After steaming briskly for three days out of Honolulu we
have finally reached a potential measurement spot. How do we know when we’ve
arrived? We know the phenomenon we want to look at: a sharp temperature front
where the surface temperature changes rapidly in the horizontal direction[1].
These fronts can lead to dramatic shifts in density horizontally, which in turn
contribute to restratifying[2] a
previously well-mixed ocean. In other words, the site of these dramatic
temperature changes indicates a location where something exciting might happen
dynamically and better understanding these processes is the primary goal of
this cruise. The North Pacific Subtropical Front spans a broad longitudinal
range, and we know roughly where it will be, but the ocean is always moving and
changing, so pinpointing the exact spot where it will be worth making extensive
measurements poses a significant challenge at the outset. Deploying an array of
10+ floats is no trivial matter, so it is important to make sure that we put
them in the right spot.
 |
| Animation of temperature near the front. Yellow/orange indicate warmer temperatures, green/blue are colder. |
Fortunately, we have a couple of tools that help us figure
out where to look. The first of these are satellite mounted sensors which allow
us to capture a regular snapshot of the surface of the ocean. For this mission,
we have been looking at data from two types of sensors: a passive microwave
sensor that enables us to get a general (low-resolution) picture even when
there is cloud coverage, and an infrared sensor that produces much higher
resolution images but only works when the region is cloud free. The first picture
here shows a false-color image animating the recent movement of the front. As
you can see, it is very dynamic, so a feature that was there two days ago may
not be there now. (A false-color image is so called because it assigns a color
value to a measurement like temperature that does not have an inherent visible
color. We use them a lot to visualize measurements because they make patterns
stand out in way that a screenful of numbers might not. If you were actually to
look at the ocean around us right now, you wouldn’t see this nice assortment of
yellows and greens). This type of image allowed us to select a preliminary
location to start measuring, but it is unlikely that the front will be in
exactly the same position.
Once we arrived “on site”, we deployed the second scouting
tool: our SWIMS towed body, which is a torpedo-shaped instrument attached to a
cable that cycles up and down between roughly 10 meters and 200 meters depth as the
ship moves, gathering salinity, temperature, pressure, oxygen, and fluorescence
data at a rate of 24 Hz (that’s 24 measurements per second, which provides
outstanding resolution in the data). Once deployed, we started along a
cruise-track suggested by the recent state of the front, and have been
attempting to drive back and forth across the front to pinpoint a location that
will lead to interesting measurements. We will also use SWIMS once we’ve
deployed the float array to gather data extensively between the floats and capture
a detailed 3D map of the frontal region.
We’ve already noted some interesting features, and
the midnight-to-noon shift has been nerding out about salinity plumes and
massive temperature shifts, so it promises to be an exciting few days. We’ll be
spending the next few blog posts talking about some of our other instruments to
paint a more complete picture of the type of data we’re gathering.
[1]
This is similar to an atmospheric front, which we hear a lot about in relation
to weather. When meteorologists talk about a cold front, we know to expect
a sudden drop in the temperature in a relatively short span.
[2]
From top to bottom, the temperature, salinity, and density of the water can
change enormously. This is what we mean when we talk about stratification. In
looking at a profile of temperature (for example) we will often see regions of
low stratification (temperature remains relatively constant) and regions of
high stratification (temperature changes dramatically in only a few meters). A
storm might mix the surface waters thoroughly, leading to low stratification.
The dramatic density differences across a front can subsequently lead to
restratification, with lower density waters sliding over high density waters. This
process is incredibly complex.
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