The Coupled Model
Intercomparison Project phase 6 (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 Hadley Centre Global Environment Model 3 – Global Coupled vn 3.1 (HadGEM3-GC3.1) model that ranges 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 constant forcing to assess drift. 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 transport), while an
eddy-present ocean resolution has a considerably reduced Antarctic
Circumpolar Current strength. All models have a reasonable representation of El Niño–Southern Oscillation. In general, the biases present after 30 years of simulations do not change character markedly over longer timescales,
justifying the experimental design.
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 to both 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 northwest Atlantic, N96ORCA1 shows a cold surface bias of up to 6 K, 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 2 K) 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 the UK Earth System Model and as a stand‐alone coupled climate model.
Plain Language Summary
In this article, a new version of the climate model currently used in the United Kingdom (HadGEM3) is presented and analyzed. The circulation of the atmosphere and the oceans is simulated on a relatively coarse spatial grid with a grid cell size of about 120 km. The advantage of using a coarse spatial grid is that less computing power (on a supercomputer) is needed compared to using a finer grid. This gives an opportunity to do many more simulations of the ways in which Earth's climate may evolve in the decades and centuries ahead. We have carefully compared a simulation of the climate around the year 2000 with climate observations from that time and with a simulation from the same model with a finer spatial grid. Our results show that our new, coarse‐grid version is representing the current climate reasonably well, for instance, with regards to climate variability in the tropics and major ocean currents. However, there are clear differences between the two models. In the coarse‐grid model, the ocean surface is too cold in the northwest Atlantic, while in the fine‐grid version it is too warm in the Southern Ocean around Antarctica. We look into explanations for these inaccuracies.
Key Points
A low‐resolution, traceable version of the current Met Office Hadley Centre climate model HadGEM3 GC3.1 is presented
The scientific performance is comparable to the medium‐resolution version, while requiring much less computational resources
In the low‐resolution version the Southern Ocean warm bias is reduced, linked with a more realistic ocean circulation
The turbulent mixing in thin ocean surface boundary layers (OSBL), which occupy the upper 100 m or so of the ocean, control the exchange of heat and trace gases between the atmosphere and ocean. Here ...we show that current parameterizations of this turbulent mixing lead to systematic and substantial errors in the depth of the OSBL in global climate models, which then leads to biases in sea surface temperature. One reason, we argue, is that current parameterizations are missing key surface‐wave processes that force Langmuir turbulence that deepens the OSBL more rapidly than steady wind forcing. Scaling arguments are presented to identify two dimensionless parameters that measure the importance of wave forcing against wind forcing, and against buoyancy forcing. A global perspective on the occurrence of wave‐forced turbulence is developed using re‐analysis data to compute these parameters globally. The diagnostic study developed here suggests that turbulent energy available for mixing the OSBL is under‐estimated without forcing by surface waves. Wave‐forcing and hence Langmuir turbulence could be important over wide areas of the ocean and in all seasons in the Southern Ocean. We conclude that surface‐wave‐forced Langmuir turbulence is an important process in the OSBL that requires parameterization.
Key Points
Climate models have biases in the depth of the ocean surface boundary layer
Langmuir turbulence is a key process mixing the ocean surface boundary layer
Langmuir turbulence deepens the layer more quickly than wind‐forced turbulence
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 “uncoupled” ocean–ice (GO5) and “coupled” atmosphere–ocean–ice–land (GC2) 1/4° global climate configurations used by the Met Office Hadley Centre. The impact of the scheme on the MLD in GO5 is up to twice as large in subtropical and mid-latitudes when the mixed layer Rossby radius is not limited to guard against CFL-type instabilities and excessively strong volume overturning. Such a limit is not found to be necessary for stable integration of the scheme in NEMO. An alternative form of the scheme is described that approximates the mixed layer Rossby radius as a function only of latitude. This formulation is more generally robust to instability and has a comparatively larger impact on the MLD than the original formulation, but yields qualitatively similar results. The global mean impact of the scheme on the MLD is found to be almost twice as large in 1° and 2° configurations of GO5 as it is in the 1/4° configuration. 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 GO5 and GC2, depending on whether the criterion is chosen to capture the actively mixing layer or the well-mixed layer. The parameterisation reduces the magnitude of deep MLD biases while increasing the magnitude of shallow biases. The globally averaged winter MLD bias is reduced from 17% to 9% of climatological values in GO5 but changes from +3% to −4% in GC2. Summer mixed layers are too shallow on average in both configurations and their average magnitude is increased by the parameterisation.
•A parameterisation of submesoscale mixed layer eddies is implemented in NEMO.•Mixed layer depth impact is examined in 30-year ocean-only and coupled integrations.•The impact varies by a factor of 2-3 when altering two model-dependent parameters.•The impact is around twice as large in non-eddying configurations.•Deep mixed layer biases are reduced but shallow biases are increased.
•Unique, year-long, high resolution glider dataset compared with 5 mixed layer models.•Model winter mixed layers are too deep, with average biases between 160 and 228 m.•After spring ...restratification, biases in MLD are small and unrelated to winter biases.•Model biases in mixed layer salinity produce non-trivial density biases, but this does not affect the subsequent spring and summer MLD and SST.
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.
Abstract 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.
The G-rich single-stranded DNA at the 3′ end of human telomeres can self-fold into G-quaduplex (GQ). However, telomere lengthening by telomerase or the recombination-based alternative lengthening of ...telomere (ALT) mechanism requires protein loading on the overhang. Using single-molecule fluorescence spectroscopy, we discovered that lengthening the telomeric overhang also increased the rate of dynamic exchanges between structural conformations. Overhangs with five to seven TTAGGG repeats, compared with four repeats, showed much greater dynamics and accessibility to telomerase binding and activity and loading of the ALT-associated proteins RAD51, WRN, and BLM. Although the eight repeats are highly dynamic, they can fold into two GQs, which limited protein accessibility. In contrast, the telomere-specific protein POT1 is unique in that it binds independently of repeat number. Our results suggest that the telomeric overhang length and dynamics may contribute to the regulation of telomere extension via telomerase action and the ALT mechanism.
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•Telomeric overhang exhibits dynamic behavior when greater than four TTAGGG repeats•Four and eight repeats yield low accessibility to WRN, BLM, Rad51, and telomerase•Five to seven repeats yield increased accessibility to loading of the same proteins•POT1 binds all repeat lengths proficiently, suggesting an active disruption of GQ
The possible G-quadruplex formation at the end of human telomeres may hinder loading of proteins. Hwang et al. show that the length of TTAGGG repeats modulates accessibility of telomerase and the alternative lengthening of telomere (ALT)-associated proteins.
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