Where the Water Flows

Jochen Horstmann releasing a drifter into the ocean. Photo: Hereon/Christian Schmid

A shipping container has gone missing and is at risk of entering a channel that sees high traffic. An oil slick might enter the protected Wadden Sea. A surfer who is unable to manoeuvre is careening out into the open waters. It is during emergencies like these when rescue teams wish to know in what direction the tides, wind and sea conditions are driving the water. Which ships, for example, must be warned of the stray container? How many coastal towns should prepare for an impending oil spill? And where exactly must the rescue helicopter fly to save the surfer?

Scientists could only imprecisely answer such questions for quite a long time: little was known concerning ocean currents, which are known as drift. Forecasts were correspondingly flawed.

Researchers have been attempting to explore patterns of movement in the sea for centuries – sometimes using surprising methods. Experts from the German Naval Observatory in Hamburg, for example, threw hundreds of bottles into the sea about 150 years ago. The form tucked inside each bottle asked those who made the discovery to inform the Naval Observatory of the bottle's location when found. This is how they would determine drift pathways. This was a first in scientific research – if an extremely imprecise approach. And a lengthy one at that: one of the bottles from Hamburg reached the coast of Australia only this spring. After 132 years at sea.

The site in Hamburg now owns the largest “message in a bottle” collection in the world. The methods have, nevertheless, been heavily refined since these beginnings: researchers have meanwhile been able to predict with increasing precision where the person, particle or object will drift. Reverse calculations are also possible now: where did the algae come from that suddenly appeared along the coast? Which ship carried the packet of cocaine that washed ashore there? And who illegally dumped paraffin into the sea a few hours ago, which is now drifting toward land? Scientists and government bodies are now able to find answers to such questions concerning the past more precisely than ever before.

Two main drivers of drift have been identified: on the one hand, the fixed sequence of ebb and flow creates ocean currents, while on the other hand, the constantly fluctuating weather drives the surface water. In addition, local factors come into play, such as in estuaries or near land barriers—for example, islands.

Heavily fragmented regions, such as the German Bight, with all the islands and tributaries, are therefore a particularly interesting research region for marine scientists: for analysing drift in the German North Sea alone, an entire network of experts has joined forces, including researchers from the Helmholtz-Zentrum Hereon. At the Institute of Coastal Research, for example, oceanographers measure how wind and sea state effect drift; mathematicians reconstruct the weather for the past sixty years through simulations in order to recognise typical current patterns; biologists observe where plankton and other marine creatures drift over the course of seasons; and technicians release small transmitter stations – called surface drifters – which are pulled along by the water in the North Sea and regularly transmit information back to land pertaining to their location.

Measurements with dirfters and marine radar

Drifters are recording interesting phenomena on currents. Photo: Hereon/Christian Schmid

"Drifters are always recording interesting phenomena on currents for us,” explains Jochen Horstmann, head of the Department of Radar Hydrography at the Hereon. “Their data, however, hardly allows us to draw conclusions on structures of currents on a large scale.” This is therefore taken care of by the institute’s radar facilities instead: high-frequency radar equipment with a range of approximately one hundred kilometres is located on the islands of Sylt and Wangerooge as well as on the coast of Büsum. The antennae measuring three metres high are constantly receiving data. The surface currents in the German Bight are calculated from this data every twenty minutes. These currents, with a resolution of up to two kilometres, are also considered an important corrective for the purely mathematical models with which researchers also work.

Dr Jochen Horstmann has been heading the Radar Hydrography Department at the Hereon's Institute of Coastal Research since 2013. Photo: Hereon/Christian Schmid

The marine radar possesses an even narrower focus: ocean currents within a radius of three kilometres are measured aboard ships, and this information is transmitted to the screen on request with only a slight time delay. Every two seconds a new image of the surface is received, with a resolution of 7.50 metres. This makes waves of different lengths visible: pulsating, their crests scurry across the monitor, and surface films emerge as dark shadows.

When Horstmann requires data with considerably higher spatial resolution, he releases a drone into the air—this is fitting, for example, near estuaries or harbour areas, as it is there that the current varies a great deal. “The seawater in the vicinity of the coast often moves at speeds of one or more metres per second and the current direction also changes quite suddenly,” explains the oceanographer. It is usually more sluggish, however, on the open sea: there it usually flows at speeds considerably less than one meter per second.

Drift effect on fish populations

Dr Ute Dawel has been employed as a scientist in the Matter Transport and Ecosystem Dynamics Department at the Institute of Coastal Research since 2016. Photo: Hereon/Jan-Rasmus Lippels

Researchers must be familiar with this mosaic of calm and extremely agitated zones if they want to correctly interpret local phenomena. Ute Daewel, for example, marine researcher at the Institute of Coastal Research, is interested in how drift effects fish populations. The current carries the eggs along after spawning – in unfavourable cases, in regions with a great number of predators, insufficient food supply or unfavourable environmental conditions. According to Daewel, “This can lead to a sharp decline in the population of certain species.”

After spawning, some fish species are dependent on the currents to transport their eggs to other regions, where the young fish can mature. Photo: iStock/yannp

If the population of an otherwise typical species of fish suddenly decreases in a region, the experts must then not only check whether this is due to an environmental problem or if fishing could be the underlying reason. The current can also severely affect animals seasonally – we only need to remember last year's “shrimp crisis”. The animals had become so rare along the German coasts that shrimp rolls in some beach resorts reached a record price of €11.50. Researchers such as Daewel assume that unfavourable current conditions could have been responsible for the decrease at the time. The animals manage comparatively better in the face of climate change.

The expert works with computational models for her studies. These models allow her to precisely track where eggs and larvae drift and how they develop, depending on water temperature and food supply. Herring, for example, typically spawn off the eastern coast of England. The current usually carries the eggs from there to the southeast, to zones with abundant food supplies along the German coast. Herring, however, spawn in winter, when the prevailing cold causes the eggs to mature even slower. If temperatures then still drop drastically, there is a risk that the larvae only hatch when, with the drift, the eggs have long since passed the nutrient-rich regions —after all, the North Sea in the German Bight is constantly turning in a counter-clockwise direction.

Einsatz von Dispergatoren bei Ölunfällen

Dr Ulrich Callies heads the Modelling for the Assessment of Coastal Systems Department at the Institute of Coastal Research. He has been employed at the research centre in Geesthacht since 1988. Photo: Hereon/Christian Schmid

Drift researchers also bring up important arguments concerning environmental issues. Using what are known as dispersants in Germany, for example, is controversial: if this chemical is sprayed onto a drifting oil slick, it dissolves into many tiny droplets that mix with the seawater and sink. The toxic slick disappears from the water's surface before it reaches land and can threaten its ecosystem. The oil particles are instead distributed in the water, penetrating all the way to the seabed and possibly seeping into the sediment, where they in turn could harm the organisms living there. “Various risks on land and at sea must be weighed before we can come to a decision on the use of dispersants,” explains Ulrich Callies, who leads the “Modelling for the Assessment of Coastal Systems” department.

He has been refining the models for drift phenomena at the Hereon for many years and is particularly interested in the behaviour of oil at sea – especially in regard to possible oil spills. “Each type of oil reacts differently when it comes into contact with air and salt water – one evaporates quickly, while others rapidly clump. The effectiveness of an applied dispersant depends heavily on the type of oil present,” explains Callies.

It is all the more difficult to predict whether the use of dispersants is actually advisable to prevent an oil spill from polluting the coast or the sensitive Wadden Sea. Callies and his colleagues therefore use computer simulations that enable them to continuously release oil into the North Sea—millions of times and during the most varying weather conditions as well as at any location. The simulations have demonstrated that if a tanker leaks directly off the coast, the dispersants no longer help against extreme contamination – the tidal movement alone would push the oil rapidly into the Wadden Sea. Far out at sea, on the other hand, conventional methods for combating oil spills are usually sufficient due to the long periods of time available. This renders any use of additional chemical pollutants in the sea unnecessary. The use of the chemical additives in a pre-defined corridor along the coast would be of interest: if oil leaks at a distance of twenty to forty kilometres from land, the chemical additives may often effectively prevent the oil slick from drifting into the Wadden Sea. In addition, the fact that the water depth in these zones already reaches values of around twenty metres speaks in favour of spraying. The water column is thus large enough for dispersants and oil particles to at least be heavily diluted. “Limited use in this area is therefore advisable to prevent worse situations from unfolding,” says Callies.

Here one can see the way in which the oil with (blue) and without (brown) application of a chemical dispersant would have been distributed after a hypothetical spill on March 15th, 2008. The hypothetical site of the accident is designated by the symbol of a wreck. It becomes clear that the oil would still be present on the open sea three days after the spill if a dispersant (with one hundred per cent efficiency) had been used. The oil, on the other hand, would have been driven by the wind towards the Wadden Sea without these measures.
Source: Data and Design: Hereon/ Map in the background: Esri, ArcGIS Online

Working with the BSH

Dr Silvia Maßmann earned her doctorate at the Alfred Wegener Institute in the field of
climate sciences and carried out oceanographic measurements in the Antarctic Ocean. She programmed models to simulate tides in the North Sea for her doctoral dissertation. Since 2010, she has been in charge of the development, validation and operation of forecasting models in the section for Operational Models at the German Hydrographic Institute and develops drift and water level forecasts. Photo: BSH

The expert is regularly in contact with the governmental bodies responsible for combatting oil at sea, such as the German Federal Maritime and Hydrographic Agency (BSH). The forecasting service there keeps an eye on the North Sea currents day and night, including with the help of Hereon data. The staff can utilise the BSH model data with a few clicks of the mouse to produce drift forecasts for the next forty-eight hours. They can subsequently track down environmental polluters by calculating which path the particles last took due to the currents.

“Our team responds to fifty such requests from police and rescue personnel per year,” explains Silvia Maßmann, who supervises the drift model at BSH. “Pressing issues are handled by the on-call water level forecasting service, even in the middle of the night, while we respond to others as quickly as possible within twenty-four hours.” She and her colleagues receive all requests that concern, for example, buoys that have broken loose during a storm, or missing persons who were last spotted at sea. Once she was contacted about a drifting naval mine near the coast – her "most unusual case," according to Maßmann. A fisherman had pulled the unexploded bomb from World War II on board, and then he threw it back into the water out of fear. He immediately contacted the waterways police, who asked Maßmann for assistance. “Luckily our forecasts at the time corresponded very precisely with the actual course so that the rescue team could quickly retrieve the bomb.”

Maßmann enters various parameters on the computer for her calculations. Where exactly, for example, did a ship leak oil? How much of the load has already leaked? What type of oil did the ship carry? The more detailed the initial data, the more precise the forecasts will be that Maßmann can provide the Central Command for Maritime Emergencies.

Her screen then shows, for example, how a bunch of black dots are set in motion off the island of Amrum, with each splash of colour symbolising a leaked quantity of oil. The cloud first drifts to the southeast, toward the island. But then the wind suddenly shifts, the tide goes out, and the oil slick is pulled out to the ocean. The coast of Amrum and its sensitive ecosystems would have been spared an environmental catastrophe in this simulation.

Thanks to Maßmann’s forecasts, the emergency response teams would have correctly positioned themselves. Their ships for combatting oil spills would have been deployed to the North Sea and their oil barriers would have been set up off the coast.

Author: Jenny Niederstadt
Published in in2science #7 (December 2018)