GeoLog

Ocean Sciences

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/.

Geosciences Column: How El Niño triggered Indonesia corals die-off

Geosciences Column: How El Niño triggered Indonesia corals die-off

In the glistening waters of Indonesia, shallow corals – the rain forests of the sea – teem with life.  Or at least they did once. Towards the end of 2015 the corals started to die, leaving a bleak landscape behind. An international team of researchers investigated the causes of the die-off. Their findings, published recently in the EGU’s open access journal, Biogeosciences, are rather surprising.

Globally, corals face tough times. Increasing ocean-water temperatures (driven by a warming climate) are disrupting the symbiotic relationship between corals and the algae that live on (and in) them.

The algae, known as zooxanthellae, provide a food source for corals and give them their colour. Changing water temperatures and/or levels, the presence of contaminants or overexposure to sunlight, put corals under stress, forcing the algae to leave. If that happens, the corals turn white – they become bleached – and are highly susceptible to disease and death.

Triggered by the 2015-2016 El Niño, water temperatures in many coral reef regions across the globe have risen, causing the National Oceanic and Atmospheric Administration (NOAA) to declare the longest and most widespread coral bleaching event in recorded history. Now into its third year, the mass bleaching event is anticipated to cause major coral die-off in Australia’s Great Barrier Reef for the second consecutive year.

The team of researchers studying the Indonesian corals found that, unlike most corals globally, it’s not rising water temperatures which caused the recent die-off, but rather decreasing sea level.

While conducting a census of coral biodiversity in the Bunaken National Park, located in the northwest tip of Sulawesi (Indonesia), in late February 2016, the researchers noticed widespread occurrences of dead massive corals. Similar surveys, carried out in the springs of 2014 and 2015 revealed the corals to be alive and thriving.

In 2016, all the dying corals were found to have a sharp horizontal limit above which dead tissue was present and below which the coral was, seemingly, healthy. Up to 30% of the reef was affected by some degree of die-off.

Bunaken reef flats. (a)Close-up of one Heliopora coerula colony with clear tissue mortality on the upper part of the colonies; (b)same for a Porites lutea colony; (c) reef flat Porites colonies observed at low spring tide in May 2014. Even partially above water a few hours per month in similar conditions, the entire colonies were alive. (d) A living Heliopora coerula (blue coral) community in 2015 in a keep-up position relative to mean low sea level, with almost all the space occupied by corals. In that case, a 15 cm sea level fall will impact most of the reef flat. (e–h) Before–after comparison of coral status for colonies visible in (c). In (e), healthy Poritea lutea (yellow and pink massive corals) reef flat colonies in May 2014, observed at low spring tide. The upper part of colonies is above water, yet healthy; (f) same colonies in February 2016. The white lines visualize tissue mortality limit. Large Porites colonies (P1, P2) at low tide levels in 2014 are affected, while lower colonies (P3) are not. (g) P1 colony in 2014. (h) Viewed from another angle, the P1 colony in February 2016. (i) Reef flat community with scattered Heliopora colonies in February 2016, with tissue mortality and algal turf overgrowth. Taken from E. E. Ampou et al. 2016.

The confinement of the dead tissue to the tops and flanks of the corals, lead the scientists to think that the deaths must be linked to variations in sea level rather than temperature, which would affect the organisms ubiquitously. To confirm the theory the researchers had to establish that there had indeed been fluctuations in sea level across the region between the springs of 2015 and 2016.

To do so they consulted data from regional tide-gauges. Though not located exactly on Bunaken, they provided a good first-order measure of sea levels over the period of time in question. To bolster their results, the team also used sea level height data acquired by satellites, known as altimetry data, which had sampling points just off Bunaken Island. When compared, the sea level data acquired by the tidal gauges and satellites correlated well.

Sea-level data from the Bitung (east North Sulawesi) tide-gauge, referenced against Bako GPS station. On top, sea level anomalies measured by the Bitung tide-gauge station (low-quality data), and overlaid on altimetry ADT anomaly data for the 1993– 2016 period. Note the gaps in the tide-gauge time series. Middle: Bitung tide-gauge sea level variations (high-quality data, shown here from 1986 till early 2015) with daily mean and daily lowest values. Bottom, a close-up for the 2008–2015 period. Taken from E. E. Ampou et al. 2016.

The data showed that prior to the 2015-2016 El Niño, fluctuations in sea levels could be attributed to the normal ebb and flow of the tides. Crucially, between August and September 2015, they also showed a sharp decrease in sea level: in the region of 15cm (compared to the 1993-2016 mean). Though short-lived (probably a few weeks only), the period was long enough that the corals sustain tissue damage due to exposure to excessive UV light and air.

