Short Time-scale Variability Research Articles

The Relationship between Convective Bursts and Warm-Core Intensification in a Nonhydrostatic Simulation of Typhoon Lionrock (2016)

Abstract Typhoon Lionrock (2016) was unusual among tropical cyclones (TCs) in that it formed east of the monsoon gyre in the western North Pacific, and moved counterclockwise. It rapidly intensified in the monsoon gyre in an environment of weak upper-tropospheric winds and vertical wind shear. This study used a 3-km mesh nonhydrostatic model to examine the warm-core intensification of Typhoon Lionrock, which was associated with cyclone-scale vigorous convection [i.e., convective bursts (CBs)]. The simulation reproduced the multiple CBs at intervals of 1 day or shorter, which were related to the diurnal cycles and other short time-scale variations in the TC convection. Each CB tended to precede peak temperature anomalies near the TC center by 0–12 h, indicating that the warm-core intensification occurred due to diabatic heating released by the vigorous eyewall convection. Notably, updrafts due to convection during the intensification phase were stronger than those occurring during the mature and decay phases, and the maximum temperature anomaly of the upper-tropospheric warm core rapidly increased during eyewall formation. In addition, this study indicated that most of the asymmetric inner-core vigorous convection associated with CBs, which was induced by the vertical wind shear, contributed to the warm-core intensification. Furthermore, the budget analysis of potential temperature within the TC inner core showed that adiabatic heating due to subsidence from near the tropopause within the eye, often following CBs, was essential in developing the eye. The lag correlation suggested the lag time between the CBs and the subsidence within the eye was 3–9 h.

Vertical mixing in a shallow tropical reservoir

This paper presents observations of diurnal cycles of stratification and vertical mixing in Kranji Reservoir, a shallow tropical reservoir with an average depth of 6.7 m located in the northwest corner of Singapore, via field measurements, focusing on a series of three 24-h self-contained autonomous microprofiler (SCAMP) measurements. This data, representing one of the most complete data sets, shows vertical mixing parameters for a tropical shallow reservoir over a diurnal cycle. Responding to diurnal cycles, the observations indicate that the thermal and flow structures are delineated by two distinct thermoclines: a three-layer structure during the daytime and a two-layer structure during the night-time when the two thermoclines merged into one. The daytime structure consisted of a thin surface mixed layer (SML) above the diurnal thermocline, an intermediate hypolimnion layer with the two thermoclines as its boundaries, and an underflow layer below the secondary thermocline. This underflow is shown to primarily arise from a horizontal temperature gradient generated by the daytime wind and stratification, and maintained by night-time differential cooling. The nocturnal structure is comprised of a SML deepened by penetrative cooling and the underflow layer. Analysis of relevant dimensionless numbers, the turbulent kinetic energy (TKE) budget, and FrT–ReT turbulent phase diagram provide a consistent picture of the different mixing mechanisms within each layer. This short time scale variability of vertical mixing as observed in Kranji Reservoir would be a factor in favouring primary production for such shallow tropical systems.

Investigating the variability of cardiac pulse artifacts across heartbeats in simultaneous EEG-fMRI recordings: A 7T study

Electroencephalography (EEG) recordings performed in magnetic resonance imaging (MRI) scanners are affected by complex artifacts caused by heart function, often termed pulse artifacts (PAs). PAs can strongly compromise EEG data quality, and remain an open problem for EEG-fMRI. This study investigated the properties and mechanisms of PA variability across heartbeats, which has remained largely unaddressed to date, and evaluated its impact on PA correction approaches. Simultaneous EEG-fMRI was performed at 7T on healthy participants at rest or under visual stimulation, with concurrent recordings of breathing and cardiac activity. PA variability was found to contribute to EEG variance with more than 500 μV2 at 7T, which extrapolates to 92 μV2 at 3T. Clustering analyses revealed that PA variability not only is linked to variations in head position/orientation, as previously hypothesized, but also, and more importantly, to the respiratory cycle and to heart rate fluctuations. The latter mechanisms are associated to short-timescale variability (even across consecutive heartbeats), and their importance varied across EEG channels. In light of this PA variability, three PA correction techniques were compared: average artifact subtraction (AAS), optimal basis sets (OBS), and an approach based on K-means clustering. All methods allowed the recovery of visual evoked potentials from the EEG data; nonetheless, OBS and K-means tended to outperform AAS, likely due to the inability of the latter in modeling short-timescale variability. Altogether, these results offer novel insights into the dynamics and underlying mechanisms of the pulse artifact, with important consequences for its correction, relevant to most EEG-fMRI applications.

Wastewater microbial community structure and functional traits change over short timescales

Wastewater contains microorganisms coming from various sources, e.g. feces discharges, soil infiltrations and sewer biofilms and sediments. The primary objective of this work was to determine if end-of-pipe wastewater microbial community structures exhibits short-timescale variation, and assess possible microbial origins. To this end, we measured hourly physicochemical characteristics of wastewater influent for 2 days and analyzed the microbial community at 4-h intervals using 16S rRNA gene amplicon sequencing. Results showed large variations in the microbial community composition at phylum and genus levels, i.e. Proteobacteria ranged from 44 to 63% of the total relative abundance and Arcobacter ranged from 11 to 22%. Diurnal patterns were observed in the alpha-diversity, beta-diversity and the prevalence of several taxa. Wastewater physicochemical characteristics explained 61% of the total microbial community variance by Canonical Correspondence Analysis (CCA), with flow rate being the main explanatory variable exhibiting a clear diurnal profile. Comparison with public databases using closed reference OTUs revealed that only 7.3% of the sequences were shared with human gut microbiota and 21.7% with soil microbiota, the majority being from the sewer biofilms and sediments. The functional trait, weighted average ribosomal RNA operon (rrn) copy number per genome, was found to be relatively high in the wastewater microbiota (average 3.6, soil 2.1, and human gut 2.6) and significantly correlated with flow, inferring active microbial enrichments in the sewer. The prevalence of Methylophilaceae, methanol oxidation genes and denitrification genes were related to high influent methanol and NO3− concentration in the influent wastewater. These functional organisms and genes indicate important carbon and nutrient removal related functions in the sewer. Together, the observed temporal patterns of the microbial community and functional traits suggest that high wastewater flow causes greater transport of active sewer microorganisms which are functionally important.

The short timescale variability of the oxygen inventory in the NE Black Sea slope water

Abstract. Warm winters have recently become common over the Black Sea, leading to the risk of shoaling oxygen penetration. The insufficient supply of oxygen to the near-bottom layer may result in a decrease in faunal abundance. However, there is a lack of data on the temporal variations in oxygen throughout the water column over short timescales of hours to weeks. In this paper, new observations over the upper part of the NE Black Sea continental slope are presented. Regular, frequent measurements were performed using a moored profiler from January to early March 2016. The profiling allowed for direct observations of the temperature in the cold intermediate layer (CIL), pycnocline structure, current velocity, and oxygen stratification and in particular, the depths of hypoxia onset. The average local oxygen inventory below a depth of 30 m was 24.9 mol m−2. Relatively high/low oxygen inventory cases were related to the thin/thick main pycnocline, which was associated with the onshore/offshore location of the Black Sea Rim Current. The pycnocline hindered the vertical transport of oxygenated water to the CIL. The vertical displacements of the hypoxia onset depth ranged from 97 to 170 m, while the shelf edge depth in this region usually ranged from 90 to 100 m. Intermittently, the hypoxia boundary depth fluctuated on two timescales: ∼17 h due to inertial oscillations and 3–7 days due to current meanders and eddies.

Gaia Data Release 2

Aims. We describe the methods used and the analysis performed in the frame of the Gaia data processing activities to produce the Gaia Data Release 2 (DR2) sample candidates with short-timescale variability together with associated parameters. Methods. The Gaia DR2 sample of candidates with short-timescale variability results from the investigation of the first 22 months of Gaia G per-CCD, GBP, and GRP photometry for a subsample of sources at the Gaia faint end (G ~ 16.5−20 mag). For this first short-timescale variability search exploiting Gaia data, we limited ourselves to the case of suspected rapid periodic variability. Our study combines fast-variability detection through variogram analysis, a high-frequency search by means of least-squares periodograms, and an empirical selection based on the investigation of specific sources seen through the Gaia eyes (e.g., known variables or visually identified objects with peculiar features in their light curves). The progressive definition, improvement, and validation of this selection criterion also benefited from supplementary ground-based photometric monitoring of a few tens of preliminary candidates with short-timescale variability, performed at the Flemish Mercator telescope in La Palma (Canary Islands, Spain) between August and November 2017. Results. As part of Gaia DR2, we publish a list of 3018 candidates with short-timescale variability, spread throughout the sky, with a false-positive rate of up to 10–20% in the Magellanic Clouds, and a more significant but justifiable contamination from longer-period variables between 19% and 50%, depending on the area of the sky. Although its completeness is limited to about 0.05%, this first sample of Gaia short-timescale variables recovers some very interesting known short-period variables, such as post-common envelope binaries or cataclysmic variables, and brings to light some fascinating, newly discovered variable sources. In the perspective of future Gaia data releases, several improvements of the short-timescale variability processing are considered, by enhancing the existing variogram and period-search algorithms or by classifying the identified variability candidates. Nonetheless, the encouraging outcome of our Gaia DR2 analysis demonstrates the power of this mission for such fast-variability studies, and opens great perspectives for this domain of astrophysics.

