by Penny Holliday, with some words by Sheldon Bacon
Today we are feeling a great sense of satisfaction for two reasons, firstly because we were treated to the most beautiful views of Greenland and the ice yesterday, and secondly because early in the afternoon we completed the final station of the section across the Labrador Sea. I don’t think that I have the words to properly describe the beauty of the views that day, so I will let my pictures tell the story. I’ve added some photos here but I also made a time-lapse movie of the view as we moved off the last station and away from Greenland through the ice. The movie is at http://youtu.be/HOI6q1VFQDA (try cutting and pasting if clicking on the link doesn’t work). It’s a few minutes long, and is most impressive if you watch it at double speed.
So apart from the joys of knowing you have the best job in the world because it takes you to places like this, we are also delighted to have successfully finished the first part of the cruise. After 53 CTD stations and 12 days we have our first results. The colourful cartoon at the end of this post shows some of our data. The image will be new to most people, so Sheldon and I have made an attempt to explain what you are seeing when you look at it.
The first thing to know is that you are seeing a vertical slice through the deep ocean, along the track shown in the map. The ocean is wide and deep, but we have greatly stretched the depth axis to show the currents we are interested in. The vertical axis is actually pressure (in decibars) but is roughly equivalent to depth in metres. There are two panels; the left on shows temperature of the water, and the right one shows how salty it is (salinity). The sections is shown as though you are looking through a giant wall of water towards the north; Canada is on the left and Greenland is on the right. I’ve shown the direction that the water moving with the cartoon arrows, and labelled the different types of water (oceanographers like to give them names that are clues as to where the water comes from).
Top left and top right in the shallow waters of the Canadian and Greenland continental shelves are the very cold, fresh waters fuelled by ice melt: both melting sea ice coming from the high Arctic, and melting land ice running off Canada and Greenland. The Canadian side is colder, thicker and faster because it’s a direct pipe taking water south out of the Arctic, coming through passages like Nares Strait, Lancaster Sound and Hudson Strait. The Greenland side is flowing north and you can see that a common feature at all depths in the Labrador Sea is that the water circulates clockwise in the basin, north on the east and south on the west.
Centred around 200-300 metres on both sides of the Labrador Sea are cores of recirculating warm and salty Atlantic waters, entering on the east side (called Irminger Water) and leaving, having been cooled and freshened (diluted), on the west side. In the top couple of hundred metres of the central Labrador Sea there sits light surface water that has been warmed by the spring sun and freshened by water mixing in from the Arctic currents around the edge of the sea. The surface water sits over something we call Labrador Sea Water; this thick layer of well-mixed water is special to this region and hugely important to the global climate system. Wintertime cold winds pull heat out of the central Labrador Sea; the newly cooled water is dense and so it sinks to deeper levels, forcing other warmer water around it to rise to the surface. The warmer water is cooled too, sinks, and more water rises to the surface. This great stirring, circular motion is called convection and is extremely efficient at mixing up a vast volume of water. In the past convection has reached 2200m, but it looks as though last winter it reached to about 1500m. Eventually this stirred up water, carrying carbon and oxygen absorbed from the air, travels all the way to the Southern Ocean as part of the global ocean circulation.
Finally, down at the bottom is the deep banana shape made by the dense waters that flow over the shallow sill in the north-east Atlantic between Greenland, Iceland and Scotland. The coldest, densest waters come from Denmark Strait, and you can see how, on their circuit of the Labrador Sea, they are warmer on the west than the east. This is caused by mixing with overlying waters, which in this case are the dense overflows from the sill east of Iceland. These NE Atlantic deep waters have taken a longer and more roundabout route to get to the Labrador Sea.
The next part of the programme is to carry out a survey of the ocean currents around the southern tip of Greenland. We will post results as soon as we get them.
Images: Photos of ice and Greenland (including a very happy Penny Holliday), a image of the temperature and salinity that we have measured in the Labrador Sea, and a map of our cruise track to yesterday.