Intra-Annual Rossby Waves Destabilization as a Potential Driver of Low-Latitude Zonal Jets: Barotropic Dynamics

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.

Similarities and Contrasts in Time-Mean Striated Surface Tracers in Pacific Eastern Boundary Upwelling Systems: The Role of Ocean Currents in Their Generation

“…These variations among regions, time periods, and tracers make it difficult to generalize our results and apply them to other EBUS regions, such as the Benguela and Canary current systems in the Atlantic. Diverse theories for the existence of striated currents [15][16][28][29][30][31][32][33][34][35][36][37] suggest the possibility of different generation mechanisms in different regions. Combined with the variety of water masses in the global ocean and their spatial properties, particularly the strength and direction of background tracer gradients, such diversity suggests large regional variations in the existence and characteristics of striated tracer fields.…”

“…Multiple theories have been proposed for their existence: artifacts of time-averaging westward-propagating mesoscale eddies [28][29][30]; beta-plumes generated through Rossby wave or eddy radiation from eastern boundary current meanders [15,16,31]; triad Rossby wave interaction and associated breaking [32][33][34]; anisotropic inverse energy cascades in geostrophic turbulence [35]; potential vorticity staircases associated with heterogeneous isopycnal mixing [36,37]; and topographic influences on potential vorticity [20,21,24], among others. Some of these theories were partly or entirely developed for EBUS [15,16,29,31].…”

Similarities and Contrasts in Stationary Striations of Surface Tracers in Pacific Eastern Boundary Upwelling Systems

“…These variations among regions and tracers make it difficult to generalize our results to other EBUS regions such as the Benguela and Canary current systems in the Atlantic. Diverse theories for the existence of striated currents [12][13][14][15][16][17][18][19][20][21][22][23] suggest the possibility of different generation mechanisms in different regions. Combined with the variety of water masses in the global ocean and their spatial properties, particularly the strength and direction of background tracer gradients, such diversity suggests large regional variations in the existence and characteristics of striated tracer fields.…”

“…These bands of alternating eastward/westward mean geostrophic currents have meridional scales of 3-5°, extend from the surface to the intermediate or deep ocean, and spread out zonally for 1000's of km [11]. Multiple theories have been proposed for their existence: artifacts of timeaveraging westward-propagating mesoscale eddies [12][13][14]; beta-plumes generated through Rossby wave or eddy radiation from eastern boundary current meanders [15][16][17]; triad Rossby wave interaction and associated breaking [18][19][20]; anisotropic inverse energy cascades in geostrophic turbulence [21]; potential vorticity staircases associated with heterogeneous isopycnal mixing [22][23], just to name a few. Some of these theories were partly or entirely developed for EBUS [13,15,16,17].…”