Rain-formed and diurnal warming-formed near-surface ocean mixed layers: impacts on SST throughout the MJO

Heating and rain freshening often stabilize the upper tropical ocean, bringing the ocean mixed layer depth to the sea surface. These thin mixed layer depths concentrate subsequent fluxes of heat, momentum, and freshwater into a thin layer, promoting rapid SST heating that is important for atmospheric convection. Shallow mixed layers also inhibit wind-driven ocean mixing and entrainment cooling of SST from below. Leading up to this study, ocean mixed layer depth and SST variability due to rainfall events, and their importance throughout the Madden-Julian Oscillation (MJO), have not been as comprehensively explored as the tropical ocean's response to heating or momentum fluxes. To gain understanding about the effects of rainfall on the upper ocean, we examine central Indian Ocean salinity and temperature microstructure measurements in the context of radar-derived rainfall maps throughout two MJO (Madden-Julian Oscillation) events during the DYNAMO experiment.

Results indicate that the ocean mixed layer was as shallow as 0-5 m during about half of the DYNAMO record throughout two MJOs. During forty-three observation days, thirty-eight near-surface mixed layer events were attributed to freshwater stabilization, called rain-formed mixed layers (RFLs). Thirty other near-surface mixed layer events were classified as diurnal warm layers (DWLs) due to daytime heating temperature stratification. As storm intensity, frequency, duration, and the ability of storms to maintain stratiform rain areas increased during disturbed and active MJO phases, shallow rain-formed mixed layers became much more common. We hypothesize that the stratiform rain components of storms helped shoal the ocean mixed layer by providing widespread, steady, long-lived freshwater fluxes. Although generally limited to rain rates ≤ 10 mm hr-1, it was demonstrated that stratiform rain can still exert a sufficiently strong buoyancy flux into the ocean for stratification, i.e. as high as maximum daytime solar heating.

RFLs and DWLs were observed to interact in two ways: 1) RFLs added salinity stratification to preexisting DWL temperature stratification, which occurred ten times; 2) RFLs lasted long enough to superheat, creating a new DWL on top of the RFL, which happened nine times. These combination stratification events were responsible for the highest SST warming rates and some of the highest SSTs leading up to the heaviest precipitation, highest wind stage of the each MJO. DWLs without RFL interaction helped produce the highest SSTs in suppressed MJO conditions when rain was infrequent and weak. These results suggest that both rain-formed and diurnal warming-formed near-surface mixed layers, their interactions, and the underlying ocean barrier layer are important for controlling SST variability and therefore MJO initiation in the central Indian Ocean.