Discription of the parallel isopycnal primitive equation OGCM (PIPE) With models for Sea Ice and Snow, Mixed Layer and Tides coupled to an wet atmospheric energy balance model with river runoff, soil and glaciers

SIGLEAvailable from TIB Hannover: RR 8522(19) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Sensitivities of the sea-ice component of two ocean general circulation models

The sensitivity of the sea-ice component of two distinct oceanic general circulation models (OGCMs) is investigated under quasi-identical forcing and boundary conditions and compared with the performance of a stand-along sea-ice model, which reproduced realistic sea-ice characteristics under the same external conditions. In contrast to most other polar studies, we are focussing on the Southern Ocean, where the presence of sea ice may modify the world ocean's deep-water formation substantially. All three sea-ice models employ a viscous-plastic constitutive law to describe the variation in internal ice stress. The initial thermodynamic formulations of the individual sea-ice models were partially different. They are substituted by the more common formulation of the stand-alone sea-ice model in order to provide higher consistency. Sensitivity studies are performed with continuous reference to the initial sea-ice model versions of the OGCMs. This study shows that the sea-ice formulations of present day global OGCMs have reached the same quality as stand-alone sea-ice models for specific regional studies, without the sacrifice of notable computation time. Furthermore, the study implicates the necessity to homogenise the treatment of sea ice for GCM-scale studies in order to prevent distorted sensitivities. (orig.)Available from TIB Hannover: RR 1347(138) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

The effect of phase changes of water on the development of volcanic plumes

A complex thermodynamic-microphysical package has been formulated that is able to deal with the microphysical processes of condensed water vapour in a volcanic plume. The microphysics follows a prognostic bulk approach for cloud water, cloud ice, rain and graupel and the interaction between them. In a standard experiment this module, applied within a new nonhydrostatic volcano plume model (ATHAM), produces reasonable concentrations of different types of hydrometeors. Under tropical conditions the plume gains three times as much water from the environment through entrainment as from the volcanic source. The formed hydrometeors are dominated by the ice phase. Thermodynamic effects of phase changes contribute about 13% to the plume's total thermal energy and therefore have a considerable effect on the vertical development of the plume. (orig.)30 refs.Available from TIB Hannover: RR 1347(270) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

An isopycnal ocean circulation model of the North Sea for dynamical downscaling

A regional version of the ocean general circulation model OPYC is presented. Lateral open boundaries and a barotropic tide model are incorporated and the model is applied for a shelf region, the North Sea. The modelled region includes the Greenland-Iceland-Norwegian Seas and the north-eastern part of the North Atlantic. The horizontal resolution is 1/2 x 1/2 with resolution enhanced to 1/10 x 1/10 in the North Sea. The vertical is discretized with 14 Lagrangian layers and the surface mixed layer. This report describes the technical layout of the model and discusses the climatological mean state of a 35-year spinup run. It demonstrates that this model setup can be used to downscale coarse resolution data of the ECMWF reanalysis project or for climate change experiments as obtained from coupled ocean-atmosphere models. In a simple downscaling example possible changes in the tides for an uniform increase in the sea level elevation of 1 metre at the lateral boundaries are estimated. (orig.)37 refs.SIGLEAvailable from TIB Hannover: RA 3251(97/E/47) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Predicting the '97 El Nino event with a global climate model

A global climate model is used for hindcasting the '97 El Ni no event. For this purpose a simple data assimilation technique has been developed which generates the thermal state of the low-latitude oceans by forcing the model with observed sea surface temperature (SST) anomalies. In this assimilation experiment, the vertical temperature profiles in the subsurface equatorial Pacific measured by the TOGA-TAO array were successfully simulated for the period 1996-1997. In a further series of eight hindcast experiments, initialized between January 1996 and October 1997, the predictive skill of the model was tested. All experiments starting in 1997 correctly simulated the evolution of the '97 El Ni no, although the amplitude was slightly underestimated. The model also predicted a dry period in the Indonesian region during fall 1997 when extensive forest fires occurred in this area. While the ocean was preconditioned to create an El Ni no already in 1996, the model correctly stayed in the cold (La Ni na) phase. All experiments initialized in '97 forecast a La Ni na event for the middle of '98. This is also suggested by the subsurface thermal structure in the tropical equatorial Pacific with a strong cold anomaly propagating eastward according to both, TOGA-TAO data and the data assimilation experiment. (orig.)18 refs.Available from TIB Hannover: RR 1347(254) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

The role of ocean dynamics for low-frequency fluctuations of the NAO in a coupled ocean-atmosphere GCM

Variability at all timescales, including low-frequency variability, is found in the North Atlantic sector in a 300-year control integration of a coupled ocean-atmosphere general circulation model (CGCM) ECHAM4/OPYC3. Statistically significant but rather weak peaks around 30 years are superimposed on the 'white-noise' power spectrum of sea level pressure and on the essentially 'red-noise' spectrum of sea surface temperature in highly active regions. Replacing the full ocean model with a 50 m fixed-depth mixed layer ocean (MLO) and coupling it to the atmosphere yields qualitatively and quantitatively very similar power spectra. Significant peaks, however, are missing. The atmospheric variability in both models is dominated by the North Atlantic Oscillation (NAO). On timescales of a decade and longer, spatial patterns bear large similarities with only a slight underestimation of amplitudes (up to 15%) in the MLO model. With respect to the importance of coupling between the ocean and the atmosphere for the longterm variations of the NAO, we find that the net surface heat flux shows general agreement in both simulations regarding spatial distributions and optimal time lags. It appears that the role of ocean dynamics is limited to a northward shift of SST anomalies (#propor to#10 ) due to contributions from advection and to a moderate increase of the atmosphere's long-term variability in subpolar regions. In view of the subordinate relevance of ocean-atmosphere coupling for the NAO it is suggested that the CGCM presented in this study mainly follows the stochastic climate model concept, i.e. the ocean integrates over the chaotic forcing imposed by the atmosphere, leaving he NAO rather unpredictable on decadal and longer timescales. (orig.)SIGLEAvailable from TIB Hannover: RR 1347(285) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Interpretation of interbasin exchange in an isopycnal ocean model

This paper reports an analysis of interbasin and interlayer exchanges in the component ocean part of the coupled ECHAM4/OPYC3 general circulation model, aimed at documenting the simulation of North Atlantic deep water (NADW) and related thermohaline circulations in the Indian and Pacific Oceans. The modeled NADW is formed mainly in the Greenland-Iceland-Norwegian Seas through a composite effect of deep convection and downward cross-isopycnal transport. The modeled deep-layer outflow of NADW can reach 16 Sv near 30 S in the South Atlantic, with the corresponding upper-layer return flow mainly coming from the ''cold water path'' through Drake Passage. Less than 4 Sv of the Agulhas ''leakage'' water contribute to the replacement of NADW along the ''warm water path''. In the South Atlantic Ocean, the model shows that some intermediate isopycnal layers with potential densities ranging between 27.0 and 27.5 are the major water source for compensating the NADW return flow and for enhancing the circumpolar deep water (CDW) flowing from the Atlantic into Indian Ocean. The modeled thermohaline circulations in the Indian and Pacific Oceans indicate that the Indian Ocean may play the major role in converting deep water into intermediate waterAvailable from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman