Seeing doldrums from space

Doldrums — the bane of sailors in ages past — are mesoscale regions of calm winds, usually seen dividing two zonal bands of convective clouds near the thermal equator. These features, together often manifest as the inter-tropical convergence zone (ITCZ), particularly over the Atlantic. The terms "ITCZ" and "doldrums" are often incorrectly used inter-changeably. With satellite observations, we show that they are in fact not the same meteorological feature. Although the doldrums seemed to have departed from current discussions, recent cross-equatorial ship-borne observations in the Atlantic have brought back attention to them and their role in shaping the distribution of convection. We use satellite measurements spanning more than 15 years to report statistics of doldrums over the Atlantic and the East Pacific. Along with their spatial extents, we document their zonal and meridional positioning as well as the seasonal and inter-annual variability therein. We also record the vertical extents of these calm horizontal winds, albeit with a shorter period of sampling. Co-located measurements of column moisture, surface rain rate and cloud liquid water provide an idea of the environmental conditions that are associated with the presence of doldrums. Particularly, we see an anomalously dry atmospheric column over the doldrums compared to that over the adjacent convergence bands, which is similar to those observed from the ship-based observations. We also find long periods (ca. 1 month) of westward propagation of doldrums, but there can be large differences in their spatio-temporal persistence among different years. Our characterization enables frameworks attempting to explain the physical mechanism of doldrums as well as their role in the mesoscale organization of the ITCZ.