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Kyla Drushka

Senior Oceanographer

Affiliate Assistant Professor, Oceanography

Email

kdrushka@apl.washington.edu

Phone

206-543-6858

Education

BS Physics, McGill University, 2004

PhD Physical Oceanography, Scripps Institution of Oceanography, 2011

Publications

2000-present and while at APL-UW

Extension of the prognostic model of sea surface temperature to rain-induced cool and fresh lenses

Bellenger, H., K. Drushka, W. Asher, G. Reverdin, M. Katsumata, and M. Watanabe, "Extension of the prognostic model of sea surface temperature to rain-induced cool and fresh lenses," J. Geophys. Res., 122, 484-507, doi:10.1002/2016JC012429, 2017.

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1 Jan 2017

The Zeng and Beljaars (2005) sea surface temperature prognostic scheme, developed to represent diurnal warming, is extended to represent rain-induced freshening and cooling. Effects of rain on salinity and temperature in the molecular skin layer (first few hundred micrometers) and the near-surface turbulent layer (first few meters) are separately parameterized by taking into account rain-induced fluxes of sensible heat and freshwater, surface stress, and mixing induced by droplets penetrating the water surface. Numerical results from this scheme are compared to observational data from two field studies of near-surface ocean stratifications caused by rain, to surface drifter observations and to previous computations with an idealized ocean mixed layer model, demonstrating that the scheme produces temperature variations consistent with in situ observations and model results. It reproduces the dependency of salinity on wind and rainfall rate and the lifetime of fresh lenses. In addition, the scheme reproduces the observed lag between temperature and salinity minimum at low wind speed and is sensitive to the peak rain rate for a given amount of rain. Finally, a first assessment of the impact of these fresh lenses on ocean surface variability is given for the near-equatorial western Pacific. In particular, the variability due to the mean rain-induced cooling is comparable to the variability due to the diurnal warming so that they both impact large-scale horizontal surface temperature gradients. The present parameterization can be used in a variety of models to study the impact of rain-induced fresh and cool lenses at different spatial and temporal scales.

Satellite and in situ salinity: Understanding near-surface stratification and sub footprint variability

Boutin, J., and 21 others, including W.E. Asher and K. Drushka, "Satellite and in situ salinity: Understanding near-surface stratification and sub footprint variability," Bull. Am. Meteor. Soc., 97, 1391-1407, doi:10.1175/BAMS-D-15-00032.1, 2016.

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1 Aug 2016

Remote sensing of salinity using satellite-mounted microwave radiometers provides new perspectives for studying ocean dynamics and the global hydrological cycle. Calibration and validation of these measurements is challenging because satellite and in situ methods measure salinity differently. Microwave radiometers measure the salinity in the top few centimeters of the ocean, whereas most in situ observations are reported below a depth of a few meters. Additionally, satellites measure salinity as a spatial average over an area of about 100 x 100 km2. In contrast, in situ sensors provide pointwise measurements at the location of the sensor. Thus, the presence of vertical gradients in, and horizontal variability of, sea surface salinity complicates comparison of satellite and in situ measurements. This paper synthesizes present knowledge of the magnitude and the processes that contribute to the formation and evolution of vertical and horizontal variability in near-surface salinity. Rainfall, freshwater plumes, and evaporation can generate vertical gradients of salinity, and in some cases these gradients can be large enough to affect validation of satellite measurements. Similarly, mesoscale to submesoscale processes can lead to horizontal variability that can also affect comparisons of satellite data to in situ data. Comparisons between satellite and in situ salinity measurements must take into account both vertical stratification and horizontal variability.

Impact of slowdown of Atlantic overturning circulation on heat and freshwater transports

Kelly, K.A., K. Drushka, L. Thompson, D. Le Bars, and E.L. McDonagh, "Impact of slowdown of Atlantic overturning circulation on heat and freshwater transports," Geophys. Res. Lett., 43, 7625-7631, doi:10.1002/2016GL069789, 2016.

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28 Jul 2016

Recent measurements of the strength of the Atlantic overturning circulation at 26°N show a 1 year drop and partial recovery amid a gradual weakening. To examine the extent and impact of the slowdown on basin wide heat and freshwater transports for 2004–2012, a box model that assimilates hydrographic and satellite observations is used to estimate heat transport and freshwater convergence as residuals of the heat and freshwater budgets. Using an independent transport estimate, convergences are converted to transports, which show a high level of spatial coherence. The similarity between Atlantic heat transport and the Agulhas Leakage suggests that it is the source of the surface heat transport anomalies. The freshwater budget in the North Atlantic is dominated by a decrease in freshwater flux. The increasing salinity during the slowdown supports modeling studies that show that heat, not freshwater, drives trends in the overturning circulation in a warming climate.

