Superimposed on the general Holocene climate evolution, the reconstructed SSST from core MD95-2011 shows pronounced quasiperiodic sub-Milankovitch-scale fluctuations at time scales of approximately 80–120, 210–320, 320–640, and 640–1280 years (Figure 8). There are obvious differences in the character of the submillennial-scale variability during the LHP compared with the HCO. The pattern of the mid-Holocene climate transition in the characteristic scales of variability longer than 1000 years have been discussed earlier by Debret et al. [2007, 2009]. The HCO and the LHP considered in the present study encompass only ∼3000 yearlong time intervals; the variations with periods longer than ∼1500 years therefore may not be resolved.
5.2.2. Quasiperiodic Variations Observed Only With High or Low Summertime Insolation
 Figures 8c, 8d, 8g, and 8h suggest the presence of prominent SSST variations at the multicentennial to millennial time scale of 640–1280 years during the LHP. Variations in this range of time scales have been reported earlier in a number of paleoclimate studies. Evidence for Holocene millennial-scale fluctuations with periodicity of around 1000 years was detected in sortable silt and δ13C records from the Gardar Drift [Chapman and Shackleton, 2000] and in the δ18O and Δ14C records from the GISP2 ice core [Stuiver and Braziunas, 1993; Stuiver et al., 1995; Grootes and Stuvier, 1997]. In addition, quasiperiodic variations in the range of 640–950 years are documented in terrestrial temperature records from the Scandinavia [Dahl and Nesje, 1996], and in the range of 640–960 years in diatom SSST records from the Reykjanes Ridge in the period from ∼9 to ∼2 kyr B.P. [Berner et al., 2008].
 SSST variations of the multicentennial scale of 210–360 years are prominent only in the HCO at the Vøring Plateau site (Figures 8a, 8b, 8e and 8f). Variability at a similar time scale of 260 years are also documented in the planktic δ18O record from the same core [Risebrobakken et al., 2003], while 140–320 year variability is documented in glacier variations in southern Norway [Matthews et al., 2000] and in δ18O from the GRIP ice core [Yiou et al., 1997]. This shorter time scale of 320 years on average is close to the one found in Δ14Cresidual record [Stuiver et al., 1998; Crosta et al., 2007] and hence potentially associated with variations in solar activity.
 Nearly centennial SSST variations of 80–120 years are documented consistently through the HCO in the MD95-2011 record (Figure 4b). Earlier investigations from the same core document variability of ∼80 years based on the planktonic δ18O record, which appears to be consistent through time [Risebrobakken et al., 2003], and 80–120 years in diatom SSST records during the Preboreal [Berner et al., 2010]. Several papers have reported variability at the time scale of around 80 years, including tree ring records from 500 AD [Briffa et al., 1992], historical and instrumental records during the last century [Schlesinger and Ramankutty, 1994], and Holocene ice core records [Chambers and Blackford, 2001]. Friis-Christensen and Lassen  discussed the variations in solar irradiance over an 80 year period, and the length of this period might indicate a possible connection to the solar Gleissberg cycle [e.g., Waple, 1999; Chambers and Blackford, 2001]. The ∼80 year variability has also been attributed to internal quasiperiodic oscillations in the atmospheric-ocean system, or to changes in the thermohaline circulation [e.g., Eddy, 1977; Ribes, 1990; Waple, 1999; Chambers and Blackford, 2001]. Changes in NADW production are suggested as an additional mechanism for internal oscillations by amplifying the solar signals and transmitting them globally [Delworth et al., 1993; Schlesinger and Ramankutty, 1994; Mann et al., 1995; Mahasenan et al., 1997].
 Attribution of quasiperiodic changes identified in proxy-based reconstructions to specific forcing process(es) is often hampered by a general complexity of the climate system and processes forming a particular proxy as well as by the lack of knowledge about past variations in solar activity. Additional complicating factors are frequently uneven time increments and timescale errors which lead to uncertainties in the spectral estimates [Mudelsee et al., 2009].
