1. Introduction
Evidence for climatic variability on a millennial scale has been found in ice cores and deep sea sediments (see Bond et al. 1997, 1999; Alley and Clark 1999 for recent reviews). Prominent layers of ice-rafted debris in North Atlantic sediment cores, presumably associated with ocean surface cooling, show that “… the 1–2 kyr cycle is a persistent feature of climate,” with 0.55 ± 0.15 cycles per kiloyear (cpky) (1400–2500 yr) as representative of the millennial climate spectrum (Bond et al. 1999; Fig. 1). Grootes and Stuiver (1997) find a broad peak at the 1500-yr period in the spectrum of δ18O data for the Greenland Ice Sheet. With regard to the generating mechanism, suggestions include solar forcing, internal dynamics of the ocean–atmosphere system and harmonics of the Milankovitch orbital frequencies; according to Bond et al. “none of those alternatives is supported thus far by particularly compelling evidence.”
Yet a different hypothesis has been proposed by Keeling and Whorf (hereafter KW; 1997, 2000); KW suggest that the millennial variability is related to extreme oceanic tides associated with orbital coincidences reoccurring at certain repeat periods; further, that strong tidal forcing causes cooling of the sea surface by increased vertical mixing. The KW proposal resembles a hypothesis put forward by Pettersson (1914, 1930). This again is based on orbital coincidences (including 1500 yr among other periods), and derives from Pettersson's discovery of a scattering of surface tides into giant internal tides (and eventually turbulent mixing) at the sills of Scandinavian fjords.
There are many options concerning the generation of millennial variability (Fig. 2). First there is the question of orbital forcing versus an internal variability inherent in ocean–atmosphere dynamics. In the former case we expect the response to be characterized by a narrow spectral line, in the latter case by a broad spectral band (as observed). With regard to orbital forcing, we distinguish between the Milankovitch terms and what are usually called the tide-producing forces; the former have periods much longer, and the latter periods much shorter, then the millennial variability. The KW proposal is for nonlinear interaction between the high-frequency tidal constituents producing low-frequency forcing.
Here we must distinguish between two distinct processes leading to low-frequency generation: repeat coincidence (RC) of orbital parameters, and harmonic beats (HB). A tidal forcing at 0.56 cpky (1795 yr) resulting from an interaction between yearly, lunar perigean, and lunar nodal forcing has been proposed by KW. We examine the set of possible nonlinear interactions and find the KW frequency to be unique among harmonic beat frequencies. This is confirmed by a numerical experiment subjecting a 275 000-yr tidal time series to various nonlinearities. Some of the harmonic beat frequencies in the cpky range have a secular change associated with the acceleration of the lunar orbit due to tidal friction.
2. External forcing versus internal variability
The most fundamental question is whether the millennial variability is associated with internal processes inherent in ocean–atmosphere dynamics, or whether it is externally forced by radiational–gravitational fluctuations inherent in the solar system. ENSO and the North Atlantic oscillation are considered to be of the former type, and much effort has gone into understanding the internal dynamics. Among external forcing we need to distinguish between orbital forcing, and the possible effect of a variable solar radiation.
A distinction between orbital forcing (whether Milankovitch or tidal) and internal variability can be made on the basis of bandwidth of the response. Orbital forcing is at precise frequencies (spectral “lines”); the frequencies are given to eight significant figures and meaningful predictions (and hindcasts) can be carried out for 10 million years. (But lines are not infinitely narrow because of tidal friction, planetary perturbations, etc.) Internal ocean–atmosphere processes are broadband, and predictions are limited to a few cycles. The experimental evidence suggests that the millennial variability is spread over a band (Fig. 1). But the evidence is not conclusive. Lines are spread into bands by poor resolution and sampling error, and bands can be collapsed into lines by “overtuning” (see Muller and MacDonald 2000). A comparison of the relative contributions to the record variance of the spectral lines and the continuum requires a comparison of line heights and continuum area (see appendix A for a discussion).
