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physical oceanography

GeoTalk: Eleanor Frajka-Williams, the 2017 Ocean Sciences Division Outstanding Early Career Scientists Awardee

GeoTalk: Eleanor Frajka-Williams, the 2017 Ocean Sciences Division Outstanding Early Career Scientists Awardee

Geotalk is a regular feature highlighting early career researchers and their work. Following the EGU General Assembly, we spoke to Eleanor Frajka-Williams, the 2017 Ocean Sciences Division Outstanding Early Career Scientists awardee. In her work, Eleanor uses real-world measurements – from ships, satellites, sea gliders and moorings – to understand how the world’s oceans work. In today’s interview we talk to her a little more about why the oceans are so fundamental to our planet’s health and some of the lesson’s she’s picked up while her career has developed.

Thank you for talking to us today! Could you introduce yourself and tell us a little more about your career path so far?

Thanks – and it’s great to be able to talk to EGU.  I’m an associate professor of physical oceanography at the University of Southampton.  I started at the University in 2012 after a couple of years as a research fellow at the National Oceanography Centre.  I originally studied applied math at university, but discovered oceanography through an undergraduate research placement and it seemed like a great way to apply math and physics to understanding the natural world.

Your research focuses on the world’s oceans, what attracted you to study the processes which govern them?

I liked the idea of studying something that was important and intense, but which we couldn’t actually see with the naked eye—because except for the sea surface, everything else is hidden.  But by collecting observations—the right set of observations—we can piece together a picture of what is happening, and maybe think about teasing apart cause and effect.  Add to that the chance to use underwater gliders, piloted remotely by satellite communications, and what’s not to like?

Deep in the bowls of the world’s oceans, huge masses of water move: cold, salty water sinks, while warmer water rises. Your work focuses on understanding how and why this happens. Can you tell us a little more about these processes?

The ocean is typically stratified, meaning that light waters overly dense waters.  The global ocean overturning circulation describes how the ocean circulation moves through the warm equatorial regions, towards the northern North Atlantic where waters are progressively cooled and transformed, to the point where they sink.  These deep waters then move south and are upwelled either around Antarctica or in distributed mixing regions around the ocean basins.

While this circulation pattern is sometimes called the ‘great ocean conveyor’, suggesting that there is a single pathway moving at a consistent speed, it’s really a set of interconnected processes including the sinking, upwelling and also interplay with the ocean gyres (wind-driven ocean currents) and between the atmosphere and ocean.

One of the most dramatic of these processes—deep ocean convection—occurs in the northern North Atlantic when cold dry winds originating over the Canadian arctic cool the surface of the ocean to the point where the waters become as dense as, or denser than, the water 1000 m deep.  During this turbulent sinking, carbon and heat are stored in the deep ocean where they may stay for centuries.

And these ocean processes also have an effect on climate too?

We expect that they do.  On long timescales (paleo-timescales), we have extensive evidence that changes in the global overturning circulation coincided with rapid changes in global temperatures.  In some cases, the shutdown of the global overturning circulation resulted from a large input of freshwater (about 100,000 km3) being dumped over the northern North Atlantic from the ice sheet melting over Canada.  This freshwater would then float on the surface of the ocean, and because it’s so buoyant, could reduce or even prevent deep convection and through it, the overturning.

In the present-day climate, we have seen mini-versions of this happening.  In the 1960s, the ‘Great Salinity Anomaly’, which should really be called the ‘Great Freshwater Anomaly’ saw the input of about 20,000 km3 of freshwater to the northern North Atlantic.  Deep convection was suppressed for several years.  Unfortunately, we don’t have any observations of what the overturning was doing at the time though the deep western boundary current (considered to be the southward flowing limb of the overturning) was still active.

It’s still a tricky problem to try to sort out, because there are limited observations and a lot of moving parts to the problem (the sinking, the southward and northward flow, and the role of the gyres or atmosphere).

If freshwater is the culprit, for a reduced overturning, we will need to keep a close eye on Greenland, which is a major reservoir of freshwater in the region.  It has been melting more quickly and some new evidence suggests that it could begin to influence (slow down) the overturning in the next 10 years.

It wasn’t just your scientific work which led to you being named OS Division Outstanding Early Career Scientists, but also your work to promote and support budding scientists. What are the most valuable lessons you’ve learnt transitioning between being a fledgling researcher to an associate professor?

Being able to support young scientists is one of the most rewarding things about my job.  It is refreshing and inspiring to work with people starting to make discoveries of their own.

Some of the lessons I’ve learned are that work-life balance is an ongoing endeavour, and it’s rare to always be ‘in balance’, but aiming for a healthy average is a good start.

I’ve also discovered that with each promotion (or each life transition, e.g. starting a family), time becomes less abundant.  So, I’ve added strategies for efficiency along the way—and of course, with more experience, tasks that took forever the first time, take a lot less time now.  And every now and then, I find it can be useful to ‘drop the ball’ and ignore those pressing administrative or other duties, and just do a bit of science.  It helps to remember what I got into it for.

Interview by Laura Roberts Artal, EGU Communications Officer

Momentous discoveries in oceanography

Earlier this month, one of our network bloggers, Matt Herod, put out a call for posts on momentous discoveries in geology as part of a well-known geoscience blog carnival, The Accretionary Wedge. With so many geoscience disciplines to choose from, and an immense wealth of exciting discoveries across the Earth sciences, choosing just one momentous discovery was no easy task. Much of my background is in oceanography though, and while momentous moments mark much of the field’s history, the CTD stands out – to me at least – as one of oceanography’s most sterling inventions.

This is a CTD (left). Often deployed off research ships with a large bundle of sampling bottles known as a rosette (right), CTDs record the conductivity (and hence, salinity), temperature and depth of water in the ocean, allowing physical oceanographers to get a good look at it’s structure and work out which water masses are moving where. (Both the CTD and rosette images are credited to NOAA)

This is a CTD (left). Often deployed off research ships with a large bundle of sampling bottles known as a rosette (right), CTDs record the conductivity (and hence, salinity), temperature and depth of water in the ocean, allowing physical oceanographers to get a good look at it’s structure and work out which water masses are moving where. (Both the CTD and rosette images are credited to NOAA)

Conductivity, Temperature and Depth recorders (CTDs) provide the data that forms the foundation of physical oceanography. Dissolved salts carry a charge and this charge can be detected using a conductivity meter, the “C” in our CTD. From this, oceanographers can determine the salinity of the water. Why do we want to know this? Salinity can be used to trace different water masses as they mix in the ocean. For example, the heat of the Mediterranean region causes large quantities of water to be evaporated, leaving a very salty sea behind. Consequently, when water leaves the Mediterranean Sea and enters the Atlantic, it has a distinctly salty signature. On entering the Atlantic, it sinks because it is more salty – and hence more dense – than the surrounding water. This body of water sits midway between the ocean surface and seafloor, so is known as Mediterranean Intermediate Water (MIW).

Temperature too, has a role to play in water density, where warm tropical waters are less dense than the cold waters at the poles. Cold water in the North Atlantic sinks to form a mass of water known as North Atlantic Deep Water (NADW), which descends from the Arctic to the seafloor and spreads south along the floor of the Atlantic Ocean. The same happens in the Southern Ocean, where cold Antarctic water sinks and spreads north, forming Antarctic Bottom Water  (AABW). Temperature can also be used as an indicator for where water masses originated, but identifying them requires a little tweaking of the data. This is because water heats up as it is compressed, so as a water mass sinks, the increasing pressure exerted by the water above it causes it to warm up and obscures its original temperature signature. Fortunately, the relationship between temperature and pressure is a well known one, and scientists can correct for this heating if they know the depth of the water mass.

