Saturday, April 25, 2015

Clivarians from Scientifico arrive at the Equator

And just like that - it's Spring. We have moved from Fall in the Southern Hemisphere to Spring in the Northern Hemisphere, skipping winter entirely.  Yesterday we, the Clivarians of P16 N arrived at the Equator.  Bruce Cowden is our chief bosen and he is a very creative person.  He began calling us the "Clivarians from the island Scientifico" as we are scientists working on a CLIVAR - related project.  So be it.  It grew from there. He had a vision of what the Clivarians were like and that vision emerged on the page through a combination of ink and water colors, as cartoons of each the Clivarians (see pics below).

The "Clivarians from Scientifico" through the eyes of Bruce Cowden
Recovery on deck of the CTD at our Equator station!

Yesterday, the Clivarians sampled right at 0 degrees, thanks to the skillful positioning of the crew, despite the strong equatorial currents raging beneath us. Check out the snapshot of the ship's ADCP from our station on the equator.  The strong surface current dominates the upper 200 meters of the water column.  Below that, the currents reverse direction and continue to flow for another 150 meters or so.  To put the numbers from that figure in perspective, the Gulf Stream flows at ~2 m/s - so these equatorial currents are right up there!
ADCP surface currents from the equator station courtesy of the Ron Brown.

Our CTD watchstanders, Annie Foppert and Alex Sanchez Franks, deployed an Argo float at this station, as well.  It emailed us that it is working too! The Argo program is a system of free-floating drifters that collect data across the world's oceans and remotely transfer the data back to shore.

Mark, Alex, and Annie deploying our Equatorial Argo float. 

There were many polywogs  or "wogs" on board (the name for someone who has yet to cross the equator), so there was much excitement about our equatorial crossing.  We took in the sights and marveled at the sea and even took a group photo. We couldn't have done what we have so far without the help of the crew.  So crew, you are Clivarians now - and thanks!

Clivarians at the equator.  Photo courtesy of Avery Edson.

 When asked about how it felt to be on the equator, the chief scientist, Jessica Cross, responded "51 stations down, 60 more to go."  Right you are, Jessica, right you are. Onward!

Tuesday, April 21, 2015

One of the most striking features that has developed in our observations since Tahiti is the shallow (~500-1000 m) lens of low oxygen water that is decreasing in concentration as we move toward the equator (see figure below).  Around 11 deg S, the water began to plummet from 100 umol/kg to nearly 10 umol/kg at times around 6 deg S.  This feature coincides with a similar low oxygen feature from this line in 2006, but it is lower in concentration, in general, than in 2006.  Why does it change from high to low oxygen, in that depth range, at 11 deg S?  Why would oxygen be lower now than in 2006?

To answer that, we have to think about what water masses are influencing us in the South Pacific.  Antarctic Intermediate Water (AAIW) is an influential water mass in the South Pacific. AAIW can be found as far north as 10-20 degrees North, but in the equatorial regions, other water masses are influential, as well, including the North Pacific Intermediate Water (NPIW) and Equatorial Pacific Intermediate Water (EqPIW).  AAIW lies above deep water but below the tropical thermocline, has been characteristically identified as a salinity minimum (34-34.5 psu) north of the Antarctic Circumpolar Current. AAIW forms in the southeast Pacific and spreads northward toward the equator bringing with it highly oxygenated waters. Prior work has observed freshening, shoaling and warming of AAIW over the past several decades. These other water masses have properties as well - for example, EqPIW is very low in oxygen and a bit saltier than AAIW (34.5-35).  EqPIW has contributions from AAIW, Pacific Deep Water, and is advected along the equator from the East Pacific toward the West Pacific.  EqPIW brings with it low oxygen waters formed in the Oxygen Minimum Zone off the coast of Peru.