More Publications

Intraseasonal variability of mixed layer depth in the tropical Indian Ocean

Keerthi, M.G., and 7 others, including K. Drushka, "Intraseasonal variability of mixed layer depth in the tropical Indian Ocean," Clim. Dyn., 46, 2633-2655, doi:10.1007/s00382-015-2721-z, 2016.

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1 Apr 2016

In this paper, we use an observational dataset built from Argo in situ profiles to describe the main large-scale patterns of intraseasonal mixed layer depth (MLD) variations in the Indian Ocean. An eddy permitting (0.25°) regional ocean model that generally agrees well with those observed estimates is then used to investigate the mechanisms that drive MLD intraseasonal variations and to assess their potential impact on the related SST response. During summer, intraseasonal MLD variations in the Bay of Bengal and eastern equatorial Indian Ocean primarily respond to active/break convective phases of the summer monsoon. In the southern Arabian Sea, summer MLD variations are largely driven by seemingly-independent intraseasonal fluctuations of the Findlater jet intensity. During winter, the Madden–Julian Oscillation drives most of the intraseasonal MLD variability in the eastern equatorial Indian Ocean. Large winter MLD signals in northern Arabian Sea can, on the other hand, be related to advection of continental temperature anomalies from the northern end of the basin. In all the aforementioned regions, peak-to-peak MLD variations usually reach 10 m, but can exceed 20 m for the largest events. Buoyancy flux and wind stirring contribute to intraseasonal MLD fluctuations in roughly equal proportions, except for the Northern Arabian Sea in winter, where buoyancy fluxes dominate. A simple slab ocean analysis finally suggests that the impact of these MLD fluctuations on intraseasonal sea surface temperature variability is probably rather weak, because of the compensating effects of thermal capacity and sunlight penetration: a thin mixed-layer is more efficiently warmed at the surface by heat fluxes but loses more solar flux through its lower base.

Understanding the formation and evolution of rain-formed fresh lenses at the ocean surface

Drushka, K., W.E. Asher, B. Ward, and K. Walesby, "Understanding the formation and evolution of rain-formed fresh lenses at the ocean surface," J. Geophys. Res., 121, 2673-2689, doi:10.1002/2015JC011527, 2016.

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1 Apr 2016

Rain falling on the ocean produces a layer of buoyant fresher surface water, or "fresh lens." Fresh lenses can have significant impacts on satellite-in situ salinity comparisons and on exchanges between the surface and the bulk mixed layer. However, because these are small, transient features, relatively few observations of fresh lenses have been made. Here the Generalized Ocean Turbulence Model (GOTM) is used to explore the response of the upper few meters of the ocean to rain events. Comparisons with observations from several platforms demonstrate that GOTM can reproduce the main characteristics of rain-formed fresh lenses. Idealized sensitivity tests show that the near-surface vertical salinity gradient within fresh lenses has a linear dependence on rain rate and an inverse dependence on wind speed. Yearlong simulations forced with satellite rainfall and reanalysis atmospheric parameters demonstrate that the mean salinity difference between 0.01 and 5 m, equivalent to the measurement depths of satellite radiometers and Argo floats, is –0.04 psu when averaged over the 20°S–20°N tropical band. However, when averaged regionally, the mean vertical salinity difference exceeds –0.15 psu in the Indo-Pacific warm pool, in the Pacific and Atlantic intertropical convergence zone, and in the South Pacific convergence zone. In most of these regions, salinities measured by the Aquarius satellite instrument have a fresh bias relative to Argo measurements at 5 m depth. These results demonstrate that the fresh bias in Aquarius salinities in rainy, low-wind regions may be caused by the presence of rain-produced fresh lenses.

Processes driving intraseasonal displacements of the eastern edge of the warm pool: The contribution of westerly wind events

Drushka, K., H. Bellenger, E. Guilyardi, M. Lengaigne, J. Vialard, and G. Madec, "Processes driving intraseasonal displacements of the eastern edge of the warm pool: The contribution of westerly wind events," Clim. Dyn., 44, 735-755, doi:10.1007/s00382-014-2297-z, 2015.

