In case you aren't familiar with the term, "hydrography" is a term
meaning "measuring the properties of the ocean." "Repeat hydrography"
is going back to where you've been before and measuring the properties
again. The program that brought us out here is tasked with going back
and re-measuring some of the longest sections cutting across the oceans
once each decade.
There are several good reasons why we do this:
First, it is important to measure the ocean repeatedly because the
ocean is changing on just about every timescale we look on. For
example:
-There are several very long term changes occurring in Earth's climate
that we can monitor with repeat hydrography. Greenhouse gases emitted
by human activities are working like a planetary blanket, preventing the
Earth from shedding as much of the energy it gets from sunlight back
into space. About ~93% of the extra heat trapped on Earth by mankind's
greenhouse gases has gone into the oceans. The atmosphere is also
warming, and a warmer atmosphere can hold more water vapor, so the
evaporation of seawater is increasing globally. This means the dry
areas of the Earth will get drier and, since what goes up must come
down, the wet areas will get wetter too. Also, about a third of all
carbon dioxide released by mankind has been absorbed by the ocean.
Carbon dioxide reacts with water molecules in seawater to form carbonic
acid. The pH of the ocean is decreasing over time globally as a result
of this simple chemical reaction. This is called "ocean acidification"
by the scientific community. There are some free and surprisingly high
production quality videos discussing and presenting perspectives ocean
acidification on the web, so if you want to know more, look for "A Sea
Change" and "Acid Test."
-There are several multi-year long cycles in Earth's climate that
repeat hydrography can potentially shed light on. For example, many
people have heard of ENSO (a.k.a. El Nino), which is a cycle of heat
build-up and release in the equatorial Pacific that takes about 2-7
years. There are similar climate-scale oscillations being studied in
the North Atlantic, over Antarctica, and broadly over the Pacific
Ocean... As an aside, several of my coworkers at Princeton are using
computer models to investigate a potential ~50 year cycle in deep water
formation around Antarctica, though I am not holding my breath that
repeat hydrography will generate enough decades of data to say anything
about that while I'm still doing science!
-Of course, the ocean is very dynamic on short daily and seasonal
timescales, but other programs look at these changes since measuring
once a decade can't tell us much about such short fluctuations.
Repeat hydrography is one our main tools for examining long timescale
changes in the ocean. Satellites can see the ocean surface, and our
floating robotic floats do a good job of plumbing the top 1000-2000
meters of the ocean. However, the deep ocean holds about 16% out of the
93% of excess heat in the ocean, and it is picking up a portion of the
extra precipitation and glacial meltwater as well. Repeat hydrography
is the only way we can currently monitor the ocean depths. Repeat
hydrography is an even more critical tool for looking at uptake of
carbon by the ocean and ocean acidification because there are no
satellites that can measure sea surface carbon concentrations and the
technology for measuring carbon from floats is still in the prototype
stage.
Another reason to do repeat hydrography is because it is a necessary
piece of the two other open ocean measurement strategies we have
mentioned: floats and satellites. As we've discussed in past blog
entries, deploying floats is one of the central tasks for this cruise,
and the floats we are deploying can measure several seawater properties
(including pH) that are not typically measured by floats. Our early
results are suggesting that these sensors are doing a great job of
capturing the variability in the ocean, but that the sensors have large
offsets in their readings. Since we deployed the floats on a repeat
hydrographic cruise, we have the best available measurements to
calibrate the float sensor readings against, so now we can apply the
corrections that will allow us to believe the ~300 profiles the floats
will provide over their several-year lifespan. Our NASA group is doing
something similar for satellite data. Satellites provide data with
unmatched coverage. They can measure the ocean globally every couple of
days, but with the limitation that the measurements only tell us about
the very surface of the ocean. Our NASA team is working to improve the
amount of information we can learn from a satellite that measures ocean
color. Scientists use the color measurement to estimate how much
chlorophyll there is in seawater. However, the critters that use
chlorophyll to make sugars from sunlight aren't always at the very
surface of the ocean, so the satellite doesn't always see them, or see
all of them. In addition, there are a number of colored chemical
compounds in seawater other than chlorophyll that can complicate the
satellite readings. Our team of NASA researchers have hitched a ride on
this repeat hydrography cruise to go out and directly measure the color
of seawater with depth, and determine exactly what gives the seawater
its color. They will then compare how the satellite measurements agree
or disagree with what they measured in this region, and use this to
improve the algorithms that estimate chlorophyll from seawater color.
Repeat hydrographic research cruises are also excellent platforms for
new measurements, prototyping measurement techniques, and collecting
samples for other research programs. Two thirds of my PhD research
relied on data collected on hydrographic cruises, and the remaining
third detailed a measurement system I developed and tested while on a
repeat hydrographic cruise. Also, right now I am working with a lab in
Princeton and in Zurich to collect seawater for nitrate and silicate
isotope measurements that these other labs will ultimately run.
Okay, I'll stop extolling the virtues of repeat hydrography now. Next
week I'll try to get to that update I promised about why I'm so excited
to be in the Southern Ocean... which will be odd because around then we
will no longer be in the Southern Ocean! As far as an update on the
progress of the cruise, we've managed to continue to get work done
intermittently despite being pummeled by an endless series of low
pressure systems. We were able to sneak in station work around the
worst of the winds and waves in large part due to Chief Scientist Lynne
Talley's careful planning and hours of staring at weather projections
with Captain DeSouza. The drama continues even now with 45 knot winds
raging outside, but we are almost out of the be-stormed latitudes.
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