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Tracking the Atlantic Multidecadal Oscillation through the last 8,000 years.

Knudsen MF, Seidenkrantz MS, Jacobsen BH, Kuijpers A - Nat Commun (2011)

Bottom Line: Understanding the internal ocean variability and its influence on climate is imperative for society.The nature and origin of the AMO is uncertain, and it remains unknown whether it represents a persistent periodic driver in the climate system, or merely a transient feature.We therefore conjecture that a quasi-persistent ∼55- to 70-year AMO, linked to internal ocean-atmosphere variability, existed during large parts of the Holocene.

View Article: PubMed Central - PubMed

Affiliation: Centre for Past Climate Studies, Department of Earth Sciences, Aarhus University, HÃøegh-Guldbergs Gade 2, Aarhus DK-8000, Denmark. mfk@geo.au.dk

ABSTRACT
Understanding the internal ocean variability and its influence on climate is imperative for society. A key aspect concerns the enigmatic Atlantic Multidecadal Oscillation (AMO), a feature defined by a 60- to 90-year variability in North Atlantic sea-surface temperatures. The nature and origin of the AMO is uncertain, and it remains unknown whether it represents a persistent periodic driver in the climate system, or merely a transient feature. Here, we show that distinct, ∼55- to 70-year oscillations characterized the North Atlantic ocean-atmosphere variability over the past 8,000 years. We test and reject the hypothesis that this climate oscillation was directly forced by periodic changes in solar activity. We therefore conjecture that a quasi-persistent ∼55- to 70-year AMO, linked to internal ocean-atmosphere variability, existed during large parts of the Holocene. Our analyses further suggest that the coupling from the AMO to regional climate conditions was modulated by orbitally induced shifts in large-scale ocean-atmosphere circulation.

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Multidecadal climate oscillations in the region bounding the North Atlantic Ocean.(a) Spectrogram delineating the temporal distribution of highly significant (orange: χ2>90%, red: χ2>95%, and dark red: χ2>99%) spectral power of the δ18O record from the Agassiz ice cap28. Distributions of highly significant spectral power are also shown for (b) the NGRIP δ18O record27, (c) the GISP2 δ18O record26, (d) the Lake Chichancanab δ18O record29, and (e) the Ti (%) record from the Cariaco Basin30. For this spectral analysis, the data were grouped in overlapping 2,000-year windows, with a spacing of 50 years, for the interval 1,000–8,000 BP. Spectral estimates for the last 1,000 years were obtained by decreasing the window size linearly from 2,000 years at 1,000 BP to 200 years at 100 BP in steps of 25 years. Only the 50- to 100-year period range is shown, because this encompasses the AMO range normally reported in the literature. The blue circle denotes the spectral peak for the instrumental AMO record, whereas the blue line shows the associated uncertainty. The green rectangles delineate the prominent burst in intensity of the Gleissberg (∼88 year) solar cycle between 4,000 and 6,250 BP34.
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f5: Multidecadal climate oscillations in the region bounding the North Atlantic Ocean.(a) Spectrogram delineating the temporal distribution of highly significant (orange: χ2>90%, red: χ2>95%, and dark red: χ2>99%) spectral power of the δ18O record from the Agassiz ice cap28. Distributions of highly significant spectral power are also shown for (b) the NGRIP δ18O record27, (c) the GISP2 δ18O record26, (d) the Lake Chichancanab δ18O record29, and (e) the Ti (%) record from the Cariaco Basin30. For this spectral analysis, the data were grouped in overlapping 2,000-year windows, with a spacing of 50 years, for the interval 1,000–8,000 BP. Spectral estimates for the last 1,000 years were obtained by decreasing the window size linearly from 2,000 years at 1,000 BP to 200 years at 100 BP in steps of 25 years. Only the 50- to 100-year period range is shown, because this encompasses the AMO range normally reported in the literature. The blue circle denotes the spectral peak for the instrumental AMO record, whereas the blue line shows the associated uncertainty. The green rectangles delineate the prominent burst in intensity of the Gleissberg (∼88 year) solar cycle between 4,000 and 6,250 BP34.