AAIW can be seen in the P16N leg 1 section plot of salinity along our cruise track so far between 500-1000 m.  Its presence can be seen through station 11 (11.5 deg S) in that depth range in both oxygen and salinity.  AAIW is typically high in oxygen, so its absence is part of the reason for the oxygen decline.  This is similar to the prior occupation in 2006.  The low oxygen water interlaced with the AAIW is most likely EqPIW in some combination with NPIW - both low in oxygen.  The oxygen is low in those water masses because it has a respiration signal which accumulated over the water masses' path as it collected falling organic-rich particles - both water masses are "older".  Older water also means it has been a long time since those water masses have been at the surface, and thus, ventilated by the atmosphere.

Because the water is warming and becoming more stratified, we would expect the already low oxygen waters to become lower in oxygen in the warming world for two reasons. First, warm water holds less gas.  Second, increased stratification will reduce the rate (flux) at which oxygen can diffuse into the low oxygen water mass from the higher oxygen water masses lying above and below the oxygen minimum.

These results are all preliminary, but exciting because the changes in oxygen are consistent with our expectations for how the system changes in a warming world.  Whether or not they are attributed to anthropogenic changes or natural variability will be a question for the scientific community to determine in the coming years.
Oxygen cross section (depth in meters vs latitude) along our cruise track for P16N (2015).  The white contours show the salinity along the same cruise track and the black dots identify the bottles sampling the water column along the way.  The depth axis has been stretched in the region of interest in order to highlight the features blogged about here. 

Friday, April 17, 2015

“The Green Flash”

You never really know what you’re going to see or experience in the world until it happens.  
One morning recently, while sampling in the CTD bay of the Ron Brown, the night watch science crew experienced the green flash in an unexpected way.  We weren’t looking for it, but we were in the right place at the right time, and experienced it. What a wondrous surprise.
There have been lots of posts at sea about the green flash – that fleeting moment when the sun's light appears green along the horizon for an instant when you have a very clear sky without many scatterers – what made this experience blog-able is that we were flashed at sunrise, when we least expected it.  While sampling from the niskin bottles filled with ocean water from more than 4000 meters below where we were standing, the weary group all cried out together in amazement various phrases of surprise, awe, and wonder– “Woo hoo!  The green flash!”, “Did you see that?!” , “Ooooh!”. 

We went on to regale our friends, colleagues, and fellow crew over breakfast, only to be met with disbelief and ridicule because, while physically possible, many of the people on this boat had logged many, many years at sea and never seen it at sunrise. It was only that we experienced it together which made it credible at all. 

So as we start this cruise, I am hoping that we have calm seas, great data, and many wondrous surprises. This cruise is a repeat hydrography section – we have been on this track before and sampled these waters before.  We even have expectations set in our minds about what we should find now after nearly 10 years between sampling this line.  So in addition to the solidification of our understanding that repeatability or predictability will provide, I also look forward to the wondrous surprises that await us. I will be blogging about them, when we do.


Thursday, April 16, 2015

Hello everyone,

Welcome to our cruise blog! This mission is part of the US GO-SHIP Repeat Hydrography Program. GO-SHIP brings together an international network of scientists interested in the physics, chemistry, and biology of the interior oceans to contribute to the global ocean and climate observing system. Through this program, we occupy a worldwide grid of hydrographic sections on approximately a decadal timescale, in an effort to study changes in physical oceanography, the carbon cycle, and marine biogeochemistry and ecosystems.

This mission will occupy the P16N section along the 152 meridian in the Pacific Ocean between Tahiti and Kodiak, Alaska. The last time that this section was occupied was in 2006, when a decadal change in the ocean carbon content was noted for the North Pacific (Byrne et al., 2010), based on another previous occupation of this line in 1991. This was the first basin-wide observation of ocean acidification. On this occupation, we are paying careful attention to the corresponding chemistry, monitoring the changes in these and other related carbon data.

Throughout our mission, we will periodically be posting here about the specific projects and scientists we have on board, representing 20 institutions, and general ship-board life. We are particularly encouraging our young sailors and students to share their experiences with you. You’ll also hear regularly from me, our mission’s chief scientist, and Samantha Siedlecki, our co-chief scientist as we report on our progress.

To get us started, we have had the best of weather that the sub-tropics can offer, complete with calm swells, sunny days, and starry nights. It makes it easy to look forward to the rest of the trip.

Fair winds and following seas,