The Hamburg atmosphere-ocean coupled circulation model - ECHO-G

ECHO-G is a global coupled atmosphere-ocean climate model whose component models are the ECHAM atmosphere general circulation model and a global version of the Hamburg Ocean Primitive Equation model, HOPE-G, which includes a dynamic-thermodynamic sea-ice model with snow cover. ECHO-G can be used in numerical studies of natural variability of the world climate and of climate changes on time-scales ranging from the component models time steps to centuries. In high latitudes, the interaction between ocean and atmosphere can be strongly affected by the sea-ice cover. In particular, the heat flux through ice and that through leads and polynyas can differ by an order of magnitude on horizontal scales much smaller than that of gridcell in global climate models. ECHO-G accounts for these effects by a separate calculation of fluxes over ice and over water when a sub-grid-scale partial ice cover is present. Since the component models are used in their stand-alone versions with only some subroutine calls added, and since the coupling interface, OASIS, is a flexible tool that allows to change the number of component models, of interpolation methods, and data exchange frequencies by keyword specification, it should be relatively easy to include other models, or change the coupling strategy. This report describes the physical and technical aspects of a specific set-up that is in use at DKRZ. (orig.)SIGLEAvailable from TIB Hannover: RR 8522(18) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

The role of the individual air-sea flux components in CO_2-induced changes of the ocean's circulation and climate

In this study we investigate the role of heat, freshwater and momentum fluxes in changing the oceanic climate and thermohaline circulation as a consequence of increasing atmospheric CO_2 concentration. Two baseline integrations with a fully coupled ocean atmosphere general circulation model with either fixed or increasing atmospheric CO_2 concentrations have been performed. In a set of sensitivity experiments either freshwater and/or momentum fluxes were no longer simulated, but prescribed according to one of the fully coupled baseline experiments. This approach gives a direct estimate of the contribution from the individual flux components. The direct effect of surface warming and the associated feedbacks in ocean circulation are the dominant processes in weakening the Atlantic thermohaline circulation in our model. The relative contribution of momentum and freshwater fluxes to the total response turned out to be less than 25%. Changes in atmospheric water vapour transport lead to enhanced freshwater input into middle and high latitudes, which weakens the overturning. A stronger export of freshwater from the Atlantic drainage basin to the Indian and Pacific ocean, on the other hand, intensifies the circulation. In total the modified freshwater fluxes slightly weaken the Atlantic thermohaline circulation. The contribution of the modified momentum fluxes has a similar magnitude, but enhances the formation of North Atlantic deep water. Salinity anomalies in the Atlantic as a consequence of greenhouse warming stem to almost equal parts from changes in net freshwater fluxes and from changes in ocean circulation caused by the surface warming due to atmospheric heat fluxes36 refs.Available from TIB Hannover: RR 1347(263) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Periodically synchronously coupled integrations with the atmosphere-ocean general circulation model ECHAM3/LSG

A new periodically synchronous coupling scheme has been applied to an atmosphere-ocean general circulation model. Due to a temporary switching off of the atmospheric model this scheme can considerably reduce computer resources of coupled model experiments. In order to evaluate the new coupling scheme the model results are compared to corresponding synchronously coupled integrations. Experiments with fixed present-day CO_2 concentration and a gradual increase of CO_2 show a good reproduction of the mean state and the climate change pattern, respectively. The deviations from the synchronously coupled experiments are in the range of the variability of the corresponding synchronously coupled runs. Due to the forcing during the ocean-only periods the short-term fluctuations are underestimated and the long-term variability is overestimated. (orig.)Available from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

The stability of the thermohaline circulation in a coupled ocean-atmosphere general circulation model

The stability of the Atlantic thermohaline circulation against meltwater input is investigated in a coupled ocean-atmosphere general circulation model. The meltwater input to the Labrador Sea is increased linearly for 250 years to a maximum input of 0.625 Sv and then reduced again to 0 (both instantaneously and slowly decreasing over 250 years). The resulting freshening forces a shutdown of the formation of North Atlantic deepwater and a subsequent reversal of the thermohaline circulation of the Atlantic, filling the deep Atlantic with Antarctic bottom water. The change in the overturning pattern causes a drastic reduction of the Atlantic northward heat transport, resulting in a strong cooling with maximum amplitude over the northern North Atlantic and a southward shift of the sea-ice margin in the Atlantic. Due to the increased meridional temperature gradient, the Atlantic intertropical convergence zone is displaced southward and the westerlies in the northern hemisphere gain strength. We identify four main feedbacks affecting the stability of the thermohaline circulation: the change in the overturning circulation of the Atlantic leads to longer residence times of the surface waters in high northern latitudes, which allows them to accumulate more precipitation and runoff from the continents, which results in an increased stability in the North Atlantic61 refs.Available from TIB Hannover: RR 1347(188) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Interannual to decadal predictability in a coupled ocean-atmosphere general circulation model

The predictability of the coupled ocean-atmosphere climate system on interannual to decadal time scales has been studied by means of ensemble forecast experiments with a global coupled ocean-atmosphere general circulation model. Over most parts of the globe the models predictability can be sufficiently explained by damped persistence as expected from the stochastic climate model concept with damping times of considerably less than a year. Nevertheless, the tropical Pacific and the North Atlantic Ocean exhibit oscillatory coupled ocean-atmosphere modes, which lead to longer predictability time scales. While the tropical mode shares many similarities with the observed ENSO phenomenon, the coupled mode within the North Atlantic region exhibits a typical period of about 30 years and relies on an interaction of the oceanic thermohaline circulation and the atmospheric North Atlantic oscillation. The model's ENSO-like oscillation is predictable up to 1/3-1/2 (2-3years) of the oscillation period both in the ocean and the atmosphere. The North Atlantic yields considerably longer predictability time scales (of the order of a decade) only for quantities describing the model's thermohaline circulation. For surface quantities and atmospheric variables only marginal predictability (of the order of a year) was obtained. The predictability of the coupled signal at the surface is destroyed by the large amount of internally generated (weather) noise. This is illustrated by means of a simple conceptual model for coupled ocean-atmosphere variability and predictability. (orig.)30 refs.Available from TIB Hannover: RR 1347(262) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

