Preprints
https://doi.org/10.5194/os-2020-102
https://doi.org/10.5194/os-2020-102

  16 Oct 2020

16 Oct 2020

Review status: a revised version of this preprint was accepted for the journal OS and is expected to appear here in due course.

Imprint of chaotic ocean variability on transports in the Southwest Pacific at interannual timescales

Sophie Cravatte1, Guillaume Serazin2, Thierry Penduff3, and Christophe Menkes4 Sophie Cravatte et al.
  • 1LEGOS, UniversitédeToulouse, IRD, CNES, CNRS, UPS, Toulouse, France
  • 2Climate Change Research Center, University of New South Wales, Sydney, Australia
  • 3Université Grenoble Alpes, CNRS, IRD, Grenoble-INP, Institut des Géosciences de l'Environnement (IGE), Grenoble, France
  • 4ENTROPIE, IRD, CNRS, UR, UNC, Ifremer, Nouméa, New Caledonia

Abstract. The Southwest Pacific Ocean sits at a bifurcation where southern subtropical waters are redistributed equatorward and poleward by different ocean currents. The processes governing the interannual variability of these currents are not completely understood. This issue is investigated using a probabilistic modeling strategy that allows disentangling the atmospherically-forced deterministic ocean variability and the chaotic intrinsic ocean variability. A large ensemble of 50 simulations performed with the same ocean general circulation model (OGCM) driven by the same realistic atmospheric forcing that only differ by a small initial perturbation is analyzed over 1980–2015. Our results show that, in the Southwest Pacific, the interannual variability of the transports is strongly dominated by chaotic ocean variability south of 20° S. In the tropics, while the interannual variability of transports and eddy kinetic energy modulation is largely deterministic and explained by El Nino Southern Oscillation (ENSO), ocean nonlinear processes still explain 10 to 20 % of their interannual variance at large-scale. Regions of strong chaotic variance generally coincide with regions of high mesoscale activity, suggesting that a spontaneous inverse cascade is at work from mesoscale toward lower frequencies and larger scales. The spatiotemporal features of the low-frequency oceanic chaotic variability are complex but spatially coherent within certain regions. In the Subtropical Countercurrent area, they appear as interannually-varying, zonally elongated alternating current structures, while in the EAC region, they are eddy-shaped. Given this strong imprint of large-scale chaotic oceanic fluctuations, our results question the attribution of interannual variability to the atmospheric forcing in the region from point-wise observations and one-member simulations.

Sophie Cravatte et al.

 
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Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version Supplement - Supplement

Sophie Cravatte et al.

Sophie Cravatte et al.

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Short summary
The various currents in the Southwest Pacific Ocean contribute to the redistribution of waters from the subtropical gyre equatorward and poleward. The drivers of their interannual variability are not completely understood, but are usually thought to be related to well-known climate modes of variability. Here, we suggest that oceanic chaotic variability alone, by definition unpredictable, explains the major part of this interannual variability south of 20° S.