Gaia Data Release 2

Context. The Gaia Data Release 2 (DR2) contains more than half a million sources that are identified as variable stars. Aims. We summarise the processing and results of the identification of variable source candidates of RR Lyrae stars, Cepheids, long-period variables (LPVs), rotation modulation (BY Dra-type) stars, δ Scuti and SX Phoenicis stars, and short-timescale variables. In this release we aim to provide useful but not necessarily complete samples of candidates. Methods. The processed Gaia data consist of the G, GBP, and GRP photometry during the first 22 months of operations as well as positions and parallaxes. Various methods from classical statistics, data mining, and time-series analysis were applied and tailored to the specific properties of Gaia data, as were various visualisation tools to interpret the data. Results. The DR2 variability release contains 228 904 RR Lyrae stars, 11 438 Cepheids, 151 761 LPVs, 147 535 stars with rotation modulation, 8882 δ Scuti and SX Phoenicis stars, and 3018 short-timescale variables. These results are distributed over a classification and various Specific Object Studies tables in the Gaia archive, along with the three-band time series and associated statistics for the underlying 550 737 unique sources. We estimate that about half of them are newly identified variables. The variability type completeness varies strongly as a function of sky position as a result of the non-uniform sky coverage and intermediate calibration level of these data. The probabilistic and automated nature of this work implies certain completeness and contamination rates that are quantified so that users can anticipate their effects. Thismeans that even well-known variable sources can be missed or misidentified in the published data. Conclusions. The DR2 variability release only represents a small subset of the processed data. Future releases will include more variable sources and data products; however, DR2 shows the (already) very high quality of the data and great promise for variability studies.

Searching for Short-timescale Variability in the Ultraviolet with the GALEX gPhoton Archive. I. Artifacts and Spurious Periodicities

Abstract In order to develop and test a methodology to search for UV variability over the entire Galaxy Evolution Explorer (GALEX) database down to the shortest timescales, we analyzed time-domain photometry of ∼5000 light curves of ∼300 bright (m FUV, m NUV ≤ 14) and blue (m FUV–m NUV < 0) GALEX sources. Using the gPhoton database tool, we discovered and characterized instrumentally induced variabilities in time-resolved GALEX photometry that may severely impact automated searches for short-period variations. The most notable artifact is a quasi-sinusoidal variation mimicking light curves typical of pulsators, seen occasionally in either one or both detectors, with amplitudes of up to 0.3 mag and periods corresponding to the periodicity of the spiral dithering pattern used during the observation (P ∼ 120 s). Therefore, the artifact may arise from small-scale response variations. Other artifacts include visit-long “sagging” or “hump” in flux, occurring when the dithering pattern is not a spiral, or a one-time change in flux level during the exposure. These instrumentally caused variations were not reported before, and are not due to known (and flagged) artifacts such as hotspots, which can be easily eliminated. To characterize the frequency and causality of such artifacts, we apply Fourier transform analysis to both light curves and dithering patterns, and examine whether artificial brightness variations correlate with visit or instrumental parameters. Artifacts do not correlate with source position on the detector. We suggest methods to identify artifact variations and to correct them when possible.

