Physical oceanography studies the physical processes connected to our oceans. Our research on physical oceanography follows three main strands: ocean-atmosphere interactions, the connection between the wind-driven and the thermohaline circulation, and nonlinear data assimilation for the ocean.

Ocean-atmosphere interactions

Understanding ocean-atmosphere interactions is crucial for understanding the functioning of both systems. For instance, the atmosphere generates large- and small-scale ocean circulations via the stress exerted by the atmospheric wind on the ocean surface. Examples are the so-called gyre circulations, basin-wide circulations with ranges up to thausants of km that are crucial for the surface heat and salt transport in the world oceans. On the other hand heat and water from the oceans is transported into the atmosphere, important for cloud formation and atmospheric frontal systems. It has been observed that sometimes an impression of the GulfStream ocean front can be found back in the atmosphere at 5 km height.

Our research in this area concentrates on ocean-atmosphere interactions in relation to the Madden-Julian Oscillation (MJO). We inverstigate the influence of rain-water layers on the ocean-atmosphere heat and humidity exchange using high-resolution atmospheric models coupled to mixed-layer ocean models. This research is conducted with Charlotte DeMott and PhD student Kyle Shackelford

The connection between the wind-driven and the global thermohaline circulation

A long standing research question in physical oceanography is how the wind-driven ocean circulation (e.g. the ocean gyres) interact with the global thermohaline circulation. Extensive studies have been performed in the Southern Ocean, i.e. the Antarctic Circumpolar Current (ACC) where a direct exchange via Ekman pumping is present. Large efforts have also been devoted to those places where oceans meet, the so-called interocean exchange regions. An example is the connection between the Indian and the South Atlantic Ocean, just south of the African continent.

Using integrated balances over different ocean basins, or parts of ocean basins, it is possible to investigate the possibility of certain circulation configurations. An example is the integrating the zonal momentum balance over an ocean basin and determine which inflow and outflow configutaions are possible or compatible with the internal circulation within the basin. This powerful tool allows us to understand better why the ocean circulations are as they are. This is because several at first sight plausible flow configurations turn out to violate e.g. the zonal or meridional integrated momentum balance.

As an example of this procedure, it turns out that a retrun flow of the thermohaline circulation via Drake Passage alone cannot fulfil the integrated balences in the South Atlantic. An inflow via the Agulhas does allow for closed balances, but only if part of that inflow leaves the South Atlantic at its southern boundary.

Understanding the Indian-South-Atlantic interocean transport

A fundamental question in physical oceanography is how the different oceans interact with each other.
Answering this question is crucial for our understanding of the ocean circulation as a whole, and the role the ocean plays in the climate system. One of these interaction points is the ocean area south of Africa where the Indian, South Atlantic and Southern Oceans meet. While it is now well established that the interocean exchange in this area is of global importance, much less is known on what the underlying processes are that determine the existence and size of this exchange.

The interocean exchange water originates in the Agulhas Retroflection area, a highly turbulent region separating the Indian and Atlantic Oceans. For this reason the exchange is highly intermittent, and has been suggested to be dominated by large Agulhas Rings shed from the retroflecting Indian Ocean Agulhas Current. That view is consistent with the estimated size of the volume transport from the Indian to the Atlantic Ocean. The volume transport of one typical Agulhas Ring with a diameter of 300 km carrying a layer of 1000 m with a speed of 5 km/day, all realistic estimates directly based on many in-situ observations and altimetry, is about 2 Sv per eddy. Since the swirl velocity in the upper layers is much larger than the propagation speed, that water is indeed trapped in the rings, as we have shown in earlier studies. Hence, 5-7 rings per year do cary about 10-14 Sv per year, very close to modern high-resolution model based estimates.

Other studies suggest that Agulhas Rings are not dominant in the exchange, although some of these studies are inaccurate. But recent very high-resolution (1-4km) modeling experiments suggest that submesoscale filaments are important contributors to the interocean exchange via direct transport and interaction with mesoscale features, although these studies do not quantify the separate contributions.

A deeper question is what the underlying cause of the interocean exchange is. Many studies in the early 2000’s established that large meanders in the Agulhas Current, so-called Natal Pulses, were strongly related to and at least partly responsible for the shedding of Agulhas Rings. However, detailed research in recent years has shed serious doubts on some of these ideas. Mooring observations of multiyear ocean observation programs suggest that even in years in which only one large meander occurs the number of Agulhas rings shed remains close to its average of 5-7 per year. Instead, these and other studies suggest that other local and remote mechanisms, such as anticyclones impinging on the Agulhas Current from the east, changing latitudes of the Westerly winds, Rossby basin modes, and perhaps even coupled climate modes, are better candidates for the control of ring shedding and interocean exchange.

We thus have to conclude that there still is a large uncertainty of how the interocean exchange is organized, and a further detailed study on the origin of the exchange is warranted and important both for understanding the cause and effect relations this highly turbulent ocean area and for turbulent ocean areas in general. The aim of this proposal is to answer the following research questions:
1) What are the controlling physical processes for the interocean exchange?
2) How do these physical processes interact to control the interocean exchange?

Two new techniques will be combined to answer these questions. The first is an ocean reanalysis obtained with a fully nonlinear data assimilation method. The nonlinearity of the data assimilation is crucial for retaining the full nonlinear evolution of the system. This reanalysis will provide an unprecedented description of the evolution of this ocean area over the last 30 years, providing an ideal starting point for the second new technique, fully nonlinear causal discovery. The nonlinear causal discovery framework will be used to identify these controlling factors and their interactions.

This is conducted with PhD student Hao-Lun Yeh. More information can be found on the Data Assimilation page.