  On small scales, the tropical atmosphere tends to be either moist or very
dry. This defines two states that, on large scales, are separated by a sharp
margin, well-identified by the anti-mode of the bimodal tropical column water
vapor distribution. Despite recent progress in understanding physical processes
governing the spatio-temporal variability of tropical water vapor, the behavior
of this margin remains elusive, and we lack a simple framework to understand
the bimodality of tropical water vapor in observations. Motivated by the
success of coarsening theory in explaining bimodal distributions, we leverage
its methodology to relate the moisture field's spatial organization to its
time-evolution. This results in a new diagnostic framework for the bimodality
of tropical water vapor, from which we argue that the length of the margin
separating moist from dry regions should evolve towards a minimum in
equilibrium. As the spatial organization of moisture is closely related to the
organization of tropical convection, we hereby introduce a new organization
index (BLW) measuring the ratio of the margin's length to the circumference of
a well-defined equilibrium shape. Using BLW, we assess the evolution of
self-aggregation in idealized cloud-resolving simulations of
radiative-convective equilibrium and contrast it to the time-evolution of the
Atlantic Inter-Tropical Convergence Zone (ITCZ) in the ERA5 meteorological
re-analysis product. We find that BLW successfully captures aspects of
convective organization ignored by more traditional metrics, while offering a
new perpective on the seasonal cycle of convective organization in the Atlantic
ITCZ.

Atmospheric and Oceanic Physics