One can expect both lines and a continuum in the spectrum of any geophysical time series. At tidal frequencies (Fig. 3) the situation is as follows: for frequencies above 1 cycle per year (cpy) the line spectrum dominates over the continuum “noise,” (the strategy of tidal analysis until the 1960s was consistent with the assumption of an infinite signal-to-noise ratio). The detection of the intertidal continuum had to await the development of modern computers (Munk and Bullard 1963; Munk and Cartwright 1966). Below 1 cpy the continuum takes over, and very long records are required to even detect the tidal lines above the continuum. So both the lines and the continuum are essential components of the spectrum at tidal frequencies. With regard to the climate spectrum, there has been remarkable success in identifying the Milankovitch orbital lines in the sedimentary record. But lines are the more appealing manifestation of variability, and the evidence is still out on what fraction of the climate variance is associated with the intertidal continuum relative to that in the line spectrum.
3. Orbital forcing
a. Milankovitch versus Darwin–Doodson
Figure 3 is a very crude attempt to pull together the work of two distinct communities: climate (left) and tides (right). The principal tidal lines are to the right and clustered about five frequency ranges: cycles per day (cpd), per month (cpm), per year (cpy), per lunar perigee (8.8 yr), and regression (18.6 yr). Higher harmonics of principal frequencies (such as 3, 4, … cpm) diminish in amplitude like ξ3, ξ4, where ξ is the parallax (Earth radius/distance, 1/59.6 for the Moon, 1/23 400 for the Sun). Multiples are associated with Kepler–Newton nonlinearity: cpm splitting of the cpd lines, cpy splitting of cpm lines, etc. The cpm fine structure of the cpd lines is further split into a cpy hyperfine structure (not shown), etc.
To the left are the so-called Milankovitch (1941) terms of orbital motion. These are associated with perturbations in the Earth–Moon–Sun system by other planets. The principal terms are precession (19 and 23 ky), obliquity (41 ky), and eccentricity (95, 100, 213, 413 kyr) [or inclination (100 kyr), see Berger (1999); Muller and MacDonald (2000)]. Here again Kepler–Newton nonlinearity leads to a fine structure of sum and difference frequencies.
Note the 1:1000 ratio between the longest “traditional” tide period of 18.6 yr and the shortest Milankovitch period of 20 000 yr, defining the boundaries to an “orbital gap.” Forcing within the gap might be attributed to high harmonics of the Kepler–Newton orbital nonlinearities in the Milankovitch terms, or low frequencies associated with Navier–Stokes nonlinearities in the tidal terms. The latter alternative has been chosen by KW.
Starting with Lord Kelvin, the traditional analysis of tide records by Darwin (Sir George, son of Charles) was entirely in the frequency domain. Fitting the data by a few dozen parameters as a basis for tide prediction was one of the success stories of the late nineteenth century. By 1921 the Doodson expansion had led to 358 terms, most of them buried in the continuum noise (Doodson 1921). Here we shall use a direct integration scheme in the time domain (far more frugal in parameter space) developed by Cartwright (Munk and Cartwright 1966; Cartwright and Tayler 1971; Cartwright and Edden 1973). (We note that Doodson and Cartwright formally include the perihelion period of 20.94 ky on the other side of the “gap.”)
The original Milankovitch formulation was entirely in the frequency domain, leading up to 47 terms in the 1970s. Fitting the sedimentary and ice records by the Milankovitch frequencies is one of the success stories of the late twentieth century. But for very long timescales subject to planetary perturbations a direct integration scheme pioneered by Laskar (1986) and Quinn et al. (1991) has led to a convenient presentation in the time domain (see also Laskar 1999; Muller and MacDonald 2000).
The transition in both the tidal and the climate analyses from the frequency to the time domain is an inevitable result of dealing with “almost (but not quite) periodic functions,” associated with weak perturbations. Records “shorter” than the perturbation timescale (100 kyr) are conveniently analyzed in the frequency domain, for longer records a comparison between orbital forcing and climate response can be conveniently performed in the time domain (Shackleton et al. 1999).
b. Insolation versus gravity
The ordinate in Fig. 3 has not been specified. For the Milankovitch forcing the ordinate is insolation (radiation in W m−2 impinging on the upper atmosphere). For the tidal forcing it is the gravity potential V, or equivalently the equilibrium height V g−1. Comparing insolation to tidal height may seem like comparing apples and oranges. But this is not so.
