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Q and A with Dr. Lisa

Archimedean Schools

October 30, 2011

  • How do eddies form and break away from the Agulhas Retroflection?
    There is much research on how eddies in the Agulhas form and effect the Agulhas retroflection, but I will try to explain in a nut shell! Eddies are very common in the ocean and result from instabilities in the circulation — the ocean is not a “linear” system, where flows are smooth, but a “turbulent” one, where flows tend to break up into eddies and filaments. Even in a western boundary current like the Agulhas (and the Gulf Stream) water parcels can leave and join the current (we call it detrainment and entrainment) along its course via eddies. In the Agulhas, eddies from the Mozambique Channel and East Madagascar Current drift westward into the current (all eddies like to drift west, because the Earth is spinning!) and cause it to meander. The meander propagates downstream with an anticlockwise eddy inshore and a clockwise eddy offshore. As the meander approaches the Retroflection it destabilizes the flow and an Agulhas Ring is formed, which is basically when the retroflection forms a closed loop, temporarily cut off from the current. This Agulhas Ring then drifts westward away from the retroflection, leaking warm and salty Indian Ocean water into the Atlantic.
  • What causes the Agulhas Retroflection?
    You guys have some tough questions! You might remember from my talk that the Agulhas is part of the Indian Ocean subtropical gyre, which is similar to the North Atlantic subtropical gyre where the Gulf Stream is the western boundary current. Subtropical gyres are driven by Earth’s large wind systems — the Trades and Westerlies — which cause a massive circulation of waters around and around each ocean basin between about 15 degrees latitude and 45 degrees latitude. The Agulhas Retroflection appears because the African continent (ending at 36°S or so) does not reach as far south as the gyre, so the Agulhas Current runs out of boundary to flow along! At first the Current continues southwest under its own inertia, until the Westerlies force it back to the east to rejoin the Indian Ocean gyre circulation — hence the retroflection is formed. But as you learned above, not all the waters from the Agulhas are retroflected back into the Indian Ocean — some leak into the Atlantic via Agulhas Rings and we call this Agulhas Leakage.
  • Have you met Robert Ballard?
    No! But I have visited his laboratory at the University of Rhode Island and seen some of the artifacts he has brought up from wrecks in the abyssal ocean.
  • Are the eddies a permanent feature of the South Atlantic or have they grown more or less noticeable over time?
    Eddies are a permanent feature of all oceans. The number of Agulhas Rings can change, effected by what we call “coupled ocean-atmosphere climate modes” like El Niño. An analogy to this would be that the number of Atlantic hurricanes each season varies dependent on large scale conditions and also El Niño. There is also evidence from ocean and climate models that the number of Rings, or the size of the Agulhas leakage, may change in the future with climate change.
  • What kept your interest in the Agulhas Current?
    The Agulhas is remote from developed nations with strong research programs like the US and Europe and hence does not receive as much attention as, for instance, the North Atlantic and Pacific. I like to work on under-studied regions, where exploration and discovery are still possible and the science is new and exciting! I also like to travel and collaborate with researchers from other countries. The community of scientists who work on the Agulhas are from all over the world — Netherlands, UK, Japan, Germany, France, and of course South Africa. Finally, the more I discover about the Agulhas, the clearer it is that it plays an important global role — it may be remote, but its impacts are not. For instance, there is evidence that if Agulhas leakage changes then the whole circulation of the Atlantic changes, which has a profound effect on global climate (at time scales of decades to centuries)!
  • How do you distinguish the start and end of a current?
    This is a good question and it is difficult to answer. Flow can be concentrated into a strong current for a number of different reasons, and then dissipate again. For instance, water feeds into the Agulhas along its whole length, so that the current gets stronger and stronger towards the tip of Africa. After the current retroflects and becomes the Agulhas Return Current, the flow gradually weakens again until it is so weak and broad that it is hard to define as a current. Oceanographers might disagree about the definition of the start and end of the current. I might define it as where the velocity drops below 10 or 20 cm/s — in other words where it is so weak that it cannot be discerned from the background eddy field. However, deep currents are very often weaker than this, but can still be discerned through their water mass properties.
  • Do all Western Boundary Currents cause similar climactic effects on their bordering continents?
    Yes. All WBCs are strong and warm and so they tend to make bordering coastal regions warmer and wetter. This is compared to Eastern Boundary Currents, which are weak and cold (like the California Current) and tend to influence the coastal regions with cooler, dryer weather.
  • Is there a specific instrument that measures Sverdrups?
    No. A Sverdrup is a measure of volume transport — in meters cubed per second — so we calculate it by measuring velocity in the ocean (m/s) and then multiplying that by a cross-sectional area (m2). Velocity is measured using an instrument like the one I brought into class, an acoustic current meter. For example, if you wanted to measure how many Sverdrups the Gulf Stream is as it flows through the Florida Straits you would first measure velocity across the Straits using an acoustic current meter which profiles the ocean while the ship is underway, called an ADCP (acoustic Doppler current profiler). The Florida Straits is only 800 m deep, so an ADCP can “see” and measure all the way to the bottom! Once you have the velocity field across the Strait you multiply by the cross-sectional area of the Strait to get the total transport in Sverdrups. I'll leave you to think about how you might calculate transport in a deeper current, or how you might calculate changes in transport over time.
  • Can climactic changes cause significant aberrations in ocean current patterns?
    Climatic changes can cause changes in ocean currents — and vice versa — changes in ocean currents can cause changes in climate! The ocean is an integral part of the climate system, it is coupled with the atmosphere, meaning that they can effect and feed back on one another. Imagine the ocean is warmer than usual — like during an El Niño when the tropical Pacific warms. More water evaporates from the warmer ocean, which also heats the atmosphere more and causes more air to rise. These effects result in more clouds and rain, which is why California is wetter during El Niño.
  • Since the Agulhas Current directly affects the Gulf Stream, can climactic changes in Europe be attributed to the Agulhas Current?
    This is another example of the fact that the ocean is an integral part of our climate system! Changes in Agulhas leakage can affect the Atlantic overturning circulation: models have shown that if the leakage of salty water from the Agulhas increases, then the formation of deep water in the North Atlantic also increases, because all that salt increases the density of the waters in the Atlantic so that they sink more easily. It takes a few decades for the signal to travel from the Agulhas into the North Atlantic and affect the overturning. However, in reality it would be difficult to attribute any change in European climate to the Agulhas alone. The ocean-atmosphere system is so interconnected, with affects and feedbacks on one another, that it is difficult to say if any one process is more or less important than another. This is why we need a lot more research!
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David Lawrence

