The North Atlantic subpolar gyre in four high resolution models

The authors present the first quantitative comparison between new velocity datasets and high-resolution models in the North Atlantic subpolar gyre [1/10° Parallel Ocean Program model (POPNA10), Miami Isopycnic Coordinate Ocean Model (MICOM), ° Atlantic model (ATL6), and Family of Linked Atlantic Ocean Model Experiments (FLAME)]. At the surface, the model velocities agree generally well with World Ocean Circulation Experiment (WOCE) drifter data. Two noticeable exceptions are the weakness of the East Greenland coastal current in models and the presence in the surface layers of a strong southwestward East Reykjanes Ridge Current. At depths, the most prominent feature of the circulation is the boundary current following the continental slope. In this narrow flow, it is found that gridded float datasets cannot be used for a quantitative comparison with models. The models have very different patterns of deep convection, and it is suggested that this could be related to the differences in their barotropic transport at Cape Farewell. Models show a large drift in watermass properties with a salinization of the Labrador Sea Water. The authors believe that the main cause is related to horizontal transports of salt because models with different forcing and vertical mixing share the same salinization problem. A remarkable feature of the model solutions is the large westward transport over Reykjanes Ridge [10 Sv (Sv ≡ 106 m3 s−1) or more

Variability of the meridional overturning circulation of the North Atlantic: Sensitivity to overflows of dense water masses

A numerical model of the Atlantic Ocean was used to study the low-frequency variability of meridional transports in the North Atlantic. The model shows a behaviour similar to those used in previous studies, and the temporal variability of certain variables compares favourably to observed time series. By changing the depth and width of the sills between the subpolar North Atlantic and the Nordic Seas, the mean horizontal and overturning circulation as well as some water mass properties are modified significantly. The reaction of meridional oceanic transports to atmospheric forcing fluctuations remains, however, unchanged. The critical role of the surface heat flux retroaction term for the meridional heat transport in stand-alone ocean models is discussed. The experiments underline the role of atmospheric variability for fluctuations of the large-scale ocean circulation on time scales from years to decades, and they support the hypothesis that the mean overturning strength is controlled by the model representation of the density of the overflow water masses

Scalability and Performance of MOM: High Resolution Simulations of the Atlantic Circulation

In the framework of FLAME (Family of Linked Atlantic Model Experiments) an eddy-permitting model of the Atlantic Ocean was used to hindcast the uptake and spreading of anthropogenic trace gases, CO2 and CFC, during the last century. The code is based on the public domain software MOM (Modular Ocean Model) Version 2.1. Towards a parallel version the code was extended for shmem and MPI message passing to achieve portability to Cray-T3E and NEC SX systems. The performance of this production code on Cray-T3E as well as NEC-SX5 and SX6 systems is discussed. To underline the need for high-resolution modeling some physical model results are presented

A monitoring design for the Atlantic meridional overturning circulation

Current hydrographic data can provide snapshots but no continuous timeseries of the meridional overturning circulation (MOC). Using output from two eddy-permitting numerical ocean models we test the feasibility of a monitoring system for the MOC in the North Atlantic. The results suggest that a relatively simple arrangement, using moorings placed across a longitude-depth section and the zonal wind stress, is able to capture most of the MOC strength and vertical structure as a function of time. Being closely related to the transport of energy to the North Atlantic, measuring the MOC would open the prospect of having continuous information about a key element of northern hemisphere climate

Windthrow damage in Picea abies is associated with physical and chemical stem wood properties

On 26 December 1999, the windstorm "Lothar" hit large parts of western and central Europe. In Switzerland, windthrow losses reached 12.7 Mio m(3) of timber, corresponding to 2.8 times the annual national timber harvest. Although these exceptional losses were due to extreme peak velocities, recent changes in tree nutrition may have increased forest susceptibility. Previous controlled environment experiments revealed that wood density (associated with wood stiffness) tends to increase in elevated CO2, and to decrease when N-availability is enhanced (e.g., by soluble N-deposition). Such changes in wood quality could theoretically influence the risk of wind damage. We used the "Lothar" windstorm as a "natural experiment" to explore links between damage and wood properties. In 104 windthrow sites across the Swiss Plateau, more than 1,600 wood cores from (1) broken, (2) uprooted and (3) still standing (not damaged) spruce trees (Picea abies) were collected in February and March 2000. Wood properties, treering width and chemistry of the wood samples were analysed. Broken trees showed wider treerings in the decade 1990-99 compared to non-broken trees (either uprooted or undamaged trees). Broken trees also showed lower non-structural carbohydrate (NSC) concentration in sapwood, reflecting active structural carbohydrate sinks associated with fast growth. There was also a trend for higher tissue N-concentrations in broken trees. No significant differences between damage types were found in wood density and wood shrinkage during desiccation. We conclude that stem breakage risk of P. abies is associated with a stimulation of growth in the past decade and with changes in tree nutritional status. However, the risk for windthrow of whole spruce trees (uprooted but not broken) was not related to the studied wood parameters