Resolving Shelf Break Exchange Around the European Northwest Shelf

Shelf seas act as a significant sink of carbon within the global ocean. This occurs as carbon is exported beneath the permanent oceanic thermocline through the downwelling circulation across the shelf break. This downwelling circulation is quantified here using two regional ocean model configurations of the European northwest shelf, with differing resolution (7‐ and 1.5‐km grid spacing). The dominant mechanisms and impact of model resolution are assessed along the length of the shelf break. The total downwelling circulation is stronger at higher resolution, due to an increased on‐shelf transport at internal depths (20–150 m) and increased off‐shelf transport at the base of the water column. At internal depths, these differences increase seasonally, influenced by stratification. Key processes in cross‐shelf exchange only begin to be resolved at O(1 km), implying that global models currently used to assess the carbon cycles will be missing these processes

Feedback of mesoscale ocean currents on atmospheric winds in high-resolution coupled models and implications for the forcing of ocean-only models

The repercussions of surface ocean currents for the near-surface wind and the air-sea momentum flux are investigated in two versions of a global climate model with eddying ocean. The focus is on the effect of mesoscale ocean current features at scales of less than 150 km, by considering high-pass filtered, monthly-mean model output fields. We find a clear signature of a mesoscale oceanic imprint in the wind fields over the energetic areas of the oceans, particularly along the extensions of the western boundary currents and the Antarctic Circumpolar Current. These areas are characterized by a positive correlation between mesoscale perturbations in the curl of the surface currents and the wind curl. The coupling coefficients are spatially non-uniform and show a pronounced seasonal cycle. The positive feedback of mesoscale current features on the near-surface wind acts in opposition to their damping effect on the wind stress. A tentative incorporation of this feedback in the surface stress formulation of an eddy-permitting global ocean-only model leads to a gain in the kinetic energy of up to 10 %, suggesting a fundamental shortcoming of present ocean model configurations

Air-sea turbulent heat flux feedback over mesoscale eddies

Air-sea turbulent heat fluxes play a fundamental role in generating and dampening sea surface temperature (SST) anomalies. To date, the turbulent heat flux feedback (THFF) is well quantified at basin-wide scales (~20 W/m2/K) but remains unknown at the oceanic mesoscale (10-100~km). Here, using an eddy-tracking algorithm in three configurations of the coupled climate model HadGEM3-GC3.1, the THFF over mesoscale eddies is estimated. The THFF magnitude is strongly dependent on the ocean-to-atmosphere regridding of SST, a common practice in coupled models for calculating air-sea heat flux. Our best estimate shows that the mesoscale THFF ranges between 35 and 45 W/m2/K globally, across different eddy amplitudes. Increasing the ratio of atmosphere-to-ocean grid resolution can lead to an underestimation of the THFF, by as much as 80\% for a 6:1 resolution ratio. Our results suggest that a large atmosphere-to-ocean grid ratio can result in an artificially weak dampening of mesoscale SST anomalies

Evaluating surface eddy properties in coupled climate simulations with 'eddy-present' and 'eddy-rich' ocean resolution

As climate models move towards higher resolution, their ocean components are now able to explicitly resolve mesoscale eddies. High resolution for ocean models is roughly classified into eddy-present (EP, 1/4°) and eddy-rich (ER, 1/12°) resolution. The cost-benefit of ER resolution over EP resolution remains debated. To inform this discussion, we quantify and compare the surface properties of coherent mesoscale eddies in high-resolution versions of the HadGEM3-GC3.1 coupled climate model, using an eddy tracking algorithm. The modelled properties are compared to altimeter observations. Relative to EP, ER resolution simulates more (+60%) and longer-lasting (+23%) eddies, in better agreement with observations. The representation of eddies in Western Boundary Currents (WBC) and the Southern Ocean compares well with observations at both resolutions. However a common deficiency in the models is the low eddy population in subtropical gyre interiors, which reflects model biases at the Eastern Boundary Upwelling Systems and at the Indonesian outflow, where most of these eddies are generated in observations. Despite a grid spacing larger than the Rossby radius of deformation at high-latitudes, EP resolution does allow for eddy growth in these regions, although at a lower rate than seen in observations and ER resolution. A key finding of our analysis is the large differences in eddy size across the two resolutions and observations: the median speed-based radius increases from 14 km at ER resolution to 32 km at EP resolution, compared with 48 km in observations. It is likely that observed radii are biased high by the effective resolution of the gridded altimeter dataset due to post-processing. Our results highlight the limitations of the altimeter products and the required caution when employed for understanding eddy dynamics and developing eddy parameterizations

Feedback of mesoscale ocean currents on atmospheric winds in high-resolution coupled models and implications for the forcing of ocean-only models

The repercussions of surface ocean currents for the near-surface wind and the air-sea momentum flux are investigated in two versions of a global climate model with eddying ocean. The focus is on the effect of mesoscale ocean current features at scales of less than 150 km, by considering high-pass filtered, monthly-mean model output fields. We find a clear signature of a mesoscale oceanic imprint in the wind fields over the energetic areas of the oceans, particularly along the extensions of the western boundary currents and the Antarctic Circumpolar Current. These areas are characterized by a positive correlation between mesoscale perturbations in the curl of the surface currents and the wind curl. The coupling coefficients are spatially non-uniform and show a pronounced seasonal cycle. The positive feedback of mesoscale current features on the near-surface wind acts in opposition to their damping effect on the wind stress. A tentative incorporation of this feedback in the surface stress formulation of an eddy-permitting global ocean-only model leads to a gain in the kinetic energy of up to 10 %, suggesting a fundamental shortcoming of present ocean model configurations

Will high-resolution global ocean models benefit coupled predictions on short-range to climate timescales?