North Atlantic interdecadal variability: A coupled air-sea mode

We describe in this paper a coupled air-sea mode in the North Atlantic with a period of about 35 years. The mode was derived from a multi-century integration with a coupled ocean-atmosphere general circulation model and involves interactions of the thermohaline circulation with the atmosphere. If, for instance, the thermohaline circulation is anomalously strong, the North Atlantic is covered by positive sea surface temperature (SST) anomalies. The atmospheric response to these SST anomalies involves a strengthened North Atlantic Oscillation, which leads to anomalous fresh water fluxes off Newfoundland and in the Greenland Sea and the generation of negative sea surface salinity (SSS) anomalies. These SSS anomalies are advected by the subpolar gyre, reaching eventually the convectively active region south of Greenland. The convection and subsequently the strength of the thermohaline circulation are weakened. This leads to a reduced poleward heat transport and the formation of negative SST anomalies, which completes the phase reversal. The mode must be regarded as an inherently coupled air-sea mode. The memory of the coupled ocean-atmosphere system, however, resides in the ocean. The similarity of the model salinity anomalies to the great salinity anomaly which originated in the late 1960s and lasted for roughly 10 years is discussed. Further, we study also the possibility of Atlantic-Pacific interactions. It appears that the Atlantic mode is introduced into the North Pacific Ocean through the atmosphere. (orig.)61 refs.Available from TIB Hannover: RR 1347(223) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Techniques for asynchronous and periodically-synchronous coupling of atmosphere and ocean models. Pt. 2 Impact of variability

A periodically-synchronous scheme suitable for coupling atmosphere and ocean models with high internal variability is presented. The performance of the scheme is tested by means of a simple zero-dimensional non-linear energy balance model with stochastic forcing. The equilibrium behaviour and the response to changes in the model parameters are analysed. The response experiments are similar to CO_2 doubling and transient CO_2 experiments. The best results are obtained using a method with weighted means of the air-sea fluxes which are calculated during the synchronously coupled periods. (orig.)10 refs.Available from TIB Hannover: RR 1347(174) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Techniques for asynchronous and periodically-synchronous coupling of atmosphere and ocean models. Pt. 1 General strategy and application to the cyclo-stationary case

Asynchronous and periodically-synchronous schemes for coupling atmosphere and ocean models are presented. The performance of the schemes is tested by simulating the climatic response to a step function forcing and to a gradually increasing forcing with a simple zero-dimensional non-linear energy balance model. Both the initial transient response and the asymptotic approach of the equilibrium state are studied. If no annual cycle is allowed the asynchronous coupling technique proves to be a suitable tool. However, if the annual cycle is retained, the periodically-synchronous coupling technique reproduces the results of the synchronously coupled runs with smaller bias. In this case it is important that the total length of one synchronous period and one ocean only period is not a multiple of 6 months. (orig.)22 refs.Available from TIB Hannover: RR 1347(169) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEEuropean Union (Euro), Brussels (Belgium)DEGerman

Detection of anthropogenic climate change using a fingerprint method

A fingerprint method for detecting anthropogenic climate change is applied to new simulations with a coupled ocean-atmosphere general circulation model (CGCM) forced by increasing concentrations of greenhouse gases and aerosols covering the years 1880 to 2050. In addition to the anthropogenic climate change signal, the space-time structure of the natural climate variability for near-surface temperatures is estimated from instrumental data over the last 134 years and two 1000 year simulations with CGCMs. The estimates are compared with paleoclimate data over 570 years. The space-time information on both the signal and the noise is used to maximize the signal-to-noise ratio of a detection variable obtained by applying an optimal filter (fingerprint) to the observed data. The inclusion of aerosols slows the predicted future warming. The probability that the observed increase in near-surface temperatures in recent decades is of natural origin is estimated to be less than 5%. However, this number is dependent on the estimated natural variability level, which is still subject to some uncertainty. (orig.)43 refs.SIGLEAvailable from TIB Hannover: RR 1347(168) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany); European Union (Euro), Brussels (Belgium)DEGerman

On multi-fingerprint detection and attribution of greenhouse gas- and aerosol forced climate change

A multi-fingerprint analysis is applied to the detection and attribution of anthropogenic climate change. While a single fingerprint, as applied in a previous paper by Hegerl et al. (1996), is optimal for detecting a significant climate change, the simultaneous use of several fingerprints allows one to investigate additionally the consistency between observations and model predicted climate change signals for competing candidate forcing mechanisms. Thus the multi-fingerprint method is a particularly useful technique for attributing an observed climate change to a proposed cause. Different model-predicted climate change signals are derived from three global warming simulations for the period 1880 to 2049. In one simulation, the forcing was by greenhouse gases only, while in the remaining two simulations the influence of aerosols was also included. The two dominant climate change signals derived from these simulations are optimized statistically by weighting the model-predicted climate change pattern towards low-noise directions. These optimized fingerprints are then applied to observed near surface temperature trends. The space-time structure of natural climate variability (needed to determine the signal-to-noise ratio) is estimated from several multi-century control simulations with different CGCMs and from instrumental data over the last 134 years. (orig.)60 refs.Available from TIB Hannover: RR 1347(207) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman