SEASONAL AND INTERANNUAL VARIABILITY OF THE NORTH PACIFIC OCEAN: MODELING RESULTS AND THEIR

SEASONAL AND INTERANNUAL VARIABILITY OF THE NORTH PACIFIC OCEAN: MODELING RESULTS AND THEIR VALIDATION THROUGH ALTIMETER DATA Stefano Pierini Dipartimento di Scienze per l’Ambiente Università di Napoli “Parthenope” (Italy)

SEASONAL VARIABILITY OF THE TROPICAL N. PACIFIC • Integrated quantities obtained from altimeter data • Numerical simulation of the same quantities / comparison • Dynamical analysis of the signals (beta-refraction) • DECADAL VARIABILITY OF THE KUROSHIO EXTENSION • Model setup and simulated variability • Comparison with altimetric measurements • for the period 1992-2004 • Analysis of the low-frequency variability • Analysis in terms of dynamical systems theory

Meridional transport fluctuations in the Pacific derived from TOPEX/Poseidon altimeter data and integrated over zonal belts (StammerJGR, 1997) (Stammer, JGR, 1997, Fig. 5c) 8 October 1992

Simulation of the gross features of the seasonal variability contained in those signals was carried out through an idealized model of the wind-driven circulation in the Pacific (Pierini JPO, 2003) r1 D1 H1 r2 D2 H2 (from ECMWF climatology)

s m = SVE s Dy=333 km ALT m=0.15 (Stammer, 1997) Integrated meridional transports m=0.12 Good validation of the seasonal variability, in consideration of the high degree of idealization of the model. Now a simple modelling tool is available for investigating the nature of the dynamics that lies behind these signals

beta-refraction Chelton and Schlax (1996) The refraction toward the equator of low latitude baroclinic Rossby wave packets is due to the strong dependence of the zonal group velocity cg on latitude: (e.g., White, 1977; Schopf et al., 1981; Philander and Yoon, 1982; Philander, 1990; Chelton and Schlax, 1996; White et al., 1998)

l Quantitative characterization of beta-refraction Philander and Yoon (JPO 1982) showed, for a given forcing frequency, that while • for latitudes higher than the critical one the • offshore scale of the response is Ri, • for lower latitudes the offshore scale is • the length covered by a Rossby wave during a period. snapshot of the modeled interface displacement: (m)

meridional dependence of a white- noise wind • same wind stress curl • different wind in the • equatorial belt Remote equatorial forcing through the same teleconnection mechanism known to act in El Niño events Spectral analysis of beta-refraction (Pierini JPO 2005) longitude-frequency map of log10 of the spectral density of the upper- layer baroclinic velocity

P5 P4 P3 A P2 B P1 Pierini (JPO 2006) Reduced-gravity primitive equations: Spatial resolution: 20 km. The domain of integration has a schematic coastline and appropriate zonal and meridional extensions (this is fundamental to insure a correct Sverdrup return transport) The time-independent wind forcing, though idealized, is chosen according to ECMWF and COADS climatologies In summary, this is a "realistic" implementation, for the North Pacific Ocean, of the classic double-gyre problem

sector B A kinetic energy B sector A sector B sector A decadal variability

This is a 10-year sequence of a typical decadal oscillation kinetic energy, B kinetic energy, A yearly paths of the Kuroshio sea surface height

Variability of the Kuroshio Extension jet, recirculation gyre, and mesoscale eddies on decadal time scales Qiu and Chen (JPO 2005) The altimeter data set was compiled by the CLSSpace Oceanographic Division of Toulouse, and merges TOPEX/Poseidon, Jason-1, and ERS-1/2 measurements from October 1992 to December 2004

unstable mode same from modeling results stable mode stable mode large variations in this interval elaboration from altimeter measurements (Qiu and Chen, 2005) positive trend within this interval

model results observations by satellite altimeters (Qiu and Chen 2005)

A B Strong stable southern recirculation gyre; the KE is in an elongated mode weakly decreasing in amplitude. The cyclonic meander produced by barotropic inst. acts as a barrier that separates the Munk- type flow south of Japan from the KE. The flux of negative rel. vorticity coming from the south cannot feed the KE, which is therefore weakly dissipating. A critical transition has occurred. The strong weakening of the first KE crest has eventually allowed the erosion of the cyclonic meander. (cm) The disruption of the stable phase has led to the intensification of the Kuroshio recirculation gyre, which now assumes a straight path that penetrates into the open sea. Proposed explanation of the self-sustained oscillation

(cm) The KE intensification continues, but it is now accompanied by the ejection of cold-core eddies detaching from the cyclonic meander, now meridionally elongated. Such process is well known to occur south of Japan. Proposed explanation of the self-sustained oscillation At t=148 y the KE energy is at its minimum. From now on the KE intensifies, being fed by the flux of negative relative vorticity coming from the south which, in this phase, is no longer trapped in the southern recirculation gyre because the cyclonic meander is too weak to act as a barrier.

b a c c b a c b a c a b c b a

region in which the stable and unstable manifolds intersect A B Strange attractor corresponding to the chaotic oscillations of the Kuroshio Extension (homoclinic orbit in phase space resulting from a global bifurcation) part of the unstable manifold part of the stable manifold saddle fixed point inside this region

Conclusions Numerical results of process-oriented model studies, successfully validated with altimeter data, have provided new insight into the dynamical functioning of relevant aspects of the seasonal and interannual variability of the North Pacific Ocean. The wind-driven seasonal variability in the eastern tropical North Pacific is found to depend substantially on a remote equatorial forcing due to a teleconnection mechanism known to play a major role in the El Niño dynamics, and on the beta-refraction of baroclinic Rossby waves. Passing to midlatitudes, an idealized Kuroshio Extension forced by a time-independent climatological wind evidences a decadal chaotic variability in significant agreement with altimeter observations for the period 1992-2004.

This suggests that purely internal nonlinear mechanismsmay be the predominant cause of the observed decadal variabilityof the jet, in contrast with the common opinion according to which the variability is basically due to wind-driven changes of the Sverdrup return flow. The validity of this hypothesis could have relevant climatological implications related to the decadal variability of the midlatitude North Pacific region and, more specifically, to the Pacific Decadal Oscillation, as it could imply a role of the ocean in inducing a decadal signal in the atmosphere. The combined use of process-oriented model studies and of altimetric measurements can constitute a valuable tool for interpreting / explaining aspects of the oceanic variability not yet fully understood

SEASONAL AND INTERANNUAL VARIABILITY OF THE NORTH PACIFIC OCEAN: MODELING RESULTS AND THEIR

SEASONAL AND INTERANNUAL VARIABILITY OF THE NORTH PACIFIC OCEAN: MODELING RESULTS AND THEIR

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