Turbulence within Rain-Formed Fresh Lenses during the SPURS-2 Experiment

Abstract

Observations of salinity, temperature, and turbulent dissipation rate were made in the top meter of the ocean using the ship-towed Surface Salinity Profiler as part of the second Salinity Processes in the Upper Ocean Regional Study (SPURS-2) to assess the relationships between wind, rain, near-surface stratification, and turbulence. A wide range of wind and rain conditions were observed in the eastern tropical Pacific Ocean near 10°N, 125°W in summer-autumn 2016 and 2017. Wind was the primary driver of near-surface turbulence and the mixing of rain-formed fresh lenses, with lenses generally persisting for hours when wind speeds were under 5 m s−1 and mixing away immediately at higher wind speeds. Rain influenced near-surface turbulence primarily through stratification. Near-surface stratification caused by rainfall or diurnal warming suppressed deeper turbulent dissipation rates when wind speeds were under 3 m s−1. In one case with 4-5 m s−1 winds, rain-induced stratification enhanced dissipation rates within the stratified layer. At wind speeds above 7-8 m s−1, strong stratification was not observed in the upper meter during rain, indicating that rain lenses do not form at wind speeds above 8 m s−1. Raindrop impacts enhanced turbulent dissipation rates at these high wind speeds in the absence of near-surface stratification. Measurements of air-sea buoyancy flux, wind speed, and near-surface turbulence can be used to predict the presence of stratified layers. These findings could be used to improve model parameterizations of air-sea interactions and, ultimately, our understanding of the global water cycle.

Spatiotemporal Variability of Rainfall and Surface Salinity in the Eastern Pacific Fresh Pool: A Joint In Situ and Satellite Analysis During the SPURS‐2 Field Campaign

“…The slight SMAP SSS negative bias compared to SPURS-2 in situ 0.1-2 m salinity data is consistent with the fact that SMAP senses lower SSS at the skin level following rainfall than would subsurface 0.1-2 m salinity in situ sensors. This supports findings from other studies that the near-surface haline stratification is prominent under frequent rainfall in the EPFP region (Reverdin et al, 2012, as well as Drushka et al, 2019Iyer & Drushka, 2021b using SPURS-2 data).…”

The Influence of Preexisting Stratification and Tropical Rain Modes on the Mixed Layer Salinity Response to Rainfall

“…Investigating this issue will provide insight into how regions of low near‐surface salinity are created and will improve our understanding of this linkage in the global water cycle. In addition to atmospheric forcing, stratification greatly influences physical processes in the near‐surface ocean: turbulence can be enhanced within and suppressed below stratified layers created by rainfall (Iyer & Drushka, 2021; Smyth et al., 1997; ten Doeschate et al., 2019; Walesby et al., 2015). Similarly, stratification caused by solar radiation can enhance near‐surface turbulence and intensify surface currents in diurnal warm layers (Brainerd & Gregg, 1993; Callaghan et al., 2014; Moulin et al., 2018; Sutherland et al., 2016).…”

Rain‐Induced Stratification of the Equatorial Indian Ocean and Its Potential Feedback to the Atmosphere

“…The nature of the atmospheric response to RL formation remains an important open question for understanding the impact of freshwater ocean surface stratification on atmospheric convection. Recently, idealized model experiments have increased understanding of RL characteristics, revealing the importance of rain rate, wind speed, and background ocean stratification in regulating RL behavior (Drushka et al, 2016;Iyer & Drushka, 2021b;Soloviev et al, 2015). While experiments investigating RLs in an idealized environment have provided insight into upper ocean response to precipitation, the collective effects of RLs under realistic, time-varying atmospheric forcing on SST patterns, surface fluxes, and feedbacks to atmospheric convection is less understood.…”