The goal of this study is to understand the mechanisms controlling the isotopic composition of the water vapor near the surface of tropical oceans, at the scale of about a hundred kilometers and a month. In the tropics, it has long been observed that the isotopic compositions of rain and vapor near the surface are more depleted when the precipitation rate is high. This is called the "amount effect." Previous studies, based on observations or models with parameterized convection, have highlighted the roles of deep convective and mesoscale downdrafts and rain evaporation. But the relative importance of these processes has never been quantified. We hypothesize that it can be quantified using an analytical model constrained by large-eddy simulations. Results from large-eddy simulations confirm that the classical amount effect can be simulated only if precipitation rate changes result from changes in the large-scale circulation. We find that the main process depleting the water vapor compared to the equilibrium with the ocean is the fact that updrafts stem from areas where the water vapor is more enriched. The main process responsible for the amount effect is the fact that when the large-scale ascent increases, isotopic vertical gradients are steeper, so that updrafts and downdrafts deplete the subcloud layer more efficiently. Plain Language Summary Water molecules can be light (one oxygen atom and two hydrogen atoms) or heavy (one hydrogen atom is replaced by a deuterium atom). These different molecules are called water isotopes, and their relative concentration in water is called the isotopic composition. The isotopic composition of the precipitation recorded in ice cores or in speleothems can be used to reconstruct past climates. However, the factors controlling the isotopic composition are complex. To better understand these factors, as a first step, we try to understand what controls the isotopic composition of the water vapor near the surface of tropical oceans. It is known to be affected by storm activity. Storms act to deplete the near-surface water vapor of the heaviest isotopes. To understand how, we use a high-resolution atmospheric model, with a horizontal grid spacing of 750 m. Such a model explicitly resolves the ascents and descents in the storms and in nearby clouds. We find that storms deplete the near-surface water vapor mainly because the ascending air export enriched water vapor from the near-surface to high levels. This conclusion is in contrast with previous studies, which highlighted more the role of descending air and of partial evaporation of the falling rain.