A beautiful day in the trade winds zone, with its typical cumulus clouds.
From the shipboard perspective, all we really see of the sea is the surface. Of course we can see into the water a short way, right close to the ship, but not very far. The horizon is 360 degrees and the great dome of sky seems endless.
Being that we are about a thousand miles from the nearest port, it is also fair to say that all we see from the ship is sea surface and sky. These are the shades of blue that I referred to in an earlier post. When this is all one has to see aside from the Knorr, our shipmates, and interesting oceanographic data (OK, we watch movies too!), it should come as no surprise that everyone becomes sensitive to the moods of the sea and sky. When you want a moment alone, the likely place to go is on deck, and there you are confronted with some new variation of the sea and sky.
We are working in what is called the trade winds zone. It is a belt of generally east and northeast winds north of the equator and east or southeast winds south of the equator that are quite steady and global in extent (in the days of sail, one’s trade depended on using routes through these reliable wind zones). In the trades zone, we might expect relatively steady 10-15 mph winds and fair skies with broken clouds. One characteristic of the fair weather cumulus clouds in the trades is that they lean over because of wind shear in the atmosphere. The blue sky dominates the evenly scattered puffy white leaning clouds that seem to all have the same base (maybe 3,000 feet above the sea).
The trade wind cumulus clouds break up the bright shades of blue on the sea surface by casting rapidly evolving shadows across the sea. The color of the sea surface certainly depends on the light reflected from the sky (a gray sky can give the ocean a grayer look) but also depends on the intrinsic color of water, which is blue.
We had few days where there seemed to be a pink hue to the sky (especially at the low sun angles of morning and late afternoon). This is likely the result of having more Saharan dust suspended in the atmosphere. It is quite common for dust storms to carry thousands of miles out over the ocean.
Sunset in the Sargasso, with hints of Saharan dust.
The color of the water depends on the angle at which you view it and the height of the sun. One of the cool things I see in the open ocean is that when you look down into the deep waters in the middle of a sunny day there is a radiation of sunbeams seemingly coming back at you from the depths. It gives the blue ocean a kind of jewel-like quality.
When we are doing work at sea, it hardly seems fair for NASA to hog the limelight. We are usually offering data from satellites, not ships, moorings, or gliders. There are partner agencies in the U.S. Government who make enormous contributions to the physical oceanography enterprise. In D.C., oceanographers know these agencies as “the four N’s” – NASA, the National Oceanic and Atmospheric Administration (NOAA), the National Science Foundation (NSF), and the Navy. Because each, in its own way, contributes to the success of physical oceanography in the USA and of SPURS in particular, I am going to try to tell you about them through their contributions and through relevant posts from the field. With this post, I am going to focus on NOAA.
The 5-cent summary is that NOAA Pacific Marine Environmental Laboratory (PMEL) is proving two moorings for SPURS and the NOAA Atlantic Oceanographic and Meteorological Laboratory (AOML) is providing enhancement to their ongoing basin-wide observing system.
One way we divided us SPURS so that we could look at all the relevant time and space scales of salinity variation (minutes to years and inches to thousands of miles), was by looking at who was strong in particular areas. NOAA is the key agency when it comes to monitoring the global ocean with measurements in the water. They maintain moorings in the tropical oceans for seasonal climate prediction, Argo floats around the globe for monitoring of upper ocean temperature and salinity profiles, a global array of surface drifters for sea surface temperature and surface velocity maps….and the list goes on.
The ocean involves so many different interacting processes that no single observing tool captures the whole picture (the variety of instrumentation in SPURS is a good example). We find that different kinds of measurements used together give a more well-rounded vision that is fuller than the simple sum of the individual parts. A focused process study like SPURS takes advantage of this, and NOAA scientists were eager to be part of it. While NOAA maintains long-term monitoring arrays that record broad fluctuations, those don’t necessarily illuminate the processes that underlie the fluctuations, so the diverse measurements in SPURS add interpretive value to NOAA’s arrays. SPURS is also a testing ground to learn what particular instruments measure well (and what they don’t), and to hone sampling strategies. When we deploy Argo floats and surface drifters in SPURS, these measurements enhance our knowledge of salinity in our study area, but the instruments will also remain in place for years to come and contribute to the Atlantic Ocean monitoring array maintained by NOAA.
