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OSNAP at AGU

OSNAP will be well represented at the 2014 AGU Fall Meeting!  Many of the scientist who have written for the blog will be giving presentations on their work.  This will be a great opportunity to get a more in depth look at ongoing OSNAP research. A list of talks and poster presentations relating to OSNAP are below.

OS41G The Atlantic Meridional Overturning Circulation, Climate Variability, and Change I
Thursday, December 18, 2014
08:00 AM – 10:00 AM
Moscone West 3009

Amy S Bower, Heather H Furey and Xiaobiao Xu
New Direct Estimates of Iceland-Scotland Overflow Water Transport Through the Charlie-Gibbs Fracture Zone, OS41G-02
08:15 AM

David Philip Marshall, Helen R Pillar, Patrick Heimbach and Helen Louise Johnson
Attributing Variability in Atlantic Meridional Overturning to Wind and Buoyancy buy cipro online Forcing, OS41G-03 (Invited)
08:30 AM

Ric Williams
Impact of Gyre-Specific Overturning Changes on North Atlantic Heat Content
08:45 AM

Martin Visbeck, Jürgen Fischer, Johannes Karstensen and Rainer Zantopp
Decadal Variations of the Atlantic Meridional Overturning Circulation
09:45 AM

 OS42B The Atlantic Meridional Overturning Circulation, Climate Variability, and Change II
Thursday, December 18, 2014
10:20 AM – 12:20 PM
Moscone West 3009

Igor Yashayaev, John Loder and Miguel Angel Morales Maqueda
Recurrence of Winter Convection in the Warming Labrador Sea and Associated Variability Downstream
11:20 AM

OS43D The Atlantic Meridional Overturning Circulation, Climate Variability, and Change III Posters
Thursday, December 18, 2014
01:40 PM – 06:00 PM
Moscone West, Poster Hall

Nicholas Foukal and Susan Lozier
Lagrangian Pathways of Temperature Anomalies from the Subtropical to the Subpolar Gyre in the North Atlantic (OS43D-1300)

 

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Mysteries of the Deep Subpolar North Atlantic

by Amy Bower and Heather Furey

Over the past several decades, oceanographers have constructed maps of the deep currents in the North Atlantic by piecing together measurements of currents and water properties from widely separated locations at different times. An example is shown in Figure 1. Such diagrams are beautiful in their simplicity and valuable for communicating the importance of northward-flowing warm and southward-flowing cold currents that together transport vast amounts of heat from the equatorial to polar regions.

Fig1

Figure 1: Schematic diagram of the major currents thought to be responsible for northward heat transport in the subpolar North Atlantic.

While useful as a summary of buy lasix canada subpolar North Atlantic overturning circulation, we need to remind ourselves that such ‘plumbing’ diagrams, if taken too literally, can give the false impression that there is very little connection between the boundaries of the ocean basin and the interior. Acoustically tracked underwater drifting buoys (called RAFOS floats) are being deployed in the deep boundary currents of the subpolar North Atlantic as part of OSNAP to investigate this connection (see previous posts for more details about the RAFOS float program in OSNAP at http://www.o-snap.org/water-goes-here-water-goes-there/ and http://www.o-snap.org/glass-floats-embark-on-a-2-year-mission-at-sea/). This Lagrangian approach to measuring ocean currents, whereby freely drifting floats reveal deep current pathways by drifting with the water (in contrast to the Eulerian approach, whereby sensors fixed in one position measure current speed and direction over a period of time) is ideal for mapping out current pathways over a large area, and with repeated deployments, we can determine how those pathways are changing in time.

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The case of the gigantic underwater cyclone

The largest water fall in the world is an underwater waterfall. Its just northwest of Iceland, and it begins with water spilling over an underwater ledge between Iceland and Greenland, the Denmark Strait. 

Hurtling down this water fall are cyclones of water, 1,500 meters high. 

Bob Pickart was the first to measure the cyclones in 2008. He measured them by accident: They spent a year whacking into 4 stationary vertical strings of instruments that he had placed in the ocean. These moorings were at the bottom of the waterfall, a checkpoint to see what it did after it spilled off. When he went to collect the data— a year’s worth —he found that it was garbled. Instead of examining a vertical slice of the ocean, the instruments had been repeatedly pushed over at an angle. What emerged from the mess was a picture of a tornado-like column of water, rushing past about once every two days. 

