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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.

Imaggeo on Mondays: Atmospheric gravity waves

Imaggeo on Mondays: Atmospheric gravity waves

From the tiny vibrations which travel through air, allowing us to hear music, to the mighty waves which traverse oceans and the powerful oscillations which shake the ground back and forth during an earthquake, waves are an intrinsic part of the world around us.

As particles vibrate repeatedly, they create an oscillation, which when accompanied by the transfer of energy, creates a wave.  The way in which waves travel varies hugely. Take for instance a ripple in a pond: vibrations there are perpendicular to the direction in which the wave is travelling – transverse waves. When a slinky moves (or sound waves), on the other hand, vibrations happen in the same direction in which the wave travels – longitudinal waves. Ocean waves are more complex. The motion there combines surface waves, created by the friction between wind and surface water, and the energy passing through the water causes it to move in a circular motion. With a little imagination, it’s not so difficult to visualise these different phenomena.

But not all waves on Earth are so intuitive.

Unlike the waves we’ve discussed up until now, internal gravity waves oscillate within a fluid medium, rather than on its surface. In the Earth’s atmosphere, internal gravity waves transfer energy from the troposphere (the layer closest to the Earth’s surface) to the stratosphere (where the ozone layer is found) to the very cold mesosphere, which starts some 50 km away from the planet’s surface. They are usually created at weather fronts: the boundary where two pockets of air at different temperatures and humidity meet. Air flowing over mountains can also generate them.

Because they propagate across layered fluids (the different layers of the atmosphere, for example), internal gravity waves can be responsible for transferring considerable amounts of energy over large distances, which is one of the main reasons why they are important in atmospheric and ocean dynamics.

But only with improved satellite and remote sensing technologies have scientists been able to observe them clearly. Today’s featured image is a great example of one such wave.  It was acquired by the European Space Agency’s Envisat satellite (which aimed to carry out the largest civilian Earth observation mission to date – launched in 2002), on September 16th 2004.

A short animation showing just how impressive these waves are when travelling across the Mozambique Channel – using data from the Meteosat 5 satellite. (Credit: Jorge Magalhaes and Jose da Silva)

The image covers an area of about 580 by 660 km and was acquired as the satellite flew over the Mozambique Channel. The two-dimensional horizontal structure of a very large-scale atmospheric internal wave can be seen in the center of the image travelling southwest. The crest length of the leading wave, in this case, extends for more than 500 km and its crest-to-crest spatial scale is approximately 10 km on average.

It is interesting to note that several (but not all) of these individual waves are made visible by characteristic cloud bands, which form as the vertical oscillations find the necessary conditions (high moisture in the atmosphere) for condensation to occur.

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/.

GeoTalk: How are clouds born?

GeoTalk: How are clouds born?

Geotalk is a regular feature highlighting early career researchers and their work. In this interview we speak to Federico Bianchi, a researcher based at University of Helsinki, working on understanding how clouds are born. Federico’s quest to find out has taken him from laboratory experiments at CERN, through to the high peaks of the Alps and to the clean air of the Himalayan mountains. His innovative experimental approach and impressive publication record, only three years out of his PhD, have been recognised with one of four Arne Richter Awards for Outstanding Early Career Scientists in 2017.

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

I am an enthusiastic atmospheric chemist  with a passion for the mountains. My father introduced me to chemistry and my mother comes from the Alps. This mix is probably the reason why I ended up doing research at high altitude.

I studied chemistry at the University of Milan where I got my degree in 2009.  During my bachelor and master thesis I investigated atmospheric issues affecting the polluted Po’ Valley in Northern Italy and since then I have always  worked as an atmospheric chemist.

I did my PhD at the Paul Scherrer Institute in Switzerland where I mainly worked at the CLOUD experiment at CERN. After that, I used the acquired knowledge to study the same phenomena, first, at almost 4000 m in the heart of the Alps and later at the Everest Base Camp.

I did one year postdoc at the ETH in Zurich and now I have my own Fellowship paid by the Swiss National Science Foundation to conduct research at high altitude with the support of the University of Helsinki.

We are all intimately familiar with clouds. They come in all shapes and sizes and are bringers of shade, precipitation, and sometimes even extreme weather. But most of us are unlikely to have given much thought to how clouds are born. So, how does it actually happen?

