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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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Location of the climate proxy records and schematic overview of the major atmospheric systems.(a) Location of proxy records used in this study. Red dots signify records for which results of the spectral analyses are included in Figure 5. For records marked with blue circles we refer to Supplementary Figures S5 and S8. (b) Modern atmospheric systems of the North Atlantic region. Not taking multi-centennial climate variations into account, our study indicates that this scenario may also represent average conditions for the period 0–3,000 years BP, where an overall 'neo-glacial' regime with more frequent meridional atmospheric circulation patterns and an ITCZ located close to the Cariaco site during Northern Hemisphere (NH) summer was prevalent. (c) ∼4,000 years BP: solar insolation was higher than present in the NH, but lower in the Southern Hemisphere (SH). Both the ITCZ and the atmospheric polar front were generally located north of its present position during the NH summer, and a zonal atmospheric circulation dominated. The position of the summer PF over Greenland was, however, roughly similar to the present location. (d) ∼7,000 years BP: significantly higher NH summer insolation than today, whereas it was lower in the SH (opposite in winter). The ITCZ was located north of its present position, with the PF also displaced well to the north in the North Atlantic sector, but, at least initially, less so in the N. American/Canadian sector, where the last remains of the Laurentide Ice Sheet (white zone) were still present in the Baffin Land region (until ∼7,000 years BP). These conditions may be representative for the average conditions between 6,000 and 8,000 BP. PF, atmospheric polar front; ITCZ, intertropical convergence zone. The yellow arrows indicate the dominant wind directions.
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f3: Location of the climate proxy records and schematic overview of the major atmospheric systems.(a) Location of proxy records used in this study. Red dots signify records for which results of the spectral analyses are included in Figure 5. For records marked with blue circles we refer to Supplementary Figures S5 and S8. (b) Modern atmospheric systems of the North Atlantic region. Not taking multi-centennial climate variations into account, our study indicates that this scenario may also represent average conditions for the period 0–3,000 years BP, where an overall 'neo-glacial' regime with more frequent meridional atmospheric circulation patterns and an ITCZ located close to the Cariaco site during Northern Hemisphere (NH) summer was prevalent. (c) ∼4,000 years BP: solar insolation was higher than present in the NH, but lower in the Southern Hemisphere (SH). Both the ITCZ and the atmospheric polar front were generally located north of its present position during the NH summer, and a zonal atmospheric circulation dominated. The position of the summer PF over Greenland was, however, roughly similar to the present location. (d) ∼7,000 years BP: significantly higher NH summer insolation than today, whereas it was lower in the SH (opposite in winter). The ITCZ was located north of its present position, with the PF also displaced well to the north in the North Atlantic sector, but, at least initially, less so in the N. American/Canadian sector, where the last remains of the Laurentide Ice Sheet (white zone) were still present in the Baffin Land region (until ∼7,000 years BP). These conditions may be representative for the average conditions between 6,000 and 8,000 BP. PF, atmospheric polar front; ITCZ, intertropical convergence zone. The yellow arrows indicate the dominant wind directions.

