We’ve shifted over to this blog on the Oxford Physics website, which allows us to have a more complete multi-media experience…
We woke this morning having left the English Channel and are now steaming across the Celtic Sea. The wind picked up overnight giving much greater swell and an early test for the team’s sea legs.
I have been thinking a lot about why we are here today, probably because the wind picked up and the swell left me feeling nauseous all day – “what am I doing here??”. To understand this, we first need a bit of background about the OSMOSIS project.
What does OSMOSIS stand for?
The project name stands for “Ocean Surface Mixing – Ocean Submesoscale Interaction Study”. This explains why we simply refer to it as OSMOSIS. Although it is not related to the chemical process of osmosis, it investigates a mixing process which happens in the upper ocean.
Surface mixing is important in the Earth’s climate system as it is where the ocean and atmosphere communicate with each other. Mixing in this area can move properties of the atmosphere down into the ocean – or properties of the ocean up into the atmosphere.
An important example of this is heat. For example, a large storm at sea might mix colder water from depth up into the surface ocean, leading to a cooler surface layer. This colder water can then chill the air above it, leading to cooler air temperatures. Another important example is CO2. In many places surface layer water has a high concentration of CO2 which it has absorbed from the atmosphere. If this surface layer water is mixed downwards it may be replaced by water which has a low concentration of CO2. This water with low concentration can then absorb more CO2 from the atmosphere, reducing the amount available to cause the greenhouse effect. However, some speculate that this mixing process has gone into reverse in other periods of Earth’s climate and so increased the greenhouse effect.
What are ‘submesoscales’ then?
The flows in the ocean have different sizes or ‘scales’. These scales include the ‘basin-scale’ which refers to currents that extend over thousands of kilometres of an ocean basin such as the Atlantic or Indian Oceans. They also include small-scale processes such as the waves you see on the surface.
In between these two scales lies the ‘mesoscale’ where the word ‘meso’ is derived from the Greek word for the middle. The mesoscale refers to the ocean eddies with a horizontal length of 50 – 150 km. The mesoscale is thought to be the most important in controlling many aspects of ocean flows and is responsible for the swirling motions in this animation from NASA.
Submesoscale then refers to the next largest size of motions, so they are ‘sub-meso’. They can be thought of as ‘eddies-on-eddies’. Oceanographers think they may be important in linking the very small scale processes with the mesoscale. Historically, the submesoscale has not been well studied because studies have tended to focus on either large or small scale features or the. The OSMOSIS project has been designed to investigate this gap in our knowledge by deploying an array in the North Atlantic which we will describe in more detail in a later post.
As for me, I eventually found my sea legs today. Once that happened it was magnificent to be out on deck with the ship riding the 5 metre waves. As we crested one wave the surface seemed as far below as it would from the deck of a ferry, but seconds later, sinking into that trough, the next wave loomed so large that we had to look upwards to its breakers. And then we crest that wave only for the next valley to reveal a lone gannet alongside skimming across the waves with its wing tips almost touching the breakers.
The crew for the first leg of the OSMOSIS cruise has gathered in Southampton to begin mobilising for the cruise. While the people have travelled from as far away as the west coast of the US, the instruments to be deployed have travelled even further. Much of the mooring equipment spent the last two years being battered by the intense conditions of the Southern Ocean, while the gliders were last seen in the Weddell Sea off the Antarctic coast.
The ship is docked in front of the National Oceanography Centre. This is a blessing for the staff of the National Marine Facilities team, that has to load so much cable for the moorings that some of the science crew wondered that the ship even remained afloat (though the hundreds of buoys on board should at least give some respite in such a journey to the bottom).
While most of the science crew concentrates on storing their instruments to be reassembled on site, the glider team of Andy and Gillian have their iRobot gliders out on deck. The price of using such advanced instruments is that they are far from the plug-and-play of consumer electronics. Getting them running involves sitting next to them on the deck typing furiously at laptops and beseeching them to speak to satellites while the crew steps over and around them as they roll kilometre after kilometre of cable down into the hold.
The first step in setting up the gliders is turning them on, which requires waving a device called a ‘wand’ ( also known as a magnet at the end of a plastic stick) near a certain part of the device. This interacts with a magnetic field inside the glider which switches it on. It is a process which remains magical to the author, a mathematician-turned-fluid dynamicist for whom electromagentics remains a mysterious force.
Over the course of the day the remainder of the science team arrives. Once convened we begin to tackle the question of how to deploy the gliders to get the most science possible from their battery life. It is a difficult point – with such new technology and an extreme environment no one can say for sure how long they will last. This discussion will continue.
The ship is due to slip away from the docks at 8.30 a.m. Tuesday morning (August 28th). There will then follow a few days of transit to the site, days of busy prep work so that we will be ready to unspool those kilometres of cable and get our instruments in the water and start observing what is happening underneath those waves.