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Abstract:
On small scales, the tropical atmosphere tends to be either moist or very dry. This denes two
states that, on large scales, are separated by a sharp margin, well identied by the antimode of the bimodal
tropical column water vapor distribution. Despite recent progress in understanding physical processes
governing the spatiotemporal 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 eld'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 toward a minimum in equilibrium. As the
spatial organization of moisture is closely related to the organization of tropical convection, we hereby
introduce a new convective organization index (BLW) measuring the ratio of the margin's length to the
circumference of a welldened equilibrium shape. Using BLW, we assess the evolution of selfaggregation
in idealized cloudresolving simulations of radiativeconvective equilibrium and contrast it to the time
evolution of the Atlantic Intertropical Convergence Zone (ITCZ) in the ERA5 meteorological reanalysis
product. We nd that BLW successfully captures aspects of convective organization ignored by more
traditional metrics, while offering a new perspective on the seasonal cycle of convective organization in the
Atlantic ITCZ
