Current drivers of ocean circulation

14 September 2015

Interview with

Dr Alex Piotrowski, University of Cambridge

Ocen circulationThe oceans act like a giant sponge, storing the extra heat and carbon dioxide we produce. They then redistribute this around the globe, influencing climate elsewhere. But what controls how water in the oceans moves, and how is this linked to climate? Jo Kerr rolled up her sleeves and went to meet Cambridge University's Alex Piotrowski to find out... 

Jo - Hi, Alex. How are you?

Alex - My name is Alexander Piotrowski. I'm a lecturer in Earth Sciences and I study past changes in ocean circulation. The global ocean circulation has been likened to a conveyor belt where deep water masses flow from the Atlantic primarily through the southern ocean, into the Indian Ocean and the Pacific Ocean. We make deep waters by either making them more salty or more cold. They become dense and they sink into the deep ocean. This is going to happen near the poles primarily. What I'm making here is average ocean water. I have a 10-litre bucket of tap water and I'm going to be adding 350 grams of salt in total. Now, I have to pour this into the tank. So, this tank is basically the average composition of the ocean and we're going to be now adding different parcels of water that have different temperatures and salinity. So therefore, different densities. What I'm making now is similar to the types of water masses you'd find at intermediate depths in the ocean. They are formed from relatively warm, salty surface waters. We're also going to make cold, salty water, and we're going to dye these in different colours so we can follow their circulation through the tank.

Jo - This is our warm, salty water.

Alex - Yes.

Jo - Okay, let's dye this red.

Alex - So I'm basically using a funnel to add the water to the tank. So that prevents it from vigorously mixing with the average seawater. So, it maintains its temperature and salinity.

Jo - So, it was flowing to the bottom and now, it started to rise up and it's actually forming an intermediate parcel of water.

Alex - So, what we can see is that basically, this red water mass is flowing along the middle of the tank. It's basically found the depth at which it has the same density as the surrounding water and is spreading along a surface of that density. In the ocean, those surfaces, we call isopycnals. We can mix along isopycnals and it's much more difficult to mix vertically. You can see this if we add some cold salty water with a different colour. What colour would you like to make the very dense water?

Jo - I think we should go for blue. At the other end of the tank, we're putting in the cold salty water and this is sinking much quicker, and it's sticking right along the bottom.

Alex - Because basically, the ocean is densely stratified. What we're adding is very dense water. It's very cold and salty. So, if you looked at the temperature and salinity of the Atlantic Ocean, you'd see a very similar picture to what we're seeing in this tank. The red water would be analogous to water mass we call north Atlantic deep water which forms in the Nordic seas near Iceland and the Labrador seas near Greenland. It is subducted down to about 3 kilometres and flows southwards. The blue water mass is analogous to Antarctic bottom water which forms near the edge of the Antarctic ice sheet by supercooling of southern ocean waters and it flows at about 4 to 5 kilometres northwards into the Atlantic ocean. We could see that there's a strong gradient in the colour between them and that basically means that these water masses are not mixing with each other.

Jo - Is this how the currents are moving around the whole ocean or is it just the Atlantic Ocean?

Alex - When we form deep waters and we circulate them through the deep ocean, we have to replace those waters with surface waters. So, the surface ocean circulation is linked to the deep ocean circulation. The surface ocean circulation includes things like the Gulf Stream which transports heat from the equatorial regions towards the poles. Deep water masses often contain a large amount of carbon and other nutrients. And so, that carbon is being kept out of contact with the atmosphere and being stored in this reservoir of deep waters. So in our analogy, the blue water mass would actually in the real ocean contain a lot of carbon. At times in the past, that may have contained more or less carbon. That carbon which would otherwise be in the atmosphere is stored in the deep ocean. So, it's linked to the carbon dioxide concentration of the atmosphere which is a known climate amplifier.

Jo - So, if we can store more carbon in the deep ocean, this is effectively influencing our climate through the atmospheric concentrations of carbon dioxide. Is that correct?

Alex - Yes, that's correct. Basically, what we'd expect to see if we look in the paleo record is that we'd see that changes in circulation would be closely linked to changes in climate. Changes in climate can affect the temperatures and salinities of the deep waters that we form. So for example, we can increase the amount of sea ice, we can change the amount of convection within the ocean, and the deep water masses would respond to that climate foresight. However, changes in the circulation through its transport of heat and the amount of carbon that's stored in deep water masses can affect climate.

Jo - Does that mean that it's not only the climate affecting the oceans, the oceans also affect the climate?

Alex - So, the Earth's climate system is not just a simple linear equation. There are feedback mechanisms between the physics of ocean circulation, the chemistry of water masses and the biology that inhabits the ocean. That's why we need to rely on both paleo proxy records to reconstruct the history of the ocean and also use computer models to understand how these feedbacks relate to each other and how they may have changed in the past, and how they may change in the future.

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