Presenter: Dipanjan Chaudhuri (Applied Physics Laboratory, University)

Description:
Precipitation affects the buoyancy of the upper ocean in two ways: (A) Rain itself makes the water fresher and lighter, and (B) Rain is often accompanied by dramatic changes in radiative and turbulent (sensible and latent) heat flux, not captured well by reanalysis products. The net heat loss (Q_N) makes the water heavier by cooling and salting the ocean surface. If these effects are similar, then the net change in buoyancy can have either sign, i.e. making the water column more or less buoyant. Here, we estimate the contribution of the thermal and haline buoyancy fluxes using in situ measurements from eighteen different moored buoys spanned over equatorial oceans (20^oS–20^oN) under the following three ranges of rainfall category: (i) Light L <2.5 mm/hr, (ii) Moderate M 2.5–10 mm/hr, and (iii) Heavy H >10 mm/hr. Heavy precipitations at the ocean surface always induce buoyant buoyancy fluxes. In contrast, the chances of getting heavy buoyancy flux under moderate and low rain rates received by the ocean are 6% and 55%, respectively. Negative Q_N values, i.e. heat losses from the ocean surface, populate 80–90% of the data set. Overall, the mean thermal buoyancy fluxes reduce the mean haline buoyancy fluxes by 10%, 70%, and 130% for H, M and L cases. These analyses suggest a 33% chance of getting a convectively unstable water column at the tropical ocean while raining.