Water circulation in the Atlantic Ocean. Temperature and salinity dictate the density of a water mass and thus, its position in the water column, with warmer, fresher water at the ocean surface and colder, more saline water at depth. (Credit: SEOS)

Simplified water circulation in the Atlantic Ocean. Temperature and salinity dictate the density of a water mass and thus, its position in the water column, with warmer, fresher water at the ocean surface and colder, more saline water at depth. (Credit: SEOS)

Depth can be determined using pressure sensors in the CTD, and it’s with these that we can work out where each of the water masses (with their unique salinity signatures) are located, and where their boundaries with other water masses are too. CTDs are lowered through the water column to determine how temperature and salinity vary with depth, but to find out how  these parameters also vary with space, oceanographers have to drop CDTs all over the ocean. Well, sample it at least. By regularly deploying CDTs at different locations, oceanographers can work out where water masses are in space, as well as depth.

Circulation patterns like the one above happen in oceans over the world and together these great moving masses of water make up a worldwide ocean circulation system known as the thermohaline conveyor belt, which influences rainfall patterns, climate, nutrient mixing, fish stocks and so much more. The uncovering the thermohaline conveyor belt in itself is a momentous discovery, but how do we know about it? The CTD. Amazing.

The global ocean conveyor belt. Blue indicates the movement of cold deep water masses and red indicates the warm surface currents that make up thermohaline circulation in the ocean. (Credit: UNEP)

The global ocean conveyor belt. Blue indicates the movement of cold deep water masses and red indicates the warm surface currents that make up the oceans’ thermohaline circulation system. (Credit: UNEP)

By Sara Mynott, EGU Communications Officer

Imaggeo on Mondays: Surface spirals

This week’s Imaggeo on Mondays is no ordinary image; it’s a snapshot of surface ocean speeds and the extent of ice cover in the North Atlantic. It was produced using a high resolution model of ocean eddies – high resolution here means that details are simplified into grids 3 km across, or one 20th of a degree. Three kilometres may sound like a pretty large area, but in oceanographic modelling, this is considered to be a fine mesh and is important for resolving small-scale processes like mesoscale eddy formation.

Mesoscale eddies are 10-250 km in diameter – again the idea that this is a small-scale process does seem a little far-fetched, but when you consider the size of the Atlantic Ocean, which covers an area over 100 million square-kilometres, the size of these eddies pale in comparison. “There are plenty of these eddies in ocean, especially in regions of high ocean currents, where the current gets unstable like in the Gulf Stream, North Atlantic Current and in the West Greenland Current”, Erik Behrens explains.

“Surface currents in the North Atlantic” by Erik Behrens. The red colours indicate speeds of about 1.5 m/s, yellow around 0.7 m/s and green 0.3 m/s. Lower speeds become transparent, and you only see the ocean floor below (dark blue). The image is intended for the public and is distributed by the EGU under a Creative Commons licence.

“Surface currents in the North Atlantic” by Erik Behrens. The red colours indicate speeds of about 1.5 m/s, yellow around 0.7 m/s and green 0.3 m/s. Lower speeds become transparent, and you only see the ocean floor below (dark blue). The image is intended for the public and is distributed by the EGU under a Creative Commons licence.

These eddies are not well captured in present climate models, but have an important role in ocean transport processes, deep ocean convection and ocean-atmospheric exchange. In order to better understand their effects, oceanographers like Behrens perform global ocean simulations, where key regions are modelled in much more detail (in this case the North Atlantic Ocean). “This simulation captures the oceanic condition quite realistically, and is therefore used for many comparisons with observations in the North Atlantic”, Behrens adds, meaning the impact of eddies both locally and on the global ocean can be studied in more detail.

The model used to produce this image is known as VIKING20, which used a data set going back to 1948. Models like this one allow marine scientists to make better interpretations of the data they’re collecting today.

Imaggeo is the EGU’s open access geosciences image repository. A new and improved Imaggeo site will be launching soon, so you will be able to peruse an even better database of visually stunning geoscience images. Photos uploaded to Imaggeo can be used by scientists, the press and the public provided the original author is credited. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. You can submit your photos here.

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