AOML started a new XBT transect between Cape Town and New York City (referred as AX08) on August 18, with XBTs deployed every 15.5 miles (25 kilometers). This is the third of five AX08 realizations that will be done on 2012. There are five realizations planned for 2013. A total of 550 XBTs are deployed on each realization.
Enhancement XBT line for SPURS.
A planned NOAA expedition to in September with some SPURS-related activity had to be postponed due to mechanical malfunction of the ship, the R/V Ron Brown.
It’s very exciting for us to help PMEL by deploying two of their “Prawler” (Profiling Crawler) moorings in SPURS. The Prawler uses the motion of the waves to provide lift for free: each time a wave lifts the mooring, the instrument holds tight to the wire with a ratchet and goes up. When the wave trough passes, the ratchet releases and the round fin keeps it from going down, so it “crawls” up the wire in steps. When it gets to the top, it free-falls back down the wire, making a profile of temperature and salinity. Since battery power is one of the main limiting factors in designing ocean instruments, the Prawler’s use of wave energy lets it work for much longer than if it had to carry a large battery pack and motor. A full and fascinating description is available here.
A Prawler on a wire.
To summarize, in order for NASA to advance the science of physical oceanography, we work closely with other federal agencies, such as NOAA, to bring the correct mix of measurements and technology to the field. SPURS is most definitely a team effort!
SPURS Chief Scientist Ray Schmitt has been thinking about the salt in the ocean for a long time. He did his PhD thesis on an unusual form of mixing called “salt fingers,” which we will discuss in a later post. This small scale mixing process led him to consider the origins of the ocean salinity contrasts that we see around the world.
It’s fairly obvious that salty waters arise from high evaporation regions and fresher waters originate from high rainfall areas or river flows into the ocean. But it turns out that accurate estimates of evaporation and rainfall over the ocean were hard to come by. For a long time, it was a relatively neglected research topic. Many meteorologists were only concerned about how much it rained on land and few seemed to care if it rained on the ocean. Pulling together the best data he could, Ray found that, in fact, the ocean completely dominated the global water cycle. The terrestrial part, so important to us on a daily basis, is a much smaller piece. The oceans hold 97 percent of the Earths free water, the atmosphere only 0.001 percent. The oceans provide 86 percent of global evaporation and receive 78 percent of all rainfall. The total of all river flows into the ocean sums to less than 10 percent of global ocean evaporation. Clearly, if one wants to find out what the water cycle is doing, one should be looking at the oceans. The traditional fixation on the terrestrial water cycle is understandable, but risks missing the big picture. It seems that the tail is wagging the dog in terms of research on the global water cycle!
A traditional view of the water cycle.
The oceanographers’ view of the water cycle.
Of course, one of the most important questions for climate change is what the water cycle will do with continued warming. Basic physics tells us that a warmer atmosphere will hold more water vapor, so an intensified water cycle is expected. Oceanographers should be able to assess any trend in the water cycle if we do a good job in monitoring ocean salinity. On land, man has altered every watershed with dams, groundwater irrigation, deforestation and human consumption. But the ocean’s mostly unaltered and its salinity field provides insight into the vast majority of the pristine natural water cycle. The ocean has its own rain gauge in the form of salinity, and our task in SPURS is to learn how to read it.
The combination of the global coverage from Aquarius for surface salinity, detailed process studies in the ocean like SPURS, and sophisticated high-resolution computer models working in concert open up the oceanic water cycle to careful scientific examination.
Aquarius salinity data from the first week of September 2012. (Credit: Oleg Melnichenko at University of Hawaii IPRC.)
A SPURS Waveglider begins its journey to study upper ocean salinity.
We are beginning to deploy the array of instruments on the ship and they are starting their year-long mission to examine the ocean salinity variations. Our challenge is to understand the detailed picture of salinity that will be painted by the various sensors and to make sense of this in the larger picture of the global water cycle.
One of the things that we worry about on the ship, as part of our daily routine, is trash. Nothing goes over the side unless it is biodegradable. We have separate trash cans for plastics, foils, and other such material that would pollute the ocean. There are cans with paper liners for food scraps and paper waste. We keep the deck clean and our eyes open for any loose cable tie or anything else that could be washed overboard. It’s our duty to pick these things up and dispose of them properly.
