The ocean is very complex and understanding the whole ocean to predict how it will respond to changing climate is very difficult. Yet this is what we must do in order to forecast hurricanes, monsoons, ice drift , sea level rise, carbon fluxes and our future climate. This is an enormous task.

We use general circulation models (GCM’s) that have to be run on a supercomputer. They work by splitting the ocean up into millions of little boxes and for each box, the computer solves some fundamental physics equations to decide what the box output into the next box. A small time step is used and the calculations repeated over and over. The smaller the boxes are the more detail can be worked out, however the smaller the boxes ,the more boxes you need. That means more calculations and more computing power is required. For example, the 1/12 degree NEMO model (an ocean GCM that has boxes that 1/12 degree of latitude and longitude by 10-100 m thick) is run on the UK supercomputer Archer using 100000s of high-performance computer cores. To put that in perspective an average laptop has 2 – 4 average performance computer cores. These models have up to half a million lines of code written by teams of scientists. Here’s an example of what the end result is for that NEMO 1/12 degree run:

These ocean models are used in a number of ways. Sometimes they are combined with atmospheric models and fed with real data to give short-term weather forecasts. Other times they are combined with atmospheric, bio-geochemical and land models and run for long periods of time with changing CO2 concentrations. These models aren’t perfect, though, we can’t just keep making the boxes smaller and smaller otherwise, we’d need to make the computers bigger and bigger. For things like climate prediction 100s of models from all over the world are used to give the probability of different outcomes.

So what about the missing detail in those not small enough boxes. By focusing on one aspect of a problem on a smaller scale, we can begin to work out the large-scale effects of some small scale processes. People like myself run small models on just 10-100s of cores that try to work out how that part of the ocean works and how it would respond to various changes and what effect on the large-scale ocean. This is called a parameterization.

One last thing. Why I titled this, the treacle ocean? That’s because in order to model the ocean we must increase its viscosity to that of treacle so when you think of ocean models. They are actually treacle oceans!

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The ocean have a major influence on our climate. The amount of heat stored in the oceans is around 1000 times greater that the atmosphere (IPCC 2007). The oceans transport heat around the world, for example the Gulf Stream brings warm waters north westwards to the UK, helping keep our temperatures warmer than other places at a similar latitude (like parts of Canada). Past climate records suggest that large changes in how water circulates through the oceans have lead to drastic changes in the climate. The ocean has what is called an overturning circulation which you can think of as a big conveyor belt.

Cold salty water sinks at high latitudes flowing through the deep oceans and gradually warming so that it rises to the surface and flows towards the poles again (this can take up to 1000 years). A warming climate increases the ocean temperature and decreases salinity though melting icecaps and glaciers This can lead to a slow down or even a “switching off” of this global overturning circulation.

The Southern Ocean is an area where that deep water can upwell to the surface by the action of strong polar winds. We’re interested in how much this drives the global circulation. Can you force the ocean conveyor belt with a pull (deep water upwelling in the Southern Ocean) rather than a push (cold salty water sinking)? Realistically there is a balance between the two. To try to understand this we use very powerful supercomputers to run complex ocean models that we call General Circulation Models (gcms). These models essentially split the ocean up into 1000s of tiny boxes and calculate a set of physical equations that describe regular changes in the oceans. But supercomputers are expensive so to reduce running time we often try to use larger boxes and then try and add in corrections that account for the smaller scale stuff that it going to be missed by using the larger boxes. In the Southern Ocean this can be a little tricky as by changing the resolution you can a vastly different circulation!

This is due to features called eddies, they’re like ocean storms and occur on scales of 10-100km. For this reason often we run regional models of just the southern ocean at higher resolution to keep costs down, but at a higher resolution (smaller grid boxes) so we can capture these eddies.

Over the past few decades an increase in the winds over the southern ocean has been observed (we think this is due to the ozone layer hole over Antarctica). Many studies have focused on how that would affect the Southern Ocean circulation. Using regional models (like my one of just the southern ocean) rely on representing what’s going on the rest of the ocean too because the behaviour the model would not be realistic if you pretend that the northern edge of the southern ocean is just a wall. Little work has been done in investigating to what extent this will affect the outcome of the model runs. That’s what I’m focusing on. I’ve gone back to basics and set up a model that is very simple to capture the essential physics of what’s going on. This tests the concept of changing the overturning circulation with only changing how my northern boundary of the model behaves. It turns out it makes quite a difference click here for a visualisation of the temperature field for a model run with an open northern condition (Process in the rest of ocean represented) and here for a model run with a closed wall at the northern boundary. The main process I’m representing the sinking of the cold salty water at high latitudes.

Figure illustrating the circulation seen in model runs (section from south to north). Red denotes a clockwise circulation while blue denotes a counter clockwise circulation. Left: open northern boundary (Lots of sinking at high latitudes represented) . Right: Closed wall (most of the circulation has disappeared leaving only an intense surface clockwise circulation).

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