Mentions: Several studies have linked precipitation changes to AMO variability7910, but the coupling between the AMO and precipitation anomalies is spatially complex; a warm AMO phase is thought to be associated with a positive precipitation anomaly in the Sahel region of Africa, whereas precipitation anomalies in, for example, Mexico appear to have been in anti-phase with the AMO and Sahel precipitation7. This is illustrated in Figure 2, which compares the AMO index to instrumental data and climate proxy records from the Atlantic region. Recent model experiments, however, have failed to identify the AMO as the primary driver of recent reductions in Sahel rainfall23. In fact, these model simulations indicate that the major climate impacts of multidecadal changes in Atlantic Ocean SST over land are to be found in North America and the northern part of South America. To investigate the existence and origin of the AMO on Holocene timescales, we carefully selected seven Holocene climate proxy records from the North Atlantic region, including five ice-core records from Greenland24252627 and the Canadian Arctic28, one lacustrine record from the Yucatan peninsula29, Mexico, and a marine record from the Cariaco Basin30, north of Venezuela (Figs 3 and 4). These records derive from sites that are located close to the current summer positions of the atmospheric polar front (PF) or the intertropical convergence zone (ITCZ). The records were chosen based on a their documented sensitivity to variations in North Atlantic SST, and an average temporal resolution of 25 years or better (Supplementary Table S1) over a time span of 8,000 years or more. The majority of these climate proxy records overlap the instrumental AMO record in time, and, although the temporal resolution for some of the proxy records is relatively low for the past 140 years, they clearly tend to co-vary with the AMO index (Fig. 2). The precipitation proxy record from Lake Chichancanab (Yucatan) co-varies with both the AMO index and a coral-based SST reconstruction from Puerto Rico, with the instrumental precipitation record from the Yucatan peninsula looking almost like an extension of the Lake Chichancanab proxy record (Fig. 2b). The comparisons show that warm AMO phases correlate with dry periods on the Yucatan Peninsula, which is in accord with numerical model simulations7 (note that the instrumental rainfall record from Yucantan has been inverted in Fig. 2b, and that high δ18O values from Lake Chichancanab correspond to dry periods). The GRIP record and, in particular, the GISP2 record also co-vary with the AMO index over the last 140 years (Fig. 2c), suggesting that the two ice-core records were sensitive to North Atlantic SST variations, with increasing SST leading to increasing δ18O values as expected. Apart from North Atlantic SST variations, climate variability observed in these proxy records may to some extent also have been driven by other Northern Hemisphere (NH), or even global-scale, climate effects that are correlated with the AMO. The potential existence of such NH teleconnections is underpinned by coupled ocean-atmosphere models520. Irrespective of this ambiguity regarding the principal driving mechanism, it remains very likely that the observed multidecadal variability, be it directly or indirectly, reflects North Atlantic SST variations related to the AMO.


