Rain-formed Fresh Lenses in SPURS-2

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Heat, freshwater and momentum enter the ocean from the atmosphere through the air-sea interface, and are then dispersed laterally and vertically by dynamical processes. Knowledge of how surface fluxes of momentum, freshwater and heat fluxes affect the upper ocean is critical for understanding the exchanges between the atmosphere and the ocean. In the tropics, heavy rainfall produces buoyant fresh "lenses" at the sea surface that can modulate upper ocean heat content, currents, and turbulent kinetic energy dissipation. It is necessary to understand how these fresh lenses form and decay in order to assess their role in coupled air-sea processes. This project will examine the impacts of these rainfall events on the salinity and turbulent kinetic energy dissipation of the upper ocean. The novel aspects of the research include making simultaneous measurements of surface and near-surface turbulence in the field, quantifying the lateral evolution of fresh lenses, and relating rain rate to surface thermal structure. The experiment will provide new insights into how the surface and near-surface ocean are linked, which has the potential to improve representation of coupled air-sea processes in weather and climate models, particularly in the rainy tropical regions. The broader impacts of the project include increased public scientific literacy and public engagement with science through hands-on outreach activities designed to engage both youth and adult members of the community. The investigators will also mentor a graduate student and three different undergraduate students, contributing to the development of a globally competitive STEM workforce. The primary objective of the project is to quantify the spatial and temporal evolution of rain-formed freshwater anomalies in the upper ocean. In particular, the aim is to understand the relationship between variability at the air-sea interface and variability in the meter just below the surface. Measurements would be collected during two field experiments in the eastern tropical Pacific Ocean in conjunction with NASA?s Salinity Processes in the Upper ocean Regional Study (SPURS-2) project. The components of the proposed experiment are: (1) tracking the horizontal evolution of fresh lenses using a balloon-mounted infrared (IR) camera system;(2) applying the "controlled flux technique", which involves heating a small patch of the sea surface with a carbon dioxide laser and computing the rate at which the heated patch cools, to infer near-surface turbulent kinetic energy dissipation;(3) making in situ measurements of turbulent dissipation using temperature and conductivity microstructure probes mounted on a towed surface salinity profiler; and (4) observing the structure of sea surface thermal variability during rain using a high-speed IR camera.Measurements will be made during the SPURS-2 field experiments, which are expected to take place in the rainy (summer) season of 2016 and 2017. The observations will be used to understand the relationship between turbulent dissipation at the air-sea interface and within the top meter of the ocean; to quantify the role of different processes in driving the build-up and decay of fresh lenses; to force and validate a turbulence model in order to assess the impacts of rainfall on the upper ocean throughout the tropics; and to investigate the relationship between rain rate and the structure of ocean skin temperature. The results from the project have the potential to significantly improve our understanding of near-surface dynamics and hence their representation in coupled models. Integrating the findings from the project into the General Ocean Turbulence Model will also lay the foundation for extrapolating the results to other rainy regions in order to assess the role of rain-formed fresh lenses in air-sea interaction throughout the tropics.