GeoLog

ocean

Knowing the ocean’s twists and turns

Knowing the ocean’s twists and turns

Navigating the ocean demands a knowledge of its movements. In the past, sailors have used this knowledge to their advantage, following the winds and the ocean currents to bring them on their way.

Prior to mutiny in 1789, Captain Bligh – on the HMS Bounty – famously spent a month attempting to pass westward through the Drake Passage, around Patagonia’s Cape Horn. Here the westerly winds were strong (as they are today) and drove the waters hard against the ship as it persisted against the flow. But they could not pass, and were forced to reach the Pacific by crossing back south of Africa, through the Indian Ocean, costing the mission many months.

It is the winds which predominantly drive the currents at the ocean’s surface. Depending on where you are on the planet, the winds blow in a variety of prevailing directions, exerting control over the surface of the oceans, over which they roll. Where the Earth’s westerlies prevail (moving eastwards, between the 30 to 60 degree latitude belt, in both hemispheres) we encounter some of the world’s fastest currents, including the Atlantic’s Gulf Stream, and the Kuroshio Current off Japan. These currents bring with them huge amounts of heat from tropical and subtropical areas; which is why Western Europe experiences much milder winters than other regions at similar latitudes (think Newfoundland, for example).

Also under the influence of the westerly winds is the world’s largest ocean current, the Antarctic Circumpolar Current, which circles Antarctica in the southern hemisphere. The Antarctic Circumpolar Current lies under the influence of the infamous Roaring Forties, Furious Fifties, and Screaming Sixties westerly wind bands, and acted as a major stretch along the historical clipper route between Europe, Australia, and New Zealand in the 19th century.

The trade winds (also known as the easterlies, circling the Earth between 0 and 30 degrees latitude, in both hemispheres) are typically weaker than the westerlies, but sufficiently strong to have enabled European expansion into the Americas over the centuries. The trades drive ocean currents such as the Canary Current and North Equatorial Current in the Atlantic Ocean, and the California Current and North Equatorial Current in the Pacific.

Also within these latitudes – particularly near the equator – are the doldrums, which are areas characterised by weak or non-existent winds. These regions became well known in the past as sailors were regularly stranded whilst crossing equatorial regions – immobile for days or weeks, resting in seas of calm – awaiting the winds to pick up and move them onwards.

As well as at the surface, the ocean is moving in its interior, with large scale sinking to depths of over 4000 meters in cold polar regions, and upwelling in the warmer tropics and subtropics. The ocean turns over on itself like a bathtub of water heated unevenly from above. Below the surface the deep waters move slowly (centimeters per second, rather than meters per second at the surface), mostly unaffected by wind. Here huge ocean scale water masses move (largely) because of density differences between regions, determined by variations in heat and salinity (salt content). Cold, salty water is dense, and sinks, while warmer water rises.

This large-scale overturning, which characterizes the movement of the world’s ocean as a whole, is known as the global conveyor belt, or the thermohaline circulation (thermo for heat, and haline for salt). Along the conveyor it takes thousands of years for water masses to complete a cycle around the planet.

But like many other features of our Earth system, it is now thought that the behaviour of the ocean’s circulation is beginning to change. Back at the surface oceanographers now expect that ocean currents will undergo substantial change in response to anthropogenic global warming. Computer simulations of the ocean and atmosphere are used to predict whether certain wind systems will strengthen or weaken in the future, and to look at the effect this might have on the underlying ocean currents.

We know from historical evidence that the strength of the ocean’s currents has varied in the past, so this coming century we can expect some changes along our ocean routes; an obvious and well highlighted example being the opening of commercial routes in the new ice-free Arctic.

Whatever the nature of the future ocean, modern technology including real-time satellite-sourced ocean data, and advanced ocean weather and wave forecasts, will allow us to constantly track changes, so that no matter the winds or current speeds, we should always be able to get where we’re going.

By Conor Purcell is a Science and Nature Writer with a PhD in oceanography.

Conor is based in Dublin, Ireland, and can be found on twitter @ConorPPurcell, with some of his other articles at cppurcell.tumblr.com. He is also the founder-editor at www.wideorbits.com.

GeoTalk: Investigating the transport of plastic pollution in the oceans

GeoTalk: Investigating the transport of plastic pollution in the oceans

Geotalk is a regular feature highlighting early career researchers and their work. In this interview we speak to Erik van Sebille, an oceanographer at the Grantham Institute at Imperial Collage London, and winner of the 2016 OS Outstanding Young Scientist Award. As an expert in understanding how oceans transport all kinds of materials, from water and heat through to plastics, Erik has gained detailed knowledge about how water masses move, particularly how they travel from one ocean basin to the next. He has applied his knowledge to understanding problems with societal impacts, such as what dynamics govern drifting debris that collects in garbage patches and the pathways of the Fukushima radioactive plume. 