This is how they form: As dense water moves over the Denmark Strait, it sinks. (It does this over the course of 100 of kilometers —though taller than Niagra falls, this under water water fall is not a steep waterfall.) As that water sinks, it starts to spin —kind of in the same way that a figure buy hydrocodone online skater hugs her arms to her chest to spin faster. The result: a towering, whirling column of water. 

If you measure its velocity, it looks like this: 

whoi_noc_avel

The red part is coming at you at 80 cm/s, the blue part is going into the screen at 30 cm/s (slower, because the whole thing is also moving southward.) 

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Glass floats embark on a 2 year mission at sea

Floats (black dots) sink to nearly 3,000 meters under the sea. They'll follow and track the deep current, wherever it goes.

Floats (black dots) sink to nearly 3,000 meters under the sea. They’ll follow and track the deep current, wherever it goes.

As we head back towards Reykjavik, 19 long cylindrical glass floats are just beginning their own journeys. 

Over the course of our month at sea, WHOI technician Elizabeth Bonk deployed floats into the ocean. Weights on the bottom carried them thousands of meters down, almost to the bottom. These floats will follow the Deep Western boundary current, mapping the pathways of the dense water as it peels away from Greenland. There is no telling where they will go. 

Sound sources on moorings will send out signals to the floats. The floats will listen, and record: The time of arrival of a sound signal indicates how far away the floats are from the sound sources. 

When the floats were originally developed, they were called SOFAR floats, named for the SOund Fixing And Ranging channel. This channel is a horizontal slice of the ocean where the sound travels at its slowest, bouncing back and forth like light in a fibre optic cable, and traveling long distances instead of just dispersing all over the place. The floats used to do the talking —sending data to a stationary source. But now, because it is easier to have them relay their information individually, the floats do the listening. Accordingly, their name is spelled backwards: RAFOS.

After two years, the sources will signal to each RAFOS float that it should sever its connection to the weight that keeps it underwater. The float will pop up to the surface, and send data back to shore. 

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Life in Knorr-world

icebucket

The internet connection on the R/V Knorr is tenable, but slow.

When I sit down at the computer, I often bring reading material to look at while web pages load. Streaming videos and downloading music is out of the question. I’ve now “watched” two awards shows, one presidential address, and multiple music videos exclusively through the lens of what people were saying about those things on Twitter and in think pieces. The song of summer — the lyrics most blasted from cars and in coffee shops and in clubs — is irrelevant here. Several among us (not me) boast that they haven’t checked Facebook since we set sail.

Still, we’re connected to the outside world. Notably, we’ve kept track of the Bardarbunga volcano in Iceland as its activity made its way to a “yellow” warning level, then up to “red.” We’re headed to Reykjavik, where we’ll dock and fly home, so if it really goes off it could, hypothetically, affect our travel plans. Each piece of Bardarbunga news, in the absence of iPhones, and instant updates, is dispensed like gossip. On the whole, the impending eruption serves as a Rorschach test. Some of us (me) look up the effects of ash on airplane engines, and make nervous jokes about how “oh well we can always leave Iceland on a boat.” Others express hope that the eruption holds off until we are back on land, and they can travel closer to it. 

The ALS “ice bucket challenge” became a thing right about the time that we left Iceland. Since the meme lives in video form, most of what I know comes from reading news articles: people dump buckets of ice on their head, and/or donate money to ALS research. 

The crew got the idea to do it on board. There’s one obvious modification: instead of using precious fresh water — which is produced on board via energy-intensive reverse osmosis — we used water straight from the North Atlantic. There is a hose on deck that conveniently dispenses salt water, at a cool 40 degrees. No one had actually seen an ice bucket challenge video, of course. But I think we figured out the gist of it alright.

What on Earth else do we do for fun without the internet?

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Through the Prince Christian Sound

Photos by Carolina Nobre

“Greenland is great because it has complex boundary currents,” explains Bob Pickart. That’s why, in the course of our trip, we’ve spent a lot of time hanging out around  Greenland: to study the currents that snake around the sloping ocean floor near it. Scientists on board the Knorr are accomplishing this with round-the-clock deployments of various instruments.

