ALBERT

Description: The space-time structure and predictability of the El Niño/Southern Oscillation (ENSO) phenomenon was investigated. Two comprehensive datasets were analyzed by means of an advanced statistical method, one based on observational data and the other on data derived from an extended-range integration performed with a coupled ocean-atmosphere general circulation model. It is shown that a considerable portion of the ENSO-related low-frequency climate variability in both datasets is associated with a cycle involving slow propagation in the equatorial oceanic beat content and the surface wind field. The existence of this cycle implies the ability of climate predictions in the tropics up to lead times of about one year. This is shown by conducting an ensemble of predictions with our coupled general circulation model. For the first time a coupled model of this type was successfully applied to ENSO predictions.

Description: We have investigated the seasonal cycle and the interannual variability of the tropical Indian Ocean circulation and the Indian summer monsoon simulated by a coupled ocean-atmosphere general circulation model in a 26- year integration. Although the model exhibits significant climate drift, overall, the coupled GCM simulates realistically the seasonal changes in the tropical Indian Ocean and the onset and evolution of the Indian summer monsoon. The amplitudes of the seasonal changes, however, are underestimated. The coupled GCM also simulates considerable interannual variability in the tropical Indian Ocean circulation, which is partly related to the El Niño/Southern Oscillation phenomenon and the associated changes in the Walker circulation. Changes in the surface wind stress appear to be crucial in forcing interannual variations in the Indian Ocean SST. As in the Pacific Ocean, the net surface beat flux acts as a negative feedback on the SST anomalies. The interannual variability in monsoon rainfall, simulated by the coupled GCM, is only about half as strong as observed. The reason for this is that the simulated interannual variability in the Indian monsoon appears to be related to internal processes within the atmosphere only. In contrast, an investigation based on observations shows a clear lead-lag relationship between interannual variations in the monsoon rainfall and tropical Pacific SST anomalies. Furthermore, the atmospheric GCM also fails to reproduce this lead-lag relationship between monsoon rainfall and tropical Pacific SST when run in a stand-alone integration with observed SSTs prescribed during the period 1970–1988. These results indicate that important physical processes relating tropical Pacific SST to Indian monsoon rainfall are not adequately modeled in our atmospheric GCM. Monsoon rainfall predictions appear therefore premature.

Description: A 26-year integration has been performed with a coupled ocean-atmosphere general circulation model (CGCM). The oceanic part resolves all three oceans in the latitude band 70°N–70°S but is dynamically active only between 30°N and 30°S. The atmosphere is represented by a global low-order spectral model. The coupled model was forced by seasonally varying insolation. Although the simulated time-averaged mean conditions in both atmosphere and ocean show significant deviations from the observed climatology, the CGCM realistically simulates the interannual variability in the tropical Pacific. In particular, the CGCM simulates an irregular ENSO with a preferred time scale of about 3 years. The mechanism for the simulated interannual variability in the tropical Pacific is related to both the “delayed action oscillator” and the “slow SST mode.” It therefore appears likely that either both modes can coexist or they degenerate to one mode within certain locations of the parameter space. This hypothesis is also supported by calculations performed with simplified coupled models, in which the atmospheric GCM was replaced by linear steady-state atmosphere models. Further, evidence is found for an eastward migration of zonal wind anomalies over the western Pacific prior to the extremes of the simulated ENSO, indicating a link to circulation systems over Asia. Because an earlier version of the CGCM did not simulate interannual variability in the tropical Pacific, additional experiments with a simplified coupled model have been conducted to study the sensitivity of coupled systems to varying mean oceanic background conditions. It is shown that even modest changes in the background conditions can push the coupled system from one flow regime into another.

Description: The influence of phytoplankton on the seasonal cycle and the mean global climate is investigated in a fully coupled climate model. The control experiment uses a fixed attenuation depth for shortwave radiation, while the attenuation depth in the experiment with biology is derived from phytoplankton concentrations simulated with a marine biogeochemical model coupled online to the ocean model. Some of the changes in the upper ocean are similar to the results from previous studies that did not use interactive atmospheres, for example, amplification of the seasonal cycle; warming in upwelling regions, such as the equatorial Pacific and the Arabian Sea; and reduction in sea ice cover in the high latitudes. In addition, positive feedbacks within the climate system cause a global shift of the seasonal cycle. The onset of spring is about 2 weeks earlier, which results in a more realistic representation of the seasons. Feedback mechanisms, such as increased wind stress and changes in the shortwave radiation, lead to significant warming in the midlatitudes in summer and to seasonal modifications of the overall warming in the equatorial Pacific. Temperature changes also occur over land where they are sometimes even larger than over the ocean. In the equatorial Pacific, the strength of interannual SST variability is reduced by about 10%–15% and phase locking to the annual cycle is improved. The ENSO spectral peak is broader than in the experiment without biology and the dominant ENSO period is increased to around 5 yr. Also the skewness of ENSO variability is slightly improved. All of these changes lead to the conclusion that the influence of marine biology on the radiative budget of the upper ocean should be considered in detailed simulations of the earth’s climate.