Giant under-sea waterfalls

by Penny Holliday

This afternoon I am writing this post hanging onto my tea mug with my right hand, my laptop with my left hand, and wedging my knees into the footwell of my desk. Yes, we are rocking and rolling with a lovely beam-on swell coming from a distant storm. It’s a magnificent day, with sunshine and white fluffy clouds, the sea looking spectacular with patches of azure blue and deep turquoise topped with bright white horses. The fulmars are circling the ship on the breeze, and looking southwest towards the sun the heaving sea is pure liquid silver. So the lack of sleep last night as my cabin rattled, squeaked and tipped from side to side is more than made up for by the views I’m seeing now. I’m also ignoring the dampness in my socks and jeans that are a result of a small greenie (a wave) that came along the starboard deck and filled my right boot while I was filming the view during CTD sampling time.

We are in the middle of the Iceland Basin, directly south of the westernmost tip of Iceland, at about 58°N. Immediately to the east of us, poised to be surveyed over the next couple of days is the warm North Atlantic Current, and more than 2km below our feet is a great river of cold water heading south along the deep flanks of the mid-Atlantic ridge. The source of the river is in the Norwegian Sea where there is a huge reservoir of cold, dense water. The dam supporting the reservoir is a rocky undersea ridge that runs between Iceland and Scotland, but the dam has some major cracks and crevices in it through which the cold water slips into the North Atlantic ocean. One of those crevices is the Faroe Bank Channel, a passage a couple of hundred kilometers long and 800m deep, and the dense water constantly pours though it at a rate of 3 million cubic metres per second. At sill of the Faroe Bank Channel, the dense water falls over the largest waterfall in the world. I suppose you could argue that it is in fact the second largest waterfall in the world – the other one being something very similar west of Iceland. The shallow ridge at the Denmark Strait (between Greenland and Iceland) is an equivalent leaking dam, holding back a reservoir of even denser water under the Iceland Sea, but allowing over 3 million cubic metres per second of the dense water to flow over the waterfall there. As these dense waters fall over the sills of the dams, we oceanographers refer to them as “overflows” – our jargon can be pleasingly descriptive at times.

Since leaving Greenland we have surveyed both of these rivers a few hundreds of kilometres downstream of the waterfalls. The characteristics of the overflow water have changed considerably since it first tumbled over those sills. Just as water falling over land waterfalls mix with the air, forming spray and bubble-filled white torrents, so the dense sea water swirls and mixes with the lighter water it is falling through. In the distance between the waterfall and the latitude of our CTD section, the volume of the rivers of overflow water has increased fourfold by pulling the other water into their unstoppable streams.

One of the fascinating aspects to these rivers of overflow water is that they balance the northward flow of the North Atlantic Current. One would not exist without the other and they form the essence of a large loop of water pulling warm water northwards at the surface and cold water southwards at depth. This great loop is what we refer to as the “overturning circulation” and one of the goals of OSNAP is to measure the amount of water carried in the loop and how it changes over time.

Images: views of the sea today (all by Penny Holliday)

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