Mentions: By applying the Lomb-Scargle Fourier transform31 and a 2,000-year running-window approach to each climate proxy record, we calculated the spectral density distributions, from which red-noise false-alarm levels31 were estimated and colour-coded. This novel combination of methods allows not only a detailed assessment of temporal variations in the cyclic behaviour of unevenly spaced data, but also a clear distinction between temporal distributions of, for example, ∼60-, 70- and 80-year oscillations. One major advantage of using the Lomb-Scargle Fourier transform is that it enables computation of spectral significances without having to interpolate the data onto a common time step. All records display a relatively narrow, dominant oscillation band between ∼55 and 70 years, as well as weaker oscillations scattered around the 70- to 125-year band. Significant spectral power overlapping the ∼88-year Gleissberg solar activity band is also observed for some of the studied records, most notably the Agassiz and GISP2 δ18O records (Fig. 5a,c), but it is only observed in discrete intervals and generally dwarfed by the spectral power observed in the 55- to 70-year band. Spectral estimates for the last 1,000 years are established by gradual narrowing of the time window, which by definition gives rise to gradual spectral blurring towards the near-present, where the oscillation bands widen. The fact that the dominant 55- to 70-year oscillation in these Holocene records is based on climate proxies, which co-vary with the AMO index over the last 140 years, have the expected sign, and which resemble the oscillations inferred from both the AMO index and model studies of SST variations, clearly indicate that this quasi-periodic oscillation reflects the Holocene AMO behaviour. This observation is in close agreement with a coral-based reconstruction of Caribbean SST variations over the past 250 years14, which shows a clear 60-year oscillation (Supplementary Fig. S1), and it is consistent with the approximately 60-year variability observed in a 1,000-year control integration of a coupled ocean-atmosphere model15. It is noticeable that the multidecadal variability related to the AMO in some records, particularly those from central Greenland, appears to be the dominant climate variability throughout the last 8,000 years, that is, the amplitude of the long-term trends is comparable to or smaller than the multidecadal-scale variability (Fig. 4).


Tracking the Atlantic Multidecadal Oscillation through the last 8,000 years.

Knudsen MF, Seidenkrantz MS, Jacobsen BH, Kuijpers A - Nat Commun (2011)

Multidecadal climate oscillations in the region bounding the North Atlantic Ocean.(a) Spectrogram delineating the temporal distribution of highly significant (orange: χ2>90%, red: χ2>95%, and dark red: χ2>99%) spectral power of the δ18O record from the Agassiz ice cap28. Distributions of highly significant spectral power are also shown for (b) the NGRIP δ18O record27, (c) the GISP2 δ18O record26, (d) the Lake Chichancanab δ18O record29, and (e) the Ti (%) record from the Cariaco Basin30. For this spectral analysis, the data were grouped in overlapping 2,000-year windows, with a spacing of 50 years, for the interval 1,000–8,000 BP. Spectral estimates for the last 1,000 years were obtained by decreasing the window size linearly from 2,000 years at 1,000 BP to 200 years at 100 BP in steps of 25 years. Only the 50- to 100-year period range is shown, because this encompasses the AMO range normally reported in the literature. The blue circle denotes the spectral peak for the instrumental AMO record, whereas the blue line shows the associated uncertainty. The green rectangles delineate the prominent burst in intensity of the Gleissberg (∼88 year) solar cycle between 4,000 and 6,250 BP34.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3105344&req=5