NOAA provides real-time Sea Surface Temperatures which identify areas at risk for coral bleaching. The Bunaken region was only put on alert in June 2016, long after the coral die-off started, therefore supporting the crucial role sea level fall played in coral mortality in Indonesia.

The link between falling sea level and El Niño events is not limited to Indonesia and the 2015-2016 event. When the researchers studied Absolute Dynamic Topography (ADT) data, which provides a measure of how sea level has change from 1992 to 2016, they found sea level falls matched with El Niño years.

The results of the study highlight that while all eyes are focused on the consequences of rising ocean temperatures and levels triggered by El Niño events, falling sea levels (also triggered by El Niño) could be having a, largely unquantified, harmful effect on corals globally.

By Laura Roberts Artal, EGU Communications Officer

References and resources

Ampou, E. E., Johan, O., Menkes, C. E., Niño, F., Birol, F., Ouillon, S., and Andréfouët, S.: Coral mortality induced by the 2015–2016 El-Niño in Indonesia: the effect of rapid sea level fall, Biogeosciences, 14, 817-826, doi:10.5194/bg-14-817-2017, 2017

Varotsos, C. A., Tzanis, C. G., and Sarlis, N. V.: On the progress of the 2015–2016 El Niño event, Atmos. Chem. Phys., 16, 2007-2011, doi:10.5194/acp-16-2007-2016, 2016.

What are El Niño and La Niña? – a video explainer by NOAA

Coral Reef Watch Satellite Monitoring by NOAA

Global sea level time series – global estimates of sea level rise based on measurements from satellite radar altimeters (NOAA/NESDIS/STAR, Laboratory for Satellite Altimetry)

El Niño prolongs longest global coral bleaching event – a NOAA News item

NOAA declares third ever global coral bleaching event – a NOAA active weather alert (Oct. 2015)

The 3rd Global Coral Bleaching Event – 2014/2017 – free resources for media and educators

What is coral bleaching? – an infographic by NOAA
The ENSO (El Niño–Southern Oscillation) Blog by Climate.gov (a NOAA resource)

Imaggeo on Mondays: the remotest place on Earth?

Imaggeo on Mondays: the remotest place on Earth?

Perhaps a bold claim, but at over 4,000 km away from Australia and 4,200 km from South Africa, Heard Island is unquestionably hard to reach.

The faraway and little know place is part of a group of volcanic islands known as HIMI (comprised of the Heard Island and McDonald Islands), located in the southwest Indian Ocean. Shrouded in persistent bad weather and surrounded by the vast ocean, Heard Island, the largest of the group, was first sighted by the merchant vessel Oriental in 1853.

Its late discovery and inaccessibility mean Heard Island is largely undisturbed by human activity (some research, surveillance, fishing and shipping take place on the island and it’s surrounding waters). It boasts a rich fauna and flora: seals, invertebrates, birds and seals call it home, as do hardy species of vegetation which grow low to the ground to avoid the fierce winds which batter the island.

Geologically speaking the islands are pretty unique too. They are the surface exposure of the second largest submarine plateau in the world, the Kerguelen Plateau. Limestones deposited some 45–50 million years ago began the process which saw the emergence of the islands from the ocean floor. Ancient volcanic activity followed, accumulating volcanic materials,  such as pillow lavas and volcanic sediments, up to 350m thick. For the last million years (or less) Heard Island has been dominated by volcanism, giving rise to the 2745m tall Big Ben and 700m tall Mt. Dixon. Eruptions and volcanic events have been observed on the island since 1947. Much of the recent volcanism in the region has centered around McDonald island, which has grown 40 km in area and 100 m in height since the 1980s.   

As the group of islands provides a remarkable setting, where geological processes and evolution (given that large populations of marine birds and mammals numbering in the millions, but low species diversity) can be observed in in real time, UNESCO declared HIMI a World Heritage Site back in 1997.

If you pre-register for the 2017 General Assembly (Vienna, 22 – 28 April), you can take part in our annual photo competition! From 1 February up until 1 March, every participant pre-registered for the General Assembly can submit up three original photos and one moving image related to the Earth, planetary, and space sciences in competition for free registration to next year’s General Assembly!  These can include fantastic field photos, a stunning shot of your favourite thin section, what you’ve captured out on holiday or under the electron microscope – if it’s geoscientific, it fits the bill. Find out more about how to take part at http://imaggeo.egu.eu/photo-contest/information/.

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