We all know that the air is full of water vapor, however, this doesn’t mean that we have clouds all the time.

When air rises in the atmosphere it cools down and after reaching a certain humidity it will start to condense and form a cloud droplet. In order to form such a droplet the water vapor needs to condense on a cloud seed that is commonly known as a cloud condensation nuclei. Pure water droplets would require conditions that are not present in our atmosphere. Therefore, it is a good assumption to say that each cloud droplet contains a little seed.

At the upcoming General Assembly you’ll be giving a presentation highlighting your work on understanding how clouds form in the free troposphere. What is the free troposphere and how is your research different from other studies which also aim to understand how clouds form?

The troposphere, the lower part of the atmosphere, is subdivided in two different regions. The first is in contact with the Earth’s surface and is most affected by human activity. This one is called the planetary boundary layer, while the upper part is the so called free troposphere.

From several studies we know that a big fraction of the cloud seeds formed in the free troposphere are produced by a gas-to-particles conversion (homogeneous nucleation), where different molecules of unknown substances get together to form tiny particles. When the conditions are favourable they can grow into bigger sizes and potentially become cloud condensation nuclei.

In our research, we are the first ones to take state of the art instrumentation, that previously, had only been used in laboratory experiments or within the planetary boundary layer, to remote sites at high altitude.

Federico has taken state of the art instrumentation, that previously, had only been used in laboratory experiments or within the planetary boundary layer, to remote sites at high altitude. Credit: Federico Bianchi

At the General Assembly you plan on talking about how some of the processes you’ve identified in your research are potentially very interesting in order to understand the aerosol conditions in the pre-industrial era (a time period for when information is very scarce). Could you tell us a little more about that?

Aerosols are defined as solid or liquid particles suspended in a gas. They are very important because they can have an influence on the Earth’s climate, mainly by interacting with the solar radiation and cooling temperatures.

The human influence on the global warming estimated by the Intergovernmental Panel for Climate Change (known as the IPCC) is calculated based on a difference between the pre-industrial era climate indicators and the present day conditions. While we are starting to understand the aerosols present currently, in the atmosphere, we still know very little about the conditions before the industrial revolution.

For many years it has been thought that the atmosphere is able to produce new particles/aerosol only if sulphur dioxide (SO2) is present. This molecule is a vapor mainly emitted by combustion processes; which, prior to the industrial revolution was only present in the atmosphere at low concentrations.

For the first time, results from our CLOUD experiments, published last year,  proved that organic vapours emitted by trees, such as alpha-pinene, can also nucleate and form new particles, without the presence of SO2. In a parallel study, we also observed that pure organic nucleation can take place in the free troposphere.

We therefore have evidence that the presence of sulphur dioxide isn’t necessary to make such a mechanism possible. Finally, with all this new information, we are able to say that indeed, in the pre-industrial era the atmosphere was able to produce new particles (clouds seeds) by oxidation of vapors emitted by the vegetation.

Often, field work can be a very rewarding part of the research process, but traditional research papers have little room for relaying those experiences. What were the highlights of your time in the Himalayas and how does the experience compare to your time spent carrying out laboratory experiments?

Doing experiments in the heart of the Himalayas is rewarding. But life at such altitude is tough. Breathing, walking and thinking is made difficult by the lack of oxygen at high altitudes.

I have always been a scientists who enjoys spending time in the laboratory. For this reason I very much liked  the time I spent in CERN, although, sometimes it was quite stressful. Being part of such a large international collaboration and being able to actively do science was a major achievement for me. However, when I realized I could also do what I love in the mountains, I just couldn’t  stop myself from giving it a go.

The first experiment in the Alps was the appetizer for the amazing Himalayan experience. During this trip, we first travelled to Kathmandu, in Nepal. Then, we flew to Luckla (hailed as one of the scariest airport in the world) and we started our hiking experience, walking from Luckla (2800 m) up to the Everest Base Camp (5300 m). We reached the measurement site after a 6 days hike through Tibetan bridges, beautiful sherpa villages, freezing nights and sweaty days. For the whole time we were surrounded by the most beautiful mountains I have ever seen. The cultural element was even more interesting. Meeting new people from a totally different culture was the cherry on the cake.

However I have to admit that it was not always as easy as it sounds now. Life at such altitude is tough. It is difficult to breath, difficult to walk and to install the heavy instrumentation. In addition to that, the temperature in your room during nights goes well below zero degrees. The low oxygen doesn’t really help your thinking, especially we you need to troubleshoot your instrumentation. It happens often that after such journey, the instruments are not functioning properly.

I can say that, as a mountain and science lover, this was just amazing. Going on a field campaign is definitely the  best part of this beautiful job.

To finish the interview I wanted to talk about your career. Your undergraduate degree was in chemistry. Many early career scientists are faced with the option (or need) to change discipline at sometime throughout their studies or early stages of their career. How did you find the transition and what advice would you have for other considering the same?

As I said before, I studied chemistry and by the end of my degree my favourite subject moved to atmospheric chemistry. The atmosphere is a very complex system and in order to study it, we need a multidisciplinary approach. This forced me to learn several other aspects that I had never been in touch with before. Nowadays, I still define myself as a chemist, although my knowledge base is very varied.

I believe that for a young scientist it is very important to understand which are his or her strengths and being able to take advantage of them. For example, in my case, I have used my knowledge in chemistry and mass spectrometry to try to understand the complex atmospheric system.

Geotalk is a regular feature highlighting early career researchers and their work.

Geosciences Column: The dangers of an enigmatic glacier in the Karakoram

Geosciences Column: The dangers of an enigmatic glacier in the Karakoram

Nestled among the high peaks of the Karakoram,  in a difficult to reach region of China, lies Kyagar Glacier. It’s trident-like shape climbs from 4800 to 7000 meters above sea level and is made up of three upper glacier tributaries which converge to form an 8 km long glacier tongue.

Until recently, it’s remoteness meant that studying its behaviour relied heavily on the acquisition of data by satellites. The installation, in 2012, of an automated monitoring station yielded photographs and other data which, combined with better satellite observations, give a detailed insight into the nature of an otherwise enigmatic glacier.

The flow of glaciers

Despite their impenetrable fortress-like appearance, glaciers are constantly on the move. Due to the force of gravity acting on the thick pack of ice, glaciers flow, albeit very, very  slowly. The ice deforms under its own enormous weight, creeping slowly down valleys and mountain sides.

The exact position of a glacier’s snout is also affected by the amount of snow that accumulates on its surface. When the rate of evaporation of snow exceeds the amount added to the glacier, it retreats. Rising global temperatures mean that glaciers worldwide are shrinking at unprecedented rates.

Kyagar Glacier on 29 March 2016, as seen from the ESA Sentinel-2A satellite. The glacier-dammed lake of approximately 5 million m3 is visible to the east of the glacier terminus. The curved scale bar up the west branch indicates the longitudinal profile used for surface velocity and elevation analysis, and the inset shows the monitoring station located about 500 m upstream of the glacier terminus. Taken from V.Round et al., 2017 (click to enlarge).

But, the remote glaciers of the Chinese Karakoram are bucking the global trend. Owing to localised increases in winter precipitation between 1999 and 2011, they are maintaining a steady ice-thickness (or even advancing slightly).

The way in which many glaciers of the central Asian mountains flow is also unique. While the majority of glaciers slide down valleys at a relatively steady rate, about 1% experience glacier surges. Long periods of quiescence where flow is extremely slow are punctuated by times (which can last months or years) of accelerated gliding and transport of material.

During active surge periods a glacier’s snout can lengthen and thicken, blocking rivers and forming ice-dammed lakes. If the dam containing the lake fails, a glacial outburst flood (GLOF) occurs, presenting a serious threat to downstream communities.

Mysterious floods

A record of devastating floods along the Yarkand River – which Kyagar Glacier feeds into – exists from as far back as the 1960s. But the origin of the floods remained a mystery for many years. While periods of thickening and advance had been recognised in Kyagar as early as in the 1920s, it wasn’t until 2012 that it was characterised as a surge-type glacier; finally establishing the link between the down valley flood events and the glacier.

