I'm Jenny Hutchings. The ice is drifting, it's cracking, breaking, sharing along cracks, diverging and converging. This spatial change in the motion where its coming together, closing, sharing, opening, we use the term kinematics to describe that motion. Other words that are useful is dynamics is the study of forces involving the moving ice. Study of forces, and mechanics is the study of the ice response to the forcing. So at the ice camp, at MOSAiC, we are going to be monitoring the kinematics of the ice motion, and we'll be trying to determine the forces involved in moving the ice. So we really need to think about the force balance on a piece of ice, and let's go back to a camp in the middle of the Arctic Ocean, and think about what this force balance is, and how it might actually be acting on the ice. So we already know that wind is effect to when that wind is applying, a stress across the surface of the ice. We also have an ocean stress that's applied on the base of the ice, and that can be thought of as a drag that opposes the wind stress in most cases, unless there's currents acting. We can think about all the forces that are acting on this piece of ice, and we can draw a diagram that represents them. Here is an example of a force balance on a piece of ice that was measured in the 1970s during the Ajax feel campaign and Kenneth Hunkins spent many days out on the ice camp, and found a time period when the wind forcing was consistent, and over a period of 60 hours, he was able to estimate that this was the balance of forces on that piece of ice. Let's walk through it so we can see how we put together as force-balance. Some things to remember is for every action, there's an equal, and opposite reaction, Newton's laws. So if there's a force in one direction, you have to have a force opposing it in the other direction, and all of the forces are going to be in balanced, if you're in equilibrium and sum to zero. Let's start with the wind stress transfer into the ice. So we can measure this in a sense, and estimates it as wind stress across that piece of ice, and the ice is moving at an angle relative to that wind stress. That angle has been found to be fairly consistent across the entire ocean. Is always at an angle to the right of the wind stress if you're moving in free drift, which means if the ice is not interacting with itself. Okay, we can also measure the water stress below the ice. So the water stress is roughly opposing the wind stress, but not quite. If we think about the next force that we can estimate, this is the Coriolis force. It is due to the rotation of the earth. So as the ice is moving, the earth is rotating underneath it. The rotation rate of the Earth increases the closer towards zero latitude you are, and this causes the Coriolis force to become larger closer to the zero latitude. So the Coriolis effect, is to turn an object in motion to the right of the force that is causing it to be in motion in the northern hemisphere. In the southern hemisphere, it is acting in the opposite direction. Notice how the ice velocity is at 90 degrees to the Coriolis force. So the Coriolis force has to be in an orthogonal direction to the drift of the ice. We have another term that's related to gravity. So Ken Hunkins realized that there was a tilt in the sea surface. The surface of the ocean tends to mound up as the ocean is rotating, and forms a dome in the Beaufort Gyre, and that dome of water has some topography associated with it, and the ice can actually move down the slope of that sea surface. So that's a gravitational term. That's the ice moving under its own weight down the surface of the sea. So he added in that term, the gravity term, the resultant of all those forces. That red line there tells him that all of those forces didn't balance to zero. They gave that red vector there. So there needs to be a force that is opposing the direction of that red vector of the same magnitude. This force is the internal ice interaction, which is represented by the ice stress. Essentially, the ice is moving against each other, ridging, cracking, breaking, bridging, and as it does that, It's dissipating energy in the form of friction, and there's a force associated with that motion of the ice against itself, and we can estimate that, and estimate what the stress field across the whole ice pack is. So finally, we have this force balance diagram showing all these forces that are driving an ice velocity, in a particular direction. So if we measure the ice velocity estimate as mass, can measure the water drag and air drag, and we know where on the planet we are. So we can determine the Coriolis parameter, which varies with latitude, and we know the gradient of the sea surface. We can build a force balance on the ice. So we will be attempting to measure the force balance at MOSAiC. In actual facts, what we'll be looking at is how stress is transferred from the atmosphere, and the ocean into the ice pack itself. We have various sets of equipment to do this. This is an example of a atmospheric flux tower that was deployed in October by Matt Shoop and Chris Cox, and is measuring very high frequency fluctuations, and eddies, and the atmospheric motion. If we couple this with an understanding of surface roughness, we can identify how that wind stress is transferred into the ice. We believe that this wind stress transfer into the ocean will be different over younger, more dynamic ice that will have a different roughness characteristic than over the older ice that we've measured it over before, and will be doing the same in the ocean. So this picture shows Tim Stanton holding the mass that he's placing under the ice where he'll make very detailed measurements occurrence, and these together with information about the temperature and salinity of the ocean. You can run a similar experiment to understand how momentum is being transferred from the ocean into the ice, and we're going to be looking at how the ice pack itself is evolving in time. So we have some understanding of how the mass, and the roughness of the ice is changing. Imagine we have an object, any object, in this case a square, is a rigid object. If we apply a force against that object, what, how does it react? So for simplest example, we push on the square, and the object will respond. A block of ice, a square of ice starts to move in the absence of friction or drag, it will keep moving in a straight line with a constant speed given by the acceleration from rest that the force imposes. Now we can imagine that that ice moves against a solid object that's immovable. The object, the square becomes constrained. It will react with a force that is equal, and opposite to the applied force pushing the ice against the wall or itself causes in reality, causes the ice to break into form. Some of the energy is dissipated in this deformation, and the force balances adjusted to v between the driving force, and the response of the ice, with the ice pack maintaining an internal stress field in reaction to the applied force, and the frictional forces in defamation of the ice. So in MOSAiC, we hope to investigate the nature of this ice stress. We're going to look at some questions like, how is the stress distributed within the ice sheets itself? Where does the stress accumulate allowing ice to crack? and how does the nature of the deformation change over the year? So how does the stress field in the eyes change over the year? We will collect a data-set with which models of sea ice defamation, and force balance can be tested.
