Equatorial Deep Jets (EDJ) are strong zonal currents in the deep tropical Atlantic Ocean (between about 400 and 2000 m depth). They alternate in direction with depth and time, with a vertical wavelength of about 500 m and a period of about 4.5 years. It has been suggested that the EDJ influence climate parameters at the ocean surface. They are also important for the distribution of tracer in the tropical Atlantic at intermediate depth, e.g. the ventilation of the eastern oxygen minimum zone. Despite this, the EDJs’ generation and maintenance mechanisms are not yet fully understood, and state-of-the-art ocean models generally lack the ability to simulate them.
There have been some attempts at theoretical explanations of the EDJ which at least resulted in partial understanding of different EDJ features. It has been shown that the EDJ resemble a resonant equatorial basin mode (composed of equatorial Kelvin and long Rossby waves) at high vertical normal modes. They are probably excited by intraseasonal variability that originates from instabilities at the surface or in the western boundary currents and propagates into the deep ocean. After they are excited, the EDJ would dissipate fast if they were not maintained by power input at their depth. A recent study suggested that this maintenance mechanism is likely provided by the deformation of intraseasonal waves by the EDJ themselves, which leads to a momentum flux that reinforces the EDJ.
In our study, we designed an idealised ocean model experiment to test this hypothesis. We used a twin setup, where one of the model runs was forced by wind stress at the surface and both intraseasonal waves and EDJ developed. From this model run, we diagnosed the momentum flux convergence associated with the deformation of intraseasonal waves by the EDJ. This intraseasonal momentum flux convergence was then used as the only forcing in the second model run.
We could show two things:
1. In our second model run (forced only by the intraseasonal momentum flux convergence associated with the deformation of intraseasonal waves by the EDJ in the first model run), EDJ again develop, with an amplitude comparable to that of the original EDJ. Our results thus show that the intraseasonal momentum flux convergence is indeed largely responsible for the EDJ maintenance at depth, strongly corroborating the theory suggested before.
2. In our model, the EDJ nonlinearly transfer energy to the time mean circulation (see also the figure). This has been suggested theoretically before, but has never been shown in action. The time mean circulation in the deep equatorial ocean is important for oxygen transport, like the EDJ themselves, but is often misrepresented in ocean models. Our results show that for a correct representation of deep tropical currents (both time mean and variable) the EDJ are essential.
Reference:
Bastin, S., M. Claus, P. Brandt, and R. J. Greatbatch (2020): Equatorial Deep Jets and Their Influence on the Mean Equatorial Circulation in an Idealized Ocean Model Forced by Intraseasonal Momentum Flux Convergence. Geophysical Research Letters, 47, doi.org/10.1029/2020GL087808.
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