Attribution of climate change in the subtropical North Atlantic Ocean: exploiting tritium-helium data and adjoint sensitivity analyses

There is uncertainity in how the North Atlantic operates in the climate system in terms of its heat content and biological export of organic matter, which ultimately affects how the ocean uptakes excess carbon from the atmosphere. For example, over the last 50 years, there has been an increase in heat content in the subtropical North Atlantic, associated with an increase in volume of both upper thermocline water (18 to 24C) and cold thermocline waters (4 to 10C), but a decrease in other neighbouring temperature classes. This proposal aims to understand how the subtropical North Atlantic operates in the climate system by analysing tritium-helium observations along 36N and 25W across the basin. The inferred tritium-helium age provides a measure of the elapsed time since the water was last in contact with the atmosphere. The measurements of tritium-helium age will be used to understand: 1. How ventilation controls the observed changes in heat content in the subtropical North Atlantic; 2. To identify the rate of export of organic matter which is diagnosed indirectly from the rate of oxygen utilisation using the tritium-helium age. The reliability of the data signals will be assessed by comparying with a circulation model and its adjoint. The sensitivity of the climate change, the export of organic matter and cycling of nutrients, will then be understood by using the same model and its adjoint, which efficiently identifies the effect of changes in external forcing and model physics.