Tuesday, March 7, 2017

Are we there yet?

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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