A simple ocean performance metrics applied to historical CMIP5 simulations

While in atmosphere models it is already common to define objective metrics to investigate how well an atmospheric model performs compared to observations, this is not too common for ocean models. Here we define a simple metrics encompassing the 3D structure of bias and absolute error to estimate the performance of ocean models and we apply it to the historical CMIP5 simulations from 1950 to 2005. Ocean model 3D temperature and salinity fields are compared to the PHC climatology for the major ocean basins. For each 3D grid point of the PHC dataset bias and absolute error of the model climatology are calculated and then volume- averaged over each ocean basin. An average CMIP5 model error is calculated for each ocean basin and used as a reference when investigating a particular model - similarly as has been done for the atmosphere by Reichler and Kim (2008) for CMIP3 models. Ocean surface temperature is generally reasonably well simulated by CMIP5 models and mean absolute errors amount to around 1 K which is comparable to the interannual variability. But in 500 to 1000 m - depending on the ocean basin and on the model - mean absolute errors of up to 4 K are detected which clearly exceed the interannual variability of generally below 1 K. For salinity mean absolute errors are in all levels clearly higher than the interannual variability. For example at the surface the mean absolute error amounts to up to 1 psu while the interannual variability is below 0.2 psu. Even if investigating biases which allows for cancelling out of errors within a basin instead of the mean absolute error this statement still holds in many cases. This means that there is a lot of scope for improvement of the simulation of the vertical structure of the ocean

Flow‐dependent stochastic coupling for climate models with high ocean‐to‐atmosphere resolution ratio

This study introduces a new flow‐dependent distribution sampling (FDDS) scheme for air–sea coupling. The FDDS scheme is implemented in a climate model and used to improve the simulated mean and variability of atmospheric and oceanic surface fields and thus air–sea fluxes. Most coupled circulation models use higher resolutions in the sea ice and ocean compared to the atmospheric model component, thereby explicitly simulating the atmospheric subgrid‐scale at the interface. However, the commonly applied averaging of surface fields and air–sea fluxes tends to smooth fine‐scale structures, such as oceanic fronts. The stochastic FDDS scheme samples the resolved spatial ocean (and sea ice) subgrid distribution that is usually not visible to a coarser‐resolution atmospheric model. Randomly drawn nodal ocean values are passed to the corresponding atmospheric boxes for the calculation of surface fluxes, aiming to enhance surface flux variability. The resulting surface field perturbations of the FDDS scheme are based on resolved dynamics, displaying pronounced seasonality with realistic magnitude. The AWI Climate Model is used to test the scheme on interannual time‐scales. Our set‐up features a high ocean‐to‐atmosphere resolution ratio in the Tropics, with grid‐point ratios of about 60:1. Compared to the default deterministic averaging, changes are largest in the Tropics leading to an improved spatial distribution of precipitation with bias reductions of up to 50%. Enhanced sea‐surface temperature variability in boreal winter further improves the seasonal phase locking of temperature anomalies associated with the El Niño–Southern Oscillation. Mean 2m temperature, sea ice thickness and concentration react with a contrasting dipole pattern between hemispheres but a joint increase of monthly and interannual variability. This first approach to implement a flow‐dependent stochastic coupling scheme shows considerable benefits for simulations of global climate, and various extensions and modifications of the scheme are possible

Long-term evolution of ocean eddy activity in a warming world

AbstractMesoscale ocean eddies, an important element of the climate system, impact ocean circulation, heat uptake, gas exchange, carbon sequestration and nutrient transport. Much of what is known about ongoing changes in ocean eddy activity is based on satellite altimetry; however, the length of the altimetry record is limited, making it difficult to distinguish anthropogenic change from natural variability. Using a climate model that exploits a variable-resolution unstructured mesh in the ocean component to enhance grid resolution in eddy-rich regions, we investigate the long-term response of ocean eddy activity to anthropogenic climate change. Eddy kinetic energy is projected to shift poleward in most eddy-rich regions, to intensify in the Kuroshio Current, Brazil and Malvinas currents and Antarctic Circumpolar Current and to decrease in the Gulf Stream. Modelled changes are linked to elements of the broader climate including Atlantic meridional overturning circulation decline, intensifying Agulhas leakage and shifting Southern Hemisphere westerlies.</jats:p

Antarctic sea ice decline delayed well into the 21st century in a high-resolution climate projection

Despite ongoing global warming and strong sea ice decline in the Arctic, the sea ice extent around the Antarctic continent has not declined during the satellite era since 1979. This is in stark contrast to existing climate models that tend to show a strong negative sea ice trend for the same period; hence the confidence in projected Antarctic sea-ice changes is considered to be low. In the years since 2016, there has been significantly lower Antarctic sea ice extent, which some consider a sign of imminent change; however, others have argued that sea ice extent is expected to regress to the weak decadal trend in the near future. In this presentation, we show results from climate change projections with a new climate model that allows the simulation of mesoscale eddies in dynamically active ocean regions in a computationally efficient way. We find that the high-resolution configuration (HR) favours periods of stable Antarctic sea ice extent in September as observed over the satellite era. Sea ice is not projected to decline well into the 21st century in the HR simulations, which is similar to the delaying effect of, e.g., added glacial melt water in recent studies. The HR ocean configurations simulate an ocean heat transport that responds differently to global warming and is more efficient at moderating the anthropogenic warming of the Southern Ocean. As a consequence, decrease of Antarctic sea ice extent is significantly delayed, in contrast to what existing coarser-resolution climate models predict. Other explanations why current models simulate a non-observed decline of Antarctic sea-ice have been put forward, including the choice of included sea ice physics and underestimated simulated trends in westerly winds. Our results provide an alternative mechanism that might be strong enough to explain the gap between modeled and observed trends alone

Multi-resolution climate modelling with the AWI Climate Model (AWI-CM)

The recently established AWI Climate Model (AWI-CM), a coupled configuration of the Finite Element Sea Ice-Ocean Model (FESOM) with the atmospheric model ECHAM6, uses a novel multi-resolution approach: Its ocean component builds on a finite element dynamical core supporting unstructured triangular surface grids, allowing to distribute the grid points in a flexible manner. This allows to concentrate resolution in dynamically important regions, with a continuous transition zone to the coarser resolution in other areas. The model is an ideal tool to study the influence of explicit resolution of smaller scales in dedicated experiments. The unique – spatially seamless – approach might also be of benefit when it comes to temporally seamless prediction, bridging the gap between numerical weather prediction and climate models. A first benchmark set-up of AWI-CM with moderate resolution in the atmosphere (T63) and 25km in key ocean areas, e.g. around the equator, achieved a similar overall simulation performance in a long control simulation compared to well-established CMIP5 models. In particular, the (isotropically) increased equatorial resolution considerably increased the realism of TIW activity and ENSO-related variability compared to standard resolutions. The potential of AWI-CM is further exploited within the EU project PRIMAVERA in the HighResMIP of CMIP6, where we plan to contribute simulations with eddy-resolving resolutions (1/12° or 9-10 km) in key areas of the global ocean, such as the Gulf Stream-North Atlantic Current region, the Agulhas retroflection zone, or the Arctic basin. First simulations show distinct improvements with respect to the development of deep temperature and salinity biases in the North Atlantic Ocean and an overall improvement of surface biases. At even higher resolutions of 4.5 km locally in the Arctic, linear kinematic features emerge in the simulated sea ice distribution with potentially strong impacts on air-sea fluxes in the coupled system. Although the tested set-ups are computationally very demanding (with numbers of grid points comparable to a regular 0.25° grid), the throughput is high at about 8 simulated years per day because of high scalability. In addition, we are about to finish the development of a finite volume version of the ocean model code (FESOM 2). It is already faster than the original FESOM version by a factor of two to three, which will further enlarge the set of computationally feasible applications

