Random scattering, refraction and focussing of ocean surface gravity waves (SGWs) by submesoscale currents result in spatial modulation - "patches'' - in the wave field. A signature of patches is that the significant wave height Hs varies by as much as 30% on horizontal scales in the range 10 to 100 km and time scales of a few hours to a day; these scales reflect those of submesoscale currents and are much smaller than those of the wind-stress forcing of SGWs which is traditionally assumed to control the spatio-temporal variability of Hs. As a result, patches pose a major challenge for the modelling and prediction of SGWs and of their impact on the ocean circulation. This project will tackle this challenge by developing new statistical models of the scattering of SGWs by submesoscale turbulence. We will use these models to explain the main features of patch variability, including a recently discovered relation between the power spectrum of Hs and the submesoscale kinetic energy spectrum, and to develop a parametrization of SGW scattering by submesoscale turbulence that we will incorporate into WAVEWATCH III. Spatial and temporal fluctuations in Hs are reflected in other properties of the SGW field including the Stokes velocity, and hence the wave-averaged vortex and Stokes-Coriolis forces which control the forcing of currents by SGWs. We will investigate the hypothesis that the interaction between waves and mean flows is strong on patch time and space scales, and develop new modeling tools tailored to these scales.

Earth's magnetic field has existed for at least 3.5 billion years and exhibits a complex spectrum of spatial and temporal variations on timescales ranging from less than seconds to millions of years. On average the field is thought to adopt a dipole-dominated configuration, which helps protect the surface environment and low-orbiting satellites from the depredations of the solar wind. Significant variations, e.g., the recent growth of a region of anomalously weak field in the southern Atlantic, and excursions and polarity reversals, may alter the shielding effect provided by the field. These surface observations document a dynamo process operating in the liquid core and provide unique insight into the dynamics and evolution of Earth's deep interior. However, data alone cannot constrain the interactions between magnetic field and flow that occur within the core: that requires an internal view of the dynamo. Understanding past field variations and making predictions about future behaviour therefore requires an intimate link between observations and simulations of the generation process. The standard picture of geomagnetic secular variation (SV) is provided by time-dependent global models of the historical, Holocene and longer term field. However, paleomagnetic data also provide evidence for Unusually Rapid Geomagnetic Events (URGEs) in the form of rapid geomagnetic intensity spikes, and directional rates of change that greatly exceed values in these models. While these URGEs are not visible in current global field models, we have recently shown that they are comparable to the fastest changes (called extremal events) produced in numerical dynamo simulations and are compatible with the physics of the dynamo process. Our results also reveal that extremal intensity and directional changes arise in different times and places and are associated with migration of distinct magnetic features at the top of the core. These findings link observations and simulations in a new and more complex view of SV, and suggest new approaches for understanding the dynamo process and our ability to predict its future variations. Progress requires moving beyond simple definitions of extremal events to investigate the spectrum of dynamical behaviour that underpins URGEs. Critical to this goal is using complementary information drawn from paleomagnetic global field models and geodynamo simulations. We propose to develop a new series of global time-dependent geomagnetic field models that can capture rapid changes. In parallel we will produce a new suite of geodynamo simulations accessing the rapidly rotating and vigorously convecting regime thought to describe the dynamics of Earth's core. Synthesis across these approaches will address the following questions: 1. What are the defining spatial and temporal characteristics of URGEs? Do they occur in preferred locations or on systematic timescales? 2. What are the physical origin(s) of URGEs? 3. Are URGEs related to excursions and reversals? 4. Are URGEs related to interactions between the core and mantle and/or stratification at the top of the core?