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1 Feb 2015

We investigate the processes responsible for the intraseasonal displacements of the eastern edge of the western Pacific warm pool (WPEE), which appear to play a role in the onset and development of El Niño events. We use 25 years of output from an ocean general circulation model experiment that is able to accurately capture the observed displacements of the WPEE, sea level anomalies, and upper ocean zonal currents at intraseasonal time scales in the western and central Pacific Ocean. Our results confirm that WPEE displacements driven by westerly wind events (WWEs) are largely controlled by zonal advection. This paper has also two novel findings: first, the zonal current anomalies responsible for the WPEE advection are driven primarily by local wind stress anomalies and not by intraseasonal wind-forced Kelvin waves as has been shown in most previous studies. Second, we find that intraseasonal WPEE fluctuations that are not related to WWEs are generally caused by intraseasonal variations in net heat flux, in contrast to interannual WPEE displacements that are largely driven by zonal advection. This study hence raises an interesting question: can surface heat flux-induced zonal WPEE motions contribute to El Niño%u—Southern Oscillation evolution, as WWEs have been shown to be able to do?

Defining Mediterranean and Black Sea biogeochemical subprovinces and synthetic ocean indicators using mesoscale oceanographic features

Nieblas, A.-E., S. Bonhommeau, K. Drushka, G. Reygondeau, V. Rossi, Hervé Demarcq, and L. Dubroca, "Defining Mediterranean and Black Sea biogeochemical subprovinces and synthetic ocean indicators using mesoscale oceanographic features," Plos One, 9, doi:10.1371/journal.pone.0111251, 2014.

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31 Oct 2014

The Mediterranean and Black Seas are semi-enclosed basins characterized by high environmental variability and growing anthropogenic pressure. This has led to an increasing need for a bioregionalization of the oceanic environment at local and regional scales that can be used for managerial applications as a geographical reference. We aim to identify biogeochemical subprovinces within this domain, and develop synthetic indices of the key oceanographic dynamics of each subprovince to quantify baselines from which to assess variability and change. To do this, we compile a data set of 101 months (2002%u20132010) of a variety of both %u201Cclassical%u201D (i.e., sea surface temperature, surface chlorophyll-a, and bathymetry) and %u201Cmesoscale%u201D (i.e., eddy kinetic energy, finite-size Lyapunov exponents, and surface frontal gradients) ocean features that we use to characterize the surface ocean variability. We employ a k-means clustering algorithm to objectively define biogeochemical subprovinces based on classical features, and, for the first time, on mesoscale features, and on a combination of both classical and mesoscale features. Principal components analysis is then performed on the oceanographic variables to define integrative indices to monitor the environmental changes within each resultant subprovince at monthly resolutions. Using both the classical and mesoscale features, we find five biogeochemical subprovinces for the Mediterranean and Black Seas. Interestingly, the use of mesoscale variables contributes highly in the delineation of the open ocean. The first axis of the principal component analysis is explained primarily by classical ocean features and the second axis is explained by mesoscale features. Biogeochemical subprovinces identified by the present study can be useful within the European management framework as an objective geographical framework of the Mediterranean and Black Seas, and the synthetic ocean indicators developed here can be used to monitor variability and long-term change.

The diurnal salinity cycle in the tropics

Drushka, K., S.T. Gille, and J. Sprintall, "The diurnal salinity cycle in the tropics," J. Geophys. Res., 119, 5874-5890, doi:10.1002/2014JC009924, 2014.

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1 Sep 2014

Observations from 35 tropical moorings are used to characterize the diurnal cycle in salinity at 1 m depth. The amplitude of diurnal salinity anomalies is up to 0.01 psu and more typically ~0.005 psu. Diurnal variations in precipitation and vertical entrainment appear to be the dominant drivers of diurnal salinity variability, with evaporation also contributing. Areas where these processes are strong are expected to have relatively strong salinity cycles: the eastern Atlantic and Pacific equatorial regions, the southwestern Bay of Bengal, the Amazon outflow region, and the Indo-Pacific warm pool. We hypothesize that salinity anomalies resulting from precipitation and evaporation are initially trapped very near the surface and may not be observed at the 1 m instrument depths until they are mixed downward. As a result, the pattern of diurnal salinity variations is not only dependent on the strength of the forcing terms, but also on the phasing of winds and convective overturning. A comparison of mixed-layer depth computed with hourly and with daily averaged salinity reveals that diurnal salinity variability can have a significant effect on upper ocean stratification, suggesting that representing diurnal salinity variability could potentially improve air-sea interaction in climate models. Comparisons between salinity observations from moorings and from the Aquarius satellite (level 2 version 3.0 data) reveal that the typical difference between ascending-node and descending-node Aquarius salinity is an order of magnitude greater than the observed diurnal salinity anomalies at 1 m depth.

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center
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