My Research: Helen Burns
Hi, I’m Helen and here I’m writing to tell you about my research. My work focuses on understanding the behaviour of the Southern Ocean. This ocean around Antarctica is a complex place; no where else in the world can an ocean current circumnavigate the globe, uninterrupted by the continents. The Southern Ocean allows all the worlds other oceans to communicate and this leads to complex physics that we’re still trying to understand. Before I go into that I will explain why studying the southern ocean matters so much.
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).