Bjerknes Compensation in the CMIP5 Climate Models

Abstract

This study examines the atmospheric and oceanic heat transports in preindustrial control and historical runs of 15 fully coupled global climate models from the CMIP5 project. The presence of Bjerknes compensation is confirmed in all models by the strong anticorrelation and approximately equal magnitude of the anomalies of these heat transports. Previous studies of Bjerknes compensation in the absence of external forcing have all shown the strongest compensation at high latitudes, where the warm ocean meets the cold Arctic atmosphere. In this study, however, it is found that many of the 15 models have a second and often dominant peak of compensation in the northern midlatitudes, where strong air–sea interaction is often associated with the midlatitude storm tracks. It has also been suggested that variations in heat transport in the ocean lead those in the atmosphere, but this work has found no clear and robust support for this, as only half the models show such a relationship. In the historical simulations where external forcings are applied, Bjerknes compensation continues to be present, but many models show pronounced trends in the heat transports. All of the models show multidecadal variability in heat transports in both preindustrial control and historical simulations. Any anthropogenic climate change signal could potentially be masked or amplified by the natural variability governed by Bjerknes compensation. Given its presence in the CMIP5 models, which are the basis of so much policy and adaptation planning, an improved understanding of Bjerknes compensation may have socioeconomic relevance for the future.