f5: Multidecadal climate oscillations in the region bounding the North Atlantic Ocean.(a) Spectrogram delineating the temporal distribution of highly significant (orange: χ2>90%, red: χ2>95%, and dark red: χ2>99%) spectral power of the δ18O record from the Agassiz ice cap28. Distributions of highly significant spectral power are also shown for (b) the NGRIP δ18O record27, (c) the GISP2 δ18O record26, (d) the Lake Chichancanab δ18O record29, and (e) the Ti (%) record from the Cariaco Basin30. For this spectral analysis, the data were grouped in overlapping 2,000-year windows, with a spacing of 50 years, for the interval 1,000–8,000 BP. Spectral estimates for the last 1,000 years were obtained by decreasing the window size linearly from 2,000 years at 1,000 BP to 200 years at 100 BP in steps of 25 years. Only the 50- to 100-year period range is shown, because this encompasses the AMO range normally reported in the literature. The blue circle denotes the spectral peak for the instrumental AMO record, whereas the blue line shows the associated uncertainty. The green rectangles delineate the prominent burst in intensity of the Gleissberg (∼88 year) solar cycle between 4,000 and 6,250 BP34.
Mentions: By applying the Lomb-Scargle Fourier transform31 and a 2,000-year running-window approach to each climate proxy record, we calculated the spectral density distributions, from which red-noise false-alarm levels31 were estimated and colour-coded. This novel combination of methods allows not only a detailed assessment of temporal variations in the cyclic behaviour of unevenly spaced data, but also a clear distinction between temporal distributions of, for example, ∼60-, 70- and 80-year oscillations. One major advantage of using the Lomb-Scargle Fourier transform is that it enables computation of spectral significances without having to interpolate the data onto a common time step. All records display a relatively narrow, dominant oscillation band between ∼55 and 70 years, as well as weaker oscillations scattered around the 70- to 125-year band. Significant spectral power overlapping the ∼88-year Gleissberg solar activity band is also observed for some of the studied records, most notably the Agassiz and GISP2 δ18O records (Fig. 5a,c), but it is only observed in discrete intervals and generally dwarfed by the spectral power observed in the 55- to 70-year band. Spectral estimates for the last 1,000 years are established by gradual narrowing of the time window, which by definition gives rise to gradual spectral blurring towards the near-present, where the oscillation bands widen. The fact that the dominant 55- to 70-year oscillation in these Holocene records is based on climate proxies, which co-vary with the AMO index over the last 140 years, have the expected sign, and which resemble the oscillations inferred from both the AMO index and model studies of SST variations, clearly indicate that this quasi-periodic oscillation reflects the Holocene AMO behaviour. This observation is in close agreement with a coral-based reconstruction of Caribbean SST variations over the past 250 years14, which shows a clear 60-year oscillation (Supplementary Fig. S1), and it is consistent with the approximately 60-year variability observed in a 1,000-year control integration of a coupled ocean-atmosphere model15. It is noticeable that the multidecadal variability related to the AMO in some records, particularly those from central Greenland, appears to be the dominant climate variability throughout the last 8,000 years, that is, the amplitude of the long-term trends is comparable to or smaller than the multidecadal-scale variability (Fig. 4).

Bottom Line: Understanding the internal ocean variability and its influence on climate is imperative for society.The nature and origin of the AMO is uncertain, and it remains unknown whether it represents a persistent periodic driver in the climate system, or merely a transient feature.We therefore conjecture that a quasi-persistent ∼55- to 70-year AMO, linked to internal ocean-atmosphere variability, existed during large parts of the Holocene.

View Article: PubMed Central - PubMed

Affiliation: Centre for Past Climate Studies, Department of Earth Sciences, Aarhus University, HÃøegh-Guldbergs Gade 2, Aarhus DK-8000, Denmark. mfk@geo.au.dk

ABSTRACT
Understanding the internal ocean variability and its influence on climate is imperative for society. A key aspect concerns the enigmatic Atlantic Multidecadal Oscillation (AMO), a feature defined by a 60- to 90-year variability in North Atlantic sea-surface temperatures. The nature and origin of the AMO is uncertain, and it remains unknown whether it represents a persistent periodic driver in the climate system, or merely a transient feature. Here, we show that distinct, ∼55- to 70-year oscillations characterized the North Atlantic ocean-atmosphere variability over the past 8,000 years. We test and reject the hypothesis that this climate oscillation was directly forced by periodic changes in solar activity. We therefore conjecture that a quasi-persistent ∼55- to 70-year AMO, linked to internal ocean-atmosphere variability, existed during large parts of the Holocene. Our analyses further suggest that the coupling from the AMO to regional climate conditions was modulated by orbitally induced shifts in large-scale ocean-atmosphere circulation.

Show MeSH
Related in: MedlinePlus