C. Severijns

A look at the ocean in the EC-Earth climate model

EC-Earth is a newly developed global climate system model. Its core components are the Integrated Forecast System (IFS) of the European Centre for Medium Range Weather Forecasts (ECMWF) as the atmosphere component and the Nucleus for European Modelling of the Ocean (NEMO) developed by Institute Pierre Simon Laplace (IPSL) as the ocean component. Both components are used with a horizontal resolution of roughly one degree. In this paper we describe the performance of NEMO in the coupled system by comparing model output with ocean observations. We concentrate on the surface ocean and mass transports. It appears that in general the model has a cold and fresh bias, but a much too warm Southern Ocean. While sea ice concentration and extent have realistic values, the ice tends to be too thick along the Siberian coast. Transports through important straits have realistic values, but generally are at the lower end of the range of observational estimates. Exceptions are very narrow straits (Gibraltar, Bering) which are too wide due to the limited resolution. Consequently the modelled transports through them are too high. The strength of the Atlantic meridional overturning circulation is also at the lower end of observational estimates. The interannual variability of key variables and correlations between them are realistic in size and pattern. This is especially true for the variability of surface temperature in the tropical Pacific (El Niño). Overall the ocean component of EC-Earth performs well and helps making EC-Earth a reliable climate model.

Dominant Modes of Variability in the South Atlantic: A Study with a Hierarchy of Ocean-Atmosphere Models

Using an atmosphere model of intermediate complexity and a hierarchy of ocean models, the dominant modes of interannual and decadal variability in the South Atlantic Ocean are studied. The atmosphere Simplified Parameterizations Primitive Equation Dynamics (SPEEDY) model has T30L7 resolution. The physical package consists of a set of simplified physical parameterization schemes, based on the same principles adopted in the schemes of state-of-the-art AGCMs. It is at least an order of magnitude faster, whereas the quality of the simulated climate compares well with those models. The hierarchy of ocean models consists of simple mixed layer models with an increasing number of physical processes involved such as Ekman transport, wind-induced mixing, and wind-driven barotropic transport. Finally, the atmosphere model is coupled to a regional version of the Miami Isopycnal Coordinate Ocean Model (MICOM) covering the South Atlantic with a horizontal resolution of 1° and 16 vertical layers. The coupled modes of mean sea level pressure and sea surface temperature simulated by SPEEDY– MICOM strongly resemble the modes as analyzed from the NCEP–NCAR reanalysis, indicating that this model configuration possesses the required physical mechanisms for generating these modes of variability. Using the ocean model hierarchy the authors were able to show that turbulent heat fluxes, Ekman transport, and wind-induced mixing contribute to the generation of the dominant modes of coupled SST variability. The different roles of these terms in generating these modes are analyzed. Variations in the wind-driven barotropic transport mainly seem to affect the SST variability in the Brazil–Malvinas confluence zone. The spectra of the mixed layer models appeared to be too red in comparison with the fully coupled SPEEDY–MICOM model due to the too strong coupling between SST and surface air temperatures (SATs), resulting from the inability to advect and subduct SST anomalies by the mixed layer models. In SPEEDY–MICOM anomalies in the southeastern corner of the South Atlantic are subducted and advected toward the north Brazilian coast on a time scale of about 6 yr.

Influence of the Meridional Overturning Circulation on Tropical Atlantic Climate and Variability

Abstract The influence of the meridional overturning circulation on tropical Atlantic climate and variability has been investigated using the atmosphere–ocean coupled model Speedy-MICOM (Miami Isopycnic Coordinate Ocean Model). In the ocean model MICOM the strength of the meridional overturning cell can be regulated by specifying the lateral boundary conditions. In case of a collapse of the basinwide meridional overturning cell the SST response in the Atlantic is characterized by a dipole with a cooling in the North Atlantic and a warming in the tropical and South Atlantic. The cooling in the North Atlantic is due to the decrease in the strength of the western boundary currents, which reduces the northward advection of heat. The warming in the tropical Atlantic is caused by a reduced ventilation of water originating from the South Atlantic. This effect is most prominent in the eastern tropical Atlantic during boreal summer when the mixed layer attains its minimum depth. As a consequence the seasonal cycle...

