On the variability of the meridional overturning circulation and its impact on ocean heat uptake

Occurrences of storms and flooding emphasize the effects of weather and climate upon our lives. The need for reliable short-term weather and long-term climate prediction are critical for global economic and societal development. With increasing concentrations of greenhouse gas polluting our Earth's atmosphere, widespread surface warming has been observed in recent decades. However, the recent unexpected hiatus in atmospheric temperature growth has emphasised the importance of understanding our environments responses to different forcing mechanisms more thoroughly, and specifically the ocean's role in our evolving climate. As a result, improved predictions of regional and global climate patterns over long periods are a distinct possibility.
The ocean's capacity to store energy far exceeds that of the atmosphere. Approximately 90% of the energy accumulated from global warming has been absorbed by the oceans. In elementary terms, the ocean's well-mixed upper layer absorbs solar radiation, and is consequently heated causing vertical temperature stratification of seawater. Changes in near-surface temperature can be transported to the deep ocean via meridional overturning circulations (MOCs). These circulations therefore potentially influence the rate and location at which heat is taken up by the ocean, the vertical distribution of the heat, and the rise in sea-level due to thermal expansion. MOCs directly influence our weather and climate: for example, heat transported into the North Atlantic reduces the severity of winters in north-west Europe.
A major challenge facing climate science is the development of a robust understanding of MOC dynamics, so that MOC fluctuations can be predicted on time-scales of decades to centuries. Recent studies highlighting a possible slowdown in the Atlantic meridional overturning circulation (AMOC) over the past century, have led some to fear a complete shutdown in the circulation which could cause regional cooling, collapse of plankton stocks and increase the severity of storms, flooding and El Nino oscillations.
My work will develop and deploy novel diagnostics to probe the behaviour of MOCs and ocean heat uptake under past and future forcing scenarios, using the current generation of UK Met-Office coupled climate models. Existing simple conceptual and numerical models of the global MOC will be used to better understand and explore factors controlling variability in ocean stratification, heat content and circulation.
Initial investigations will address the AMOC, in an attempt first to understand eastern and western boundary densities. Following the work of Bell and Burton, I will then re-construct AMOC to investigate its variability, and consider the main factors contributing to transport. Subsequently, my research will quantify the extent to which the Sverdrup relation between surface wind and depth-integrated ocean transport determines the east-west structure of ocean heat content, and then seek to establish diagnostic relationships between the time-mean position of isotherms and surface wind and heat fluxes.
These diagnostics will then be used in climate change simulations to investigate historical changes in ocean heat uptake and provide improved interpretation of predicted future changes. Further I hope to perform a thorough statistical comparison of diagnostic performance on simulation output with that on real-world observations of ocean heat uptake. My ambition is that the diagnostic framework developed in my PhD project will contribute directly to the interpretation of UK Met-Office climate forecasts, the assessment of climate model performance, and the design and development of improved models for climate prediction.