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

The low-resolution version of HadGEM3 GC3.1: development and evaluation for global climate

A new climate model, HadGEM3 N96ORCA1, is presented that is part of the GC3.1 configuration of HadGEM3. N96ORCA1 has a horizontal resolution of ~135 km in the atmosphere and 1° in the ocean and requires an order of magnitude less computing power than its medium-resolution counterpart, N216ORCA025, while retaining a high degree of performance traceability. Scientific performance is compared both to observations and the N216ORCA025 model. N96ORCA1 reproduces observed climate mean and variability almost as well as N216ORCA025. Patterns of biases are similar across the two models. In the north-west Atlantic, N96ORCA1 shows a cold surface bias of up to 6K, typical of ocean models of this resolution. The strength of the Atlantic meridional overturning circulation (16 to 17 Sv) matches observations. In the Southern Ocean, a warm surface bias (up to 2K) is smaller than in N216ORCA025 and linked to improved ocean circulation. Model El Niño/Southern Oscillation and Atlantic Multidecadal Variability are close to observations. Both the cold bias in the Northern hemisphere (N96ORCA1) and the warm bias in the Southern hemisphere (N216ORCA025) develop in the first few decades of the simulations. As in many comparable climate models, simulated interhemispheric gradients of top-of-atmosphere radiation are larger than observations suggest, with contributions from both hemispheres. HadGEM3 GC3.1 N96ORCA1 constitutes the physical core of the UK Earth System Model (UKESM1) and will be used extensively in the Coupled Model Intercomparison Project 6 (CMIP6), both as part of UKESM1 and as a stand-alone coupled climate model

UKCP18 marine report

Changes in global and regional sea level arise from a wide variety of geophysical processes that operate on different time and space scales (the sea level “jigsaw puzzle”). Global mean sea level (GMSL) rise occurs from thermal expansion of seawater and the addition of water to the ocean from the loss of land-based ice and water. Changes in land-based ice and water storage result in spatial patterns of regional sea level change through the associated impact on Earth’s gravity field and other effects. Local changes in seawater density and ocean circulation also give rise to a spatial pattern of change, which varies markedly among climate models, and is therefore highly uncertain. In addition, the ongoing response of the Earth system to the last deglaciation brings about a spatial pattern of regional sea level change across the UK that is dominated by the effect of vertical land motion. At local scales, the impacts of coastal sea level change typically arise primarily from extreme water level events. These deviations from the regional mean water level are often associated with storm surges and extreme wave conditions combined with the local tide. The UKCP18 sea level work focuses on 21st century projections of: (i) regional time-mean sea level; (ii) changes in surge extremes; (iii) potential changes in tide and surge characteristics; and (iv) changes in local wave climate. In addition, we present exploratory projections of regional time-mean sea level change out to 2300. All projections are rooted in, or traceable to, CMIP5 climate model simulations under the RCP climate change scenarios

GOSI9: UK Global Ocean and Sea Ice configurations

The UK Global Ocean and Sea Ice configuration version 9 (GOSI9) is a new traceable hierarchy of three model configurations at 1, and based on version 4.0.4 of the NEMO code. GOSI9 has been developed as part of the UK's Joint Marine Modelling Programme (JMMP), a partnership between the Met Office, the National Oceanography Centre, the British Antarctic Survey, and the Centre for Polar Observation and Modelling. Following a seamless approach, it will be used for a variety of applications across a wide range of spatial and temporal resolutions: short-range coupled numerical weather prediction (NWP) forecasts, ocean forecasts, seasonal and decadal forecasts, and climate and Earth system modelling. The GOSI9 configurations are described in detail with a special focus on the updates since the previous version (GO6-GSI8). Results from 30-year ocean–ice integrations forced by CORE2 fluxes are presented for the three resolutions, and the impacts of the updates are assessed using the integrations. The upgrade to NEMO 4.0.4 includes a new sea ice model SI3 (Sea Ice modelling Integrated Initiative) and faster integration achieved through the use of partially implicit schemes that allow a significant increase in the length of the time step. The quality of the simulations is generally improved compared to GO6-GSI8. The temperature and salinity drifts are largely reduced thanks to the upgrade to NEMO 4.0.4 and the adoption of fourth-order horizontal and vertical advections helping to reduce the numerical mixing. To improve the representation of the Southern Ocean, a scale-aware form of the Gent–McWilliams parameterization and the application of a partial-slip lateral boundary condition on momentum in the Southern Ocean have been added, resulting in a stronger and more realistic Antarctic Circumpolar Current (ACC) transport and a reduction in the temperature and salinity biases along the shelf of Antarctica. In the Arctic, the representation of sea ice is improved, leading to a reduction in surface temperature and salinity biases. In particular, the excessive and unrealistic Arctic summer sea ice melt in GO6-GSI8 is significantly improved in GOSI9 and can be attributed to the change in the sea ice model and to the higher albedos that increased sea ice thickness