Even for a traditional tide prediction some radiational terms need to be included to give sensible results. For example, the annual and semiannual tides are dominated by nongravitational effects such as the thermal expansion of the water column and, more important, land-and-sea breeze and other wind forcing. The “response method” of tide prediction introduces a “radiational tide potential” (closely related to insolation) in addition to the traditional gravity potential (Munk and Cartwright 1966). In this sense there is a continuity from the shortest semidiurnal tide to the longest Milankovitch orbital forcing. Given the solar mass and radiance, the response of sea level to gravitational and radiational forcing is surprisingly competitive (appendix B).
A quite different connection between tides and climate is associated with a thermal response to gravitational forcing. This is the route taken by KW (2000): “… variations in the strength of oceanic tides cause periodic cooling of surface ocean water by modulating the intensity of vertical mixing that brings to the surface colder water from below,” in line with Garrett's (1979) suggestion of an 18.6-yr cycle in surface temperature. [But a “bidecadal” climate signal appears to be an irregular long-term ENSO cycle and not of tidal origin (Ghil and Vautard 1991; Mann and Park 1994; but see Cerveny and Shaffer 2001).] A more likely scenario makes the connection in terms of the poleward heat transport (rather than surface temperature) as the pertinent climate variable. Interior mixing powered by tides significantly modulates the meridional overturning circulation and thus the equator-to-pole heat transport (Munk and Wunsch 1998). The scenario involves high nonlinearities (mixing by internal wave breaking) on a millennial timescale (bottom water renewal), but we have made no attempt at a quantitative analysis.
4. Nonlinear generation of low frequencies
We now explore tidal forcing in the millennial gap at frequencies well below the frequencies of the fundamental tidal constituents. This is the main topic of Keeling and Whorf (1997, 2000).
Times of extreme tide-producing forces requires the following three conditions to occur nearly simultaneously: (i) Earth at perihelion, (ii) longitude of the Moon's perigee near perihelion or aphelion, and (iii) longitude of the Moon's node near perihelion or aphelion. Pettersson (1930) refers to “parallactic tides” as representative of a near overlap; they are the subject of a monograph by Wood (1986) who refers to them as the “perigean tides.” An example going back to the classics is the metonic cycle of 19 tropical years, which is very close to 254 tropical months (19.0002 tropical years). For even longer intervals one can find even closer overlaps and more extreme tides. These perigean tides are characterized by being of short duration (a fraction of a tidal cycle) and of only slightly greater amplitude than ordinary high tides.
Perigean tides are appropriate if we wish to predict spilling over a sea wall; the prediction of eclipses are of this type since they involve the precise alignment of several orbital parameters. And this would be the proper metric for climate variability if, for example, the loss of radiation during solar eclipses were a significant factor in the radiation balance of the Earth.
A different physics is based on the beat frequencies between the harmonics of the tidal frequencies. Some of the harmonics are densely packed with opportunities for small difference frequencies, as will be shown. The generation of sum and difference frequencies is the familiar tool of the signal processing community.
We shall use the terms “repeat coincidence (RC)” and “harmonic beats (HB)” to refer to the two foregoing procedures. Both procedures are based on a near commensurability of tidal frequencies or their harmonics, but are otherwise quite distinct. Here RC is the appropriate language for eclipse problems; we believe that HB is the appropriate formalism for considering tidal forcing of millennial climate variability (if indeed there is significant tidal forcing).
Pettersson (1930) ascribes climate variability to a number of coincidences such as the metonic cycle and longer periods “… up to 1850 years, the longest that I have been able to find” but gives no indication of how he came up with 1850 years. Keeling and Whorf (1997) speak of the “… slight degree of misalignment and departures from the closest approach of the Earth with the Moon and Sun at the time of extreme tide raising forces.” Using such RC language to discuss climate problems confuses the issue. There is further confusion in that the 1795-yr period proposed by KW is actually of the HB type.
