Variability on decadal scales in Pacific sea surface temperatures and atmosphere ocean interaction in the coupled general circulation model ECHAM4/OPYC3

The work presented in this thesis consists of both practical development and analytical investigation of a run with the coupled climate model ECHAM4/OPYC3. The practical aspect involved the development of a coupling interface for synchronizing ECHAM and OPYC and performing the regridding of the surface fluxes and SST. The numerical treatment of the regridding problem is innovative in coupled climate modelling. A meshwidth refinement strategy in analogy to interpolatory subdivision, widespread in computer aided geometric design (CAGD), was developed which preserves positivity for fields such as river discharge. It includes special treatment of the irregular domain given by ocean grid points only and employs open boundaries at the latitudinal extremities. The annual mean flux adjustment used was based on ideas by Oberhuber (pers. comm.), in collaboration with whom a scheme for heat flux adjustment for sea ice covered grid points was implemented. The coupled model simulation for the Pacific reproduces the mean climate simulated by the uncoupled model as discussed in Roeckner et al. (1996b). Temperature, zonal wind, 500 hPa height and preciptation annual mean and seasonal results deviate little from the uncoupled reference. For sea surface temperature (SST) the seasonal cycle of warm pool and cold tongue extent is similar to observations, apart from cooling in the north-eastern warm pool area about one month prematurely and a premature termination of the cold tongue cold phase. Surface heat fluxes and wind stress agree with the Oberhuber (1988) and ECMWF reanalysis climatologies, respectively, if the uncertainties in the heat flux climatology, e.g. over the warm pool domain, are taken into account. Exceptions are excessive heat flux in areas with stratus over the eastern subtropical oceans and the substantial over-estimation of boreal summer south-eastern trades on the equator. Mixed layers are predicted successfully. The tropical thermocline and currents due to the use of isopycnic coordinates are located correctly, but currents are artificially broadened and attain only about half the observed maximum speeds. Sea ice seasonal evolution agrees with observations except for underestimated Antarctic winter sea ice cover. (orig.)Available from TIB Hannover: RR 9(59) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

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

ENSO response to greenhouse warming

The response of the tropical Pacific climate system and its interannual variability to enhanced greenhouse warming was investigated by means of integrations with a global climate model. The climate model is the first one applied to transient greenhouse warming simulations which simulates the El Ni no/southern oscillation (ENSO) phenomenon, the leading mode of interannual variability, realistically. The long-term changes in the mean state of the tropical Pacific climate system are similar to those observed during present-day El Ni nos. Furthermore, the changes in the mean state lead to changes in the statistics of the interannual variability. An ENSO mode exists under enhanced greenhouse conditions also, but it becomes more energetic relative to present, so that variations from year to year become more extreme. In particular, the cold phases of the ENSO cycle amplify considerably, while the statistics of the warm phases do not change significantly. It is shown that changes in the ocean dynamics associated with a sharper thermocline lead to the enhanced interannual variability. Our results have strong implications not only for the global climate system but also for the ecology of the tropical Pacific and the societies and economies of many countries. (orig.)20 refs.Available from TIB Hannover: RR 1347(251) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman