Mats Bentsen

Coordinate Transformation on a Sphere Using Conformal Mapping

When setting up global ocean circulation models one faces the problem of including the Arctic Ocean where the traditional spherical coordinate system has a singularity at the pole. In addition, in regional model applications one has to deal with open boundaries where assumptions are made about the normally poorly known boundary conditions. Here an analytical reversible coordinate transformation on a sphere that preserves the orthogonality and the shape of infinitesimal figures is presented. Starting from a standard spherical coordinate system, the transformation is able to map the North and South Poles to two arbitrary locations of the earth and this is readily done with the aid of a conformal mapping in the extended complex plane. The resulting coordinate system will have enhanced resolution along the geodesic curve between the new poles. Examples are given where the transformation is used to strongly increase the resolution in a particular region of interest in the model domain.

Sea Surface Temperature of the mid-Piacenzian Ocean: A Data-Model Comparison

The mid-Piacenzian climate represents the most geologically recent interval of long-term average warmth relative to the last million years, and shares similarities with the climate projected for the end of the 21st century. As such, it represents a natural experiment from which we can gain insight into potential climate change impacts, enabling more informed policy decisions for mitigation and adaptation. Here, we present the first systematic comparison of Pliocene sea surface temperature (SST) between an ensemble of eight climate model simulations produced as part of PlioMIP (Pliocene Model Intercomparison Project) with the PRISM (Pliocene Research, Interpretation and Synoptic Mapping) Project mean annual SST field. Our results highlight key regional and dynamic situations where there is discord between the palaeoenvironmental reconstruction and the climate model simulations. These differences have led to improved strategies for both experimental design and temporal refinement of the palaeoenvironmental reconstruction.

Simulated North Atlantic-Nordic Seas water mass exchanges in an isopycnic coordinate OGCM

[1] The variability in the volume exchanges between the North Atlantic and the Nordic Seas during the last 50 years is investigated using a synoptic forced, global version of the Miami Isopycnic Coordinate Ocean Model (MICOM). The simulated volume fluxes agree with the existing observations. The net volume flux across the FaroeShetland Channel (FSC) is positively correlated with the net flux through the Denmark Strait (DS; R = 0.74 for 3 years low pass filtering), but negatively correlated with the net flux across the Iceland-Faroe Ridge (IFR; R = 0.80). For the Atlantic inflow across the FSC and IFR, the correlation is R = 0.59. For the transports through the FSC and DS, the simulation suggests that an atmospheric pattern resembling the North Atlantic Oscillation is the main driving force for the variations, involving Ekman fluxes and barotropic adjustment. The model also shows a 0.7 Sv reduction of the Atlantic inflow to the Nordic Seas since the late 50’s. INDEX TERMS: 4215 Oceanography: General: Climate and interannual variability (3309); 4255 Oceanography: General: Numerical modeling; 9315 Information Related to Geographic Region: Arctic region; 1635 Global Change: Oceans

Gulf Stream Variability in Five Oceanic General Circulation Models

Five non-eddy-resolving oceanic general circulation models driven by atmospheric fluxes derived from the NCEP reanalysis are used to investigate the link between the Gulf Stream (GS) variability, the atmospheric circulation, and the Atlantic meridional overturning circulation (AMOC). Despite the limited model resolution, the temperature at the 200-m depth along the mean GS axis behaves similarly in most models to that observed, and it is also well correlated with the North Atlantic Oscillation (NAO), indicating that a northward (southward) GS shift lags a positive (negative) NAO phase by 0–2 yr. The northward shift is accompanied by an increase in the GS transport, and conversely the southward shift with a decrease in the GS transport. Two dominant time scales appear in the response of the GS transport to the NAO forcing: a fast time scale (less than 1 month) for the barotropic component, and a slower one (about 2 yr) for the baroclinic component. In addition, the two components are weakly coupled. The GS response seems broadly consistent with a linear adjustment to the changes in the wind stress curl, and evidence for baroclinic Rossby wave propagation is found in the southern part of the subtropical gyre. However, the GS shifts are also affected by basin-scale changes in the oceanic conditions, and they are well correlated in most models with the changes in the AMOC. A larger AMOC is found when the GS is stronger and displaced northward, and a higher correlation is found when the observed changes of the GS position are used in the comparison. The relation between the GS and the AMOC could be explained by the inherent coupling between the thermohaline and the wind-driven circulation, or by the NAO variability driving them on similar time scales in the models.

The sensitivity of the present‐day Atlantic meridional overturning circulation to freshwater forcing

[1] Mounting evidence indicates that the Atlantic Meridional Overturning Circulation (AMOC) was strongly reduced during cold climate episodes in the past, possible due to freshwater influx from glacial melting. It is also expected that the freshwater input to high northern latitudes will increase as human-induced global warming continues, with potential impacts on the AMOC. Here we present results from a 150 years sensitivity experiment with the Bergen Climate Model (BCM) for the present-day climate, but with enhanced runoff from the Arctic region throughout the integration. The AMOC drops by 30% over the first 50 years, followed by a gradual recovery. The simulated response indicates that the present-day AMOC might be robust to the isolated effect of enhanced, high-latitude freshwater forcing on a centennial time scale, and that the western tropical North Atlantic may provide key information about the long-term variability, and by that monitoring, of the AMOC.

Ocean General Circulation Modelling of the Nordic Seas

The complexity of the state-of-the-art Ocean General Circulation Models (OGCMs) has increased and the quality of the model systems has improved considerably over the last decades. The improvement is caused by a variety of factors ranging from improved representation of key physical and dynamical processes, parallel development of at least three classes of OGCM systems, accurate and cost-effective numerical schemes, an unprecedented increase in computational resources, and the availability of synoptic, multidecadal atmospheric forcing fields. The implications of these improvements are that the present generation of OGCMs can, for the first time, complement available ocean observations and be used to guide forthcoming ocean observation strategies. OGCMs are also extensively used as laboratories for assessing cause-relationships for observed changes in the marine climate system, and to assess how the ocean system may change in response to, for instance, anomalous air-sea fluxes of heat, freshwater, and momentum. The Nordic Seas are a particularly challenging region for OGCMs because of characteristic length scales of only a few to about 10 km, a variety of complex and interrelated ocean processes, and extreme air-sea fluxes. This paper gives an overview of the status of the prognostic modelling of the Nordic Seas marine climate system. To exemplify the status, we present output from two widely different state-of-the-art OGCM systems. We also address processes that are still inadequately described in the current generation of OGCMs, thus providing guidelines for the future development of model systems particularly tailored for the Nordic Seas region.

Transient response of the Atlantic Meridional Overturning Circulation to enhanced freshwater input to the Nordic Seas–Arctic Ocean in the Bergen Climate Model

The transient response of the climate system to anomalously large freshwater input to the high latitude seas is examined using the newly developed Bergen Climate Model. A 150-yr twin-experiment has been carried out, consisting of a control and a freshwater integration. In the freshwater integration, the freshwater input to the Arctic Ocean and the Nordic Seas is artificially increased by a factor of 3, or to levels comparable to those found during the last deglaciation. The obtained response shows a reduced maximum strength of the Atlantic Meridional Overturning Circulation (AMOC) over the first 50 yr of about 6 Sv (1 Sv =106 m3 s—1), followed by a gradual recovery to a level comparable to the control integration at the end of the period. The weakened AMOC in the freshwater integration is caused by reduced deep-water formation rates in the North Atlantic subpolar gyre and in the Nordic Seas, and by a reduced southward flow of intermediate water masses through the Fram Strait. The recovery of the AMOC is caused by an increased basin-scale upwelling in the Atlantic Ocean of about 1 Sv, northward transport of saline waters originating from the western tropical North Atlantic, and a surface wind field maintaining the inflow of Atlantic Water to the Nordic Seas between the Faroes and Scotland. Associated with the build-up of more saline waters in the western tropical North Atlantic, a warming of ~0.6 °C over the uppermost 1000 m of the water column is obtained in this region. This finding is consistent with paleo records during the last deglaciation showing that the tropics warmed when the high latitudes cooled in periods with reduced AMOC. Furthermore, the results support the presence of a coupled North-Atlantic-Oscillation-like atmosphere’sea-ice’ocean response mode triggered by the anomalous freshwater input. Throughout most of the freshwater integration, the atmospheric circulation is characterized by anomalously low sea level pressure in the Nordic Seas and anomalously high sea level pressure over Spain. This forces the North Atlantic Drift to follow a more easterly path in the freshwater integration than in the control integration, giving an asymmetric sea surface temperature response in the northern North Atlantic, and thereby maintaining the properties of the AtlanticWater entering the Nordic Seas between the Faroes and Scotland throughout the freshwater integration.

