Modes of internal thermohaline variability in a single-hemispheric ocean basin

Raa, L.A. te; Dijkstra, Henk A.

doi:10.1357/002224003322384906

Cited by 22 publications

(16 citation statements)

References 35 publications

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“…O'Brien et al, 1995) are points of reference for many paleo-proxy studies. In addition, a 320 yr and a 550 yr cycle have been repeatedly observed in proxy records as well as output of general circulation models (GCM) with reduced complexity (Weijer and Dijkstra, 2003;Weber et al, 2004;Te Raa and Dijkstra, 2003). However, none of the four modes arose more prominently than others within our global study.…”

Section: Spectral Divorce At Mid-holocenesupporting

confidence: 55%

“…These modes are generated by the advection of buoyancy anomalies around the overturning loop, both in a single-hemispheric basin leading to centennial modes or throughout the global ocean responsible for millennial cycles. The most negative eigenvalues (strongest damping) was found for centennial oscillations (Weijer and Dijkstra, 2003;Te Raa and Dijkstra, 2003). However, such modes could be activated if fluctuations in radiative energy input are included into simulation studies (Weber et al, 2004).…”

Section: Interactive Discussionmentioning

confidence: 99%

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Mid-Holocene regional reorganization of climate variability

Wirtz

Bernhardt

Lohmann

et al. 2009

Preprint

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Abstract. We integrate 130 globally distributed proxy time series to refine the understanding of climate variability during the Holocene. Cyclic anomalies and temporal trends in periodicity from the Lower to the Upper Holocene are extracted by combining Lomb-Scargle Fourier-transformed spectra with bootstrapping. Results were cross-checked by counting events in the time series. Main outcomes are: First, the propensity of the climate system to fluctuations is a region specific property. Many records of adjacent sites reveal a similar change in variability although they belong to different proxy types (e.g., δ18O, lithic composition). Secondly, at most sites, irreversible change occured in the Mid-Holocene. We suggest that altered ocean circulation together with slightly modified coupling intensity between regional climate subsystems around the 5.5 kyr BP event (termination of the African Humid Period) were responsible for the shift. Fluctuations especially intensified along a pan-American corridor. This may have led to an unequal crisis probability for early human civilizations in the Old and New World. Our study did not produce evidence for millennial scale cyclicity in some solar activity proxies for the Upper Holocene, nor for a privileged role of the prominent 250, 550, 900 and 1450 yr cycles. This lack of global periodicities corroborates the regional character of climate variability.

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Section: Spectral Divorce At Mid-holocenesupporting

confidence: 55%

Section: Interactive Discussionmentioning

confidence: 99%

Mid-Holocene regional reorganization of climate variability

Wirtz

Bernhardt

Lohmann

et al. 2009

Preprint

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show abstract

“…These modes are generated by the advection of buoyancy anomalies around the overturning loop, both in a single-hemispheric basin leading to centennial modes or throughout the global ocean responsible for millennial cycles (Broecker et al, 1985;Stocker et al, 1992;Weijer and Dijkstra, 2003). The most negative eigenvalues (strongest damping) were found for centennial oscillations (Weijer and Dijkstra, 2003;Te Raa and Dijkstra, 2003). In simulation studies, such modes…”

Section: Overturning Eigenmodesmentioning

confidence: 99%

Mid-Holocene regional reorganization of climate variability: Analyses of proxy data in the frequency domain

Wirtz

Lohmann

Bernhardt

et al. 2010

Palaeogeography, Palaeoclimatology, Palaeoecology

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Recurrent shifts in Holocene climate define the range of natural variability to which the signatures of human interference with the Earth system should be compared.Characterization of Holocene climate variability at the global scale becomes increasingly accessible due to a growing amount of paleoclimate records for the last 9 000-11 000 years. Here, we integrate 124 proxy time series of different types (e.g., δ 18 O, lithic composition) and apply a modified Lomb-Scargle spectral analysis. After bootstrapping the data in moving time windows we observe an increased probability for generation or loss of periodic modes at the mid-Holocene. Spatial autocorrelation of spectral changes robustly reveals that this (in)activation of modes was organized in regional clusters of subcontinental size. Within these clusters, changes in spectral properties are unexpectedly homogeneous, despite different underlying climatolog- Preprint submitted to Elsevier A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPTical variables. Oscillations in the climate system were amplified especially at the upwelling areas and dampened in the North Atlantic. We cross-checked the spectral analysis by counting events in the time series and tested against possible dating errors in individual records or against an overestimation of singular events. A combination of different mechanisms may have affected the coupling intensity between climate subsystems, turning these more or less prone to oscillations.

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“…In addition, there is a class of rapid oscillations in a loop model, so that a closed circulation cell sped up and slowed down in a balance between fluid inertia and forcing whose phases lead or lag the velocity and thus produce the oscillation (Quon and Ghil, 1995; Dewar and Huang, 1996; Huang and Dewar, 1996). Oscillations have also been found in ocean climate numerical models (Gargett and Ferron, 1996; Zhang et al, 2002; te Raa and Dijkstra, 2002, 2003a,b; te Raa, 2003). In many cases, a simple explanation of the mechanism is lacking.…”

Section: Introductionmentioning

confidence: 80%

Laboratory observations and simple models of slow oscillations in cooled salt-stratified bodies

Whitehead¹,

Raa²,

Tozuka³

et al. 2005

Tellus A

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A laboratory basin of saltwater with a freshwater layer at the top is exposed to cooling from above and inflow of freshwater from a layer that lies alongside. It exhibits oscillations of temperature, salinity, and layer depth when certain parameters are set. A simple analytical model illustrates the basic mechanism. Upon cooling, the layer gradually becomes denser. Ultimately, a density inversion at the interface between the layer and deeper fluid causes a gravitational instability. The layer is mixed with the deeper water and it vanishes. The emplacement of a new thin layer from freshwater inflow at the top repeats the cycle. Layered numerical models duplicate this cycle. Examples incorporate transport of heat out of the top and into the bottom of the layer. In addition, there is salinity transport up through the bottom from turbulent mixing. They reproduce the qualitative structure of the experimental data, but the many simplifications in the model do not permit precise quantitative agreement.

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Modes of internal thermohaline variability in a single-hemispheric ocean basin

Cited by 22 publications

References 35 publications

Mid-Holocene regional reorganization of climate variability

Mid-Holocene regional reorganization of climate variability

Mid-Holocene regional reorganization of climate variability: Analyses of proxy data in the frequency domain

Laboratory observations and simple models of slow oscillations in cooled salt-stratified bodies

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