Doldrums rediscovered in storm-resolving simulations

Near surface winds, navigation directions and the doldrums over the tropical Atlantic. Maury, M. F., The physical geography of the sea and its meteorology (1864). Parts of plate VIII are shown.

New storm-resolving simulations, showing how resolved convection, and its associate circulations, interact with and form the larger scale circulations within the Atlantic intertropical convergence zone (ITCZ) are presented by Daniel Klocke, Matthias Brueck, Cathy Hohenegger and Bjorn Stevens in a recently published article. One of the most prominent features emerging from those simulations are the doldrums - a largely forgotten region of calm and variable winds in the deep tropics which extends across vast parts of the tropical Atlantic ocean.

The doldrums were important to maritime travel before the invention of steam ships and were prominent in early wind atlases by Maury (the figure) and Köppen. Scientific focus shifted in the 20th century to the convective systems embedded within the doldrums forming the ITCZ, to the extent that now many scientists confuse the doldrums with the horse latitude. Comparing their simulation results to the early wind atlas and ship measurements from the 19th century, the authors found an astonishing correspondence. Using six orders of magnitude less data, the ship based point observations unwittingly describe the small scale variability of the simulated winds as they appear to have blown 150 years ago and still appear to blow. In contrast and despite the large-scale extent of the doldrums, models with coarse resolutions fail to reproduce the doldrums because parameterisations for convection fail to reproduce the associated wind systems. The study thus suggests that it is essential to represent the storm-scale dynamics to correctly represent the large-scale structure of the tropical dynamics, which current parameterisations of convection are not able to do. The authors encourage further research in this area, to break the deadlock in simulating the tropical circulation systems.

The simulations, given their grid spacing (2.4 km), simulation domain (3000x9000 km) and integration period (2 months) open exciting new opportunities as they are able to both explicitly capture small-scale features of the atmospheric flow, like the gust wind of a convective storm, and the whole atmospheric circulation over the tropical Atlantic on timescales interesting for climate studies. The huge amount of simulation output is stored at DKRZ and will be made available by the authors upon requests. The simulations were performed with the ICON model of the German Weather Service and the Max Planck Institute for Meteorology (MPI-M) in the context of MPI-M's NARVAL-I and II airborne measurement campaigns.The simulations were made feasible through a cooperation between DWD and MPI via the two projects HErZ and HD(CP)2.

Publication:
Klocke, D., Brueck, M., Hohenegger, C. and Stevens, B. (2017): Rediscovery of the doldrums in storm-resolving simulations over the tropical Atlantic. Nature Geoscience. doi: 10.1038/s41561-017-0005-4

Contact:

Dr. Daniel Klocke
Hans Ertel Center for Weather Research
Deutscher Wetterdienst
Phone: +49 69 8062-3051
Email: daniel.klocke@we dont want spamdwd.de

Dr. Cathy Hohenegger
Max Planck Institute for Meteorology
Phone: +49 40 41173 302
Email: cathy.hohenegger@we dont want spammpimet.mpg.de

Dr. Matthias Brueck
Max Planck Institute for Meteorology
Phone: +49 40 41173 308
Email: matthias.brueck@we dont want spammpimet.mpg.de