But Greenland is also great because it is very scenic. After days of hovering to the east of Greenland, the science crew took the day off as we steamed through the Prince Christian Sound.

bridge

misty_iceberg

A narrow channel called the Prince Christian Sound splits the very southern bit of Greenland from the rest of the remote land mass. For most of the year, it is clogged with pack ice. For now, its clear —save for icebergs, and little chunks of ice called “growlers” — allowing water, and a few ships, to flow through. The water in this channel is a mere trickle compared to the North Atlantic, so we’re not here to study it, just to use it as a throughway. (It would be an equally long trip to travel around the tip of the country.) 

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Deploying tripods on the continental shelf

Dan explains the tk to CTD watch stander Shannon Rauch

WHOI scientist Daniel Torres explains the instruments on board the tripod to CTD watch stander Shannon Rauch.

The so-called “tripods” on board the Knorr look like lunar landers, or blobs of giant pollen, depending on how you tilt your head. They’re a kind of mooring: a package of instruments that sits on the ocean floor, measuring the water above and around it.

Deploying moorings, is the primary point of our trip. Tripods are moorings that go in shallow water — about 200 m. A weight in the middle of the tripod sinks the thing to the bottom of the ocean.

In two years, when Bob Pickart and his team return to this spot, a sound signal will release the tripod from the weight; those yellow floats will carry it up to the surface. Hopefully, they’ll be packed with data. 

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Brian Hogue prepares attaches a CTD to the rope that will float above the tripod

The Payload

An instrument called an Acoustic Doppler Current Profiler (ADCP) sends up sound signals to measure the velocity of currents flowing above it. Sound signals go out at a particular frequency, bounce off the zooplankton and other particles that travel along with the current, and then return to the ACDP back at a different frequency depending on the speed of the flow. This is called the doppler effect; it is the same concept that makes radar guns that police use to catch speeding drivers. From ADCP data, scientists will know how fast the water in the ocean above the tripod is moving. 

Every 15 minutes, the CTDs — one attached to the tripod, and one attached to the string above it — will collect conductivity (used to determine how salty the water is), temperature, and density data. They’ll take these stats every 15 minutes — in contrast to to the 32 measurements per second that the CTDs deployed on the rosette take. But these mooring order viagra australia measurements happen over two years, providing a record of what happens over the course of several seasons. 

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Resolutions

At all hours of the day and night, the team of 6 watch standers have been taking turns hauling a heavy piece of equipment into and out of the ocean. The Knorr stops, the watch standers on duty go through an elaborate ritual of deploying a “CTD rosette.” The instrument collects data on the water as it descends and then ascends — sometimes 3 kilometers deep. Its a process that can take hours. When its done, the Knorr moves to the next designated spot in the line of CTD stops, and the whole thing starts over again. Three days of that — 21 stops — provides a snapshot of a section of the ocean that looks like this: Data from three days of CTD

The varying color shows the varying salinity of the water. That purple color depicts fresh, cold, Arctic water, melted from icepacks North of us — with contributions from more local glaciers. The bright red: warm, salty, water, traveling up from the subtropics via the gulf-stream. Lower down, in green and yellow, is a third current. All this water is making its way around the tip of Greenland as “boundary currents” at the moment. It’s headed North West for a bit before they take a turn and go South. 

The CTD data is collected in between stops to place buy phentermine online moorings — long strings of instruments, shown in these pictures as lines and dots. These will take a snapshot of the water every two hours, and only take data where instruments (dots) have been placed.  In contrast, the CTD rosette collects data every few meters as the rosette makes its descent, collecting information along lots of points — which makes for a spatially high-resolution picture of the water.

To get an idea of what a mooring snapshot will look like, Bob Pickart modified the snapshot provided by the CTD rosette, making a map using only measurements that were taken approximately where the mooring instruments will be. That modified map, with reduced resolution, looks like this:

That data translated into what a mooring would see

That the picture provided by the “moorings” captures the important contours of the CTD rosette data is encouraging. “You see something like this and you can’t help but get very excited,” says Pickart. 

So, why use moorings at all?

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How do we measure currents?

By Nick Foukal

As our chief scientist Bob Pickart says, we’re out here on the ocean to build a “picket fence.” Practically though, the pickets in this fence – moorings – are needle-thin on the scale of the ocean. We only have about 40 “needles” (deployed by this leg of OSNAP and others) to span the entire North Atlantic and the flow we’re measuring is about 15 times stronger than all the rivers in the world combined. This is a tall task, so how do we do it?