First, could you introduce yourself and tell us a little more about your career path so far?

I am a physicist by training, with an PhD in Physical Oceanography from Utrecht University in the Netherlands. After finishing my PhD in 2009, I did a two-year postdoc at the University of Miami. In 2011, I became a Fellow and lecturer at the University of New South Wales in Sydney, Australia. And then in early 2015 I came back to Europe, as a lecturer at Imperial College London. So I’ve been moving around a bit, living and working in three different continents in the past 5 years. It’s been a fantastic journey and I’m really happy to have lived in such beautiful and fun places.

During EGU 2016, you received the Outstanding Young Scientist Award from the Ocean Sciences Division. You presented your recent work on modelling the global distribution of floating plastic pollution in the oceans. How big a problem does plastic pollution present to our oceans and why should people care?

It’s shocking how much plastic there is in the ocean. Quite literally these days, it’s hard to go to a place in the ocean and not find tiny pieces of plastic. In nearly every surface trawl, sediment sample, or biopsy we take, we find plastic.

However, while we find  plastic everywhere, we have no idea what its global extent is. There are really only two numbers that are known with some confidence in the global ocean plastic budget: the total amount of plastic floating at the surface today is in the order hundreds of thousands of tonnes. And the total amount of plastic going into the ocean in a single year is in the order of 10 million metric tonnes. So the flux is 2 orders of magnitude larger than the stock. In other words, more than 99% of the plastic in the ocean is not at the surface!

How, exactly, do you go about building the  models which help you investigate where the plastic in the ocean waters is?

My research tries to find out where all this plastic is, by tracking it virtually in high-resolution Ocean General Circulation Models such as NEMO.  NEMO is a large European computer simulation that replicates the movement of ocean water around the globe. Within this oceanic flow field, we’re literally tracking billions of virtual plastic particles, from their sources on land as they are carried around by the ocean currents.

The difficult bit is to make the virtual particles behave like plastic. In order to realistically simulate the pathways and fate of the plastic, we need to simulate fragmentation (how plastics break up), ingestion (animals who eat plastic), biofouling (how algae grow on the plastic), beaching (how plastic particles end up on coastlines) and a dozen other processes that happen to plastic in the real ocean. That’s what my team and I are working on!

Then, once we can track the plastic within models with reasonable accuracy, we can start asking important questions like: Where are ecosystems most at risk? Whose plastic ends up where? And where can we best clean up the plastic?

Erik, along with colleague David Fuchs, created Plastic Adrift.com. A page which models the journey of plastics in the oceans. The research used to create the page is described in this IOP paper: http://iopscience.iop.org/article/10.1088/1748-9326/7/4/044040/meta;jsessionid=3C17B7D3F10B29C6CCF1BD2BA132BF76.c5.iopscience.cld.iop.org

Erik, along with colleague David Fuchs, created Plastic Adrift.org. A page which models the journey of plastics in the oceans. The research used to create the simulation is described in this IOP paper.

So, are you at a stage where you can reliably track particles of plastic in your simulation? And if so, what can you tell us about the distribution of plastic across the world’s oceans?

No, we’re not nearly there yet. We’re just beginning with this exciting project, which was awarded a large European Research Council Grant this year. Ask me again in five years 😉

The outlook isn’t positive, so, how can we go about mitigating the problem?

The situation is pretty dire, indeed. Global plastic production has increased exponentially over the last decades, and there is no reason to think that exponential growth will slow. So the main aim should be to prevent plastic from going into the ocean in the first place.

Last May, I was invited to the UK Parliament to give oral evidence to a Select Committee about my thoughts on a country-wide ban on microbeads used in cosmetics (an issue which has been in the news recently). Such a ban is now supported by the UK Government, which is fantastic news. But microbeads from cosmetics represent only 0.1% of all plastic entering the ocean from the UK. There is really much more work to do. We need better filtering of plastic particles and fibres in sewage treatment plants. We need much better recycling techniques. We need innovative new plastics that are less harmful.

And we need a better understanding of how the plastic in the ocean interacts with marine life, from charismatic megafauna down to phytoplankton and microbes. In particular, I call on EGU’s ocean biogeochemistry community to take up the challenge of understanding the interactions between plastic particulates and biofouling. There’s such an enormous knowledge gap there, and we need all the help we can get.

