This joint proposal to U.S. National Science Foundation's Directorate for Geosciences and U.K. Natural Environment Research Council aims to investigate how the Oyashio Extension frontal variability in the Northwest Pacific Ocean influences the large-scale atmospheric circulation by accumulating the interaction between the individual weather system and underlying ocean front. The atmospheric storm track exhibits the local maximum strength in the Northwest Pacific over the strong ocean fronts driven by collocated maximum baroclinicity, which is in turn maintained by huge heat and moisture supplied by the ocean. While significant advances have been achieved in the past decade or so on our understanding of ocean front's impact on the atmosphere for the mean climate, there are still many crucial questions yet to be answered, especially related to impact of ocean frontal variability on the atmospheric circulation variability. A particular goal of this proposal is to unveil the link between the local air-sea interaction in weather scale near the Oyashio Extension and its cumulative impact on the large-scale atmospheric circulation and climate variability. Specific emphases will be placed on the seasonality of this link by contrasting the early and late winter, and also the asymmetry/nonlinearity in the large-scale atmospheric response to warm and cold SST anomalies induced by a shift of the Oyashio Extension front to the north and south, respectively. These challenging goals will be addressed by combining analyses of observational and reanalysis datasets and targeted climate model experiments using the Variable Resolution Community Atmosphere Model v.6 with Spectral Element dynamical-core, a state-of-the art atmospheric general circulation model, which will be configured with a very high-resolution over the North Pacific and lower resolution elsewhere globally to realistically simulate the frontal air-sea interaction over the Oyashio Extension as well as the feedback with the large-scale circulation at a manageable computational cost. Furthermore, the role of local ocean coupling will be investigated by comparing the atmosphere-only simulations with those coupled to the 1-dimensional column ocean model.

Monsoon systems influence the water supply and livelihoods of over half of the world. Observations are too short to provide estimates of monsoon variability on the multi-year timescale relevant to the future or to identify the causes of change on this timescale. The credibility of future projections of monsoon behavior is limited by the large spread in the simulated magnitude of precipitation changes. Past climates provide an opportunity to overcome these problems. This project will use annually-resolved palaeoenvironmental records of climate variability over the past 6000 years from corals, molluscs, speleothems and tree rings, together with global climate-model simulations and high-resolution simulations of the Indian, African, East Asia and South American monsoons, to provide a better understanding of monsoon dynamics and interannual to multidecadal variability (IM). We will use the millennium before the pre-industrial era (850-1850 CE) as the reference climate and compare this with simulations of the mid- Holocene (MH, 6000 years ago) and transient simulations from 6000 year ago to ca 850 CE. We will provide a quantitative and comprehensive assessment of what aspects of monsoon variability are adequately represented by current models, using environmental modelling to simulate the observations. By linking modelling of past climates and future projections, we will assess the credibility of these projections and the likelihood of extreme events at decadal time scales. The project is organized around four themes: (1) the impact of external forcing and extratropical climates on intertropical convergence and the hydrological cycle in the tropics; (2) characterization of IM variability to determine the extent to which the stochastic component is modulated by external forcing or changes in mean climate; (3) the influence of local (vegetation, dust) and remote factors on the duration, intensity and pattern of the Indian, African and South American monsoons; and (4) the identification of palaeo-constraints that can be used to assess the reliability of future monsoon evolution.

