Abyssal circulation

Density section of the South Pacific from the World Ocean Circulation Experiment

The deep ocean can store vast amounts of heat and carbon, making it an important regulator of climate on centennial, millennial, and longer time scales, for example playing a key role in glacial cycles. How much heat and carbon the deep ocean exchanges with the atmosphere is controlled by the large-scale overturning circulation that consists of deep and bottom water formation at high latitudes and intricate upwelling pathways at lower latitudes. The goal of this work is to understand better how the upwelling of bottom waters out of the abyss works. These waters form around Antarctica and spread into the deepest parts of the ocean. To return to the surface, they must slowly be warmed by the action of small-scale mixing. Our work explores how this warming happens if the mixing is — as observations suggest — largely confined to a few hundred meters above the ocean bottom. An interesting consequence of this bottom-intensification of mixing is that it drives downwelling instead of upwelling in part of the water column. We seek to understand how these bottom layer dynamics shape the large-scale circulation and in the net generate upwelling.

Mixing on slopes

Observational estimates of bottom-intensified mixing (Polzin et al., 1997, Science)

It is becoming increasingly clear that strong mixing near the ocean bottom plays a crucial role in the large-scale overturning circulation of the ocean. This mixing is thought to be generated by tidal and geostrophic flow over a rough ocean bottom, which excites internal waves that tend to break and generate turbulence within a few hundred meters of the bottom. Observations are scant, however, so the physics of these bottom boundary layers are not very well constrained. In this work, we use simple dynamical models to elucidate how near-bottom mixing is generated, how hydrodynamic instabilities might modify the dynamical balances in these bottom layers, and what the impact might be on the large-scale circulation.

Submesoscale turbulence

Surface temperature field from an idealized simulation of submesoscale turbulence

Heat and carbon are taken up at the sea surface, but how do they make it into the interior ocean? It has has recently become clear that the vertical circulation at fronts that are about 1–10 km in scale can very effectively exchange fluid between the surface and interior ocean. Our aim here is to understand how these submesoscale fronts are created and how the associated circulation is energized. We have tested different theories in observations collected by research vessels, developed simple dynamical models that capture the essence of the observations, and we have probed the small-scale physics with idealized numerical simulations. The ultimate goal of this work is to understand whether and how the small-scale fronts and their vertical exchange feed back on the large-scale structure of the ocean, and whether that modifies how the ocean responds to changes in atmospheric forcing.

SWOT

Surface currents in a high-resolution numerical simulation of the subtropical North Atlantic

The sea surface is deflected by ocean currents; it rises and falls by up to a meter between ocean eddies. These deflections have been measured routinely from space since the 1990s, and this 20-year legacy of satellite altimetry has revolutionized our conception of the ocean as a strongly turbulent fluid. But current satellite altimeters only measure sea surface height along their ground track, and they are limited in resolution to about 100 km — they can only resolve the largest eddies. Now NASA’s Surface Water and Ocean Topography (SWOT) mission is developing a new altimeter, which is expected to obtain measurements at higher resolution, higher accuracy, and in a two-dimensional swath along its track. This promises to reveal new physics: smaller eddies and fronts, but also internal waves excited by tides. We are doing high-resolution modelling to anticipate what the altimeter will see, to help interpret the data once the satellite is in orbit, and to understand what we can learn about the physics.

Seismic ocean thermometry

Time series of temperature change in the deep Indian Ocean generated from earthquake data

The majority of the energy trapped on the planet by increasingly abundant greenhouse gases is taken up by the ocean, but monitoring the resulting temperature change is challenging because the warming rates are small. Given the current energy imbalance, the deep ocean is excepted to warm at a rate of about 0.02 Kelvin per decade. Such a change is much smaller than local natural fluctuations due to mesoscale eddies, internal waves, and other processes, posing a challenging sampling problem. We developed a method to detect these small changes using the long-range transmission of sound waves that are generated by earthquakes. The sound speed in seawater depends on temperature, so a warmer ocean means that these seismic sound waves are received slightly earlier.

Waves and turbulence in the atmosphere

Atmospheric inertia–gravity waves around Amsterdam Island, visualized by their effect on cloud formation

Much like the ocean, the mid-latitude atmosphere has a field of strong macro-turbulence — which we usually call “weather.” The dynamics of this macro-turbulence at scales larger than about 500 km are well-understood, but what dynamics dominate at smaller scales has long been debated. Are these mesoscale winds strongly turbulent? Are they constrained by rotation and stratification? Or are they wave-like? Applying a method that we originally developed for the interpretation of ship-track data of ocean currents to aircraft observations of winds and temperature, we were able to show that these flows follow predictions from linear wave theory, supporting the hypothesis that they are dominated by inertia–gravity waves. This reveals interesting parallels with the ocean, which has long been known to be filled by such waves.