It may be surprising to some of you how much plastic is already in the ocean. It’s both amazing and depressing. If one stands by the ship’s rail and watches the water go by, as one wants to do in an idle moment, it is quite shocking to realize that more man-made objects can be observed than natural ones (like fish). A 2005 report from the U.N. Environment Program estimated that, on average, more than 13,000 visible pieces of plastic litter were floating on any square kilometer of ocean.
A piece of plastic debris floating near the Knorr.
Given the fact that plastic takes so long to break down, it should come as no surprise that this problem is still getting worse by the year, despite decades of effort to reduce the sources of pollution.
As a physical oceanographer, I know that the major ocean basins have gyre circulations at mid-latitude. The water moves in a great loop around the center of the ocean basins in the northern and southern hemispheres. Surface waters tend to converge toward the center of these gyres and trash of all sorts concentrates in these spots, far away from the coasts of the continents. In the North Pacific, there is the so-called “Great Garbage Patch” between California and Hawaii. Similar, but less well-known patches occupy dynamically similar regions in the other oceans. Nikolai Maximenko at University of Hawaii, a NASA-funded physical oceanographer studying the Pacific circulation, uses satellite and drifter data to understand this surface circulation in great detail. He made news with his analysis of the fate of tsunami debris from the 2011 Japan disaster.
Here in the Sargasso Sea, we have yet to see some giant patch of garbage (well, except for my desk in the upper lab!), but plastic does abound. I thought that I should testify as to my personal experience of observing so much plastic along our path. It’s more than I remember seeing 30 years ago, when I last sailed for an extended period in the Sargasso Sea. Its certainly not a scientific observation, but it reminds me that we are the creators and users of these materials and we should be the stewards of their disposal as well. So, pick up and recycle any loose plastic you see! It could wind up in the ocean.
Your SPURS blogger, Eric Lindstrom, showing off the NASA logo on the surface buoy.
The central mooring at the SPURS site is a critical piece of gear. It will provide us with a time series of upper ocean properties at one location over the entire year. We’ll build the other SPURS measurements around this spot on this and future voyages. We’ll “fly” the gliders in patterns centered on this location.
Our first order of business is to survey the bottom depth in the vicinity of the proposed mooring location (near 25N, 38W). The water depth we are aiming for is near 17,390 feet (5,300 meters).
Tom, working on the survey of bottom depths prior to mooring deployment.
Seabeam maps the bottom along a swath 8 miles wide.
The mooring is anchored to the bottom (with a 10,000 pound anchor). A large, heavily-instrumented buoy at the surface holds the entire string of instruments below. Just above the anchor is an acoustic release mechanism that can disengage the mooring from the anchor on command from the ship next year. Above the release are 80 glass floats (inside hardhats) that serve to float the bottom of the mooring to the surface after release.
80 glass floats in hardhats go at the bottom of the surface mooring.
The glass floats at the bottom of the WHOI mooring, trailing behind the R/V Knorr.
It’s a process of many hours to deploy the mooring. The ship will position itself some miles from the proposed anchoring site (depending on wind and currents) and start steaming toward the spot very slowly. The length of mooring and gear are then deployed over the stern starting with the top of the mooring, the surface buoy. After that various current meters, salinity and temperature sensors are attached in turn with various lengths of chain and shackles. As they are joined, they are in turn lowered over the stern and the surface buoy begins to distance itself in the ship’s wake. About 8 hours after the start of the deployment, the 16,000 feet of mooring is laid out on the surface behind the ship, and all that’s left on deck is the anchor.
At this point, location is everything. If timed correctly, the ship will be some distance past the location mooring intended to land on the bottom (say 10 percent of the water depth). If so, it is time to drop the anchor. As it falls, the length of mooring will drag it back toward the spot it will finally come to rest. We will see the surface buoy begin to rush swiftly back toward the ship (hopefully finishing up at its intended target location).
Lifting the WHOI buoy for deployment.
The buoy is away!
The mooring wire and equipment are gradually added.
Such work has been done thousands of times over the decades, but every deployment presents its own challenges of ocean bottom topography, wind, currents, and equipment. The length of the mooring needs to precisely cut for the water depth in which it is anchored. If it is too long, the mooring swings around too much at the surface. If it is too short, the mooring may be under too much stress or snap.