Physical characterization of the deep-space debris WT1190F: A testbed for advanced SSA techniques

We report on extensive BVRcIc photometry and low-resolution (λ/Δλ∼250) spectroscopy of the deep-space debris WT1190F, which impacted Earth offshore from Sri Lanka, on 2015 November 13. In spite of its likely artificial origin (as a relic of some past lunar mission), the case offered important points of discussion for its suggestive connection with the envisaged scenario for a (potentially far more dangerous) natural impactor, like an asteroid or a comet.Our observations indicate for WT1190F an absolute magnitude Rc=32.45±0.31, with a flat dependence of reflectance on the phase angle, such as dRc/dϕ∼0.007±2 mag deg−1. The detected short-timescale variability suggests that the body was likely spinning with a period twice the nominal figure of Pflash=1.4547±0.0005s, as from the observed lightcurve. In the BVRcIc color domain, WT1190F closely resembled the Planck deep-space probe. This match, together with a depressed reflectance around 4000 and 8500 Å may be suggestive of a “grey” (aluminized) surface texture.The spinning pattern remained in place also along the object fiery entry in the atmosphere, a feature that may have partly shielded the body along its fireball phase perhaps leading a large fraction of its mass to survive intact, now lying underwater along a tight (∼1×80 km) strip of sea, at a depth of 1500 m or less.Under the assumption of Lambertian scatter, an inferred size of 216±30/α/0.1 cm is obtained for WT1190F. By accounting for non-gravitational dynamical perturbations, the Area-to-Mass ratio of the body was in the range (0.006⩽AMR⩽0.011) m2 kg−1.Both these figures resulted compatible with the two prevailing candidates to WT1190F’s identity, namely the Athena II Trans-Lunar Injection Stage of the Lunar Prospector mission, and the ascent stage of the Apollo 10 lunar module, callsign “Snoopy”. Both candidates have been analyzed in some detail here through accurate 3D CAD design mockup modelling and BRDF reflectance rendering to derive the inherent photometric properties to be compared with the observations.

Stochastic marine ice sheet variability

It is well known that deterministic two-dimensional marine ice sheets can only be stable if the grounding line is positioned at a sufficiently steep, downward sloping bedrock. When bedrock conditions favour instabilities, multiple stable ice sheet profiles may occur. Here, we employ continuation techniques to examine the sensitivity of a two-dimensional marine ice sheet to stochastic noise representing short time scale variability, either in the accumulation rate or in the sea level height. We find that in unique regimes, the position of the grounding line is most sensitive to noise in the accumulation rate and can explain excursions observed in field measurements. In the multiple equilibrium regime, there is a strong asymmetry in transition probabilities between the different ice sheet states, with a strong preference to switch to the branch with a steeper bedrock slope.

Short-timescale γ-Ray Variability in CTA 102

Abstract The flat-spectrum radio quasar CTA 102 experienced a prolonged state of enhanced activity across the entire observed electromagnetic spectrum during 2016–2017, most pronounced during a major outburst between 2016 December and 2017 May. Fermi-LAT observed a flux of (2.2 ± 0.2) × 10−5 photons cm−2 s−1 at energies above 100 MeV on 2017 April 19 during a single orbit. We report here the detection of significant (4.7σ) flux variations down to timescales of ∼5 minutes during this orbit. The measured variability timescale is much shorter than the light-travel time across the central black hole (∼70 minutes) indicating a very compact emission region within the jet, similar to that seen in IC 310, Mrk 501, or PKS 1222+21 from MAGIC observations. This short-timescale variability is unexpected since the γ-ray spectrum shows no sign of attenuation due to pair creation in interactions with photons from the broad emission line region, and therefore must be assumed to originate far from the black hole. The observed fast variability could either indicate the dissipation of magnetic islands or protons in a collimated beam from the base of the jet encountering the turbulent plasma at the end of the magnetic nozzle.

Short time-scale optical variability properties of the largest AGN sample observed with Kepler/K2

We present the first short time-scale ($\sim$hours to days) optical variability study of a large sample of Active Galactic Nuclei (AGN) observed with the Kepler/K2 mission. The sample contains 252 AGN observed over four campaigns with $\sim 30$ minute cadence selected from the Million Quasar Catalogue with R magnitude $< 19$. We performed time series analysis to determine their variability properties by means of the power spectral densities (PSDs) and applied Monte Carlo techniques to find the best model parameters that fit the observed power spectra. A power-law model is sufficient to describe all the PSDs of our sample. A variety of power-law slopes were found indicating that there is not a universal slope for all AGN. We find that the rest-frame amplitude variability in the frequency range of $6\times10^{-6}-10^{-4}$ Hz varies from $1-10$ % with an average of 1.7 %. We explore correlations between the variability amplitude and key parameters of the AGN, finding a significant correlation of rest-frame short-term variability amplitude with redshift. We attribute this effect to the known "bluer when brighter" variability of quasars combined with the fixed bandpass of Kepler data. This study also enables us to distinguish between Seyferts and Blazars and confirm AGN candidates. For our study we have compared results obtained from light curves extracted using different aperture sizes and with and without de-trending. We find that limited de-trending of the optimal photometric precision light curve is the best approach, although some systematic effects still remain present.

LINETGIS analysis of lightning flash density in Serbia based on ten years data

Lightning detection networks installed throughout the world provide data for obtaining flash density maps and information about lightning discharges characteristics. According to the IEC 62858 Standard, lightning data for at least ten years is required to ensure that short time scale variations in lightning parameters are accounted for. LINET (European Lightning Detection Network) was installed in Serbia in 2008, so its data for the last ten years period are used for the analysis in this paper. LINETGIS is a new software application based on geographical information system (GIS) technology in order to obtain regional flash density maps of Serbia. LINETGIS application may be used for up-to-date regional flash density maps of Serbia, but also for any other GIS covered area.

White dwarf variability with gPhoton: pulsators

We present results from a search for short time-scale white dwarf variability using \texttt{gPhoton}, a time-tagged database of \textit{GALEX} photon events and associated software package. We conducted a survey of $320$ white dwarf stars in the McCook-Sion catalogue, inspecting each for photometric variability with particular emphasis on variability over time-scales less than $\sim 30$ minutes. From that survey, we present the discovery of a new pulsating white dwarf: WD 2246-069. A Ca II K line is found in archival ESO spectra and an IR excess is seen in WISE $W1$ and $W2$ bands. Its independent modes are identified in follow-up optical photometry and used to model its interior structure. Additionally, we detect UV pulsations in four previously known pulsating ZZ Ceti-type (DAVs). Included in this group is the simultaneous fitting of the pulsations of WD 1401-147 in optical, near-ultraviolet and far-ultraviolet bands using nearly concurrent Whole Earth Telescope and \textit{GALEX} data, providing observational insight into the wavelength dependence of white dwarf pulsation amplitudes.

Outflow-confined H ii Regions. II. The Early Break-out Phase

Abstract In this series of papers, we model the formation and evolution of the photoionized region and its observational signatures during massive star formation. Here, we focus on the early breakout of the photoionized region into the outflow cavity. Using results of 3D magnetohydrodynamic-outflow simulations and protostellar evolution calculations, we perform a post-processing radiative transfer. The photoionized region first appears at a protostellar mass of in our fiducial model and is confined to within 10–100 au by the dense inner outflow, which is similar to some of the observed very small hypercompact H ii regions. Since the ionizing luminosity of the massive protostar increases dramatically as the Kelvin–Helmholtz (KH) contraction proceeds, the photoionized region breaks out to the entire outflow region in ≲10,000 year. Accordingly, the radio free–free emission brightens significantly in this stage. In our fiducial model, the radio luminosity at 10 GHz changes from at to at , while the infrared luminosity increases by less than a factor of two. The radio spectral index also changes in the break-out phase from the optically thick value of ∼2 to the partially optically thin value of ∼0.6. Additionally, we demonstrate that short-timescale variation in the free–free flux would be induced by an accretion burst. The outflow density is enhanced in the accretion burst phase, which leads to a smaller ionized region and weaker free–free emission. The radio luminosity may decrease by one order of magnitude during such bursts, while the infrared luminosity is much less affected because internal protostellar luminosity dominates over accretion luminosity after the KH contraction starts. Such a variability may be observable on timescales as short 10–100 year if accretion bursts are driven by disk instabilities.

Modelling broadband scatter from periodic sea surfaces

Surface waves are the primary source of short time-scale variation of the channel impulse response in underwater transmission scenarios when source and receiver positions are fixed. This study proposes an exact method for modeling the single surface bounce path of the channel impulse response, which contains most of short time-scale effect. This method is designed for one dimensional, periodic wave profiles, and solves the scattering problem as a series of motionless surfaces. To create a time series for each surface, the proposed method solves the Helmholtz equation at many frequencies, performing three steps at each frequency. First, the point source is decomposed into a series of plane waves, each of which scatter from the surface at a finite number of Bragg angles. Second, the amplitudes of these discrete scattered plane waves are found with an exact method. Finally, the surface response for a three-dimensional point source is recovered with a plane wave synthesis of these scattered plane waves. The results of this study are intended as a reference to inform the interpretation of surface scattering results from more common underwater acoustic models.

Parametrizing modified gravity for cosmological surveys

One of the challenges in testing gravity with cosmology is the vast freedom opened when extending General Relativity. For linear perturbations, one solution consists in using the Effective Field Theory of Dark Energy (EFT of DE). Even then, the theory space is described in terms of a handful of free functions of time. This needs to be reduced to a finite number of parameters to be practical for cosmological surveys. We explore in this article how well simple parametrizations, with a small number of parameters, can fit observables computed from complex theories. Imposing the stability of linear perturbations appreciably reduces the theory space we explore. We find that observables are not extremely sensitive to short time-scale variations and that simple, smooth parametrizations are usually sufficient to describe this theory space. Using the Bayesian Information Criterion, we find that using two parameters for each function (an amplitude and a power law index) is preferred over complex models for 86% of our theory space.

Short time-scale variables in the Gaia era: detection and characterization by structure function analysis

We investigate the capabilities of the ESA Gaia mission for detecting and character- izing short timescale variability, from tens of seconds to a dozen hours. We assess the efficiency of the variogram analysis, for both detecting short timescale variability and estimating the underlying characteristic timescales from Gaia photometry, through extensive light-curve simulations for various periodic and transient short timescale variable types. We show that, with this approach, we can detect fast periodic variabil- ity, with amplitudes down to a few millimagnitudes, as well as some M dwarf flares and supernovae explosions, with limited contamination from longer timescale variables or constant sources. Timescale estimates from the variogram give valuable informa- tion on the rapidity of the underlying variation, which could complement timescale estimates from other methods, like Fourier-based periodograms, and be reinvested in preparation of ground-based photometric follow-up of short timescale candidates evi- denced by Gaia. The next step will be to find new short timescale variable candidates from real Gaia data, and to further characterize them using all the Gaia information, including color and spectrum.

A long-term study of AGN X-ray variability

Variability in the X-rays is a key ingredient in understanding and unveiling active galactic nuclei (AGN) properties. In this band flux variations occur on short time scales (hours) as well as on larger times scales. While short time scale variability is often investigated in single source studies, only few works are able to explore flux variation on very long time scales.This work provides a statistical analysis of the AGN long term X-ray variability. We study variability on the largest time interval ever investigated for the 0.2-2 keV band, up to $\sim 20$ years rest-frame for a sample of 220 sources. Moreover, we study variability for 2,700 quasars up to $\sim 8$ years rest-frame in the same (soft) band.We build our source sample using the 3XMM serendipitous source catalogue data release 5, and data from ROSAT All Sky Survey Bright and Faint source catalogues. In order to select only AGN we use the Sloan Digital Sky Survey quasar catalogues data releases 7 and 12. Combining ROSAT and XMM-Newton observations, we investigate variability using the structure function analysis which describes the amount of variability as a function of the lag between the observations.Our work shows an increase of the structure function up to 20 years. We do not find evidence of a plateau in the structure function on these long time scales.The increase of the structure function at long time lags suggests that variability in the soft X-rays can be influenced by flux variations originated in the accretion disk or that they take place in a region large enough to justify variation on such long time scales.

High-resolution variability of the enrichment of fluorescence dissolved organic matter in the sea surface microlayer of an upwelling region

Enrichment of fluorescence dissolved organic matter (FDOM) in the sea surface microlayer (SML) provides insights into biogeochemical processes occurring at the sea surface, including cycling of organic matter, photochemistry, and air-sea gas exchange. We present data concerning the variability of FDOM enrichment in the SML during upwelling events in the Baltic Sea (Cruise M117). Our results show that FDOM is frequently enriched (75% of all samples) and that enrichment factors are significantly higher in SMLs located in regions with upwelling (pooled median = 1.4) compared to a non-upwelling region (median = 1.1). The enrichment factor of FDOM showed short time-scale variability, changing by 6% within ten-minute intervals. Larger variabilities (standard deviation up to ±0.14 µg L–1 compared to background of ±0.01 µg L–1) occurred when fronts were present and when the SML was mixed with underlying bulk water. Small-scale patchiness, indicated by changes in the variability of FDOM enrichment in SML, was a common feature of the sea surface. Wind speed played a potential role in controlling the enrichment of FDOM in the SML, but the effects of solar radiation on photochemical processes, mixing and upwelling of water masses, and biological processes as a source of FDOM also influence enrichment at this critical interface between ocean and atmosphere.