Given your experience advising the UK government on a matter as significant as plastic pollution in the oceans, how important do you think it is for early career scientists to play a role in advising policy-makers when it comes to environmental issues?

Meet Erik! Credit: Erik van Sebille

Meet Erik! Credit: Erik van Sebille

I think it is extremely important to make sure that your research gets out to the people who can use it to make decisions. Politicians and other stakeholders are always keen to hear about the latest science; they don’t have time and expertise to read through all of the scientific literature so it is partly up to us scientists to point them to the latest findings. It doesn’t matter whether you are an early career researcher or a seasoned senior professor, if you are funded by public money then you have a duty to give results back to society.

For the past twelve months the EGU has been working on developing its science for policy programme. ‘Science for policy’ involves applying scientific knowledge to the decision-making process to strengthen the resulting policies. If like Erik, this is an area you are interested in, or one where your research findings could make a difference, why not visit our policy pages on the website? They include  a range of resources aimed at informing scientists about the world of science policy and initiatives to help you get involved.

Erik, thank you for talking to us today. Our final question of the interview is, perhaps a little simplistic given the scale of the problem, but is there anything everyone could be doing at home to minimise the amount of plastic that makes its way to the oceans?

I think it starts with awareness. Be aware what you do with your used plastics. Don’t just chuck it out. And discuss the issue with your family and friends. I think that a great deal of progress can be made simply by being more careful how we discard our plastic waste.

Last chance to enter the EGU Photo Contest 2016!

Last chance to enter the EGU Photo Contest 2016!

If you are pre-registered for the 2016 General Assembly (Vienna, 17 -22 April), you can take part in our annual photo competition! Winners receive a free registration to next year’s General Assembly! But hurry, there are only a few days left to enter!

Every year we hold a photo competition and exhibit in association with our open access image repository, Imaggeo and our annual General Assembly. There is also a moving image competition, which features a short clip of continuous geoscience footage. Pre-registered conference participants can take part by submitting up to three original photos and/or one moving image on any broad theme related to the Earth, planetary and space sciences.

How to enter

You will need to register on Imaggeo to upload your image, which will also be included in the database. When you’ve uploaded it, you’ll have the option to edit the image details – here you can enter it into the EGU Photo Contest – just check the checkbox! The deadline for submissions is 1 March.

Imaggeo on Mondays: Sunset over the Labrador Sea

Ruby skies and calm waters are the backdrop for this week’s Imaggeo image – one of the ten finalist images in this year’s EGU Photo contest.

 Sunset over the Labrador Sea. Credit: Christof Pearce (distributed via  imaggeo.egu.eu)

Sunset over the Labrador Sea. Credit: Christof Pearce (distributed via imaggeo.egu.eu)

“I took the picture while on a scientific cruise in West Greenland in 2013,” explains Christof Pearce, a postdoctoral researcher at Stockholm University. “We spent most of the time inside the fjord systems around the Greenland capital, Nuuk, but this specific day we were outside on the shelf in the open Labrador Sea. The black dot on the horizon toward the right of the image is a massive iceberg floating in the distance.”

Pearce took part in a research cruise which aimed to obtain high-resolution marine sedimentary records, which would shed light on the geology and past climate of Greenland during the Holocene, the epoch which began 11,700 years ago and continues to the present day.

A total of 12 scientists and students took part in the Danish-Greenlandic-Canadian research cruise in the Godthåbsfjord complex and on the West Greenland shelf. By acquiring cores of the sediments at the bottom of the sea floor, the research team would be able to gather information such as sediment lithology, stable isotopes preserved in fossil foraminifera – sea fairing little creatures – which can yield information about past climates, amongst other data. One of the main research aims was to learn more about the rate at which the Greenland Ice Sheet melted during the Holocene and how this affected local climate conditions and the wider climate system.

“The picture was taken approximately 25 kilometres off the shore of west Greenland coast. In this region the water depth is ca. 500 meters,” describes Pearce. “At this location we deployed a so-called gravity corer and took a 6 meter long sediment core from the ocean floor. Based on radiocarbon measurements – by measuring how much carbon 14 is left in a sample, the age of the sampled units can be known – we now know that these 6 meters correspond to approximately 12000 years of sedimentation, and thus it captures a history of climate and oceanography from the last ice age all the way to present day.”

 

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

Follow

Get every new post on this blog delivered to your Inbox.

Join other followers: