Lagrangian decomposition of the Ocean Circulation Observed from Space (LOCOS)

The global ocean overturning circulation shapes our climate and its variability. However, our understanding of how it redistributes heat from an observational perspective is still very limited.

This proposed project thus aims at analysing the pathways of the overturning circulation from a Lagrangian framework using satellite data.

This will make it possible to trace the origin and fate of the water masses that carry and redistribute the heat and the freshwater around the global ocean.

Trajectory calculations will make it possible to decompose the pathways into different components depending on their individual beginning and destiny.

The present availability of over 30-years of satellite data opens up for analyses that until recently only been possible using model data.

Gridded data is simple to analyse in the Eulerian framework, where one can just compares a variable at a given time and position. This gives, however, a rather limited view.

Here, we will instead analyse the ocean circulation in the Lagrangian framework, where one “observes” the changes along the flow, which gives a more complete view of the ocean circulation and not just its local picture.

The analysis here will thus differ from the standard methods when e.g., measuring the strength of e.g., the Atlantic Meridional Overturning Circulation (AMOC).

In this Lagrangian approach we will decompose the overturning circulation in the different constituent water masses and examine how these contribute and mix with each other and how this circulation is a part of the Global overturning circulation (sometimes referred to as the Ocean Conveyor Belt or the Thermohaline Circulation).

The climate change associated with the ocean and atmospheric circulation and its variability are also generally analysed in the Eulerian framework, where one does not take account of that it takes a certain time for the water to flow from one point to another. The AMOC is thus often falsely described as a change that would instantly be in equilibrium.

Here we will instead analyse the change along the paths, which thus takes account gradually how the ocean changes.Particular emphasis will be on the complexity of the AMOC, which is generally too simply described as just a meridional flow.

In Berglund et al. (2022), we found that the northward warm and saline water in the North Atlantic needs to spiral down in the North Atlantic Subtropical Gyre as it gradually mixes with the cold and fresher water from the Subpolar Gyre before it can continue northward as part of the North Atlantic Current.

These model findings were, however, rather incomplete and highly dependent on the model resolution and parameterisations.

Here we will study this with satellite-based data, which will both improve our understanding of these processes as well as clarify the shortcomings and limitations of the ocean circulation models.

This in turn will help the development of the models and in particular the sub-grid parameterisation that models always need to include the effects of the unresolved scales.

For these analyses of the satellite-based data we will apply methods that we have developed through the years on how to analyse model data.

On of the main tools for this will be our Lagrangian trajectory model TRACMASS, which tracks water mass parcels in contrast to other trajectory models traces that instead tracks single particles of 0-dimension. This makes it possible trace mass, heat and salt transports between different regions in the ocean.

The code will in the beginning of the project be adapted to the various satellite-based data sets, which will be used in the project.Our Lagrangian trajectory model TRACMASS will be adapted to several satellite-based data sets.

This adaptation to satellite-based data will be made available on our TRACMASS web page and on the GitHub repository to users.