Intra-annual Rossby waves destabilization as a potential driver of Low-Latitude Zonal Jets – Barotropic dynamics

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  • 1 LEGOS - Université de Toulouse, CNRS, CNES, IRD, Toulouse, France
  • 2 LOPS - Université de Bretagne Occidentale, Ifremer, CNRS, IRD, Brest, France
  • 3 LEGOS - Université de Toulouse, CNRS, CNES, IRD, Toulouse, France
  • 4 Department of Earth System Science, Stanford University, Stanford, California
  • 5 LEGOS - Université de Toulouse, CNRS, CNES, IRD, Toulouse, France
  • 6 LOPS - Université de Bretagne Occidentale, Ifremer, CNRS, IRD, Brest, France
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Abstract

At low latitudes in the ocean, the deep currents are shaped into narrow jets flowing eastward and westward, reversing periodically with latitude between 15°S and 15°N. These jets are present from the thermocline to the bottom. The energy sources and the physical mechanisms responsible for their formation are still debated and poorly understood. This study explores the role of the destabilization of intra-annual equatorial waves in the jets formation process, as these waves are known to be an important energy source at low latitudes. The study focuses particularly on the role of barotropic Rossby waves as a first step towards understanding the relevant physical mechanisms. It is shown from a set of idealized numerical simulations and analytical solutions that Non-Linear Triad Interactions (NLTI) play a crucial role in the transfer of energy towards jet-like structures (long waves with short meridional wavelengths) that induce a zonal residual mean circulation. The sensitivity of the instability emergence and the scale selection of the jet-like secondary wave to the forced primary wave is analyzed. For realistic amplitudes around 5-20 cm s−1, the primary waves that produce the most realistic jet-like structures are zonally-propagating intra-annual waves with periods between 60 and 130 days and wavelengths between 200 and 300 km. The NLTI mechanism is a first step towards the generation of a permanent jet-structured circulation, and is discussed in the context of turbulent cascade theories.

Corresponding author: Audrey Delpech, audrey.delpech@legos.obs-mip.fr

Abstract

At low latitudes in the ocean, the deep currents are shaped into narrow jets flowing eastward and westward, reversing periodically with latitude between 15°S and 15°N. These jets are present from the thermocline to the bottom. The energy sources and the physical mechanisms responsible for their formation are still debated and poorly understood. This study explores the role of the destabilization of intra-annual equatorial waves in the jets formation process, as these waves are known to be an important energy source at low latitudes. The study focuses particularly on the role of barotropic Rossby waves as a first step towards understanding the relevant physical mechanisms. It is shown from a set of idealized numerical simulations and analytical solutions that Non-Linear Triad Interactions (NLTI) play a crucial role in the transfer of energy towards jet-like structures (long waves with short meridional wavelengths) that induce a zonal residual mean circulation. The sensitivity of the instability emergence and the scale selection of the jet-like secondary wave to the forced primary wave is analyzed. For realistic amplitudes around 5-20 cm s−1, the primary waves that produce the most realistic jet-like structures are zonally-propagating intra-annual waves with periods between 60 and 130 days and wavelengths between 200 and 300 km. The NLTI mechanism is a first step towards the generation of a permanent jet-structured circulation, and is discussed in the context of turbulent cascade theories.

Corresponding author: Audrey Delpech, audrey.delpech@legos.obs-mip.fr
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