As the importance of the ocean in the weather and climate system is increasingly recognised, operational systems are now moving towards coupled prediction not only for seasonal to climate timescales but also for short-range forecasts. A three-way tension exists between the allocation of computing resources to refine model resolution, the expansion of model complexity/capability, and the increase of ensemble size. Here we review evidence for the benefits of increased ocean resolution in global coupled models, where the ocean component explicitly represents transient mesoscale eddies and narrow boundary currents. We consider lessons learned from forced ocean/sea-ice simulations; from studies concerning the SST resolution required to impact atmospheric simulations; and from coupled predictions. Impacts of the mesoscale ocean in western boundary current regions on the large-scale atmospheric state have been identified. Understanding of air-sea feedback in western boundary currents is modifying our view of the dynamics in these key regions. It remains unclear whether variability associated with open ocean mesoscale eddies is equally important to the large-scale atmospheric state. We include a discussion of what processes can presently be parameterised in coupled models with coarse resolution non-eddying ocean models, and where parameterizations may fall short. We discuss the benefits of resolution and identify gaps in the current literature that leave important questions unanswered

Temperature

KEY HEADLINES: • The first MCCIP ARC in 2006 reported following what was then the warmest year globally in 2005 (0.26°C higher than the 1981-2010 average). • Since 2005, new global record temperatures have been set in 2010 and then in each successive year 2014, 2015 and 2016. In these last three record years the global average temperature anomaly was 0.31,0.44, 0.56°C higher than the 1981-2010 average. • 2014 was a record warm year for coastal air and sea temperatures around the UK. Between 1984 and 2014 coastal water temperatures rose around the UK at an average rate of 0.28 °C/decade. The rate varies between regions, the slowest warming was in the Celtic Sea at 0.17 °C/decade and the maximum rate was in the Southern North Sea at 0.45 °C/decade. • There is also variability over shorter time periods. In all regions of UK seas there was a negative trend in the 10-year period between 2003 and 2013. This is due to variability within the ocean /atmosphere system which is natural. • There is a trend towards fewer in-situ observations, and this will ultimately influence the confidence in future assessments. • Some gridded datasets can offer alternatives to single point observations, but to understand the patterns of ocean variability, the quality information from ocean timeseries cannot yet be replaced by surface observations or autonomous data collection. • The first MCCIP report card in 2006 used the UKCIP projections from 2002 which had a very limited representation of the SST. • The latest updates to the UK Climate Projections shelf seas models were published in 2016 and projected increases in sea surface temperature for 2069-89 relative to 1960-89 of over 3 °C for most of the North Sea, English Channel, Irish and Celtic Seas. For the deeper areas to the north and west of Scotland out towards Rockall and in the Faroe Shetland Channel the increase in temperature is projected to be closer to 2 °C. • Over the last 10 years there has been a steady improvement in the scientific basis underlying centennial sea temperature projections for the seas around the UK, and significant progress in the field of seasonal and decadal projections. • The scientific basis to such projections and predictions will continue to improve over the next 10 years, with increasing resolution, treatment of climate uncertainties, and methodology. Over the centennial scale the difference between emissions scenarios are still the source of the largest uncertainties. • Development of North West European Shelf (NWS) modelling systems driven by seasonal forecasting systems may allow NWS temperature prediction over the monthly to decadal period

Impact of ocean resolution on coupled air-sea fluxes and large-scale climate

Air-sea fluxes are a crucial component in the energetics of the global climate system. The largest air-sea fluxes occur in regions of high sea surface temperature variability, such as ocean boundary, frontal currents and eddies. In this paper we explore the importance of ocean model resolution to resolve air-sea flux relationships in these areas. We examine the sea surface temperature-wind stress relationship in high-pass filtered observations and two versions of the Met Office climate model with eddy-permitting and eddy-resolving ocean resolutions. Eddy-resolving resolution shows marginal improvement in the relationship over eddy-permitting resolution. However, by focussing on the North Atlantic we show that the eddy-resolving model has significant enhancement of latent heat loss over the North Atlantic Current region, a long-standing model bias. While eddy-resolving resolution does not change the air-sea flux relationship at small scale, the impact on the mean state has important implications for the reliability of future climate projections

Description of the resolution hierarchy of the global coupled HadGEM3-GC3.1 model as used in CMIP6 HighResMIP experiments

CMIP6 HighResMIP is a new experimental design for global climate model simulations that aims to assess the impact of model horizontal resolution on climate simulation fidelity. We describe a hierarchy of global coupled model resolutions based on the HadGEM3-GC3.1 model that range from an atmosphere-ocean resolution of 130 km-1° to 25 km-1/12°, all using the same forcings and initial conditions. In order to make such high resolution simulations possible, the experiments have a short 30 year spinup, followed by at least century-long simulations with both constant forcing (to assess drift and the focus of this work), and historic forcing. We assess the change in model biases as a function of both atmosphere and ocean resolution, together with the effectiveness and robustness of this new experimental design. We find reductions in the biases in top of atmosphere radiation components and cloud forcing. There are significant reductions in some common surface climate model biases as resolution is increased, particularly in the Atlantic for sea surface temperature and precipitation, primarily driven by increased ocean resolution. There is also a reduction in drift from the initial conditions both at the surface and in the deeper ocean at higher resolution. Using an eddy-present and eddy-rich ocean resolution enhances the strength of the North Atlantic ocean circulation (boundary currents, overturning circulation and heat transports), while an eddy-present ocean resolution has a considerably reduced Antarctic Circumpolar Current strength. All models have a reasonable representation of El Nino – Southern Oscillation. In general the biases present after 30 years of simulations do not change character markedly over longer timescales, justifying the experimental design