Revisiting the Doldrums: New Insights from the ORCESTRA Campaign

The Intertropical Convergence Zone (ITCZ) is a central component of the atmospheric general circulation and is traditionally described as a region of mean surface convergence and high precipitation rates. The ITCZ is also associated with the doldrums, regions of low wind speeds and variable wind directions, although the exact relationship between the two remains unclear. Reexamining this relationship, we show that the doldrums are largely confined to the area between the edges of the ITCZ, which are characterised by enhanced surface convergence. Although this is a region of high time-averaged precipitation, low wind speed events only occur in the absence of precipitation. This suggests that the traditional explanation of the doldrums being the result of ascending air motion is incorrect. We therefore investigate the vertical structure of the doldrums using data collected during the ORCESTRA (Organized Convection and EarthCARE Studies over the Tropical Atlantic) field campaign. ORCESTRA took place in the tropical Atlantic in August and September 2024 and consisted of eight sub-campaigns. Here, we focus on the sampling of vertical air motion that we measured with dropsondes in the deep tropics in general and in the doldrums in particular. In combination with limited-area simulations that provide daily hindcasts of atmospheric conditions in the campaign region, we use these data to characterise for the first time the vertical structure of vertical air motion within regions of low wind speeds. This study can also be seen as an example of a more fundamental goal of the ORCESTRA campaign, which is to gain a better understanding of the mesoscale structure of the ITCZ and the importance of its significant day-to-day variability.