UK Global Ocean GO6 and GO7: a traceable hierarchy of model resolutions

Versions 6 and 7 of the UK Global Ocean configuration (known as GO6 and GO7) will form the ocean components of the Met Office GC3.1 coupled model and UKESM1 earth system model to be used in CMIP61 simulations. The label “GO6” refers to a traceable hierarchy of three model configurations at nominal 1, 1∕4 and 1∕12° resolutions. The GO6 configurations are described in detail with particular focus on aspects which have been updated since the previous version (GO5). Results of 30-year forced ocean-ice integrations with the 1∕4° model are presented, in which GO6 is coupled to the GSI8.1 sea ice configuration and forced with CORE22 fluxes. GO6-GSI8.1 shows an overall improved simulation compared to GO5-GSI5.0, especially in the Southern Ocean where there are more realistic summertime mixed layer depths, a reduced near-surface warm and saline biases, and an improved simulation of sea ice. The main drivers of the improvements in the Southern Ocean simulation are tuning of the vertical and isopycnal mixing parameters. Selected results from the full hierarchy of three resolutions are shown. Although the same forcing is applied, the three models show large-scale differences in the near-surface circulation and in the short-term adjustment of the overturning circulation. The GO7 configuration is identical to the GO6 1∕4° configuration except that the cavities under the ice shelves are opened. Opening the ice shelf cavities has a local impact on temperature and salinity biases on the Antarctic shelf with some improvement in the biases in the Weddell Sea

The Met Office Forecast Ocean Assimilation Model (FOAM) using a 1/12‐degree grid for global forecasts

The Met Office Forecast Ocean Assimilation Model (FOAM) ocean–sea-ice analysis and forecasting operational system has been using an ORCA tripolar grid with 1/4° horizontal grid spacing since December 2008. Surface boundary forcing is provided by numerical weather prediction fields from the operational global atmosphere Met Office Unified Model. We present results from a 2-year simulation using a 1/12° global ocean–sea-ice model configuration while keeping a 1/4° data assimilation (DA) set-up. We also describe recent operational data assimilation enhancements that are included in our 1/4° control and 1/12° simulations: a new bias-correction term for sea-level anomaly assimilation and a revised pressure correction algorithm. The primary effect of the first is to decrease the mean and variability of sea-level anomaly increments at high latitudes, whereas the second significantly reduces the vertical velocity standard deviation in the tropical Pacific. The level of improvement achieved with the higher resolution configuration is moderate but consistently satisfactory when measured using neighbourhood verification metrics that provide fairer quantitative comparisons between gridded model fields at different spatial resolutions than traditional root-mean-square metrics. A comparison of the eddy kinetic energy from each configuration and an observation-based product highlights the regions where further system developments are most needed

A comparison of five surface mixed layer models with a year of observations in the North Atlantic

Five upper ocean mixed layer models driven by ERA-Interim surface forcing are compared with a year of hydrographic observations of the upper 1000 m, taken at the Porcupine Abyssal Plain observatory site using profiling gliders. All the models reproduce sea surface temperature (SST) fairly well, with annual mean warm biases of 0.11  ° C (PWP model), 0.24  ° C (GLS), 0.31  ° C (TKE), 0.91  ° C (KPP) and 0.36  ° C (OSMOSIS). The main exception is that the KPP model has summer SSTs which are higher than the observations by nearly 3 ° . Mixed layer salinity (MLS) is not reproduced well by the models and the biases are large enough to produce a non-trivial density bias in the Eastern North Atlantic Central Water which forms in this region in winter. All the models develop mixed layers which are too deep in winter, with average winter mixed layer depth (MLD) biases between 160 and 228 m. The high variability in winter MLD is reproduced more successfully by model estimates of the depth of active mixing and/or boundary layer depth than by model MLD based on water column properties. After the spring restratification event, biases in MLD are small and do not appear to be related to the preceding winter biases. There is a very clear relationship between MLD and local wind stress in all models and in the observations during spring and summer, with increased wind speeds leading to deepening mixed layers, but this relationship is not present during autumn and winter. We hypothesize that the deepening of the MLD in autumn is so strongly driven by the annual cycle in surface heat flux that the winds are less significant in the autumn. The surface heat flux drives a diurnal cycle in MLD and SST from March onwards, though this effect is much more significant in the models than in the observations. We are unable to identify one model as definitely better than the others. The only clear differences between the models are KPP’s inability to accurately reproduce summer SSTs, and the OSMOSIS model’s more accurate reproduction of MLS

The impact of a parameterisation of submesoscale mixed layer eddies on mixed layer depths in the NEMO ocean model

