A Cold Lid on a Warm Ocean: Indian Ocean Surface Rain Layers and Their Feedbacks to the Atmosphere

Ocean surface rain layers (RLs) form when relatively colder, fresher, less dense rain water stably stratifies the upper ocean. RLs cool sea surface temperature (SST) by confining surface evaporative cooling to a thin near-surface layer, and generate sharp SST gradients between the cool RL and the surrounding ocean. In this study, ocean-atmosphere coupled simulations of the November 2011 Madden-Julian Oscillation (MJO) event are conducted with and without RLs to evaluate two pathways for RLs to influence the atmosphere. The first, termed the "SST gradient effect," arises from the hydrostatic adjustment of the boundary layer to RL-enhanced SST gradients. The second, termed the "SST effect," arises from RL-induced SST reductions impeding the development of deep atmospheric convection. RLs are found to sharpen SST gradients throughout the MJO suppressed and suppressed-to-enhanced convection transition phases, but their effect on convection is only detected during the MJO suppressed phase when RL-induced SST gradients enhance low-level convergence/divergence and broaden the atmospheric vertical velocity probability distribution below 5 km. The SST effect is more evident than the SST gradient effect during the MJO transition phase, as RLs reduce domain average SST by 0.03 K and narrow vertical velocity distribution, thus delaying onset of deep convection. A delayed SST effect is also identified, wherein frequent RLs during the MJO transition phase isolate accumulated subsurface ocean heat from the atmosphere. The arrival of strong winds at the onset of the MJO active phase erodes RLs and releases subsurface ocean heat to the atmosphere, supporting the development of deep convection. Rain water is less dense than near-surface ocean water. For this reason, rain water can float on the ocean surface following rain events and form a "rain layer" in the upper ocean. Rain layers that form in the tropical Indian Ocean reduce sea surface temperature (SST) and shield the subsurface ocean below the rain layer from the atmosphere, thus altering heat exchange between the ocean and atmosphere. In this study, we conduct model experiments over the tropical Indian Ocean to investigate rain layer feedbacks to the atmosphere. We identify two potential rain layer feedback mechanisms, one in which rain-enhancement of SST gradients contributes to the formation of clouds and precipitation in the atmosphere, and a second in which rain-driven SST reduction suppresses the development of clouds and precipitation. Our results indicate that rain-driven SST reduction is the dominant immediate feedback, as clouds and precipitation are reduced in the presence of rain layers. A delayed rain layer feedback is also identified, wherein rain layers insulate subsurface ocean heat from the atmosphere. When rain layers are destroyed by wind-driven mixing, the stored ocean heat is released to the atmosphere, and supports the development of clouds and precipitation. Cold and statically stable freshwater rain layers (RLs) warm the subsurface ocean by reducing nocturnal vertical mixing RL-induced surface cooling and sub-surface warming first delay, but then enhance the development of deep convection within the Madden-Julian Oscillation (MJO) RL regulation of convective intensity may thus play a role in setting MJO period and propagation speed