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

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

Location of the climate proxy records and schematic overview of the major atmospheric systems.(a) Location of proxy records used in this study. Red dots signify records for which results of the spectral analyses are included in Figure 5. For records marked with blue circles we refer to Supplementary Figures S5 and S8. (b) Modern atmospheric systems of the North Atlantic region. Not taking multi-centennial climate variations into account, our study indicates that this scenario may also represent average conditions for the period 0–3,000 years BP, where an overall 'neo-glacial' regime with more frequent meridional atmospheric circulation patterns and an ITCZ located close to the Cariaco site during Northern Hemisphere (NH) summer was prevalent. (c) ∼4,000 years BP: solar insolation was higher than present in the NH, but lower in the Southern Hemisphere (SH). Both the ITCZ and the atmospheric polar front were generally located north of its present position during the NH summer, and a zonal atmospheric circulation dominated. The position of the summer PF over Greenland was, however, roughly similar to the present location. (d) ∼7,000 years BP: significantly higher NH summer insolation than today, whereas it was lower in the SH (opposite in winter). The ITCZ was located north of its present position, with the PF also displaced well to the north in the North Atlantic sector, but, at least initially, less so in the N. American/Canadian sector, where the last remains of the Laurentide Ice Sheet (white zone) were still present in the Baffin Land region (until ∼7,000 years BP). These conditions may be representative for the average conditions between 6,000 and 8,000 BP. PF, atmospheric polar front; ITCZ, intertropical convergence zone. The yellow arrows indicate the dominant wind directions.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: Location of the climate proxy records and schematic overview of the major atmospheric systems.(a) Location of proxy records used in this study. Red dots signify records for which results of the spectral analyses are included in Figure 5. For records marked with blue circles we refer to Supplementary Figures S5 and S8. (b) Modern atmospheric systems of the North Atlantic region. Not taking multi-centennial climate variations into account, our study indicates that this scenario may also represent average conditions for the period 0–3,000 years BP, where an overall 'neo-glacial' regime with more frequent meridional atmospheric circulation patterns and an ITCZ located close to the Cariaco site during Northern Hemisphere (NH) summer was prevalent. (c) ∼4,000 years BP: solar insolation was higher than present in the NH, but lower in the Southern Hemisphere (SH). Both the ITCZ and the atmospheric polar front were generally located north of its present position during the NH summer, and a zonal atmospheric circulation dominated. The position of the summer PF over Greenland was, however, roughly similar to the present location. (d) ∼7,000 years BP: significantly higher NH summer insolation than today, whereas it was lower in the SH (opposite in winter). The ITCZ was located north of its present position, with the PF also displaced well to the north in the North Atlantic sector, but, at least initially, less so in the N. American/Canadian sector, where the last remains of the Laurentide Ice Sheet (white zone) were still present in the Baffin Land region (until ∼7,000 years BP). These conditions may be representative for the average conditions between 6,000 and 8,000 BP. PF, atmospheric polar front; ITCZ, intertropical convergence zone. The yellow arrows indicate the dominant wind directions.
Mentions: Several studies have linked precipitation changes to AMO variability7910, but the coupling between the AMO and precipitation anomalies is spatially complex; a warm AMO phase is thought to be associated with a positive precipitation anomaly in the Sahel region of Africa, whereas precipitation anomalies in, for example, Mexico appear to have been in anti-phase with the AMO and Sahel precipitation7. This is illustrated in Figure 2, which compares the AMO index to instrumental data and climate proxy records from the Atlantic region. Recent model experiments, however, have failed to identify the AMO as the primary driver of recent reductions in Sahel rainfall23. In fact, these model simulations indicate that the major climate impacts of multidecadal changes in Atlantic Ocean SST over land are to be found in North America and the northern part of South America. To investigate the existence and origin of the AMO on Holocene timescales, we carefully selected seven Holocene climate proxy records from the North Atlantic region, including five ice-core records from Greenland24252627 and the Canadian Arctic28, one lacustrine record from the Yucatan peninsula29, Mexico, and a marine record from the Cariaco Basin30, north of Venezuela (Figs 3 and 4). These records derive from sites that are located close to the current summer positions of the atmospheric polar front (PF) or the intertropical convergence zone (ITCZ). The records were chosen based on a their documented sensitivity to variations in North Atlantic SST, and an average temporal resolution of 25 years or better (Supplementary Table S1) over a time span of 8,000 years or more. The majority of these climate proxy records overlap the instrumental AMO record in time, and, although the temporal resolution for some of the proxy records is relatively low for the past 140 years, they clearly tend to co-vary with the AMO index (Fig. 2). The precipitation proxy record from Lake Chichancanab (Yucatan) co-varies with both the AMO index and a coral-based SST reconstruction from Puerto Rico, with the instrumental precipitation record from the Yucatan peninsula looking almost like an extension of the Lake Chichancanab proxy record (Fig. 2b). The comparisons show that warm AMO phases correlate with dry periods on the Yucatan Peninsula, which is in accord with numerical model simulations7 (note that the instrumental rainfall record from Yucantan has been inverted in Fig. 2b, and that high δ18O values from Lake Chichancanab correspond to dry periods). The GRIP record and, in particular, the GISP2 record also co-vary with the AMO index over the last 140 years (Fig. 2c), suggesting that the two ice-core records were sensitive to North Atlantic SST variations, with increasing SST leading to increasing δ18O values as expected. Apart from North Atlantic SST variations, climate variability observed in these proxy records may to some extent also have been driven by other Northern Hemisphere (NH), or even global-scale, climate effects that are correlated with the AMO. The potential existence of such NH teleconnections is underpinned by coupled ocean-atmosphere models520. Irrespective of this ambiguity regarding the principal driving mechanism, it remains very likely that the observed multidecadal variability, be it directly or indirectly, reflects North Atlantic SST variations related to the AMO.

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