Research Fellows Directory
Dr Helen Johnson
University of Oxford
My research is focused on understanding the Atlantic ocean’s overturning circulation, its connection to the high-latitude forcing regions, and its response to natural and human induced change.
The overturning circulation (or “thermohaline circulation”) carries huge amounts of heat northwards in the Atlantic, most of which is released to the atmosphere at mid latitudes, influencing the atmospheric storm track that determines the weather over western Europe. As a result, the overturning circulation has a significant impact on climate, keeping temperatures here warmer than the average for their latitude. It also has profound implications for African rainfall, as well as the potential to influence hurricane statistics, via its effect on tropical and subtropical sea surface temperatures. It is an important part of the carbon cycle, naturally sequestering carbon in the deep ocean when the cold water formed at high latitudes sinks.
Despite its importance, we do not fully understand what determines the average strength of the overturning circulation, its variability, or its adjustment to change. Evidence from palaeoclimate records suggests that it has been radically different in the past, and state-of-the-art numerical climate models predict that, as a result of global warming, it may weaken significantly in the future, perhaps over the course of just a few decades.
I work to improve our understanding of the overturning circulation, combining computer models of a range of complexities with fluid dynamics theory, and with observations collected from ships and ocean moorings. Understanding the dynamics of high latitude regions is a key piece of the puzzle. A thorough physical understanding of the links between the high-latitude oceans and the overturning circulation will be essential if we are to design an effective ocean monitoring system and predict the response of the ocean circulation to global warming, and the resulting impact on climate.