We now develop the two formalisms quantitatively (the reader already persuaded may wish to go directly to section 6).
a. Harmonic beats and repeat coincidences
Thus δf and δT are both of order ϵ, whereas the amplitude loss relative to perfect constructive interference is of the order of ϵ2.
Nearly commensurate frequencies (ϵ → 0) are prerequisite for both HB and RC events. Small integers n favor HB events, large integers make sense only for RC. HB events are of low frequency δf ≈ ϵnf and long duration [order (δf)−1]. For RC events ϵ enters as an amplitude parameter: the defect (relative to perfect constructive interference) is of order ϵ2. Frequencies are of order f/n and remain finite as ϵ → 0; durations are short, a fraction of f−1.
b. Squaring, cubing, … and other powers
c. Clipping
The two operations xp(t) and
d. A numerical example
The repeat period of 20 time units is exact (frequencies are commensurate) and clearly seen in the clipped spectrum. This requires severe clipping of an appropriately limited record portion. Taking ϵ = 0.01, fa = 3.05, fb = 5, fm = 0.975, fc = 4.025, na = 5, nb = 3, Eq. (4.7) gives event times fmt = ℓ = −2, −1, 0, 1, 2 in agreement with Fig. 4. For earlier and later times the amplitude drops below 0.9 in accordance with Eq. (4.8). There are high events at subsequent times, but these are shifted in phase and drop out from a harmonic analysis on a single time base. Within the interval −2.5 < t < +2.5 (Fig. 4b), soft clipping produces HB, severely hard clipping produces RC events.
5. The Saros cycle and other repeat coincidences
6. Keeling–Whorf harmonic beat frequency
We have searched all (21)4 cases for which n2, n3, n4, and n5 have values from −10 to +10 for δf < 0.001 cpy. The only solutions are {0, 0, 0, 0} and the above combination of {0, −1, 6, 6}. The KW cycle is remarkable for involving relatively low harmonics. Keeling and Whorf (2000) note that this 0.5702 cpky line lies within a broad peak 0.4–0.7 cpky found in glacial-Holocene petrologic events (Fig. 1). A second peak is centered at 0.21 cpky (KW have a special argument for a 0.215 cpky frequency). We refer here to a suggestion by Wunsch (2000) that a sharp line at 0.689 cpky (1452 yr) reported by Mayewski et al. (1997) is an alias associated with sampling at multiple intervals of the “calendar year” of exactly 365 days.
We need a systematic way to evaluate HB frequencies based on a suitable list of derived constituents.
7. The Doodson climate vector
This leads to a number of interesting results: (i) using the complete set of 484 Doodson vectors rather than the 6 primary tidal frequencies reduces the required nonlinearity from p = 13 to p = 4. (ii) Some of the interacting constituents include the strong semidiurnal and diurnal frequencies [see Eq. (7.5)], and (iii) these include diurnal and annual frequencies with significant radiational components.
This is as far as (perhaps further than) we shall want to go with tidal numerics. The complexity argues for an analysis in the time domain, especially since some of the frequencies will drift over the time span considered here.
8. Secular change
9. A numerical experiment
We wish to gain some further insight by applying the nonlinear filters xp(t) and Cq[x(t)] on a time series x(t) that bears resemblance to the actual tides. For p = 1 and q sufficiently negative we are back to the unfiltered time series.
We use the development in the time domain of the tidal spherical harmonics
We arbitrarily chose the location of Honolulu, Hawaii (21.3°N, 202.2°W), to compute the equilibrium tide at 10-min intervals for a period of 275 601 yr. This included all of the f1 to f5 constituents; the perihelion term in f6 was omitted (thus avoiding perihelion splitting). Terdiurnal tides 3f1 are included, so that the highest linear frequency is 3 cpday. When raised to the fifth power this yields frequencies up to 15 cpday, which are adequately sampled at 10-min intervals.
A Hanning filter with a 2-day window, subsampled at daily intervals, was used to find filter these 10-min values. The record of 275 601 yr of daily averages was then broken into 5 overlapping segments of 91 867 yr. A sample of the record is shown in Fig. 7.