Delayed Antarctic sea-ice decline in high-resolution climate change simulations

Despite global warming and Arctic sea-ice loss, on average the Antarctic sea-ice extent has not declined since 1979 when satellite data became available. In contrast, climate model simulations tend to exhibit strong negative sea-ice trends for the same period. This Antarctic sea-ice paradox leads to low confidence in 21st-century sea-ice projections. Here we present multi-resolution climate change projections that account for Southern Ocean mesoscale eddies. The high-resolution configuration simulates stable September Antarctic sea-ice extent that is not projected to decline until the mid-21st century. We argue that one reason for this finding is a more realistic ocean circulation that increases the equatorward heat transport response to global warming. As a result, the ocean becomes more efficient at moderating the anthropogenic warming around Antarctica and hence at delaying sea-ice decline. Our study suggests that explicitly simulating Southern Ocean eddies is necessary for providing Antarctic sea-ice projections with higher confidence

Twenty first century changes in Antarctic and Southern Ocean surface climate in CMIP6

Two decades into the 21st century there is growing evidence for global impacts of Antarctic and Southern Ocean climate change. Reliable estimates of how the Antarctic climate system would behave under a range of scenarios of future external climate forcing are thus a high priority. Output from new model simulations coordinated as part of the Coupled Model Intercomparison Project Phase 6 (CMIP6) provides an opportunity for a comprehensive analysis of the latest generation of state‐of‐the‐art climate models following a wider range of experiment types and scenarios than previous CMIP phases. Here the main broad‐scale 21st century Antarctic projections provided by the CMIP6 models are shown across four forcing scenarios: SSP1‐2.6, SSP2‐4.5, SSP3‐7.0 and SSP5‐8.5. End‐of‐century Antarctic surface‐air temperature change across these scenarios (relative to 1995–2014) is 1.3, 2.5, 3.7 and 4.8°C. The corresponding proportional precipitation rate changes are 8, 16, 24 and 31%. In addition to these end‐of‐century changes, an assessment of scenario dependence of pathways of absolute and global‐relative 21st century projections is conducted. Potential differences in regional response are of particular relevance to coastal Antarctica, where, for example, ecosystems and ice shelves are highly sensitive to the timing of crossing of key thresholds in both atmospheric and oceanic conditions. Overall, it is found that the projected changes over coastal Antarctica do not scale linearly with global forcing. We identify two factors that appear to contribute: (a) a stronger global‐relative Southern Ocean warming in stabilisation (SSP2‐4.5) and aggressive mitigation (SSP1‐2.6) scenarios as the Southern Ocean continues to warm and (b) projected recovery of Southern Hemisphere stratospheric ozone and its effect on the mid‐latitude westerlies. The major implication is that over coastal Antarctica, the surface warming by 2100 is stronger relative to the global mean surface warming for the low forcing compared to high forcing future scenarios

Ocean Model Formulation Influences Transient Climate Response

The transient climate response (TCR) is 20% higher in the Alfred Wegener Institute Climate Model (AWI-CM) compared to the Max Planck Institute Earth System Model (MPI-ESM) whereas the equilibrium climate sensitivity (ECS) is by up to 10% higher in AWI-CM. These results are largely independent of the two considered model resolutions for each model. The two coupled CMIP6 models share the same atmosphere-land component ECHAM6.3 developed at the Max Planck Institute for Meteorology (MPI-M). However, ECHAM6.3 is coupled to two different ocean models, namely the MPIOM sea ice-ocean model developed at MPI-M and the FESOM sea ice-ocean model developed at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). A reason for the different TCR is related to ocean heat uptake in response to greenhouse gas forcing. Specifically, AWI-CM simulations show stronger surface heating than MPI-ESM simulations while the latter accumulate more heat in the deeper ocean. The vertically integrated ocean heat content is increasing slower in AWI-CM model configurations compared to MPI-ESM model configurations in the high latitudes. Weaker vertical mixing in AWI-CM model configurations compared to MPI-ESM model configurations seems to be key for these differences. The strongest difference in vertical ocean mixing occurs inside the Weddell and Ross Gyres and the northern North Atlantic. Over the North Atlantic, these differences materialize in a lack of a warming hole in AWI-CM model configurations and the presence of a warming hole in MPI-ESM model configurations. All these differences occur largely independent of the considered model resolutions