&amp;lt;p&amp;gt;&amp;lt;span&amp;gt;&amp;lt;span&amp;gt;&amp;lt;span&amp;gt;A parameterisation scheme for restratification of the mixed layer by submesoscale mixed layer eddies is implemented in the NEMO ocean model. Its impact on the mixed layer depth (MLD) is examined in 30-year integrations of &amp;quot;uncoupled&amp;quot; ocean-ice and &amp;quot;coupled&amp;quot; atmosphere-ocean-ice-land global climate configurations used by the Met Office Hadley Centre. The specification of the mixed-layer Rossby radius in the scheme is shown to affect its impact on the MLD in the 1/4 degree uncoupled configuration by up to a factor of 2 in subtropical and mid-latitudes. This factor has been limited in the extent to which small mixed-layer Rossby radii are utilised to guard against CFL-type instabilities in the scheme, but such a limit was not found to be necessary for this implementation. An alternative form of the scheme is described that approximates the mixed-layer Rossby radius as a function only of latitude. This form is shown to yield similar results to the original formulation for an appropriate choice of parameters. The global mean impact of the scheme on the MLD is found to be almost twice as large in the 1 degree and 2 degree uncoupled configurations as it is in the 1/4 degree configuration, although the parameterised vertical buoyancy fluxes have closer agreement. This is shown to be the result of the scheme overcompensating for the decay in strength of resolved mixed layer density fronts in this model with decreasing horizontal grid resolution. The MLD criterion defining the depth scale of the scheme is shown to affect its global mean impact on the MLD by nearly a factor of 3 in the 1/4 degree uncoupled and coupled configurations, depending on whether the criterion is chosen to capture the actively mixing layer or well-mixed layer. Climatological MLD biases are improved overall in both cases, substantively reducing deep winter biases whilst slightly increasing shallow summer biases.&amp;lt;/span&amp;gt;&amp;lt;/span&amp;gt;&amp;lt;/span&amp;gt;&amp;lt;/p&amp;gt; </jats:p

Preparing NEMO4.2, the new NEMO modelling framework for the next generation HPCinfrastructures

&amp;lt;p&amp;gt;Nowadays one of the main challenges in scientific computational field is developing the next&amp;lt;br&amp;gt;generation of HPC technologies, applications and systems towards exascale. This leads to focus the&amp;lt;br&amp;gt;efforts on the development of a new, efficient, stable and scalable NEMO reference code with&amp;lt;br&amp;gt;improved performances adapted to exploit future HPC technologies in the context of CMEMS systems.&amp;lt;br&amp;gt;On the main factors that limit the current scalability is an inefficient exploitation of the single node&amp;lt;br&amp;gt;performance. Different technical solutions have been tested to fully exploit memory hierarchies and&amp;lt;br&amp;gt;hardware peak performance. Between all, the fusion of DO loops together and by dividing the&amp;lt;br&amp;gt;computation over tiles are the two optimization strategies more efficiently take advantage of the&amp;lt;br&amp;gt;cache memory organization. This work focuses on the first one.&amp;lt;br&amp;gt;The loop fusion is a transformation which takes two adjacent loops that have the same iteration space&amp;lt;br&amp;gt;traversal and combines their bodies into a single loop. This optimization improves data locality so&amp;lt;br&amp;gt;giving a better exploitation of the cache memory and a reduction of the memory footprint because the&amp;lt;br&amp;gt;temporary arrays can be replaced with scalar values.&amp;lt;br&amp;gt;Performance tests have been executed on a domain size of 3002x2002x31 grid points running 1-year&amp;lt;br&amp;gt;GYRE_PISCES simulations with IO disabled on the Zeus Intel Xeon Gold 6154 machine, available at&amp;lt;br&amp;gt;CMCC. An increasing number of cores - from 504 to 2016 &amp;amp;#8211; have been used to test experiments with&amp;lt;br&amp;gt;the different HPC options.&amp;lt;br&amp;gt;The analysis focused on the routines where the optimizations have been applied. The use of the&amp;lt;br&amp;gt;extended halo introduces a penalty in the execution time that grows as the number of processes&amp;lt;br&amp;gt;increases and generally the use of loop fusion optimization slightly improves the performance. For&amp;lt;br&amp;gt;many routines, as subdomains get smaller, the improvements due to optimizations are less significant.&amp;lt;br&amp;gt;The simultaneous application of all optimizations leads to an improvement between 10% and 50%&amp;lt;br&amp;gt;(except for lateral diffusion). Looking at the total elapsed time, the new HPC optimizations speed up&amp;amp;#160;&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;the elapsed time of a factor 1.25x. Unfortunately, non-optimized routines mitigate this improvement.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;The same scalability test has been repeated running 1-month ORCA025 simulations with the output&amp;lt;br&amp;gt;set to be produced at the end of the run. The results show that the use of loop fusion optimization&amp;lt;br&amp;gt;slightly improves the performance. The use of tiling in ORCA025 introduces less benefits with&amp;lt;br&amp;gt;reference to GYRE. The simultaneous application of all optimizations doesn't lead many benefits in&amp;lt;br&amp;gt;ORCA025 since the improvement concerns only a subset of routines with the Tracer lateral diffusion&amp;lt;br&amp;gt;routine getting worse in all cases.&amp;lt;br&amp;gt;In conclusion the impact of the new optimized code behaves differently depending on the&amp;lt;br&amp;gt;configuration. The overhead introduced by the extended halo implies a computation time cost that the&amp;lt;br&amp;gt;proposed optimizations are able to regain difficultly. Tiling is the aspect with the highest impact in&amp;lt;br&amp;gt;these optimizations (especially in GYRE) and loop fusion has in general a low impact. The optimizations&amp;lt;br&amp;gt;should be applied to all the rest of the code to obtain more benefits.&amp;lt;/p&amp;gt;</jats:p