Nick Foukal at work at the CTD watch station.

Nick Foukal at work at the CTD watch station.

The key is the ocean’s density, which is a function of the water’s temperature, salinity and pressure (pressure increases as one goes deeper). As luck would have it, many of the instruments we are deploying on this cruise measure those three variables. And from those data we can construct a map of density across the OSNAP line. 

For the most part, the densest water in the ocean is near the bottom. Heavy things sink: that much is fairly intuitive. But there are also areas where the density contours (also called isopycnals) are sloped, and these are locations where the flow tends to be strongest. Dense water wants to flow under less dense water, but due to the rotation of our Earth – we’re looking at a huge distance, almost 10% of the circumference of the planet – instead of that dense water flowing toward the less dense water, it often flows 90° to the right of where it should (in the southern hemisphere the water flows 90° to the left). This is what oceanographers call ‘geostrophic flow’, and most of the ocean obeys the rules of geostrophic flow (areas that don’t always obey geostrophy include coastal regions, the bottom and the very surface). It’s like attempting to push someone overboard and the person ending up on the deck next to you – not exactly intuitive. If that’s hard to understand, you’re not alone: as I enter my fourth year of graduate school in oceanography, I’m still trying to come to grips with this phenomenon… that is the geostrophy one, not the man overboard experiment – I’m pretty sure I know how that one would end up. 

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Night watch by the light of the supermoon

The Knorr never sleeps. So neither do the scientists on it — or at least not all at the same time. Between sunrise and sunset, two “watch standers” and one marine tech are engaged in a data-collection all-nighter.

Sunday was the fifth night shift of our cruise, the night of the full, close-to-Earth moon, and the night that I chose to sit in. Close to Greenland, we were cruising over the continental shelf: the water is a little shallower here, the currents more interesting, and the work of the watch standers faster-paced.

CTD

Preparing to drop the rosette into the ocean at the beginning of the night.

The primary job of the watch standers is to usher a heavy and expensive instrument — called a CTD rosette, for its shape — to the bottom of the ocean. When watch standers Robert Daniels and Mironov Illarion take over at 8 PM, the thing is about 1,000 meters underwater, and rising at 60 meters/minute. Every couple hundred meters it stops, they hit a button that says “fire bottle” on the screen, and it closes one of its grey tubes to add 10 liters of ocean water to its load. Then one of them will press “talk” on a  speaker box and tell an operator to bring the rosette up to a new height. Along its journey, sensors on the rosette measure the salinity, temperature, and density of the water: the “fingerprint” of a current. A lab tech, Elizabeth Bonk, will analyze the salt samples collected by the bottles.

This procedure — a CTD cast — is a staple of oceanography. Illarion and Daniels have both done the casts before — Illarion in the Arctic, and Daniels off the Bering straight. The CTD data they collect here will contribute to a high-resolution snapshot of what the current around Greenland is doing.

Creating this snapshot is, for now, an act of construction work. Illarion and Daniels have showed up to work in steel-toe boots; they add hard hats and orange life-vests to their safety ensemble. (Illarion is also wearing neon yellow overalls, his own sartorial contribution to the “safety” look.) When the rosette returns to the surface, we go outside, and stand on the deck. It is 50 degrees, plus wind chill. Nicholas Matthews, the marine tech on duty tells me to stand inside the garage or put on a hard hat — so he doesn’t get in trouble.

The rosette, on its wire line, emerges from the water. The watch standers hook the metal frame of the rosette with poles longer than they are tall. From a control room above, an operator recoils the wire, and retracts the metal beam that it is hooked on so the rosette swings onto the deck of the boat, and lowers onto a little wooden platform. The watch standers strap it in, and then, with the pull of a lever, the platform moves the rosette from the deck into a little garage, where I am standing. Beneath the light of a heat lamp, Illarion sprays the frame down with a fresh water hose. He and Daniels collect salt samples from each of the six bottles that they fired during the cast.

We go back inside, and then, maybe half an hour buy lasix online later return to the deck-garage to do the reverse: the watch standers move the rosette onto the deck, unstrap it, and the operator lifts it into the ocean. From the control room, we wait for this piece of machinery — which, for all the world, looks like Iron Man’s heart or a slice of a particle collider — to make its way to the bottom of the sea.

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