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.

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.…”

Atmospheric Rivers Contribute to Summer Surface Buoyancy Forcing in the Atlantic Sector of the Southern Ocean

“…This may be a result of the WG being a moving platform, entering an area affected by rain that relates to a decrease in salinity, and then leaving the localized area affected by rainfall‐induced surface freshening, raising the salinity once again. Iyer and Drushka (2021) observed such rainfall‐induced patchiness in surface salinity as their ship entered an area (∼7 km in length) with ongoing precipitation (up to 60 mm hr −1 ). The salinity at 23 cm, which is the closest to our measurement depth (∼30 cm), varied significantly horizontally, up to ±1 PSU at a high frequency in both temporal and spatial space (5–10 min and less than kilometer scale).…”

“…(2016) showed that sea‐ice formation/melt and freshwater flux from precipitation played a critical role in lowering the density of upwelled waters, aiding in driving meridional overturning circulation (open ocean precipitation south of 50°S adds 24.2 km 3 freshwater to the ocean every day, compared to 43.2 km 3 from sea‐ice melt). Further, precipitation over the ocean can create surface “rain lenses,” which are very shallow layers with a reduced salinity (ten Doeschate et al., 2019; Shcherbina et al., 2019; Iyer & Drushka, 2021). These layers are highly stratified and therefore suppress turbulence close to the surface, which in turn could inhibit the transfer of turbulent energy from the surface to the deeper parts of the mixed layer (Iyer & Drushka, 2021).…”

“…Further, precipitation over the ocean can create surface “rain lenses,” which are very shallow layers with a reduced salinity (ten Doeschate et al., 2019; Shcherbina et al., 2019; Iyer & Drushka, 2021). These layers are highly stratified and therefore suppress turbulence close to the surface, which in turn could inhibit the transfer of turbulent energy from the surface to the deeper parts of the mixed layer (Iyer & Drushka, 2021). Given that ARs are predicted to increase in frequency and carry more moisture over the ocean due to anthropogenic warming (Payne et al., 2020), understanding how they currently impact surface ocean buoyancy is an important step toward quantifying the impact ARs will have on the surface ocean and its circulation in the future.…”

Temperature and Salinity Anomalies in the Sea Surface Microlayer of the South Pacific During Precipitation Events

“…Half‐hourly precipitation (GPM IMERG Final Precipitation L3, 0.1° × 0.1°) images were analyzed (Huffman et al., 2019). Although integrated multi‐satellite retrievals for GPM (IMERG) algorithms have been improved to resolve diurnal cycles (Tan et al., 2019), there are still limitations in resolving small‐scale features of rainfall events (Iyer and Drushka (2021b) and references herein). For this study, we chose the half‐hour IMERG product, because it has high temporal resolution consistent with our observation time of less than 6 hr.…”