To minimize the risk of dangerous climate change associated with increasing concentrations of atmospheric greenhouse gases (GHG), as part of ongoing international efforts, the 2008 Climate Change Act requires that the UK reduces its GHG emissions by at least 80% by 2050, compared to 1990 levels. To support such legislation, methods must be developed to reduce uncertainty on existing national GHG emissions estimates and monitor the efficacy of emissions reduction strategies. In 2010, CO2 represented about 85% of total UK GHG emissions, with the remainder largely from methane (CH4) and nitrous oxide (N2O). In 2010, the main UK sources of CO2 were energy supply, road transport, business, and residential; the main sources of CH4 were agriculture and landfill with small sources from gas leakage and coal mines; and the main sources of N2O were agriculture, industrial process, and road transport. There are substantial associated uncertainties with sectoral estimates of these emissions, particularly for N2O. The main focus of Greenhouse gAs Uk and Global Emissions (GAUGE) is to quantify UK budgets of CO2, CH4, and N2O from different sectors, and to improve global GHG budgets. The UK study will focus on fossil fuels and agriculture, the two largest sources of the three GHGs. We will achieve this by combining atmospheric measurements with computer models of the atmosphere, which describe the movement of GHGs after emission. We already have a reasonable idea of where GHGs are emitted but the size of the emissions typically has a large associated error. Depending on the emission type it may also have a substantial seasonal cycle (e.g., agriculture). It is therefore important we make regular GHG measurements at different times of the year and in different places. The UK research aircraft will provide the broad-scale 3-D perspective on the inflow and outflow of UK GHG budgets, complementing information from existing tall towers. The network of tall towers measure GHGs at 100-200m above the surface to ensure that the sampled air is representative of larger areas, and the towers are intentionally sited to provide estimates of GHG emissions in the Devolved Administrations. As part of GAUGE we will add to this network with a tower in the Scottish borders that provides substantially more information about the north of England, Scotland, and the North Sea; a tower over SE England, downwind of London; and we will support existing instruments on the BT tower in central London. The SE London tower and the BT tower together will allow us to provide the first multi-year record of urban emissions from a megacity. We will use GHG isotopes to improve understanding of the fossil fuel sources. A detailed study of agricultural GHG emissions will be conducted over East Anglia, allowing us to quantify the importance of this sector in the UK GHG budget. Weekly measurements aboard a North Sea ferry will provide constraints on UK GHG fluxes by regularly sampling transects of UK outflow. Satellite observations of GHGs offer a unique global perspective, linking UK emissions to the rest of the world, and we will work with NASA to develop and apply new observations to quantify global GHG budgets on a sub-UK spatial scale. Embedded in this long-term measurement strategy will be a measurement intensive to quantify London GHG emissions, where we will use the UK research aircraft to sample profiles of upwind/downwind air, validate dedicated satellite observations, and link urban measurements with downwind in situ and tall tower measurements. In GAUGE we bring together computer models of the atmosphere, and a team of world-leading modellers, in order to relate observed variations of GHGs to estimates of the underlying emissions. Statistical approaches will be used to find emissions that best agree with the measurements, taking account of model and data uncertainties. The main outcome from GAUGE will be robust GHG emission estimates from the UK and from the world.

Predictions of future climate, essential for safeguarding society and ecosystems, are underpinned by numerical models of the Earth system. These models are routinely tested against, and in many cases tuned towards, observations of the modern Earth system. However, the model predictions of the climate of the end of this century lie largely outside of this evaluation period, due to the projected future CO2 forcing being significantly greater than that seen in the observational record. Indeed, recent work reconstructing past CO2 has shown that the closest analogues to the 22nd century, in terms of CO2 concentration, are tens of millions of years ago, in 'Deep-Time'. The Palaeoclimate Modelling Intercomparison Project (PMIP) provides a framework (but no funding!) by which the palaeoclimate modelling community assesses state-of-the-art climate models relative to past climate data. Traditionally, PMIP has focussed on the relatively recent mid-Holocene (6,000 years ago) and Last Glacial Maximum (21,000 years ago), but these time periods have even lower CO2 than modern (~280 and ~180 ppmv respectively, c.f. ~400 ppmv for the modern). Recently, PMIP has expanded into other time periods, most notably the mid-Pliocene (3 million years ago), but even then, CO2 was most likely less than modern values (~380 ppmv). The modelling community would clearly benefit from an intercomparison of 'Deep-Time' climates, when CO2 levels were close to those predicted for the end of this century. We will organise and provide funding for 2 workshops, with the aim of producing papers describing the experimental design and outputs from a new climate Model Intercomparison Project - "DeepMIP", focussing on past climates in which atmospheric CO2 concentrations were similar to those projected for the end of this century. The papers will evaluate the models relative to past geological data, and aim to understand the reasons for the model-model differences and model-data (dis)agreements, providing information of relevance to the IPCC. A previous NERC grant, NE/K014757/1, is currently aiming to assess climate sensitivity (the response of surface air temperature to a doubling of atmospheric CO2), through geological time. That project is focussing on many time periods, but with only one model. This IOF will complement that project, and bring added-value, by focussing on one particular time period, but with many models. As such we will address the crucial issue of model-dependence.

