Stefano Pierini

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

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 a...

A Nonlinear Theory of the Kuroshio Extension Bimodality

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 ...

Modeling the Oceanic Circulation in the Area of the Strait of Sicily: The Remotely Forced Dynamics

In order to describe aspects of the baroclinic dynamics in the region of the Strait of Sicily a high-resolution multilayer numerical model has been implemented in a central Mediterranean region including the Tyrrhenian and the Ionian Seas. Three layers have been considered representing water of Atlantic origin (MAW), the Levantine Intermediate Water (LIW), and deep water of the Mediterranean. Quasi-stationary circulations representing the local manifestation of the large-scale Mediterranean conveyor belt are obtained [after an adjustment time of O(2 months)] by imposing steady fluxes along the remote open boundaries, in the absence of meteorological forcings. These circulations can be interpreted as possible dynamic scenarios of the seasonal variability in the Strait of Sicily. In the numerical simulations an inflow of MAW and an outflow of LIW through the Strait of Sardinia, an outflow of MAW and an inflow of LIW through the Ionian boundary, and an outflow of MAW through the Corsica channel are imposed, resulting in a vanishing total net transport in each layer. For realistic values of these transports the model captures the main features of the observed circulation, such as (i) the separation of the Algerian Current into two branches, one directed toward the Tyrrhenian Sea and the other entering the strait; (ii) a secondary bifurcation of MAW within the strait giving rise to a southward-moving current that follows the Tunisian continental slope and to a current that flows southeastward along the southern Sicilian coast and then northward along the southern Italian coasts (the so-called Atlantic‐Ionian Stream); (iii) a bifurcation of LIW at the strait level leading to a main current directed toward the Strait of Sardinia and to a weaker current that, after having crossed the strait, bends eastward and enters the Tyrrhenian Sea. Sensitivity experiments carried out by imposing different boundary fluxes have shed light on the functioning of the MAW and LIW bifurcations. First of all, for a given net transport of MAW and LIW through the strait (imposed indirectly by the boundary fluxes), the ratio Rmaw between the transport of MAW entering the Tyrrhenian Sea and that entering the strait is found to be virtually independent of the boundary-imposed Algerian Current transport. It is, on the contrary, determined by a local dynamic control, which selects the value Rmaw 0.43 for a net MAW/LIW strait transport of 61 Sv, in excellent agreement with observations. Second, for decreasing baroclinic transports the ratio Rmaw is found to decrease up to the limiting value Rmaw 0.2 (corresponding to the linear regime) for transports ,O(0.1 Sv). Finally, Rmaw is found to be very sensitive to the barotropic transport T through the strait, whereas the corresponding ratio for the LIW, Rliw, is virtually independent of T. For T 5 20.5 Sv, Rmaw 1.1 while for T 51 0.5 Sv, Rmaw decreases by one order of magnitude: Rmaw 0.1. In other words, a weakening of the LIW (or a strengthening of the MAW) net transport through the strait reduces the relative intensity of the Tyrrhenian branch of MAW, and vice versa. On the other hand, for values of T within the same range one always findsRliw 20.3. It thus appears that the local control exerted by the topography through the LIW potential vorticity budget forces the transport of the Tyrrhenian branch of LIW to be always ;1/3 of that directed toward the Strait of Sardinia.

A Model for the Alboran Sea Internal Solitary Waves

Abstract The propagation into the Alboran Sea of the interface depression generated at the Strait of Gibraltar by the interaction of the semidiurnal tidal current with the main (Camarinal) sill is studied numerically by using a unidirectional model with two horizontal space dimensions. An initial waveform within the strait is determined whose evolution corresponds with the train of internal solitary waves detected in the Alboran Sea by means of current measurements at different depths, as described by Kinder. The agreement is good for two points located at about 45 km east of the strait, the first aligned with the strait axis and the second 20 km to the north. Moreover, the shape and amplitude of this initial condition are in agreement with a typical interface depression generated in the strait by the interaction of the tidal current with the sill. A series of numerical experiments with different initial conditions is also performed to study the sensitivity of the model results to changes in the initial i...

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

AbstractIn 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,...

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

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 resonan...

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

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 correct...

Kuroshio Extension Bimodality and the North Pacific Oscillation: A Case of Intrinsic Variability Paced by External Forcing

AbstractA previous North Pacific Ocean circulation model forced by climatological winds is extended here to include a time-dependent North Pacific Oscillation (NPO) forcing. The Kuroshio Extension (KE) decadal bimodal cycle (which is a self-sustained intrinsic relaxation oscillation in the climatologically forced case) is now excited by the NPO forcing. Both the timing of the cycles and the Rossby wave teleconnection mechanism that is found to govern the synchronization from 1993 to 2012 are in good agreement with altimeter observations. Sensitivity numerical experiments are carried out by varying the zonal location and amplitude of the NPO forcing, and the lateral eddy viscosity. The emergence of the KE bimodality with a correct timing is found to be extremely sensitive to changes in the dissipative parameterization; the implications of such sensitivity for deficiencies found in more realistic North Pacific Ocean general circulation models are discussed. The dynamical mechanism that emerges from this stu...

Measuring the Impact of Observations on the Predictability of the Kuroshio Extension in a Shallow-Water Model

AbstractIn this paper, sequential importance sampling is used to assess the impact of observations on an ensemble prediction for the decadal path transitions of the Kuroshio Extension. This particle-filtering approach gives access to the probability density of the state vector, which allows the predictive power—an entropy-based measure—of the ensemble prediction to be determined. The proposed setup makes use of an ensemble that, at each time, samples the climatological probability distribution. Then, in a postprocessing step, the impact of different sets of observations is measured by the increase in predictive power of the ensemble over the climatological signal during one year. The method is applied in an identical-twin experiment for the Kuroshio Extension using a reduced-gravity shallow-water model. This study investigates the impact of assimilating velocity observations from different locations during the elongated and the contracted meandering states of the Kuroshio Extension. Optimal observation lo...

A model for the spreading and sinking of the Deep Ice Shelf Water in the Ross Sea

Spreading and sinking of the Deep Ice Shelf Water (DISW) in the Ross Sea are analysed using in situ observations and the results of a nonlinear, reduced gravity, layered numerical model, which is able to simulate the motion of a bottom trapped current over realistic topography. The model is forced by prescribing thickness and density of the DISW layer at the southern model boundary as well as ambient density stratification above the DISW layer. This density structure is imposed using hydrographic data acquired by the Italian PNRA-CLIMA project. In the model water of the quiescent ambient ocean is allowed to entrain in the active deep layer due to a simple entrainment parameterization. The importance of forcing the model with a realistic ambient density is demonstrated by carrying out a numerical simulation using an idealized ambient density. In a more realistic simulation the path and the density structure of the DISW vein flowing over the Challenger Basin are obtained and are found to be in good agreement with data. It is found that entrainment, which is particularly active in regions of strong topographic variation, significantly influences the pattern followed by the DISW layer. The evolution of the DISW layer beyond the continental shelf, i.e., in a region where the paucity of experimental data does not allow for a detailed description of the deep ocean dynamics, is also investigated.