Rectification of the Bias in the Wavelet Power Spectrum

Liu, Yonggang; Liang, X. San; Weisberg, Robert H.

doi:10.1175/2007jtecho511.1

Cited by 440 publications

(375 citation statements)

References 32 publications

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“…The well-studied transition in 1976/1977 of Pacific climate represents a relatively small change when compared to many such shifts observed throughout the Fig. 9; Liu et al, 2007;Torrence and Compo, 1998) all independently confirm the oscillatory nature of the proxy rainfall record. MTM and wavelet analyses reveal persistent concentrations of power at decadal periodicities, significant above an AR(1) red noise model (p < 0.05 for the wavelet and p < 0.01 in the MTM).…”

Section: Characterizing the Rainfall Reconstructionsupporting

confidence: 60%

“…On century scales, there is no persistent or consistent relationship between stalagmite δ 18 O variability and the solar variability reconstruction of Vieira et al (2011). Periods of high solar irradiance (e.g., solar maxima centered at 1600 CE) and low solar irradiance (e.g., Maunder Minimum centered at (Liu et al, 2007;Torrence and Compo, 1998). Black contours denote power above the 95 % confidence interval when using an AR(1) red noise model; crosshatching represents the "cone of influence" (Torrence and Compo, 1998).…”

Section: Stalagmite Variability: External Forcing Vs Internal Variabmentioning

confidence: 99%

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Persistent decadal-scale rainfall variability in the tropical South Pacific Convergence Zone through the past six centuries

et al. 2014

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Abstract. Modern Pacific decadal variability (PDV) has global impacts; hence records of PDV from the preinstrumental period are needed to better inform models that are used to project future climate variability. We focus here on reconstructing rainfall in the western tropical Pacific (Solomon Islands; ∼ 9.5 • S, ∼ 160 • E), a region directly influenced by PDV, using cave deposits (stalagmite). A relationship is developed between δ 18 O variations in the stalagmite and local rainfall amount to produce a 600 yr record of rainfall variability from the South Pacific Convergence Zone (SPCZ). We present evidence for large (∼ 1.5 m), abrupt, and periodic changes in total annual rainfall amount on decadal to multidecadal timescales since 1423 ± 5 CE (Common Era) in the Solomon Islands. The timing of the decadal changes in rainfall inferred from the 20th-century portion of the stalagmite δ 18 O record coincides with previously identified decadal shifts in PDV-related Pacific ocean-atmosphere behavior (Clement et al., 2011;Deser et al., 2004). The Solomons record of PDV is not associated with variations in external forcings, but rather results from internal climate variability. The 600 yr Solomon Islands stalagmite δ 18 O record indicates that decadal oscillations in rainfall are a persistent characteristic of SPCZ-related climate variability.

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Section: Characterizing the Rainfall Reconstructionsupporting

confidence: 60%

Section: Stalagmite Variability: External Forcing Vs Internal Variabmentioning

confidence: 99%

Persistent decadal-scale rainfall variability in the tropical South Pacific Convergence Zone through the past six centuries

et al. 2014

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“…The first study we performed to measure the surface rotation was a time-period analysis using a wavelet decomposition (Torrence & Compo 1998), improved for the low-frequency region as in Liu et al (2007), and adapted to our asteroseismic purposes following Mathur et al (2010b). This study allows us to track the temporal evolution of any modulation in the light curve that could be related to the rotation period and hence check whether it is not caused by a sudden event in the time series (normally related to a instrumental perturbation).…”

Section: Time-frequency Analysis and Projected Power Spectrummentioning

confidence: 99%

Rotation and magnetism ofKeplerpulsating solar-like stars

García

Ceillier

Salabert

et al. 2014

A&A

275

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Kepler ultra-high precision photometry of long and continuous observations provides a unique dataset in which surface rotation and variability can be studied for thousands of stars. Because many of these old field stars also have independently measured asteroseismic ages, measurements of rotation and activity are particularly interesting in the context of age-rotation-activity relations. In particular, age-rotation relations generally lack good calibrators at old ages, a problem that this Kepler sample of old-field stars is uniquely suited to address. We study the surface rotation and photometric magnetic activity of a subset of 540 solar-like stars on the mainsequence and the subgiant branch for which stellar pulsations have been measured. The rotation period was determined by comparing the results from two different analysis methods: i) the projection onto the frequency domain of the time-period analysis, and ii) the autocorrelation function of the light curves. Reliable surface rotation rates were then extracted by comparing the results from two different sets of calibrated data and from the two complementary analyses. General photometric levels of magnetic activity in this sample of stars were also extracted by using a photometric activity index, which takes into account the rotation period of the stars. We report rotation periods for 310 out of 540 targets (excluding known binaries and candidate planet-host stars); our measurements span a range of 1 to 100 days. The photometric magnetic activity levels of these stars were computed, and for 61.5% of the dwarfs, this level is similar to the range, from minimum to maximum, of the solar magnetic activity. We demonstrate that hot dwarfs, cool dwarfs, and subgiants have very different rotation-age relationships, highlighting the importance of separating out distinct populations when interpreting stellar rotation periods. Our sample of cool dwarf stars with age and metallicity data of the highest quality is consistent with gyrochronology relations reported in the literature.

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“…To perform a time-frequency analysis, we used the wavelets tool developed by Torrence & Compo (1998), including the correction by Liu et al (2007), and adapted for asteroseismology by Mathur et al (2010b). It consists of looking for the correlation between the time series and a mother wavelet with a given period.…”

Section: Time-frequency Analysis: Magnetic Cycle Proxymentioning

confidence: 99%

Magnetic activity of F stars observed byKepler

Mathur

García

Ballot

et al. 2014

A&A

162

195

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Context. The study of stellar activity is important because it can provide new constraints for dynamo models when combined with surface rotation rates and the depth of the convection zone. We know that the dynamo mechanism, which is believed to be the main process that rules the magnetic cycle of solar-like stars, results from the interaction between (differential) rotation, convection, and magnetic field. The Kepler mission has already been collecting data for a large number of stars during four years allowing us to investigate magnetic stellar cycles. Aims. We investigated the Kepler light curves to look for magnetic activity or even hints of magnetic activity cycles. Based on the photometric data we also looked for new magnetic indexes to characterise the magnetic activity of the stars. Methods. We selected a sample of 22 solar-like F stars that have a rotation period shorter than 12 days. We performed a timefrequency analysis using the Morlet wavelet yielding a magnetic proxy for our sample of stars. We computed the magnetic index S ph as the standard deviation of the whole time series and the index S ph , which is the mean of standard deviations measured in subseries of length five times the rotation period of the star. We defined new indicators, such as the contrast between high and low activity, to take into account the fact that complete magnetic cycles are not observed for all the stars. We also inferred the Rossby number of the stars and studied their stellar background. Results. This analysis shows different types of behaviour in the 22 F stars. Two stars show behaviour very similar to magnetic activity cycles. Five stars show long-lived spots or active regions suggesting the existence of active longitudes. Two stars in our sample seem to have a decreasing or increasing trend in the temporal variation of the magnetic proxies. Finally, the last group of stars shows magnetic activity (with the presence of spots) but no sign of cycle.

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Rectification of the Bias in the Wavelet Power Spectrum

Cited by 440 publications

References 32 publications

Persistent decadal-scale rainfall variability in the tropical South Pacific Convergence Zone through the past six centuries

Persistent decadal-scale rainfall variability in the tropical South Pacific Convergence Zone through the past six centuries

Rotation and magnetism ofKeplerpulsating solar-like stars

Magnetic activity of F stars observed byKepler

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