Going with the Flow

Ocean circulation plays an enormous role in stabilizing the global climate and transporting nutrients and resources. This criticality drives scientists to better understand what influences ocean circulation, and in turn, its biological, chemical and physical impact.

Dr. Hussein Aluie, professor in the Department of Mechanical Engineering at the University of Rochester, is seeking to answer just that. He and his team, the Turbulence and Complex Flow Group, are using their background in astrophysics and fluid mechanics to better understand how the ocean circulates on a global scale and if this supports or challenges longstanding theories.

Members of the unit also shared their perspectives on the work, including master’s student Siyu Xue, PhD student Mehrnoush Kharghani, and Benjamin A. Storer, research associate and lead developer for the computational code used in this research.

To get started, can you provide some background on your work and the research your lab focuses on?

Aluie: Absolutely. Our work is quite varied. It's transdisciplinary in a way because what we study is fluid dynamics in a very general sense of the word. And we've applied this knowledge and understanding of fluid mechanics to space weather, nuclear fusion, ocean circulation, and most recently, to analyze atmospheric circulation.

The main thrust of my group, though, is to try to understand how the ocean flows. The ocean is a very important component of the Earth's climate system because there's two systems that regulate our climate: the atmosphere and the ocean. Without these fluid systems that flow, you can't have a habitable planet because you need to redistribute the excess heat from the equator to the poles. There's so much we don't know about how it's moving and circulating and the mechanisms that govern that. This is important because we want to be able to predict the circulation under a changing climate, how it would respond and affect the climate system.

What are the challenges to understanding ocean weather patterns, besides the obvious one of scope?

Aluie: We have a lot of data being generated from primarily satellite observations, in-situ observations and sophisticated models, but how can you understand the data? Ocean circulation is highly non-linear and a multi-scale system. Large-scale circulation over the entire globe is like a conveyor belt. And then we move down to gyres and eddies in the ocean. And even further down, there are sub-mesoscales, fronds, Langmuir circulation and waves that we see at the beach. We can look at the flow and see how the ocean is moving, but this encompasses all those components.

Traditional tools have included Fourier analysis, which is the go-to approach to disentangle scales. The problem with this method is that you go into a certain region and take a box, typically a few hundred kilometers on the edges. The Fourier tool disentangles the weather systems from the smaller scales, but as soon as you pick a box, you're introducing something artificial. You're saying, “I'm only looking into this window and I cannot see the bigger picture.” You lose spatial information. You're missing the gyre scales, you're missing the overturning circulation, the planetary circulation. And that's a major limitation.

What makes your approach to understanding ocean circulation unique?

Aluie: Of course, there's a lot already known, but there are still fundamental gaps. We look at things from the hierarchy of scales. We’ve been developing an approach that goes one step further than Fourier. We call it a core-screening approach that allows us to do something similar, disentangling length scales, but it allows us to do so without having to pick this box. This allows us to acknowledge continental boundaries and to look at things in physical space.

Storer: One big difference is the scale of it. Not only do we study the entire global ocean, but we analyzed the entire range of physical scales: from the grid-scale up to the circumference of the Earth [~10km up to 40,000 km], on four years of high-resolution (~8km) ocean data. This work is also different in that it preserves location. Traditional analysis approaches, while very powerful and with a well-established history in the literature, can only tell you how much energy transfer is happening at a given scale, and nothing about where that energy transfer is happening.

Xue: We’re expanding theory on turbulence to a broader context—the entire Earth System, the ocean and the atmosphere, even topography. And doing this research within our department is unique in that it’s different from pure statistical analysis. There is also a lot of collaboration within the group, which accelerates research progress.

Aluie: This also means we’re taking a mechanical, or deterministic approach, as opposed to a statistical one. A statistical understanding uses correlations. You have a certain number of states your system could be in based on initial conditions, but there are some uncertainties. A statistical approach evolves a collection of future models and gives you the probabilities of each outcome. These methods are commonly used because nobody can guarantee that what they're predicting will get manifested with a hundred percent certainty.

Our deterministic approach considers that we ultimately only have one Earth and one climate system. We don't have a collection of Earths where we can weigh the probabilities. We’re not trying to predict things in the statistical sense, but to understand how the gears are cranking. We’re trying to understand the fundamental processes governing the circulation. What's affecting what and how?

And what are your findings?

Aluie: From this most recent work, the finding was that the weather systems in the ocean, which are the most energetic flows that pervade the ocean circulation, have a typical size of a hundred kilometers. They transfer energy or communicate directly with the large-scale circulation, the planetary-scale circulation or gyre-scale circulation at scales a few thousand kilometers in size. This was unexpected in that people did not theorize or hypothesize that such an exchange would occur in the past.

Why not? Because I mentioned that people have been working in boxes. A lot of work that has different scales coupled together have relied on these windows, which make you miss the big picture.

We also expanded existing knowledge on the Intertropical Convergence Zone (ITCZ), which is a rainy band near the equator. We found that it has a very important impact on the flow in the ocean by acting as a hydraulic jump. An easy way to think about it is if you have a river flowing and suddenly there’s a rock—after the water goes over the rock, you have an eddy.

And that's what we found happens in the ocean at these large scales. It's not as severe as the eddy in the river, but if the flow is going from the equator towards the north, it encounters the ITCZ. It's as if there’s a barrier, slowing the flow. And then behind it, there is a circulation like an eddy, rolling the opposite way.

What are the implications of this work?

Aluie: It's important for us to have confidence when people make climate predictions, and to understand the fundamental underlying processes. My hope is that by delving into this, the mechanics of the climate system will help as a guide to understand, explain and have faith in predictions and models.

Kharghani: The implications of this work are significant for understanding and predicting oceanic transport, which greatly affects marine ecosystems and climate. However, gaps remain in our understanding of the complex interactions between different scales.

Aluie: Models are always incomplete. You have a grid, but you can’t have knowledge of things that happen at scale smaller than your grid cell. Since you don't know what's happening at smaller scales, understanding how they communicate with and impact each other at the fundamental level is important to guide sub-grid modeling and in turn, large-scale or predictive-modeling frameworks.

What’s next for this research?

Storer: The oceans are incredibly complex energetic systems and there's still much to study. What about the exchange between kinetic and potential energy? How does it depend on scale or season? Understanding this is key to helping measure and close the ocean’s energy budget. Also, we can create maps of scale-transfer of kinetic energy, so how can we use that to inform parameterizations? Could we, for instance, use these results to train neural networks to be able to predict the energy exchange to improve the accuracy of simulations?

Aluie: One of the things that we're really excited about is that we've demonstrated that this method can disentangle different scales and how climate scales communicate with weather systems. We’re starting to apply that to the atmosphere because weather is more intuitive for us. What about this large-scale planetary circulation in the atmosphere and how it impacts weather systems that we experience?

And hopefully, taking an even further step back and thinking about the Earth’s climate system as a whole, you have the atmosphere, you have the ocean, you have the radiation system, you have ice. We’d like to apply our approach to disentangle them, peeling layers of the onion and understanding how they all connect with each other.