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Tropical instability waves (TIWs) are identified in three multiyear equatorial mooring records in Pacific and Atlantic cold tongues to evaluate how TIWs modulate turbulence. At 0°, 140°W in the Pacific, TIWs are present in 43% of observations, and are associated with elevated vertical shear and a 40% average increase in turbulence dissipation rates (ϵ) above the Equatorial Undercurrent. Zonal shear is greatest when currents are southward while buoyancy is greatest later in the TIW cycle, leading to greater potential for instability and elevated turbulence before and during the southward flow maximum. This suggests that TIW vortex stretching contributes to enhanced shear and turbulence. In the Atlantic, TIWs are found in 38% of observations at 0°, 23°W and 16% of observations at 0°, 10°W. TIWs at 23°W increaseϵby 18% where turbulence is likely modulated by vortex stretching and, near the surface, by the seasonally wind‐forced equatorial roll. At 23°W and 140°W, TIWs with strong meridional velocity fluctuations are associated with the strongest turbulence. Contributions of seasonal variations are removed by considering only periods when TIWs are climatically active. During these periods, mean values ofϵin the presence of strong TIWs are elevated by 61% at 140°W, 29% at 23°W, and 36% at 10°W. At 10°W, where our identification scheme may include wind‐forced oscillations in the same frequency band, increases inϵare not consistent in the presence of TIWs and do not contribute significantly to multiyear averages.

 

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⁻¹and 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⁻¹winds, 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.

 

The freshwater input from rain to the surface ocean is a key component of the global water cycle. Frequent rainfall in the inter‐tropical convergence zone creates regions of strong surface stratification and low salinity, which vary seasonally. We evaluate how variations in rain type and preexisting upper ocean stratification influence the timing and duration of the salinity response to rainfall using the General Ocean Turbulence Model. A series of model simulations was run by prescribing three typical background stratification conditions and idealized rain and wind forcing that was consistent with observed convective, stratiform, and mixed convective and stratiform rainfall. Background stratification was assessed using underway CTD observations and rain forcing was identified from mooring observations collected in the eastern tropical Pacific during the second Salinity Processes in the Upper Ocean Regional Study. Model results show that strong stratification, whether preexisting or from convective rainfall, inhibits downward mixing of freshwater and allows near‐surface salinity anomalies to persist following rain. In contrast, when stratiform rain precedes convective rain, salinity anomalies are quickly mixed downward and longer lasting deeper in the mixed layer. This implies that accurately quantifying the salinity structure following rain should consider preexisting stratification and the type of rainfall. Furthermore, patterns of rainfall and stratification likely affect the bias between salinity observations at the surface and deeper in the mixed layer. Because satellite rain data do not correctly represent the small scales of rain forcing, the small‐scale surface salinity response to rain cannot be predicted from satellite data.

 

The Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) took place from 7 January to 11 July 2020 in the tropical North Atlantic between the eastern edge of Barbados and 51∘ W, the longitude of the Northwest Tropical Atlantic Station (NTAS) mooring. Measurements were made to gather information on shallow atmospheric convection, the effects of aerosols and clouds on the ocean surface energy budget, and mesoscale oceanic processes. Multiple platforms were deployed during ATOMIC including the NOAA RV Ronald H. Brown (RHB) (7 January to 13 February) and WP-3D Orion (P-3) aircraft (17 January to 10 February), the University of Colorado's Robust Autonomous Aerial Vehicle-Endurant Nimble (RAAVEN) uncrewed aerial system (UAS) (24 January to 15 February), NOAA- and NASA-sponsored Saildrones (12 January to 11 July), and Surface Velocity Program Salinity (SVPS) surface ocean drifters (23 January to 29 April). The RV Ronald H. Brown conducted in situ and remote sensing measurements of oceanic and atmospheric properties with an emphasis on mesoscale oceanic–atmospheric coupling and aerosol–cloud interactions. In addition, the ship served as a launching pad for Wave Gliders, Surface Wave Instrument Floats with Tracking (SWIFTs), and radiosondes. Details of measurements made from the RV Ronald H. Brown, ship-deployed assets, and other platforms closely coordinated with the ship during ATOMIC are provided here. These platforms include Saildrone 1064 and the RAAVEN UAS as well as the Barbados Cloud Observatory (BCO) and Barbados Atmospheric Chemistry Observatory (BACO). Inter-platform comparisons are presented to assess consistency in the data sets. Data sets from the RV Ronald H. Brown and deployed assets have been quality controlled and are publicly available at NOAA's National Centers for Environmental Information (NCEI) data archive (https://www.ncei.noaa.gov/archive/accession/ATOMIC-2020, last access: 2 April 2021). Point-of-contact information and links to individual data sets with digital object identifiers (DOIs) are provided herein. 

Abstract. The science guiding the EUREC4A campaign and its measurements is presented. EUREC4A comprised roughly 5 weeks of measurements in the downstream winter trades of the North Atlantic – eastward and southeastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, EUREC4A marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or the life cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso- (200 km) and larger (500 km) scales, roughly 400 h of flight time by four heavily instrumented research aircraft; four global-class research vessels; an advanced ground-based cloud observatory; scores of autonomous observing platforms operating in the upper ocean (nearly 10 000 profiles), lower atmosphere (continuous profiling), and along the air–sea interface; a network of water stable isotopologue measurements; targeted tasking of satellite remote sensing; and modeling with a new generation of weather and climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that EUREC4A explored – from North Brazil Current rings to turbulence-induced clustering of cloud droplets and its influence on warm-rain formation – are presented along with an overview of EUREC4A's outreach activities, environmental impact, and guidelines for scientific practice. Track data for all platforms are standardized and accessible at https://doi.org/10.25326/165 (Stevens, 2021), and a film documenting the campaign is provided as a video supplement.