The Meandering Margin of the Moist Meteorological Tropics

Bimodality in the frequency distribution of atmospheric column-integrated water vapor (CWV) indicates a well-defined moist regime in the Tropics, above a marginal value of about 48 kg m^-2 in current climate (roughly 80% of column saturation). In maps, this margin is a sinuous contour that bounds broad plateaus with quasi-uniform airmass properties. Within these, convective storms of much smaller scale occur sporadically. Here we show satellite data composites around the moving margin, revealing its sharpness: for instance, expected precipitation doubles within 100 km, associated with a steep enhancement and deepening of convective cloudiness. Transient patches and filaments of the moist airmass cause consequential precipitation events within and beyond the Tropics. Distinguishing synoptic flows that cross the margin from flows that move the margin appears possible with a novel satellite-based estimate of Lagrangian CWV tendency. Climate models do not reliably reproduce the observed distribution, so the moist mode’s maintenance and transition processes need further elucidation. The role of these mode-and-margin dynamics in low frequency variability and climate is blurred by conventional time averaging, but next-generation data diagnostics can account for them, if models can reproduce the phenomenon in the first place.