Stable water isotopes and large-scale vertical motions in the tropics

A complete understanding of the interaction between convection and the large-scale circulation in the tropics remains an outstanding problem. Although there is evidence that the vertical structure of convective heating has great influence in the large-scale response and that this structure also presents considerable geographical variations, more need to be done. One of the questions that are still unanswered is how the vertical structure of the convective heating, or, similarly, of vertical velocity, varies across the tropical Pacific. Here it is suggested that some light can be shed on this debate by considering stable water isotopes. Because these tend to be progressively less abundant with increasing height, precipitation associated with top heavy profiles should be expected to be more depleted than that associated with bottom heavy profiles. This claim is verified with a variety of data: first, using observations from IAEA/WMO Global Network of Isotopes in Precipitation stations; then, using a simple model based on the budget of water isotopes in precipitation; finally, using a more complex isotope-enabled general circulation model. Evidence provided by these sources confirms that different structures of vertical velocities are associated with different isotopic abundances, with top heavy profiles giving rise to more depleted rainfall. Finally, the data from over the Pacific, although scarce, seem to suggest that precipitation in the eastern part is more enriched than in the western, thus hinting at velocity profiles over the East being more bottom heavy than over the West Pacific.