Effects of diapycnal and isopycnal mixing on the ventilation of CFCs in the North Atlantic in an isopycnic coordinate OGCM

Simulated distributions of the chlorofluorocarbons CFC-11 and CFC-12 are used to examine the ventilation of the North Atlantic Ocean in a global version of the Miami Isopycnic Coordinate Ocean Model (MICOM). Three simulations are performed: one with a diapycnal diffusivity Kd = 3 × 10−7/N m2 s−1 and an isopycnal diffusive velocity (i.e., diffusivity divided by the size of the grid cell) vtrac = 0.01 m s−1 (Exp. 1); Exp. 2 is as Exp. 1 but with Kd = 5 × 10−8/N m2 s−1 plus increased bottom mixing; and Exp. 3 is as Exp. 2 but with vtrac = 0.0025 m s−1. The main features of the simulated ventilation are strong uptake of the CFCs in the Labrador, Irminger and Nordic Seas, and a topographically aligned geostrophically controlled southward transport of CFC-enriched water in the Atlantic. It is found that the Overflow Waters (OW) from the Nordic Seas, the penetration of the western boundary currents, the ventilation of the subtropical surface waters, the vertical density stratification and the meridional overturning are all critically dependent on the applied isopycnal and diapycnal diffusivities, with Exp. 3 (Exp. 1) yielding the most (least) realistic results. Furthermore, it is the combined rather than the isolated effect of the isopycnal and diapycnal diffusivities that matter. For instance, the strength of the simulated Meridional Overturning Circulation (MOC) is similar in Exps. 1 and 3, but the simulated CFC-distributions are far too diffusive in Exp. 1 and fairly realistic in Exp. 3. It is demonstrated that the simulated distributions of transient tracers like the CFCs can be used to set the strength of the applied isopycnal mixing parameterization, a task that is difficult to conduct based on the simulated hydrography alone.

Simulating transport of non-Chernobyl 137Cs and 90Sr in the North Atlantic–Arctic region

The spatial and temporal distributions of the anthropogenic radionuclides (137)Cs and (90)Sr, originating from nuclear bomb testing and the Sellafield reprocessing plants in the Irish Sea, are simulated using a global version of the Miami Isopycnic Coordinate Ocean Model (MICOM). The physical model is forced with daily atmospheric re-analysed fields for the period 1950 to present. Comparison of temporal evolution of observed and simulated concentrations of (137)Cs have been conducted for the regions east of Scotland, west of central Norway and at the entrance of the Barents Sea. It follows that the radionuclides from the Sellafield discharge reach the Barents Sea region after 4-5 years, in accordance with observations. The simulation provides a detailed distribution and evolution of the radionuclides over the integration time. For the Atlantic waters off the coast of Norway and in the southern Barents Sea, the atmospheric fallout dominates over the Sellafield release up to the mid 1960s and from the early 1990s, whereas Sellafield is the main source for the two radionuclides in the 1970s and 1980s. It is furthermore argued that model systems like the one presented here can be used for future prediction of radioactive contaminations in the Nordic Seas and the Arctic Ocean, for instance under various global warming scenarios.

Seasonal-to-decadal predictions with the ensemble Kalman filter and the Norwegian Earth System Model: a twin experiment

Here, we firstly demonstrate the potential of an advanced flow dependent data assimilation method for performing seasonal-to-decadal prediction and secondly, reassess the use of sea surface temperature (SST) for initialisation of these forecasts. We use the Norwegian Climate Prediction Model (NorCPM), which is based on the Norwegian Earth System Model (NorESM) and uses the deterministic ensemble Kalman filter to assimilate observations. NorESM is a fully coupled system based on the Community Earth System Model version 1, which includes an ocean, an atmosphere, a sea ice and a land model. A numerically efficient coarse resolution version of NorESM is used. We employ a twin experiment methodology to provide an upper estimate of predictability in our model framework (i.e. without considering model bias) of NorCPM that assimilates synthetic monthly SST data (EnKF-SST). The accuracy of EnKF-SST is compared to an unconstrained ensemble run (FREE) and ensemble predictions made with near perfect (i.e. microscopic SST perturbation) initial conditions (PERFECT). We perform 10 cycles, each consisting of a 10-yr assimilation phase, followed by a 10-yr prediction. The results indicate that EnKF-SST improves sea level, ice concentration, 2 m atmospheric temperature, precipitation and 3-D hydrography compared to FREE. Improvements for the hydrography are largest near the surface and are retained for longer periods at depth. Benefits in salinity are retained for longer periods compared to temperature. Near-surface improvements are largest in the tropics, while improvements at intermediate depths are found in regions of large-scale currents, regions of deep convection, and at the Mediterranean Sea outflow. However, the benefits are often small compared to PERFECT, in particular, at depth suggesting that more observations should be assimilated in addition to SST. The EnKF-SST system is also tested for standard ocean circulation indices and demonstrates decadal predictability for Atlantic overturning and sub-polar gyre circulations, and heat content in the Nordic Seas. The system beats persistence forecast and shows skill for heat content in the Nordic Seas that is close to PERFECT.