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Suneil Iyer and Kyla Drushka

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.

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Suneil Iyer, Kyla Drushka, and Luc Rainville

Abstract

As part of the second Salinity Processes in the Upper Ocean Regional Study (SPURS-2), the ship-towed Surface Salinity Profiler (SSP) was used to measure near-surface turbulence and stratification on horizontal spatial scales of tens of kilometers over time scales of hours, resolving structures outside the observational capabilities of autonomous or Lagrangian platforms. Observations of microstructure variability of temperature were made at approximately 37 cm depth from the SSP. The platform can be used to measure turbulent kinetic energy dissipation rate when the upper ocean is sufficiently stratified by calculating temperature gradient spectra from the microstructure data and fitting to low-wavenumber theoretical Batchelor spectra. Observations of dissipation rate made across a range of wind speeds under 12 m s−1 were consistent with the results of previous studies of near-surface turbulence and with existing turbulence scalings. Microstructure sensors mounted on the SSP can be used to investigate the spatial structure of near-surface turbulence. This provides a new means to study the connections between near-surface turbulence and the larger-scale distributions of heat and salt in the near-surface layer of the ocean.

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Kyla Drushka, Janet Sprintall, Sarah T. Gille, and Irsan Brodjonegoro

Abstract

The subsurface structure of intraseasonal Kelvin waves in two Indonesian Throughflow (ITF) exit passages is observed and characterized using velocity and temperature data from the 2004–06 International Nusantara Stratification and Transport (INSTANT) project. Scatterometer winds are used to characterize forcing, and altimetric sea level anomaly (SLA) data are used to trace the pathways of Kelvin waves east from their generation region in the equatorial Indian Ocean to Sumatra, south along the Indonesian coast, and into the ITF region.

During the 3-yr INSTANT period, 40 intraseasonal Kelvin waves forced by winds over the central equatorial Indian Ocean caused strong transport anomalies in the ITF outflow passages. Of these events, 21 are classed as “downwelling” Kelvin waves, forced by westerly winds and linked to depressions in the thermocline and warm temperature anomalies in the ITF outflow passages; 19 were “upwelling” Kelvin waves, generated by easterly wind events and linked to shoaling of the thermocline and cool temperature anomalies in the ITF. Both downwelling and upwelling Kelvin waves have similar vertical structures in the ITF outflow passages, with strong transport anomalies over all depths and a distinctive upward tilt to the phase that indicates downward energy propagation. A linear wind-forced model shows that the first two baroclinic modes account for most of the intraseasonal variance in the ITF outflow passages associated with Kelvin waves and highlights the importance of winds both in the eastern equatorial Indian Ocean and along the coast of Sumatra and Java for exciting Kelvin waves.

Using SLA as a proxy for Kelvin wave energy shows that 37% ± 9% of the incoming Kelvin wave energy from the Indian Ocean bypasses the gap in the coastal waveguide at Lombok Strait and continues eastward. Of the energy that continues eastward downstream of Lombok Strait, the Kelvin waves are split by Sumba Island, with roughly equal energy going north and south to enter the Savu Sea.

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Kyla Drushka, Janet Sprintall, Sarah T. Gille, and Susan Wijffels

Abstract

The boreal winter response of the ocean mixed layer to the Madden–Julian oscillation (MJO) in the Indo-Pacific region is determined using in situ observations from the Argo profiling float dataset. Composite averages over numerous events reveal that the MJO forces systematic variations in mixed layer depth and temperature throughout the domain. Strong MJO mixed layer depth anomalies (>15 m peak to peak) are observed in the central Indian Ocean and in the far western Pacific Ocean. The strongest mixed layer temperature variations (>0.6°C peak to peak) are found in the central Indian Ocean and in the region between northwest Australia and Java. A heat budget analysis is used to evaluate which processes are responsible for mixed layer temperature variations at MJO time scales. Though uncertainties in the heat budget are on the same order as the temperature trend, the analysis nonetheless demonstrates that mixed layer temperature variations associated with the canonical MJO are driven largely by anomalous net surface heat flux. Net heat flux is dominated by anomalies in shortwave and latent heat fluxes, the relative importance of which varies between active and suppressed MJO conditions. Additionally, rapid deepening of the mixed layer in the central Indian Ocean during the onset of active MJO conditions induces significant basin-wide entrainment cooling. In the central equatorial Indian Ocean, MJO-induced variations in mixed layer depth can modulate net surface heat flux, and therefore mixed layer temperature variations, by up to ~40%. This highlights the importance of correctly representing intraseasonal mixed layer depth variations in climate models in order to accurately simulate mixed layer temperature, and thus air–sea interaction, associated with the MJO.