Tropical Pacific–Driven Decadel Energy Transport Variability

The atmospheric energy transport variability associated with decadal sea surface temperature variability in the tropical Pacific is studied using an atmospheric primitive equation model coupled to a slab mixed layer. The decadal variability is prescribed as an anomalous surface heat flux that represents the reduced ocean heat transport in the tropical Pacific when it is anomalously warm. The atmospheric energy transport increases and compensates for the reduced ocean heat transport. Increased transport by the mean meridional overturning (i.e., the strengthening of the Hadley cells) causes increased poleward energy transport. The subtropical jets increase in strength and shift equatorward, and in the midlatitudes the transients are affected. NCEP–NCAR reanalysis data show that the warming of the tropical Pacific in the 1980s compared to the early 1970s seems to have caused very similar changes in atmospheric energy transport indicating that these atmospheric transport variations were driven from the tropical Pacific. To study the implication of these changes for the coupled climate system an ocean model is driven with winds obtained from the atmosphere model. The poleward ocean heat transport increased when simulated wind anomalies associated with decadal tropical Pacific variability were used, showing a negative feedback between decadal variations in the mean meridional circulation in the atmosphere and in the Pacific Ocean. The Hadley cells and subtropical cells act to stabilize each other on the decadal time scale.

Simulations of Hydrographic Properties in the Northwestern North Atlantic Ocean in Coupled Climate Models

Abstract The performance of coupled climate models (CCMs) in simulating the hydrographic structure and variability of the northwestern North Atlantic Ocean, in particular the Labrador and Irminger Seas, has been assessed. This area plays an important role in the meridional overturning circulation. Hydrographic properties of the preindustrial run of eight CCMs used in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) are compared with observations from the World Ocean Circulation Experiment Repeat section 7 (WOCE AR7). The mean and standard deviation of 20 yr of simulated data are compared in three layers, representing the surface waters, intermediate waters, and deep waters. Two models simulate an extremely cold, fresh surface layer with model biases down to −1.7 psu and −4.0°C, much larger than the observed ranges of variability. The intermediate and deep layers are generally too warm and saline, with biases up to 0.7 psu and 2.8°C. An analysis of the maximum mixed layer...

The future of Antarctica's surface winds simulated by a high‐resolution global climate model: 2. Drivers of 21st century changes

Antarcticas katabatic winds are among the strongest near-surface winds on Earth, and among the most consistent ones. As these winds are primarily due to the strong surface cooling, greenhouse warming of the surface may act to reduce the strength of these winds as well as their consistency. Here we use the atmospheric component of the global climate model EC-Earth in prescribed sea surface temperature (SST) simulations of the present day (2002–2006) and future (2094–2098) climates, using two model resolutions: (1) T159L62 (~100 km, 62 vertical levels), and (2) T799L91 (~20 km, 91 vertical levels) to investigate changes in Antarcticas surface winds and the reasons thereof. Circumpolar westerlies over the Southern Ocean strengthen and shift poleward because of the deepening of the circumpolar trough and the associated increase in Southern Annular Mode (SAM), especially in high resolution, causing weaker coastal easterlies. Generally, surface wind speeds over the Antarctica mainland exhibit a small decrease. According to the simulations, the temperature deficit (or inversion strength) and associated katabatic forcing exhibit only minor changes over the continent. Changes in the surface winds over Antarcticas slopes can thus be attributed mainly to changes in the synoptic forcing (large-scale pressure gradient). Hence, with modeled 21st century changes in the katabatic forcing being small, changes in zonal and meridional surface winds in and around Antarctica are largely decoupled from those over the Southern Ocean and are governed by changes in synoptic forcing and large-scale pressure gradients. As a result, these changes are largely independent on model resolution.

The future of Antarctica's surface winds simulated by a high‐resolution global climate model: 1. Model description and validation

One of the key components of Antarcticas harsh climate is its renowned katabatic winds, which are among the fiercest surface winds on Earth. Caused primarily by strong surface cooling over the sloping ice surface, these semipermanent winds result primarily from the strong surface temperature inversion and associated temperature deficit between the surface layer and the free atmosphere aloft. Katabatic winds exert a strong effect on the mass budget of the Antarctic ice sheet by affecting snowdrift (sublimation) and by (partially) regulating the net atmospheric moisture transports toward the Antarctic. It has been suggested that greenhouse warming may lead to reduced surface cooling and weakened katabatic winds. This is tested by using a global climate model (EC-Earth) in prescribed sea surface temperature simulations of the present-day (2002–2006) and future (2094–2098) climates. Because simulated topographically induced katabatic winds are likely to depend on the model grid, we employ two model resolutions: (1) T159L62 (~100 km, 62 vertical levels) and (2) T799L91 (~20 km, 91 vertical levels). It is shown here that present-day surface winds over Antarctica in high resolution are generally stronger than in low resolution, especially in the escarpment region with its steep orography. Simulated surface winds are generally underestimated with respect to observations, in particular the strongest winds (occurring over steep slopes), and especially in low resolution. The seasonal cycle in surface winds is simulated fairly accurately. Surface temperatures are also relatively well simulated (when corrected for elevation differences), especially in high resolution.