Milankovich's theory revisited
In accordance with Milankovich's theory, most climate transitions that have occurred over the past millions of years have been recognized as resulting from variations in orbital parameters of the Earth, namely eccentricity, axial tilt, and precession, due to their synchronism[1]. However, understanding the underlying physical mechanisms is encountering considerable problems so that how variations in Earth’s orbit pace the glacial-interglacial cycles of the Quaternary are probably one of the greatest mysteries of modern climate science.
Changes in the forcing are too small to explain the observed climate variations as simple linear responses[2]. Difficulties reach their culmination when the Mid-Pleistocene Transition (MPT) is considered, that is a fundamental change in the behavior of glacial cycles during the Quaternary glaciations. The transition happened approximately 1.2 million years ago, in the Pleistocene epoch. Before the MPT, the glacial cycles were dominated by a 41-year periodicity coherent with the Milankovitch forcing from axial tilt. After the MPT the cycle durations have increased, with an average length of approximately 100 years coherent with the Milankovitch forcing from eccentricity. However, the intensity of the forcing resulting from the eccentricity is much lower than that induced by the axial tilt.
As suggested by some authors,[3][4] non-linear interactions occur between small changes in the Earth's orbit and internal oscillations of the climate system. Consequently, to strictly apply the Milankovitch's theory, a mediator involving positive feedbacks must be found, endowing the climate response with a resonant feature. Supported by both observational and theoretical considerations, a recent work approaches the Milankovitch's theory in a new way in which the solar and orbital forcing of the climate system occurs under the mediation of very long-period Rossby waves winding around the subtropical gyres. Due to their specific properties, the so-called Gyral Rossby Waves (GRWs) are resonantly forced in subharmonic modes. This means that the forcing efficiency strongly depends on the deviation between the forcing period and the closest natural period of GRWs among the different subharmonic modes.[5]
As we will see, the mediation of orbital and solar forcing by GRWs can explain most of the observed climatic phenomena that have occurred over the last millions of years. Involving only the intrinsic properties of GRWs, this new concept complements Milankovitch's theory while avoiding the many opportunistic assumptions intended to lift the veil on the many vagaries of the climate system.
Mid-Pleistocene transition (MPT)[edit]
The MPT finds a straightforward explanation when the mediation of GRWs is considered because of the variations in eccentricity forcing that has occurred nearly 1.2 Ma ago (see the figure a) obtained from [6]). Indeed, the amplitude of orbital forcing related to the obliquity, whose period is 41 Ka, being much higher than that related to the eccentricity, the dominant ice age-interglacial period was 41 Ka before the MPT. Since the MPT, the dominant period has been coherent with that of eccentricity while it was approaching to the resonance period related to the subharmonic mode n11, namely 98.3 Ka. The forcing period becoming remarkably close to one of the natural periods of GRWs, the climate system has been resonantly forced, which means that the orbital forcing efficiency has increased drastically. It has continued to increase since 1.4 Ma BP, which suggests that the tuning is still improving.[7].
Adjustment of the radius of subtropical gyres during the MPT[edit]
Before the MPT, the closest natural period was related to the subharmonic mode n10, namely 49.2 Ka. The transition of the period from 41 Ka, when the subharmonic mode n10 was tuned to the forcing period before the MPT, to the natural period of 49.2 Ka at present required an adjustment of the gyre. Indeed, the resonance which was exerted at 41 Ka is now done at 98.5 Ka, remarkably close to the forcing period, therefore no longer requiring a particular adjustment of the perimeter of the gyres.
By considering together paleothermometers at two sites in the Tasman Sea, it has been shown that during late Pliocene, a poleward displacement of the subtropical front compared to modern occurred between 40 and 44° S [8]. A 4° drift of the subtropical front towards the south has increased the circumference by almost 20%, which reflects the adjustment of the gyre to increase the natural period of the subharmonic mode n10 from 41 to 49.2 Ka during the MPT.
