Skip to main page content
Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
, 94 (16), 8321-8

Possible Forcing of Global Temperature by the Oceanic Tides

Affiliations

Possible Forcing of Global Temperature by the Oceanic Tides

C D Keeling et al. Proc Natl Acad Sci U S A.

Abstract

An approximately decadal periodicity in surface air temperature is discernable in global observations from A.D. 1855 to 1900 and since A.D. 1945, but with a periodicity of only about 6 years during the intervening period. Changes in solar irradiance related to the sunspot cycle have been proposed to account for the former, but cannot account for the latter. To explain both by a single mechanism, we propose that extreme oceanic tides may produce changes in sea surface temperature at repeat periods, which alternate between approximately one-third and one-half of the lunar nodal cycle of 18.6 years. These alternations, recurring at nearly 90-year intervals, reflect varying slight degrees of misalignment and departures from the closest approach of the Earth with the Moon and Sun at times of extreme tide raising forces. Strong forcing, consistent with observed temperature periodicities, occurred at 9-year intervals close to perihelion (solar perigee) for several decades centered on A.D. 1881 and 1974, but at 6-year intervals for several decades centered on A.D. 1923. As a physical explanation for tidal forcing of temperature we propose that the dissipation of extreme tides increases vertical mixing of sea water, thereby causing episodic cooling near the sea surface. If this mechanism correctly explains near-decadal temperature periodicities, it may also apply to variability in temperature and climate on other times-scales, even millennial and longer.

Figures

Figure 1
Figure 1
Global surface temperature anomaly (combined land and marine) from 1855 through mid-1995 in degrees C (ref. and P. D. Jones, personal communication). Monthly averages are shown as dots. The solid line is a spline fit (15) of these data with a standard error, σ, of 0.107°C.
Figure 2
Figure 2
Spline fits of the global temperature anomaly of Fig. 1 together with associated bandpasses. (Top) Global trends depicted by three superimposed spline fits (15), one consisting of a very stiff spline fit to yearly averages (dotted curve 1, σ of 0.106°C), and two consisting of splines of lesser stiffness fit to monthly averages (dashed curve 2 and solid curve 3, σs of 0.161 and 0.148°C, respectively). (Middle) Decadal bandpass (curve 3 minus curve 2). (Bottom) Low frequency bandpass (curve 3 minus curve 1).
Figure 3
Figure 3
Comparison of mean sunspot number (ref. , -month running mean, upper curve) with the decadal bandpass of global surface temperature in degrees C, of Fig. 2 (Middle).
Figure 4
Figure 4
Maximum entropy spectrum of the global surface temperature anomaly of Fig. 1. The logarithm of power is plotted versus frequency in cycles per year. Above the 24 most prominent spectral peaks are shown their periods, in years.
Figure 5
Figure 5
Spectral bandpasses of the global temperature anomaly based on the maximum entropy spectrum of Fig. 4. (Top) Decadal (sum of two oscillations with periods of 9.31 and 10.27 years). (Middle) Low frequency (sum of nine oscillations with periods from 6.05 to 31.4 years, inclusive). (Bottom) Broad bandpass, including higher frequencies (sum of 24 spectral oscillations as described in text).
Figure 7
Figure 7
Comparison of prominent 6- and 9-year tidal events, shown by vertical hatched lines, as in Fig. 6, with fluctuations in temperature shown by time plots derived spectrally or from spline fits. (Curve 1, Decadal) Decadal spectral bandpass as in Fig. 5 (Top). (Curve 2, Low Frequency/Spectral) Low frequency spectral bandpass as in Fig. 5 (Middle). (Curve 3, Low Frequency/Spline) Low frequency spline plot as in Fig. 2 (Bottom). (Curve 4, High Frequency) High frequency spline plot derived by subtracting curve 1 of Fig. 2 (Top) from the spline of Fig. 1.
Figure 6
Figure 6
Timing of prominent lunisolar tide raising forces from A.D. 1850 to 2000 according to Wood (ref. , table 16). Each event is plotted as a vertical solid line whose length, above a threshold, is an approximate measure of the strength of forcing, expressed by the quantity γ, in degrees of arc per day, as described in the text. All events with γ greater than 17.02° d−1 are shown, resulting in approximately one displayed event per year. Selected dominant sequences of 18.03-year events, labeled A-D, subdominant sequences, B* and C*, and an equinoctial sequence, EN, are identified by arcs connecting these tidal events and by dots or solid triangles (see text). Vertical hatched lines indicate times of events grouped 6 or 9 years apart, as described in the text.
Figure 8
Figure 8
Timing of tidal forcing from A.D. 1600 to 2140, plotted with arcs connecting events of each prominent 18-year tidal sequence, as in Fig. 6. Also plotted are times of cool episodes seen in climate data identified by Jones and Bradley as of possibly global significance (ref. , pp. 658–659). A more recent global cool interval near 1970 is also shown, as discussed in the text. Dominant tidal Sequences A–D are labeled as in Fig. 6. Two earlier Sequences, Y and Z, and a later Sequence E are also labeled. Although climactic events of the dominant sequences from 1700 and 1974 are at 93-year intervals, there are irregularities in the time intervals between earlier and later climactic events, yielding an average near-centenniel interval of about 90 years.

Similar articles

See all similar articles

Cited by 3 PubMed Central articles

LinkOut - more resources

Feedback