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Jacob. M. Steinberg, Sylvia T. Cole, Kyla Drushka, and Ryan P. Abernathey

Abstract

Oceanic mesoscale motions including eddies, meanders, fronts, and filaments comprise a dominant fraction of oceanic kinetic energy and contribute to the redistribution of tracers in the ocean such as heat, salt, and nutrients. This reservoir of mesoscale energy is regulated by the conversion of potential energy and transfers of kinetic energy across spatial scales. Whether and under what circumstances mesoscale turbulence precipitates forward or inverse cascades, and the rates of these cascades, remain difficult to directly observe and quantify despite their impacts on physical and biological processes. Here we use global observations to investigate the seasonality of surface kinetic energy and upper ocean potential energy. We apply spatial filters to along-track satellite measurements of sea surface height to diagnose surface eddy kinetic energy across 60-300 km scales. A geographic and scale dependent seasonal cycle appears throughout much of the mid-latitudes, with eddy kinetic energy at scales less than 60 km peaking 1-4 months before that at 60-300 km scales. Spatial patterns in this lag align with geographic regions where an Argo-derived estimate of the conversion of potential to kinetic energy are seasonally varying. In mid-latitudes, the conversion rate peaks 0-2 months prior to kinetic energy at scales less than 60 km. The consistent geographic patterns between the seasonality of potential energy conversion and kinetic energy across spatial scale provide observational evidence for the inverse cascade, and demonstrate that some component of it is seasonally modulated. Implications for mesoscale parameterizations and numerical modeling are discussed.

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Kyla Drushka, William E. Asher, Janet Sprintall, Sarah T. Gille, and Clifford Hoang

Abstract

Surface salinity variability on O(1–10) km lateral scales (the submesoscale) generates density variability and thus has implications for submesoscale dynamics. Satellite salinity measurements represent a spatial average over horizontal scales of approximately 40–100 km but are compared to point measurements for validation, so submesoscale salinity variability also complicates validation of satellite salinities. Here, we combine several databases of historical thermosalinograph (TSG) measurements made from ships to globally characterize surface submesoscale salinity, temperature, and density variability. In river plumes; regions affected by ice melt or upwelling; and the Gulf Stream, South Atlantic, and Agulhas Currents, submesoscale surface salinity variability is large. In these regions, horizontal salinity variability appears to explain some of the differences between surface salinities from the Aquarius and SMOS satellites and salinities measured with Argo floats. In other words, apparent satellite errors in highly variable regions in fact arise because Argo point measurements do not represent spatially averaged satellite data. Salinity dominates over temperature in generating submesoscale surface density variability throughout the tropical rainbands, in river plumes, and in polar regions. Horizontal density fronts on 10-km scales tend to be compensated (salinity and temperature have opposing effects on density) throughout most of the global oceans, with the exception of the south Indian and southwest Pacific Oceans between 20° and 30°S, where fronts tend to be anticompensated.

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Steven A. Rutledge, V. Chandrasekar, Brody Fuchs, Jim George, Francesc Junyent, Brenda Dolan, Patrick C. Kennedy, and Kyla Drushka

Abstract

A new, advanced radar has been developed at Colorado State University (CSU). The Sea-Going Polarimetric (SEA-POL) radar is a C-band, polarimetric Doppler radar specifically designed to deploy on research ships. SEA-POL is the first such weather radar developed in the United States. Ship-based weather radars have a long history, dating back to GATE in 1974. The GATE radars measured only reflectivity. After GATE, ship radars also provided Doppler measurements. SEA-POL represents the next advancement by adding dual-polarization technology, the ability to transmit and receive both horizontal and vertical polarizations. This configuration provides information about hydrometeor size, shape, and phase. As a result, superior rain-rate estimates are afforded by the dual-polarization technology, along with hydrometeor identification and overall improved data quality. SEA-POL made its first deployment as part of the Salinity Processes in the Upper Ocean Regional Study, second field phase (SPURS-2) fall 2017 cruise to the eastern tropical Pacific, sailing on the R/V Roger Revelle. SPURS-2 was a field project to investigate the fate of freshwater deposited on the ocean’s surface. Oceanographers are keenly interested in how fast these freshwater patches mix out by wind and upper-ocean turbulence, as the less dense rainfall sitting atop the salty ocean inhibits mixing through increased stability. To this end, during SPURS-2, SEA-POL produced rain maps identifying the location of freshwater lenses on the ocean’s surface thereby providing context for measurements of SST and salinity. Examples of SEA-POL polarization measurements are also discussed to assess microphysical processes within oceanic convection. Future ocean-based field campaigns will now benefit from SEA-POL’s advanced dual-polarization technology.

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Charlotte A. DeMott, Aneesh Subramanian, Shuyi Chen, Kyla Drushka, Yosuke Fujii, Adrienne Sutton, Janet Sprintall, and Dongxiao Zhang
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