Pliocene-Pleistocene Transition[edit]
The MPT sparked a sustained creativity on the part of climatologists to find a plausible explanation. However, a resonance phenomenon comparable to what occurred in Mid-Pleistocene is highlighted at the hinge of the Pliocene and the Pleistocene, the periods being 10 times higher than during the MPT [5].
This transition is more difficult to highlight than the previous one because of the periods mentioned, but the observed phenomena are similar. In both cases, the variation of the forcing period causes the GRWs to resonate, which has the effect of suddenly increasing the forcing efficiency.
Whether it is Mid-Pleistocene or Pliocene-Pleistocene transition, two subharmonic modes compete with respect to orbital forcing. They are n10 and n11 whose natural periods are 49.2 and 98.3 Ka during the first transition, and n13 and n14 whose natural periods are 0.39 and 1.18 Ma during the second. In both cases, the natural period of the highest mode tunes transiently, but optimally, to the forcing period when both become remarkably close. Then, the two subharmonic modes swap the dominant mode, as attested by the nearly symmetrical opposite variation in the forcing efficiencies [5]. The change in the dominant period occurred approximately 2.2 Ma ago when the amplitude of the 0.39 Ma period oscillation collapsed in favor of the 1.18 Ma period oscillation. Nowadays the transition is occurring in the other direction since the amplitude of the 1.18 Ma period oscillation is collapsing in favor of the 0.39 Ma period oscillation, both forcing having approximately the same amplitude. This transition involves a weak adjustment in the circumference of the gyre because the deviation between the forcing and natural periods of the lower subharmonic mode is small, 0.41 and 0.39 Ma, respectively.
References[edit]
- ↑ Hays, J. D.; Imbrie, John; Shackleton, N. J. (1976-12-10). "Variations in the Earth's Orbit: Pacemaker of the Ice Ages". Science. 194 (4270): 1121–1132. doi:10.1126/science.194.4270.1121. ISSN 0036-8075. PMID 17790893.
- ↑ Rial, J (2000-12-01). "Understanding nonlinear responses of the climate system to orbital forcing". Quaternary Science Reviews. 19 (17–18): 1709–1722. doi:10.1016/S0277-3791(00)00087-1.
- ↑ Ghil, Michael (1994-10-01). "Cryothermodynamics: the chaotic dynamics of paleoclimate". Physica D: Nonlinear Phenomena. 77 (1–3): 130–159. doi:10.1016/0167-2789(94)90131-7.
- ↑ Gildor, Hezi; Tziperman, Eli (2000-12-01). "Sea ice as the glacial cycles' Climate switch: role of seasonal and orbital forcing". Paleoceanography. 15 (6): 605–615. doi:10.1029/1999PA000461.
- ↑ 5.0 5.1 Pinault, Jean-Louis (2021-01-01). "Resonantly Forced Baroclinic Waves in the Oceans: A New Approach to Climate Variability". Journal of Marine Science and Engineering. 9 (1): 13. doi:10.3390/jmse9010013.
Text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
- ↑ Berger, A.; Loutre, M.F. (1991-01-01). "Insolation values for the climate of the last 10 million years". Quaternary Science Reviews. 10 (4): 297–317. doi:10.1016/0277-3791(91)90033-Q.
- ↑ Pinault, Jean-Louis (2018-09-19). "Modulated Response of Subtropical Gyres: Positive Feedback Loop, Subharmonic Modes, Resonant Solar and Orbital Forcing". Journal of Marine Science and Engineering. 6 (3): 107. doi:10.3390/jmse6030107. ISSN 2077-1312.
- ↑ McClymont, Erin L.; Elmore, Aurora C.; Kender, Sev; Leng, Melanie J.; Greaves, Mervyn; Elderfield, Henry (2016-06-01). "Pliocene‐Pleistocene evolution of sea surface and intermediate water temperatures from the southwest Pacific". Paleoceanography. 31 (6): 895–913. doi:10.1002/2016PA002954. ISSN 0883-8305. PMC 4950126. PMID 27478302.CS1 maint: PMC format (link)
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