The balance between the production of organic carbon during phytoplankton photosynthesis and its consumption by bacterial, zooplankton and phytoplankton respiration determines how much carbon can be stored in the ocean and how much remains in the atmosphere as carbon dioxide. The amount of organic carbon stored in the ocean is as large as the amount of carbon dioxide in the atmosphere, and so is a key component in two global carbon cycle calculations needed to avoid a global temperature rise of more than 1.5 degrees C: the calculation of the technological and societal efforts required to achieve net zero carbon emissions and the calculation of the efficiency of ocean-based engineering approaches to directly remove carbon dioxide from the atmosphere. Yet, despite its vital role, our ability to predict how ocean carbon storage will change in the future is severely limited by our lack of understanding of how plankton respiration varies in time and space, how it is apportioned between bacteria and zooplankton and how sensitive it is to climate change-induced shifts in environmental conditions such as increasing temperature and decreasing oxygen. This woeful situation is due to the significant challenge of measuring respiration in the deep-sea and the uncoordinated way in which these respiration data are archived. This project will directly address these two problems. We will take advantage of our leadership and participation in an international programme which deploys thousands of oceanic floats measuring temperature, oxygen and organic carbon in the global ocean, in an international team of experts focused on quantifying deep-sea microbial respiration, and our experience of collating international datasets, to produce an unprecedented dataset of bacterial and zooplankton respiration. We will derive estimates of respiration based on data from floats, so that together with estimates derived from recently developed methods including underwater gliders, the new database will include respiration measurements calculated over a range of time and space scales. Crucially, respiration rates will be coupled with concurrent environmental data such as temperature, oxygen and organic carbon. This dataset will enable us to quantify the seasonal and spatial variability of respiration and derive equations describing how respiration changes with the proportion of bacteria and zooplankton present and with the chemical and physical properties of the water. These equations can then be used in climate models to better predict how respiration and therefore ocean carbon storage will change in the future with climate-change induced shifts in temperature, oxygen, organic carbon and plankton community. We will take part in a hybrid hands-on and online international training course on observations and models of deep-water respiration targeted to early career researchers from developing and developed countries to showcase the useability of the respiration database and the global array of oceanic floats. We will also prepare Science Festival exhibits on observing life in the deep ocean for schoolchildren. The deliverables of the project - a unique global open-access database of respiration measurements, new equations describing the sensitivity of respiration to changing temperature and oxygen suitable for climate models and online training materials for early career researchers - are of benefit to scientists who aim to predict how a changing climate will affect the storage of carbon in the ocean, educators who train the next generation of ocean scientists and practitioners, policy makers who need to quantify nationally determined contributions to actions limiting global warming, and scientists, engineers, lawyers, governing bodies and commercial companies designing, evaluating and implementing ocean-based carbon dioxide removal technologies.

Submesoscale flows and near-inertial motions are ubiquitous features of the upper ocean. Recently developed theories have posited that the interaction of submesoscale flows and near-inertial motions could play an important role in closing the energy budgets of both the balanced circulation and the unbalanced waves. These theories have yet to be fully tested in the field due to the challenges of observing both types of motions in a relatively controlled setting. The northern Gulf of Mexico is a natural laboratory that is ideal for doing this. Here, the Mississippi-Atchafalaya river plume forms a rich field of submesoscale eddies and fronts, which in the summer are driven by a land-sea breeze that forces inertial motions at near resonance. The proposed research will involve intensive field campaigns utilizing novel observational techniques that will be closely integrated with idealized and realistic numerical simulations to study the interaction of submesoscale eddies and near-inertial motions in the Mississippi-Atchafalaya river plume. The objective will be to characterize, quantify, and understand the energy exchanges between balanced and unbalanced motions and the turbulent cascade that can result from the interaction. More specifically, the numerical and observational experiments will be designed to test the hypotheses that periodic straining of isopycnals by inertial motions in fronts, subduction of low potential vorticity water, and the propagation, trapping, and reflection of near-inertial waves interacting with submesoscale eddies facilitate the energy exchange and result in enhanced mixing throughout the water column. The proposed research tackles one of the outstanding questions in physical oceanography of how the kinetic energy in the balanced circulation is dissipated, in particular through wave-mean flow interactions. This question has been studied mostly theoretically for idealized flow configurations, but it has not been explored observationally in a controlled setting, as proposed here. The research will integrate observations with submesoscale-resolving simulations and large-eddy simulations, a combination that has proven to be effective in understanding the fundamental physics of complex, multi- scale observed flows.

Our understanding of the way in which rocks record the geomagnetic field is based on an analytical theory which makes the assumption that particles are uniformly magnetized (i.e., they are single domain, SD). But, it has long been known that most rocks contain grains referred to as pseudo-single-domain (PSD); these are too big to be uniformly magnetized, and are unsuited for analytical theory. Recent breakthroughs in numerical modeling have opened new avenues for understanding the nature of PSD grains. Making it possible to estimate the temporal and thermal stability of PSD particles. This approach has gained new urgency given recent results on PSD samples which call into question their reliability for paleointensity research. The opportunity now exists to combine experimental and numerical approaches for a radical new understanding of paleomagnetic recording, with the potential to transform how paleointensity research is done.