Friday, July 1, 2011

Just below the surface

- Graig Sutherland [NUI Galway]

By now you, our faithful blog readers, have been introduced to marine aerosols, eddy correlation fluxes, and gas transfer rates and now I’m going to tell you a little bit about why I’m here. My name is Graig Sutherland and I’m a PhD candidate from the National University of Ireland, Galway (NUIG) studying the physical oceanography in the upper ocean. Surprisingly the upper ocean, even though it’s the most accessible part of the ocean, is one of the most under-sampled regions in physical oceanography. I’m interested in the physics of the upper ocean and how this interacts with the atmosphere. To measure this we use an autonomous vertical profiler ASIP (Air-Sea Interaction Profiler), which was developed by my supervisor Dr. Brian Ward (now at NUIG). ASIP is an ideal instrument for upper ocean physics in that it profiles upwards, making measurements at sub centimetre scales, through the water column to the water surface. ASIP measures temperature, salinity (inferred from the conductivity of seawater), the dissipation due to turbulence (inferred from the small-scale shear), water velocity, ambient sound, and solar radiation coming through the water surface. These measurements can be related to effects from the atmosphere (such as wind, gas transfer, etc.) and vice versa (i.e. effects from the atmosphere can be related to physical aspects of the upper ocean).
The high precision sensors fitted on ASIP are extremely fragile and offer some challenges in how we get ASIP into the water without breaking everything. This is no small feat as ASIP is 2.5 metres long and weighs in at 100 kilograms (that’s 8 feet 4 inches and 220 lbs respectively). To achieve this we need to go out in a small boat and gently place ASIP into the water. The first photo (courtesy of Phil Bresnahan) shows ASIP (with the yellow top) getting ready to go to sea. This is usually a good time (as long as nothing breaks!) and there is never a shortage of volunteers to go out for a little scientific expedition on a small boat. Once in the water ASIP is all on it’s own to make measurements. It descends through the water with the work of three thrusters which pull it down to a pre-programmed depth at which point it freely rises to the surface. Once ASIP reaches the surface it tells us it’s location via iridium satellite so we can track it until it’s time to go out and recover it.
With just over two weeks to go there is ample opportunity to investigate air-sea interactions and to keep looking at what is going on just beneath the surface.

Oh, the beauty of a CTD cast!

- Cristina Schultz [National Space Institute, Brazil] (figures by Ryan Kennedy)

I believe all oceanographic research cruises must count on its magic to keep up the good work. It consists of sensors of conductivity, needed for the calculation of salinity, temperature, depth (hence the name CTD), and any other sensors able to help with the understanding of the vertical structure of the ocean beneath the ship. Here we are also measuring oxygen concentration and fluorescence (which helps to see where the phytoplankton are). Attached to the CTD are 24 bottles to collect water samples at different depths, and the combination of the bottles with the sensors is what we know as the Rosette system.

At sea it is easier to picture many of the phenomena that we so much study while in land: the waves, wind-forcing the water in many directions, temperature gradients forcing heat transfer between air and sea. But even when we are so used to it that the days go by following the motion of the waves instead of the clock, what lies under our feet would still be a mystery if it weren’t for the CTD.

The understanding of the big picture is a task for the CTD. Which water masses are swimming between the surface and the sea floor can easily be identified by its sensors, and the variation of these water masses is important for air-sea interaction. All that motion going on right below our feet is available for us in real-time through a computer monitor as the CTD swims down into the cold waters, rapidly increasing in pressure, to give us such valuable data.

It is surely not the only way to measure vertical structure (as you will see on Graig's post, coming soon), but it is used as a guide for the other measurements going on at the cruise. Once the CTD has explored the water column’s gradients we can evaluate how certain properties should behave and at what depth other measurements can get what they are looking for.

On our cruise we are not torturing the CTD with hours of cold water and intense pressure until it almost reaches the bottom. We are doing casts only to depths of 100 meters (length of an American football field). By profiling to this depth we can see what we call the 'mixed layer': a layer of water where temperature and salinity (from which we can get density values) are quite homogeneous. This layer’s characteristics can be determined by temperature gradients between air and sea, and wind forcing. In the fluorescence signal, we see how the attenuation of light by the water makes it hard for phytoplankton to synthesize chlorophyll. Oxygen (and also CO2) is affected by the values of salinity, which “competes” for space in the water, temperature (the colder, the more gas can be dissolved) and by phytoplankton respiration and photosynthesis. So yep! A lot of processes are seen with much excitement on the screen!

Today, as we moved north from where we were yesterday, the mixed layer got a little bit deeper (it is still around 20 meters), but the phytoplankton, that had been hiding below it, probably protecting themselves from too much turbulence or light, have approached the surface by inhabiting the mixed layer.

We do daily casts at 1 pm, and a cast every 6 hours when on station. So, on 24 hours of station we can get 5 casts, and that helps to see how heat transfer and wind can change the mixed layer on a daily basis at the given latitude.

Interesting changes perceived by our casts will be reported to the blog as soon as we see them!


- Brett Stacy [Humboldt State University]

When one imagines the open ocean, they may picture it as a constantly dynamic and often ferocious landscape. With huge crashing waves, high speed winds, and ship-dwarfing storms. Even looking at the background to this blog gives a viewer that impression. One of the many goals of this cruise is to document one of these features of the ocean, moreover the effect of one of these features on the state of the sea surface: The wind and the white caps they induce.

White caps hold special significance to our world. Besides being a surfer’s worst enemy and a kite-surfer’s best friend, white caps have a profound influence on, you guessed it, climate change. They contribute to gas exchange between the ocean and atmosphere, they affect the reflectivity of solar radiation reaching the earth’s surface, and they serve as a visual byproduct of the transfer of energy from wind to ocean surface currents.

Throughout this cruise, my job is to quantify the proportion of white caps covering the sea surface in order to later relate their influence on relative flux measurements between and DMS made by others on the ship. How do I do this? Using state of the art cameras mounted on the highest point of the ship, images of the sea surface and the white caps on it are automatically collected once every second during the daylight period. These images are then processed to discover what proportion of their contents include white caps. The process includes converting the image to black and white, with a threshold for the color white determined by observation of the color characteristically exclusive to white caps. The final result is a White Cap Fraction (WCF) for each image.

Being the ocean’s personal photographer has been a blast. Participating on the Knorr in general has been great. This cruise is my first time on a significant (more than one day) research voyage and so far it’s been an adventure. Every day something interesting happens and new challenges arise bringing everyone on the ship closer together. It has only been one week on the Knorr since leaving Woods Hole, but already it feels like we are all part of a big family. Every ones’ positivity and enthusiasm makes life at sea fun and easy-going. I can’t wait to see how our family develops and what exciting events are in store for us at higher latitudes!