Rediscovery of the doldrums in storm-resolving simulations over the tropical Atlantic

Abstract

The doldrums — a zone of calm and variable winds in the deep tropics between the trades — were of key importance to nineteenth century maritime travel. As a result, the region was a focus in atmospheric science at that time. However, as sailing ships were replaced by steamboats, scientific interest shifted to the heavy precipitating storms within the doldrums: the deep convective systems of the intertropical convergence zone. Now, in storm-system-resolving simulations over a period of two months that cover a large part of the tropical Atlantic, the doldrums are one of the most prominent features. The doldrums are substantially less pronounced in coarser-resolution simulations that use a parameterization for convection, despite their large-scale extent. We conclude that explicitly representing the storm scale dynamics and their coupling to the surface wind on the storm-system scales helps to maintain the systems of winds that define the doldrums. We suggest that the lack of these wind systems could explain the persistent tropical precipitation biases in climate models.

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Fig. 1: Snap-shot of wind speed at 10 m height in the storm-resolving simulations.
Fig. 2: A composite view of the ITCZ.
Fig. 3: Variability of the wind direction in storm-resolving simulations and a model with parameterized convection.
Fig. 4: Wind directions and calms visualized following Maury42.

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Acknowledgements

This research was carried out in the Hans Ertel Center for Weather Research (HErZ). This German research network of universities, research institutions and the German Weather Service (DWD) is funded by the BMVI (Federal Ministry of Transport and Digital Infrastructure). M.B. is partly supported by the High Definition Clouds and Precipitation for advancing Climate Prediction (HD(CP)2) project funded by the German Ministry for Education and Research (01LK1501B). The simulations were conducted on the super computer system of the European Centre for Medium-Range Weather Forecasts (ECMWF) and the simulation data is stored at the the German Climate Computing Center (DKRZ). The authors are very grateful to the librarians at DWD for their support with the historic literature.

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Correspondence to Daniel Klocke.

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Klocke, D., Brueck, M., Hohenegger, C. et al. Rediscovery of the doldrums in storm-resolving simulations over the tropical Atlantic. Nature Geosci 10, 891–896 (2017). https://doi.org/10.1038/s41561-017-0005-4

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