October 30, 2011

  • How much food and water does the ship carry for a voyage? How long before it needs new supplies, and how do you get them onboard?
    Research Vessel Melville is an “ocean class” vessel, which means she can be out at sea for six to eight weeks at a time, with enough fuel, water, and food to support the 23 crew and up to 30 scientists. For numbers we will have to ask the Captain, so why don't you put together a few questions for him?
  • Does the temperature change you are seeing affect coral reefs closer to the tropics, like the ones in the Caribbean?
    Warming of the world’s oceans related to global climate change does effect coral reefs and there have been bleaching events in the Caribbean. There are a number of scientists at RSMAS who study climate change effects on corals, including ocean acidification, but this is not something I research myself.
  • How does the Agulhas affect migrating animal life? Do you see any Great White SHarks in your research? How about seals or other marine mammals?
    The Agulhas has a dramatic effect on the migration of sardine, which annually travel northward in great schools, against and inshore of the Current, to feed and spawn off the Kwa-Zulu Natal coast of South Africa. This event is called the Natal Sardine Run but very little is understood about why the fish migrate and exactly how the Agulhas effects the success of their migration. South African waters are famous for sharks, but I have not seen any from the ship. Some of our science team will be going recreational diving after our cruise to see the Great Whites. Sometimes we see jellyfish, tuna, or whales in the Agulhas, but the waters are not that productive — like the Gulf Stream the waters are very blue, a beautiful deep blue — which is a sign that they are clear and largely free of plankton and marine life. Such waters are called oligotrophic.
  • What are the sea conditions like where you are? Is it rough?
    Sea conditions vary from one day to the next and we hope we will have enough calm seas to conduct our science! We will be in the region in local summer time, when storms are less likely and less severe than in winter. However, the Agulhas is notorious for large waves, because often the wind blows in the opposite direction to the strong current, creating big, steep waves which have been known once or twice to break oil tankers in half! Yes, really. But RV Melville is shorter than a tanker and can ride the waves well, so we're not worried for our lives - only for our stomachs and getting the science done. Wish us luck!
  • How does the temperature and clarity of the water compare to the gulfstream? If you had to pick a spot on the Gulfstream analogous to where you will be doing your research, where would it be? Off the Carolinas? Further south or further north?
    As I said above, the waters of the Agulhas do look rather similar to those of the Gulf Stream, but tend to be a few degrees C warmer. Since we will be in the Agulhas at about 34 degrees latitude South, a similar spot in the Gulf Stream would be at 34 degrees North. I’ll let you look up whereabouts that would be on the east coast and let me know.
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Mast Academy