In order to manage the hazard presented by future GLOFs,  it is important to fully understand the surge dynamics of Kyagar.  Using a combination of satellite images and data, as well as images and weather records made by the automated observation station, a team of researchers have been able to establish the speed at which Kyagar moved between 2011 and 2016.

The study period also coincided with a recent surge cycle at Kyagar, giving the scientists their first detailed glimpse of how Kyagar moves and forms hazardous ice-dammed lakes.

Kyagar’s surges and GLOFs

Before 2012, Kyagar was in a quiescent phase (which had lasted at least 14 years). During that time the glacier snout was thinning and ice was built up in an area towards the top of the glacial tongue, forming a reservoir.

Glacier surface elevation changes during the surge from subtraction of two TanDEM-X DEMs. (Left) During the quiescent period, snow accumulate in an area towards the top of the glacial tongue, while the snout thinned. (Right) During the surge period this pattern was reversed. Modified from V. Round et al., 2017 (click to enlarge).

Gradually after that, the thickness of ice at the snout began to increase, as ice moved from the reservoir higher up in the glacier where it had accumulated previously.

The velocity with which the glacier moved forward also increased. Between April and May 2014 speeds doubled compared to the maximum speeds recorded before then. Despite a few fluctuations, speeds continued to increase overall, peaking in mid 2015. In that time, Kyagar gained over 60m of ice at its snout.

Photographs from the monitoring station revealed that a lake began to form upstream from the glacier’s terminus in December 2014. It grew steadily throughout the spring and summer and drained, abruptly, through channels carved out below the glacier in July 2015.

By September 2015 the lake began to fill again. Ten months later, in July 2016 it reached its peak volume of 40 million ㎥ (equivalent to the amount of water held in 16,000 olympic sized swimming pools) and drained suddenly shortly after. It refilled over the course of the next month, reaching a volume of 37 million ㎥ , and once again drained abruptly in August 2016.

Radar backscatter images of the glacier terminus showing the lake (a) 11 days before drainage, (b) just after the start of drainage, and (c) after the lake drainage. Lake drainage clearly occurred through subglacial channels, rather than through dam collapse or overtopping. Images from TanDEM-X data provided by DLR. Taken from V.Round et al., 2017 (click to enlarge).

What causes Kyagar to surge?

Not unlike other surge-type glaciers, Kyagar seems to have an inefficient drainage system at its base. It is particularly poor at transporting ice from its reservoir to its snout in a regular manner. Instead, it does is cyclically, through surges.

During quiescent periods interconnected tunnels at the base of the glacier carry water away efficiently. During surge periods, the tunnels turn to cavities which are poorly connected by very narrow passages, meaning material isn’t carried away from the glacier easily. This leads to a pressure build-up, and only when the pressure is high enough, is water released, lubricating the glacier bed and encouraging sliding over large areas.

Scientists aren’t certain what causes Kyagar to behave in this way. It is likely a combination of factors: the glacier tongue is relatively flat compared to the steeper slopes in the accumulation area, while the underlying geology and regional climatic conditions also play a role.

What does the future hold?

Historical records of glacial advance and lake formation at Kyagar suggest surge periods occur every 15 to 20 years. Unless there are major changes to the rate at which snow accumulates on the glacier, the nest quiescent period is expected to last until, at least, 2030.

The current risk of GLOFs remains high and will remain so for the next few years, as the glacier snout is still slightly higher than normal and transport of ice from the reservoir is ongoing.

Whether a lake will form (and how large it will grow) during future surge periods depends on the height of the ice dam and how efficiently water is drained away through the subglacial channels.

Regular satellite images, taken during summer periods, are needed to continually assess the risk of GLOFs and to prepare downstream communities.

By Laura Roberts Artal, EGU Communications Officer

References

Round, V., Leinss, S., Huss, M., Haemmig, C., and Hajnsek, I.: Surge dynamics and lake outbursts of Kyagar Glacier, Karakoram, The Cryosphere, 11, 723-739, doi:10.5194/tc-11-723-2017, 2017.

Gardelle, J., Berthier, E., Arnaud, Y., and Kääb, A.: Region-wide glacier mass balances over the Pamir-Karakoram-Himalaya during 1999–2011, The Cryosphere, 7, 1263-1286, doi:10.5194/tc-7-1263-2013, 2013.

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