For a segment length of 91 867 yr the frequency resolution is roughly 0.01 cpky and the KW frequency of about 0.5 cpky is well resolved. Figure 8 shows the spectrum over the lower ranges of frequency that are of interest here. The regressional term f5 is prominent. Some of the terms listed in Table 4 can be recognized. The Cartwright term f4 − 2f5 = 5.6 cpky (179 yr) first appears in the squared record, and its harmonic in the cubed record.
To detect features in the millennial band required further filtering. The side bands of the f5 peak caused by the discontinuities of the record segments obliterated all low frequencies in the raw analysis (not shown). The spectral level at the low frequencies was reduced by 6 orders of magnitude by applying a Hanning data window to each segment, but at the expense of a reduced resolution (widening of the f4 peak). Without such severe tapering the peaks associated with HB frequencies around 1 cpky could not be detected.
The KW is prominent at p ≥ 4, as expected. Using a test signal we have determined that the apparent broadening is consistent with the expected effects of windowing; there is no evidence here for anything but a single line (and no perihelion splitting since f6 was omitted from the generated time series). A number of spectral lines at and above 1 cpky have not been identified.
The lower panel shows the spectrum clipped at various levels (see Fig. 7). Only the f5 peak is noticeable; rather disappointingly there is no evidence for any of the RC terms, even at severe clipping. We suspect some of the RC frequencies would show up in an analysis of relatively short record lengths, as indicated in Fig. 4.
10. Discussion
Keeling and Whorf (1997, 2000) propose that millennial climate variability is associated with tidal forcing. There is indeed a resemblance between the measured spectrum (Fig. 1) and the spectrum of the tide potential as seen through a nonlinear polynomial filter (Fig. 8). But the recorded band structure (rather than line structure) is more consistent with nonorbital generation, such as instabilities in the ocean–atmosphere dynamics, a variable solar radiation, etc.
Assuming orbital forcing, one needs to take into account that the millennial frequency is 5 octaves above the highest Milankovitch frequencies, and 5 octaves below the lowest tidal frequencies. To penetrate the gap one needs high Milankovitch harmonics or high tidal subharmonics. The KW proposal is for nonlinear generation of the tidal subharmonics.
We consider two processes by which tidal orbits can produce low-frequency forcing. Keeling and Whorf refer to the occurrence of repeat coincidence in the orbital parameters. Eclipse “cycles” are associated with such RC events. Extreme high tides occur at long intervals, and the more extreme the tide the longer the interval. The problem here is that RC events are of very short duration (like eclipses). Beat frequencies between neighboring harmonics persist over the time interval of the interference pattern, and are a more likely cause of climate variability.
Given sufficiently high nonlinearities (hard clipping or high-power polymonials), the many harmonics generated are sufficiently densely distributed that there will be some combinations forming low difference frequencies. This is a consequence of Dirichlet's Theorem (Hardy and Wright 1960, p. 375); it remains to be seen whether the penalty function (6.4) associated with the orbital constants of the solar system differs from what is expected from an equivalent set of random numbers.
In their earlier paper, KW (1997) featured decadal and centennial repeat coincidences. We suggest that harmonic beat frequencies are more likely candidates (Table 4); f4 − 2f5 (178 yr) and its harmonic 2(f4 − 2f5) (89 yr) are particularly prominent in the numerical experiments (Fig. 8). Any supporting evidence from the climate record would strengthen the KW case for tidal generation of millennial variability.
From orbital considerations, KW incisively argue for a 1795-yr period, and this corresponds to the combination (−f3 + 6f4 + 6f5) of the annual, lunar perigee and nodal frequencies. Our (somewhat brute force) search through the frequency and time domains reveals no other combination of comparably low harmonics to yield millennial frequencies. Even so, this requires the nonlinear fourth power interaction of tidal constituents. The equivalent tidal amplitude of the millennial term is estimated at 0.04 mm. It is difficult to suppose that this is of sufficient amplitude, and associated with sufficient climate perturbation, to account for the millennial variability. In conclusion, we favor processes with millennial time constants (not yet identified) inherent in ocean–atmosphere dynamics as the source of the millennial climate variability; millennial variability in solar radiation (not yet discovered) is a possibility. But low beat frequencies between tidal harmonics (rather than repeat coincidence, the traditional view) cannot be ruled out by any evidence known to us; if indeed these are a factor, the low amplitude notwithstanding, then the {−1, 6, 6} combination proposed by KW is the most likely candidate.
David Cartwright, Chris Garrett, and Carl Wunsch encouraged us to think about this problem. We are grateful to them and to Jeff Severinghaus, Charles Keeling, and Ralph Keeling for discussions (not implying consensus). The reviewers made a number of very helpful suggestions. Walter Munk holds the Secretary of the Navy Chair in Oceanography. Steven Jayne was supported by the Contract JPL 1218134 with the University of Colorado. His computations were performed at the National Center for Atmospheric Research under support of the National Science Foundation. Breck Betts and Andrea Santos assisted in the preparation of the manuscript.
REFERENCES
Alley, R. B., , and P. U. Clark, 1999: The deglaciation of the northern hemisphere: A global perspective. Ann. Rev. Earth Planet Sci, 27 , 149–182.
Berger, W. H., 1999: The 100-kyr ice-age cycle: Internal oscillation or inclinational forcing? J. Earth Sci, 88 , 305–316.
Bills, B., , and R. D. Ray, 1999: Lunar orbital evolution: A synthesis of recent results. Geophys., Res. Lett, 26 , 3045–3048.
Bond, G., and and Coauthors, 1997: A pervasive millenial-scale cycle in North Atlantic holocene and glacial climates. Science, 278 , 1257–1266.
Bond, G., , W. Showers, , M. Eliot, , M. Evans, , R. Lotti, , I. Hajdas, , G. Bonani, , and S. Johnson, 1999: Mechanism of Global Climate Change at Millenial Time Scales. Geophys. Monogr., No. 112, Amer. Geophys. Union, 394 pp.
Cartwright, D. E., 1974: Years of peak astronomical tides. Nature, 248 , 656–657.
Cartwright, D. E., , and R. J. Tayler, 1971: New computations of the tide-generating potential. Geophys. J. Roy. Astron. Soc, 23 , 45–74.
Cartwright, D. E., , and A. C. Edden, 1973: Corrected tables of tidal harmonics. Geophys. J. Roy. Astron. Soc, 33 , 253–264.
Cerveny, R. S., , and J. A. Shaffer, 2001: The moon and El Niño. Geophys. Res. Lett, 28 , 25–28.
Dickey, J. O., and and Coauthors, 1994: Lunar laser ranging: A continuing legacy of the Apollo program. Science, 265 , 482–490.
Doodson, A. T., 1921: The harmonic development of the tide-generating potential. Proc. Roy. Soc. London, 100A , 305–329.
Garrett, C. J. R., 1979: Mixing in the ocean interior. Dyn. Atmos. Oceans, 3 , 239–265.
Ghil, M., , and R. Vautard, 1991: Interdecadal oscillations and the warming trend in global temperatures. Nature, 350 , 324–327.
Grootes, P. M., , and M. Stuiver, 1997: Oxygen 18/16 variability in Greenland snow and ice with 103- to 105-year time resolution. J. Geophys. Res, 102 , 26455–26470.
Halley, E., 1695: Some account of the ancient state of the city of Palmyra, with short remarks upon the inscriptions found there. Philos. Trans. Roy. Soc. London, 19 , 160–175.
Hardy, G. H., , and E. M. Wright, 1960: An Introduction to the Theory of Numbers. 4th ed. Clarendon Press, 421 pp.
Keeling, C. D., , and T. P. Whorf, 1997: Possible forcing of global temperature by oceanic tides. Proc. Natl. Acad. Sci, 94 , 8321–8328.
Keeling, C. D., , and T. P. Whorf, 2000: The 1800-year oceanic tidal cycle: A possible cause of rapid climate change. Proc. Natl. Acad. Sci, 97 , 3814–3819.
Laskar, J., 1986: Secular terms of classical planetary theories using the results of general theory. Astron. Astrophys, 157 , 59–70.
Laskar, J., 1999: The limits of Earth orbital calculations for geological time-scale use. Philos. Trans. Roy. Soc. London, 357A , 1735–1759.
Mann, M., , and J. Park, 1994: Global-scale modes of surface temperature variability on interannual to century time scales. J. Geophys. Res, 99 , 25819–25834.
Mayewski, P. A., , L. D. Meeker, , M. S. Twickler, , S. Whitlow, , Q. Yang, , W. B. Lyons, , and M. Prentice, 1997: Major features and forcing of high-latitude northern hemisphere atmospheric circulation using a 110 000-year-long glaciochemical series. J. Geophys. Res, 102 , 26345–26366.
Milankovitch, M., 1941: Kanon der Erdbestrahlung und seine Anwendung auf das Eiszeitproblem (Canon of isolation and the Ice-Age problem). Special Publication, Vol. 132, Section of Mathematical and Natural Sciences, Vol. 33, Royal Serbian Academy, 484 pp. Translated by the Program for Scientific Translation, Jerusalem, Israel, Rep. TT 67-51410, 1969. [Available from U.S. Dept. of Commerce, Clearinghouse for Federal, Scientific and Technical Information, Springfield, VA 22151.].
Muller, R. A., , and G. J. MacDonald, 2000: Ice Ages and Astronomical Causes. Springer-Verlag, 318 pp.
Munk, W. H., , and E. C. Bullard, 1963: Patching the long-wave spectrum across the tides. J. Geophys. Res, 68 , 3627–3634.
Munk, W. H., , and D. E. Cartwright, 1966: Tidal spectroscopy and prediction. Philos. Trans. Roy. Soc. London, 259A , 533–581.
Munk, W. H., , and C. Wunsch, 1998: Abyssal Recipes II: Energetics of tidal and wind mixing. Deep-Sea Res, 45 , 1977–2010.
Pettersson, O., 1914: Climatic variation in historic and prehistoric time. Sven. Hydrogr.-Biol. Komm. Skr, 5 .26 pp.
Pettersson, O., 1930: The tidal force. Geogr. Ann, 12 , 261–322.
Quinn, T. R., , S. Tremaine, , and M. Duncan, 1991: A three million year integration of the Earth's orbit. Astron. J, 101 , 2287–2305.
Shackleton, N. J., , S. J. Crowhurst, , G. P. Weedon, , and J. Laskar, 1999: Astronomical calibration of Oligocene-Miocene time. Philos. Trans. Roy. Soc. London, 357A , 1907–1929.
Wolfram, S., 1999: The Mathematica Book. 3d ed. Wolfram Media/Cambridge University Press, 1403 pp.
Wood, F. J., 1986: Tidal Dynamics. Reidel, 558 pp.
Wunsch, C., 2000: On sharp spectral lines in the climate record and the millennial peak. Paleoceanography, 15 , 417–424.
APPENDIX A
Spectral Lines and the Continuum
The situation is not clear for the Milankovitch range of frequencies.
APPENDIX B
Gravitational and Radiational Forcing
Power spectra of hematite-stained grains from deep sea sediment cores in the subpolar North Atlantic. The time series spanning 80 kyr is a sensitive measure of ice-rafting episodes associated with ocean surface cooling [Bond et al. 1997 (bottom), 1999 (top)]
Citation: Journal of Climate 15, 4; 10.1175/1520-0442(2002)015<0370:MCVITA>2.0.CO;2
Sketch of some of the issues involved in the generation of millennial climate variability
Citation: Journal of Climate 15, 4; 10.1175/1520-0442(2002)015<0370:MCVITA>2.0.CO;2
Cartoon of climate variability. Vertical lines represent the combined gravitational–radiational forcing at various orbital frequencies. There is a gap of orbital forcing between periods of 18.6 and 20 000 yr. The dotted band underlying the climate line spectrum designates the continuous spectrum, with the millennial band indicated. The line spectrum and the continuum have different dimensions, both drawn to arbitrary scale. The relative heights of the tidal lines (right) show a gravitational bias; for radiational forcing the annual lines are enhanced
Citation: Journal of Climate 15, 4; 10.1175/1520-0442(2002)015<0370:MCVITA>2.0.CO;2
An artificial example of the generation of HB and RC. Two sinusoids of amplitude 1/2 and frequencies fa = 3.05 and fb = 5 cycles per unit time produces an interference pattern that repeats in T = 20 time units for every 61 fa-cycles and 100 fb-cycles; there is also a “pseudo” RC of T = 1 time unit for 3 fa-cycles and 5 fb-cycles. The record is “clipped” at q = −1 (unclipped), −0.75, 0 and +0.9 amplitude units, as shown. (a) Fourier transforms for a record length of 100 time units sampled at 0.05 time units. Some of the harmonics nafa + nbfb are identified. (bottom) At frequencies below 1 cycle the spectrum consists of (i) HB frequencies down to 5fa − 3fb = 0.25 cycles, and (ii) harmonics 1, 2, … , 20 of the RC frequency T−1 = 0.05 cycles. The pseudo RC associated with T−1 = 0.99 cycles is limited to severe clipping of short record lengths, −2.5 < t < 2.5
Citation: Journal of Climate 15, 4; 10.1175/1520-0442(2002)015<0370:MCVITA>2.0.CO;2
(Continued)
Citation: Journal of Climate 15, 4; 10.1175/1520-0442(2002)015<0370:MCVITA>2.0.CO;2
Generation of the Saros RC period of 18.03 yr. Constructive interference between the nodical and synodic oscillations result in high values at 0.95-yr intervals. Staring with perfect overlap with unit amplitude, a near overlap of 0.9999 is exceeded 3 times in the first 20 yr, and a value of 0.99999 only once, after 18.03 yr (upper panel), showing the detailed oscillation over an interval of 0.0004 yr, or about 3½ h. Bottom panels show the envelope of a beat frequency of 1 cycle in 122 yr between the 223d harmonic of the nodical month and the 242d harmonic of the synodic month
Citation: Journal of Climate 15, 4; 10.1175/1520-0442(2002)015<0370:MCVITA>2.0.CO;2
Zero-crossing events cos(2πn1 ḟ1t2 − ϕ) for n1 = 1 and f1 = −2 × 10−3 cycles ky−2 associated with tidal friction. The transient duration is roughly 40 n1 ky−1
Citation: Journal of Climate 15, 4; 10.1175/1520-0442(2002)015<0370:MCVITA>2.0.CO;2
Equilibrium tide for Hawaii (21.3°N, 202.2°W) for (top) a month and (bottom) 30 yr. Monthly, annual, and regressional variability can be seen
Citation: Journal of Climate 15, 4; 10.1175/1520-0442(2002)015<0370:MCVITA>2.0.CO;2
(top) Spectra of the equilibrium time series raised to powers p = 1 to 5 and (bottom) clipped as indicated. The lowest spectra (linear, no clipping) are identical. Lunar regression (f5) appears in the linear record. A HB frequency f4 − 2f5 is generated at p = 2, and twice that frequency at p = 3. The KW line −f3 + 6f4 + 6f5 is generated for p = 4. Clipping does not generate any identifiable HB events
Citation: Journal of Climate 15, 4; 10.1175/1520-0442(2002)015<0370:MCVITA>2.0.CO;2
Tidal constituents. Periods are in days and tropical kiloyears; frequencies in cpy. Tropical month is sometime called “sidereal” month. The perihelion term is not ordinarily among the tidal constituents but is included in the Doodson expansion. The last three constituents are referred to as the Milankovich terms
Certain derived tidal constituents. Periods in days, frequencies in cycles per tropical year
Parameters evaluated from Eq. (4.11). The column marked * is associated with the Saros cycle
The HB frequencies δf = n2f2 +···+ n5f5 (cpky) for stated values of ni. The top four lines give conventional tidal frequencies. The next five lines give differences between frequency pairs, so chosen to yield the lowest δf for the smallest possible Σ|ni|. The combination n4 = 10, n5 = −21 is featured by Cartwright (1974). The only efficient triplet is the adjusted KW (2000) combinations. Any lower frequencies require very high harmonics. The “penalty function” (PF) is defined in Eq. (6.4)
