Does rainfall create buoyant forcing at the ocean surface?

Abstract

Rain affects the buoyancy of the upper ocean in two ways: The freshwater flux in rain makes the water fresher and lighter, stabilizing the ocean (a negative buoyancy flux). The convective systems that produce rain are often accompanied by cold, dry air, often called 'cold pools', and reduced short-wave radiation, which makes the water colder and heavier, destabilizing the ocean (a positive buoyancy flux). We estimate net buoyancy fluxes using in situ measurements from twenty-two moored buoys in the equatorial oceans under different rainfall categories. We find that buoyancy fluxes tend to destabilize the ocean during light rain (0.2-4 mm/hr) and stabilize the ocean during heavy rain (>4 mm/hr). Furthermore, buoyancy fluxes during rain tend to be more positive at night than during the day, with nighttime rain twice as likely to cause instability compared to daytime rain, even at the same rainfall intensity. Average buoyancy fluxes across the tropics during rain can have either sign. These findings challenge the common assumption that rainfall makes the ocean surface lighter and provide a starting point for focusing on the overall effect of precipitation on the ocean.