Dr. Femke de Jong

‘Ocean currents are the engine of the climate’

Research interest

My research is mostly based in the North Atlantic Subpolar Gyre, investigating processes like dense water overflows, deep convection and boundary current – interior exchange. I am particularly interested in the relation between these processes and the Atlantic Meridional Overturning Circulation (AMOC).

I am a sea going oceanographer with a preference for hydrographic and Lagrangian observations, though I often use models and theory to better understand these observations.

Vidi project: From ice to deep ocean, where does the freshwater go?

Deep water formation in the ocean basins around Greenland is an important process within the Atlantic circulation. It is the vertical link between the northward flow of warm, buoyant water in the upper ocean and the southward flow of cold, dense water in the deep ocean. This deep water formation is a consequence of the weak local stratification and strong surface buoyancy forcing. Input of freshwater, which has a much lower density then seawater, is predicted to increase the stratification to the point where deep water formation stops and the Atlantic circulation slows down.

In climate models, the input of freshwater from the Arctic and Greenland’s melting ice cap slows down the Atlantic Ocean circulation significantly, with consequences for climate over north-western Europe. However, the range in predicted responses of the circulation to increasing freshwater input is large. Model predictions rely on the models’ representation of freshwater exchange between the Greenland boundary current system and the deep ocean. These processes occur on scales smaller than the typical model grid scale, making results highly dependent on model resolution.

Observational studies of freshwater exchange have focused on the west Greenland shelf, where ocean eddies have been observed to transport freshwater from the boundary current into the interior. Multiple lines of evidence suggest that freshwater exchange occurs along the 3700 km long east Greenland shelf as well, but this process is yet unstudied. This project aims to study key areas and mechanisms of liquid freshwater exchange around Greenland. And to quantify the impact of these freshwater fluxes on the ocean circulation and evaluate models on their ability to represent the exchange and impact.

The fieldwork of the project consists of surface drifter deployments on the East Greenland shelf in the summers of 2019 and 2020. The trajectories of the drifters will show possible pathways of freshwater out of the boundary current towards the convective regions. We will also study stratification and the impact of freshwater on convection in the Irminger Sea.

NACLIM/OSNAP Irminger Current array

Oceanographic research carried out in the North Atlantic Ocean is of great importance to understand the role of the ocean in our climate and future climate change. As part of an international collaboration between the US, UK, Germany, France, the Netherlands, Canada and China the Netherlands Institute for Sea Research (NIOZ) deployed a moored array on the western flank of the Mid Atlantic Ridge. These 5 moorings (four tall and one short) sample the northward flow of warm, saline water in the Irminger Current in the upper water column as well as the dense North East Atlantic Deep Water (also called Iceland Scotland Overflow Water) in the lower water column.

The complete OSNAP array will provide a high-resolution time series of full water-column transs-baisn fluxes of volume, heat and freshwater. This will provide a measure of the strength and variability of the Atlantic Meridional Overturning Circulation in the Subpolar North Atlantic.

This video shows our research on the currents in the subpolar North Atlantic. More information can be found on the OSNAP webpage at www.o-snap.org. Here you can find video's of the NACLIM project.

Pathways to Denmark Strait

The water mass that overflows the shallow sill of Denmark Strait (aptly named Denmark Strait Overflow Water or DSOW) is the densest constituent of North Atlantic Deep Water, which forms the deep southward return flow in the Meridional Overturning Circulation. There are two main hypothesized sources of DSOW, the Greenland Sea via the East Greenland Current, or the Iceland Sea via the North Icelandic Jet (or possibly a both). 

RAFOS floats were deployed in the Iceland Sea to elucidate the sources of DSOW and the pathways of dense water to the Denmark sill. The first batch of 26 floats was deployed in July 2013, the second batch of 26 floats was deployed in July 2014. These floats can be tracked using the signals of six sound sources moored in the Iceland Sea.

(Funded by NSG grant# 1259210: de Jong & Bower, in collaboration with Henrik Soiland of the Institute of Marine Research, Bergen, Norway.)

Meso-scale eddies in the Labrador Sea

The Labrador Sea is one of few regions where deep convection occurs. During strong winters the surface waters are cooled to the point where they become dense enough to sink and mix with deeper waters. This process forms a cold water mass called Labrador Sea Water that extends from the surface down to 1 to 2 km deep. During spring and summer the central Labrador Sea restratifies. The warm, saline water responsible for the restratification is thought to originate from the Irminger Current along the west Greenland shelf. Due to a sudden steepening of the shelf the current becomes unstable and sheds warm-core eddies. These eddies can transport heat and salt to the interior basin.

Using observations from a mooring we estimated the heat transport of eddies into the Labrador Sea. We found that these eddies vary strongly in size, with radii from 11 to 35 km. Seasonally varying properties add to the large range in transport estimates.