Depletion of the stratospheric ozone layer has been at the forefront of environmental concern over the last 40 years. The layer shields Earth's surface from certain wavelengths of harmful ultraviolet (UV) radiation that would otherwise be detrimental to human and plant health. Ozone also absorbs terrestrial infra-red (IR) radiation meaning it is a greenhouse gas, and changes in its abundance can therefore impact climate. The primary cause of ozone depletion is the release of halogens (chlorine and bromine) from long-lived anthropogenic compounds, such as chlorofluorocarbons (CFCs) and halons. Production of these ozone-depleting compounds is now controlled by the UN Montreal Protocol, but they were once widely used in refrigeration and fire suppression units, among other applications. Due to the success of the Protocol, the stratospheric abundance of chlorine and bromine is now declining, albeit slowly, and the ozone layer is widely expected to 'recover' to levels observed pre-1980 in the middle to latter half of this century. However, a key uncertainty, highlighted in the WMO/UNEP 2014 Assessment of Stratospheric Ozone Depletion, is the increasing emissions of uncontrolled chlorine-containing Very Short-Lived Substances (Cl-VSLS) which can also reach the stratosphere and cause ozone loss. The most abundant Cl-VSLS is dichloromethane (CH2Cl2), whose tropospheric abundance has increased by >60% over the last decade. CH2Cl2 is human-produced and in the Northern Hemisphere, close to industrial sources, long-term observations show a mean CH2Cl2 growth rate of ~8%/year. The precise cause of these increases is unknown. However, emissions of CH2Cl2 (and other Cl-VSLS) are known to be relatively large over Asia, and in the absence of policy controls on production, atmospheric concentrations are expected to continue to increase in coming years. Our recent modelling work has shown (i) that the contribution of Cl-VSLS to stratospheric chlorine has already doubled in the last decade alone, and (ii) that sustained CH2Cl2 growth could delay the recovery of the Antarctic Ozone Hole by up to several decades. This would significantly offset some of the gains achieved by the Montreal Protocol, and because the Ozone Hole influences surface climate of the Southern Hemisphere in several ways, could affect forward predictions of climate change. This project (ISHOC) establishes a new task force comprised of world-leading chemistry-climate modelling groups. We will perform the first concerted multi-model assessment of the threat posed to stratospheric ozone from CH2Cl2 growth. Lancaster University will lead the model intercomparison in collaboration with the University of Cambridge, and an international consortium of 9 partners. We will develop a series of growth scenarios describing possible future trajectories of CH2Cl2 in the atmosphere. Each of the models in our consortium will perform forward simulations considering these scenarios and the output will be analysed to determine (a) the expected delay to ozone recovery in different regions of the stratosphere due to CH2Cl2 growth and (b) the subsequent implications for climate and surface UV. The results from ISHOC will provide powerful new insight into the role of compounds not controlled by the Montreal Protocol in ozone depletion, which will be highly relevant to future international assessments of ozone and climate change (e.g. WMO/UNEP and IPCC reports). While the focus of ISHOC is on CH2Cl2, the task force will remain active beyond the project to examine future threats to ozone from other uncontrolled Cl-VSLS (e.g. CHCl3, C2H4Cl2) as they emerge. Indeed, our ongoing work suggests that emissions of these Cl-VSLS are also increasing.