How does stably-stratified shear-driven turbulence mix our oceans and estuaries?

This research is ultimately motivated by reducing the harmful consequences of climate change on society, in the UK and worldwide. The root of the problem is global warming, caused by the greenhouse effect of carbon dioxide from fossil fuels. As our atmosphere warms, so do our oceans, which directly affects biodiversity and causes sea levels to rise. As our oceans warm, the balance of forces that keep them in constant motion changes too, disrupting their worldwide circulation. This disruption is worrying, both in the short and long term, because the present circulation patterns perform at least two functions vital to our hospitable climate. First, vertical currents store excess heat and carbon deep into the ocean (slowing global warming). Second, North-South currents redistribute tropical heat to more temperate regions (reducing extreme weather and climate). Therefore, a weakening of these currents could accelerate climate change, with long-lasting societal consequences. To mitigate this, scientists try to predict how the world's climate will evolve by using advanced mathematical and computer models of the ocean circulation. However, these models and their predictions need to be improved to be of greater benefit to society and decision-makers. A serious cause of uncertainty in these models lies in the mixing between water currents that have different salinity or temperature (and thus density). Currents of different densities organise into vertically-stacked (or "stably-stratified") layers which flow past one another at different speeds (creating a "shear" flow). These flows are always turbulent, which means that a vast number of tiny chaotic "eddies" mix the salinity and temperature of much larger layers in complex and unpredictable ways. This fundamental but extremely challenging process of turbulent mixing in stably-stratified shear flows needs to be better understood. To do this, I will employ the following scientific approach in three steps. First, I will use an accurate, reduced-scale model of such flows in the laboratory. This has two great benefits: it gives full control over the flow geometry, the density difference, flow speed, etc, allowing me to test and understand the influence of each parameter separately; and it allows me to use high-tech measurements to quantify the chaotic eddies and their mixing better than ever before. Second, I will interpret these new laboratory measurements with mathematical models of turbulent mixing to generalise (or "extrapolate") my findings to real-scale flows found in the ocean. This crucial step relies on the power of "dimensional analysis" in fluid dynamics, which is also routinely used by engineers to develop new aircraft or ship designs from smaller-scale laboratory prototypes. Third, I will verify the validity of my real-scale predictions by comparing them to actual measurements taken from ships (which are usually sparse, expensive, and less accurate). This step is similar to engineers performing a full-scale test before production, except that we have no control over the ocean. Although challenging, this "validation" step will help ensure that my whole approach succeeds in providing climate scientists with more accurate models for ocean mixing. In addition to the long-term effects of global warming, I will also apply the above three steps to a shorter-term consequence: saltwater intrusions in estuaries. Sea level rise, more frequent droughts, extreme storm surges, and stronger tides will all increase the gradual encroachment of seawater in densely-populated deltas (including the important Thames Basin in the UK). The upstream intrusion of a dense saltwater layer beneath the fresh river water, and their vertical mixing reduce the availability of surface freshwater, with devastating consequences for coastal communities already felt around the world. I will develop more accurate models of mixing in saltwater intrusions to help mitigate this urgent problem.