 In order to identify the possible forcing mechanisms for the quasi-cyclical variations found in the MD95-2011 SSST record we used the ice core 10Be-based Holocene record of TSI (total solar irradiance) of Steinhilber et al. . Wavelet analysis applied to this series (Figure 10) identifies quasiperiodic variations at a broad range of time scales, with most of the series variance concentrated in the higher-frequency (subcentennial) band. Analysis suggests a lack of any stationary variability evident throughout the entire record but rather the presence of intermittent variations at different timescales. The wavelet coherence approach [Torrence and Compo, 1998; Grinsted et al., 2004] performed on the reconstructed WA-PLS SSST and TSI series (not shown) revealed statistically significant coherent variations in the bands of 400–600 years during the HCO, and 260–450 and 640–900 years during the LHP. The results are visualized in Figure 11. To band-pass filter the signals in the respective frequency ranges we used the scales-averaged wavelet power following the technique described by Torrence and Compo .
Figure 11. Normalized wavelet band-pass-filtered series of WA-PLS reconstructed SSST from the MD95-2011 site (black) and the reconstructed total solar irradiance (dashed gray; see text for details). For the late Holocene period (LHP), the frequency bands (a) 640–900 years and (b) 260–450 years are shown, and for the Holocene Climate Optimum (HCO), the band (c) 400–600 years is shown.
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 Figure 11a shows nearly synchronous variations in TSI and MD95-2011 SSST at the time scale of 640–900 years, indicating that this submillennial-scale mode of variability, evident during the LHP, is directly associated with a varying solar forcing. The variations at the shorter scale of 260–450 years, similar to the 320–640 year mode of variability found in the reconstructed SSST, display a lagged response to solar forcing with a phase locked behavior (Figure 11b). The observed lag of about 100 years is rather consistent through time and points to the existence of a feedback mechanism in the climate system triggered by variations in the solar constant. Notable is that this mode of SSST variability at the core site is modulated by solar irradiance during the LHP only.
 Modeling studies [e.g., Renssen et al., 2005, 2006] proposed that the principal mechanism for centennial-scale cooling events in response to negative TSI anomalies involves a lasting reorganization of the oceanic circulation and deep convection shutdown in the Nordic seas, followed by sea ice expansion. A potential atmospheric mechanism amplifying cooling is related to a decrease in lower stratospheric ozone formation, resulting in amplified stratospheric cooling and leading to contraction of the Hadley Cells and expansion of the polar cells in the troposphere [Haigh, 1996].
 The lag in SSST cooling events, in turn, could be attributed to the thermal inertia of the oceans as well as a probabilistic character of the deep convection failure in response to a reduced TSI [Renssen et al., 2006]. It also implies that not only the magnitude but notable the duration of the TSI anomaly that increases the probability of the deep convection shutdown, so even moderate variations in TSI on longer scales are capable of generating pronounced, lasting anomalies in SSST.
 During the HCO the coherent variations in total solar irradiance and MD95-2011 SSST are revealed at the scale 400–600 years and show a variable phasing. We hypothesize that this could be related to the generally warmer climate of the HCO, which lowers the probability of drastic changes in the oceanic circulation in response to TSI anomalies. It leads to a less consistent response to changing solar irradiance during the periods with high orbital forcing [Renssen et al., 2005, 2006].
 The most prominent cooling events of the last 3000 years are recorded during the HCI at 2300 years B.P. and the LAI at 500 years B.P. These events coincide and are nearly in phase with minimum submillennial and multicentennial TSI variations. A striking feature of these events is the abruptness with which they commence, in line with the mechanism proposed by Renssen et al.  involving the abrupt shutdown of the THC and associated sea ice expansion. The HCI shows a 1.5°C SSST cooling within about a half a century, whereas the LIA starts with a SSST fall of 1.5°C within a decade. During these two periods the core site was under the direct influence of cold Arctic waters. We note that the onset of the LIA recorded in the analyzed reconstruction leads by approximately 40–50 years the shift in NAO, from a consistently positive to a more variable state, similar to the one observed at present [Trouet et al., 2009]. The inferred lag is in agreement with the modeled delayed response of atmospheric NAO to changes in solar insolation and SSST [Swingedouw et al., 2010]. We note that neither the HCI nor the LIA were uniform periods, and both periods are characterized with two cold peaks at the start and end of the periods, with a warmer interval in the middle.