October 30, 2011

  • How much food and water does the ship carry for a voyage? How long before it needs new supplies, and how do you get them onboard?
    Research Vessel Melville is an “ocean class” vessel, which means she can be out at sea for six to eight weeks at a time, with enough fuel, water, and food to support the 23 crew and up to 30 scientists. For numbers we will have to ask the Captain, so why don't you put together a few questions for him?
  • Does the temperature change you are seeing affect coral reefs closer to the tropics, like the ones in the Caribbean?
    Warming of the world’s oceans related to global climate change does effect coral reefs and there have been bleaching events in the Caribbean. There are a number of scientists at RSMAS who study climate change effects on corals, including ocean acidification, but this is not something I research myself.
  • How does the Agulhas affect migrating animal life? Do you see any Great White Sharks in your research? How about seals or other marine mammals?
    The Agulhas has a dramatic effect on the migration of sardine, which annually travel northward in great schools, against and inshore of the Current, to feed and spawn off the Kwa-Zulu Natal coast of South Africa. This event is called the Natal Sardine Run but very little is understood about why the fish migrate and exactly how the Agulhas effects the success of their migration. South African waters are famous for sharks, but I have not seen any from the ship. Some of our science team will be going recreational diving after our cruise to see the Great Whites. Sometimes we see jellyfish, tuna, or whales in the Agulhas, but the waters are not that productive — like the Gulf Stream the waters are very blue, a beautiful deep blue — which is a sign that they are clear and largely free of plankton and marine life. Such waters are called oligotrophic.
  • What are the sea conditions like where you are? Is it rough?
    Sea conditions vary from one day to the next and we hope we will have enough calm seas to conduct our science! We will be in the region in local summer time, when storms are less likely and less severe than in winter. However, the Agulhas is notorious for large waves, because often the wind blows in the opposite direction to the strong current, creating big, steep waves which have been known once or twice to break oil tankers in half! Yes, really. But RV Melville is shorter than a tanker and can ride the waves well, so we're not worried for our lives — only for our stomachs and getting the science done. Wish us luck!
  • How does the temperature and clarity of the water compare to the Gulfstream? If you had to pick a spot on the Gulfstream analogous to where you will be doing your research, where would it be? Off the Carolinas? Further south or further north?
    As I said above, the waters of the Agulhas do look rather similar to those of the Gulf Stream, but tend to be a few degrees C warmer. Since we will be in the Agulhas at about 34° latitude South, a similar spot in the Gulf Stream would be at 34° North. I’ll let you look up whereabouts that would be on the east coast and let me know.
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South Broward

November 7, 2011

  • Although studying the Agulhas current is an aspect of physical oceanography, what kind of impact will the studies ahve on the biological community of South Africa?
    Marine biologists are always concerned with ocean currents and water properties (like temperature, salinity, and oxygen), because these represent the habitat for marine animals, impacting their growth, migration, and recruitment. In particular, this study of the Agulhas Current will be of interest to marine biologists in South Africa because they will know more about the currents we measure on and off the eastern continental shelf of South Africa. Such currents can drive upwelling which brings nutrients to the surface to feed plankton and fish. Also, the Agulhas Current likely affects the so called Natal Sardine Run, which is an annual migration of sardine to the north where they spawn in warmer waters. Since the sardine have to swim against the Current to travel north, meandering of the path of the Current probably impacts the success of their migration, although we do not yet have the data to show this.
  • Do warm core rings in the Agulhas current have faster currents than surrounding waters?
    Agulhas Rings spin off from the Retroflection into the Atlantic and can have currents over 1 meter per second, which is five to ten times the background flow in the southeastern Atlantic.
  • How is this project generally funded? By RSMAS or by private funding?
    This project is funded by the National Science Foundation, a government funding agency which specializes in funding fundamental research. In other words, it specializes in funding innovative research for the public good, and not research that may lead to personal or monetary gain. In its most basic sense, this is research that will lead to an improved understanding of our planet, its atmosphere, ocean, land, and ice systems, and how we can best preserve and even manage them for future generations. The NSF, together with NOAA (National Oceanic and Atmospheric Administration), and the Navy (Office of Naval Research) are responsible for funding almost all oceanographic research in the US.
  • Since Doppler shift measures the ocean’s velocity, doesn't the ship's movement affect the readings since it is moving as well?
    This is a very good question! The Doppler current meters which we deploy over the side on the moorings (like the one I brought with me to class) are not affected by the ship's motion. However, there are two ADCPs (Acoustic Doppler Current Profilers) mounted in the ship’s hull which are affected by the velocity, and the pitch, roll and yaw, or attitude, of the ship. Hence, we need a way to independently measure the motion of the ship. This is done using satellite GPS (Global Positioning System) received by a four antenna array mounted on the flying bridge. GPS is so accurate that the difference in distance of the four antennas from the satellite gives us the ship’s attitude. Then the difference in position over time gives us the ship's speed and direction.
  • Does the current affect any local fisheries in the area?
    Because of the fast currents, western boundary regions do not support good fisheries and the Agulhas is no exception. The South African fishery is largely on the east coast, in the southeast Atlantic, where the cool and sluggish northward Benguela Current is fed by strong coastal upwelling. However, there is also a small fishery over the Agulhas Bank, which is a wide, shallow area of ocean to the south of the African continent. The lower Agulhas runs along the eastern margin of the Agulhas Bank and therefore can sometimes stir the waters there if it meanders on or offshore, affecting the temperature of the waters and causing upwelling or downwelling. This can have a small affect on the fishery.