Atlantic Equatorial Deep Jets: Maintenance mechanism and energy transfer to mean currents – Swantje Bastin (GEOMAR Helmholtz Centre for Ocean Research)

Swantje Bastin is a PhD Student at the GEOMAR Helmholtz Centre for Ocean Research in Kiel. She participated in the ESM Summer School in 2019 in Bad Aibling. As part of the Research Unit Ocean Dynamics, Swantje Bastin focuses on Equatorial Deep Jets (EDJ). In this article, she explains the impact of EDJ’s on its environment and how it is possible to drive realistic EDJ in an idealized ocean model.

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 the 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 cannot generally simulate them.

There have been some attempts at theoretical explanations of the EDJ which at least resulted in a 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.

 

We also took the chance to ask Swantje more about her and her current and future research:

- How did the experience in the ESM summer school help you with your work and career?

For my PhD, I am working with ocean models only, but as I plan to apply some of my results for the improvement of coupled forecast models, it is also important for me to gain some understanding of other earth system model components (atmosphere, land…) and the coupling mechanisms. The ESM summer school helped me with this. 

- What are you currently working on?

I am currently investigating the exact scales of the ocean currents I am working on. Because they are located at great depths, measurements have been relatively scarce. In the recent years, more and more data have been collected, such that I can use them now to obtain a more accurate idea of those deep ocean currents. 

- How has COVID-19 changed your daily life/working situation?

I am rather fortunate, because I have not experienced any severe impairment of my work (I work with computer models, so that’s perfectly doable from home office). My daily life suffers from lack of non-virtual contacts, as I am sure everyone feels at the moment...

- How would you describe your job or reasearch to an eight year old?

Imagine the Atlantic Ocean, between Africa and South America, at the equator where the weather is very hot. In the water, if you dived about 1km deep, you would find currents that sometimes flow eastward (towards Africa) and at other times reverse direction and flow westward (towards South America). They do this change of flow direction every few years. Whether they flow westward or eastward, influences the weather on the land around, which is of course important for the people who live there. The problem is that, because it is in reality very difficult to dive 1km deep, we do not know exactly how the currents look and how they work. So this is what I am trying to find out. 

- What are your plans after you finished your PhD?

I would very much like to stay in research and continue with some ideas that came up during my PhD work. But we’ll have to see whether that is possible or I have to think of something else…