimodal cycle are found to be in significant agreement with altimeter observations for the period 1992–2004. A complex dynamical mechanism supporting this internal oscillation, and involving the bimodal behavior of the Kuroshio south of Japan, is proposed and discussed. On the basis of these modeling results and of their validation with altimeter data, it is hypothesized that the observed bimodal decadal variability of the Kuroshio Extension is basically due to a self-sustained internal oscillation related to the instability of the Kuroshio south of Japan without any crucial intervention of wind-driven Sverdrup transport fluctuations and of topographic interactions, although such effects certainly play an important role in shaping the finer structure of Kuroshio Extension changes. Finally, in a preliminary analysis of the variability in the framework of nonlinear dynamical systems theory it is suggested that the strange attractor corresponding to the modeled low-frequency variability is associated with a homoclinic orbit produced by a global bifurcation; moreover, transitions between oscillations of different character found for slightly different values of the lateral eddy viscosity and forcing amplitude are conjectured to be due to heteroclinic connections.

Decadal variability of the Kuroshio Extension

2020 ◽  
Author(s):  
Guidi Zhou ◽  
Xuhua Cheng
Keyword(s):  
Decadal Variability ◽  
Kuroshio Extension ◽  
Relaxation Oscillation ◽  
Mesoscale Eddy ◽  
Height Anomaly ◽  
Intrinsic Variability ◽  
Mean Flow ◽  
The Real ◽  
Eddy Activity ◽  
The Kuroshio

<p>The decadal variability of the Kuroshio Extension (KE) is investigated using altimeter observations (AVISO) and the output of an ocean model (OFES). It is shown that the KE decadal variability is manifested in its strength, latitudinal position, and zonal extent, as well as the associated mesoscale eddy activity. Two differences between the two datasets are identified: (a) In OFES, the eddy activity positively correlates with the KE mode index when it leads by a few years, whereas in AVISO the two are negatively and concurrently correlated. (b) In OFES, the positive KE mode is associated with large meanders of the Kuroshio south of Japan, but in AVISO they are irrelevant. These differences indicate that the generation mechanism of KE's decadal variability is different in OFES and the real ocean. The sea surface height anomaly (SSHA) is then decomposed into major components including the wind-driven Rossby waves and residual (intrinsic) variability. The relationship between the two components are virtually the same in OFES and in AVISO, showing a negative correlation when the wind-driven part leads by a few years. Further diagnostics based on OFES reveals that the residual SSHA originates from the downstream region over the Shatsky Rise, slowly propagates westward, and is driven by eddy potential energy transfer. The OFES results partly conform to the intrinsic relaxation oscillation theory put forth by idealized model analyses, but in the latter the SSHA signal originates from the upstream Kuroshio. A new mechanism is then proposed for OFES: the decadal variability of the KE is first a result of the intrinsic relaxation oscillation probably excited by wind forcing, which regulates the strength of the KE’s inflow and thus modulates the downstream topography interaction, resulting in different downstream mesoscale eddy activity that further feeds back on the mean-flow. The mechanism for the real ocean is also reassessed.</p>

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