Multi-resolution simulations with the AWI Climate Model (AWI-CM)

The recently established AWI Climate Model (AWI-CM), a coupled configuration of the Finite Element Sea Ice-Ocean Model (FESOM) with the atmospheric model ECHAM6, uses a novel multi-resolution approach: Its ocean component builds on a finite element dynamical core supporting unstructured triangular surface grids, allowing to distribute the grid points in a flexible manner. This allows to concentrate resolution in dynamically important regions, with a continuous transition zone to the coarser resolution in other areas. The model is an ideal tool to study the influence of explicit resolution of smaller scales in dedicated experiments. The unique – spatially seamless – approach might also be of benefit when it comes to temporally seamless prediction, bridging the gap between numerical weather prediction and climate models. A first benchmark set-up of AWI-CM with moderate resolution in the atmosphere (T63) and 25km in key ocean areas, e.g. around the equator, achieved a similar overall simulation performance in a long control simulation compared to well-established CMIP5 models. In particular, the (isotropically) increased equatorial resolution considerably increased the realism of TIW activity and ENSO-related variability compared to standard resolutions. The potential of AWI-CM is further exploited within the EU project PRIMAVERA in the HighResMIP of CMIP6, where we plan to contribute simulations with eddy-resolving resolutions (1/12° or 9-10 km) in key areas of the global ocean, such as the Gulf Stream-North Atlantic Current region, the Agulhas retroflection zone, or the Arctic basin. First simulations show distinct improvements with respect to the development of deep temperature and salinity biases in the North Atlantic Ocean and an overall improvement of surface biases. At even higher resolutions of 4.5 km locally in the Arctic, linear kinematic features emerge in the simulated sea ice distribution with potentially strong impacts on air-sea fluxes in the coupled system. Although the tested set-ups are computationally very demanding (with numbers of grid points comparable to a regular 0.25° grid), the throughput is high at about 8 simulated years per day because of high scalability. In addition, we are about to finish the development of a finite volume version of the ocean model code (FESOM 2). It is already faster than the original FESOM version by a factor of two to three, which will further enlarge the set of computationally feasible applications

PRIMAVERA: High-Resolution Climate Processes: Benefits of locally refined ocean resolution

Ocean model biases such as the North West corner cold bias connected to the location of the Gulf Stream path, the warm bias in upwelling zones, the warm bias in the Southern Ocean, and model drift like the deep ocean warm bias which tends to peak in around 800 to 1000 m depth in the Atlantic Ocean are issues common among state-of-the-art ocean models. These issues are often amplified when the ocean model is coupled to an atmosphere model to perform climate simulations. Furthermore, unrealistic freezing of the Labrador Sea is an issue in various climate models. With the unstructured mesh approach in our Finite Element Sea ice Ocean Model (FESOM) we are able to systematically investigate the benefits of local refinement of the ocean model grid both in an uncoupled set-up (sea-ice ocean only) as well as in a fully coupled climate model (atmosphere- land-sea ice-ocean). While the horizontal ocean model resolution is 25 km on average in the finer grids, we refine the grids in some key areas to up to 5 km. Therefore we can explicitly resolve ocean eddies and simulate eddy-mean flow interactions in these key areas. The atmosphere-land component of our AWI-CM (Alfred Wegener Institute Climate Model) is ECHAM6-JSBACH developed at the Max-Planck-Institute for Meteorology in Hamburg, Germany. Here we present results of century-long uncoupled and coupled simulations on ocean model grids with different local refinements while keeping the atmosphere resolution constant in the coupled simulations. Results indicate that high horizontal resolutions in key regions such as the Gulf Stream / North Atlantic Current area or the Agulhas Stream can reduce biases such as the North West corner cold bias and the deep ocean model drift