Variability in North Pacific intermediate and deep water ventilation during Heinrich events in two coupled climate models

https://doi.org/10.1016/j.dsr2.2011.12.002Get rights and content

Abstract

The responses of North Pacific intermediate and deep water ventilation and ocean biogeochemical properties to northern North Atlantic glacial freshwater perturbations are evaluated with a coupled atmosphere–ocean general circulation model MIROC and an earth system model of intermediate complexity LOVECLIM. When the Atlantic meridional overturning circulation (AMOC) is weakened as a result of the North Atlantic freshwater discharge, both models simulate subthermocline and intermediate water warming in the Pacific Ocean. The sensitivities of the Pacific meridional overturning circulation (PMOC) to AMOC weakening differ significantly between the two models. MIROC simulates a small enhancement of the deep sinking branch of the PMOC in the North Pacific. On the contrary, the LOVECLIM freshwater experiment exhibits intensified deep water formation in the North Pacific, associated with a maximum transport change of 19 Sv. Despite the significant differences in ocean circulation response, both models successfully reproduce high-oxygen and low-nutrient conditions of intermediate and deep waters, in accordance with sediment core based paleoproxy reconstructions from the North Pacific and Bering Sea during Heinrich event 1. Emergence of younger intermediate and deep water in the North Pacific can be partly attributed to an overall enhanced mixing as well as intensified overturning circulation of the subpolar North Pacific. Our models simulate broad features observed in several paleoproxy data of the Pacific Ocean: biological production decrease in northern Japan, cooling in the western North Pacific Ocean, and the southward shift of the Pacific intertropical convergence zone.

Introduction

Ice core records from Greenland reveal a high degree of millennial-scale variability during the last glacial period (Dansgaard et al., 1993). Some of the associated warm–cold transitions that occurred during this period were accompanied by huge iceberg discharges into the northern Atlantic region; these are known as Heinrich events (Bond and Lotti, 1995, Heinrich, 1988). It has been argued that freshwater discharge caused by melting icebergs reduces the surface water density in the North Atlantic Ocean, thereby weakening and shoaling the Atlantic meridional overturning circulation (AMOC) (Sarnthein et al., 1994). Evidence for an AMOC collapse that occurred at the beginning of the last glacial termination 17,500 years ago and coincided with Heinrich event 1 (H1) is suggested by Pa/Th data from the subtropical North Atlantic Ocean (McManus et al., 2004). Numerous paleoclimatic records from both hemispheres have shown that Heinrich events in conjunction with AMOC cessation exerted a powerful influence on the global climate during glacial times.

The influence of Heinrich events on the Pacific climate is well established. Spatio-temporal reconstructions of sea surface temperature (SST) in the Pacific Ocean during H1 (Kiefer and Kienast, 2005) demonstrate a high degree of coherence between the Atlantic and Pacific oceans. Prominent decrease in SST during H1 is found in planktonic foraminiferal Mg/Ca values in the western North Pacific (Sagawa and Ikehara, 2008) and alkenone temperature reconstructions from the Okhotsk Sea (Harada et al., 2006, Harada et al., 2012). During this period, benthic and planktonic radiocarbon age differences indicate the existence of younger intermediate and deep waters in the western North Pacific (Ahagon et al., 2003, Okazaki et al., 2010, Sagawa and Ikehara, 2008) and older waters in the eastern North Pacific at intermediate depths (Marchitto et al., 2007, Mix et al., 1999, Stott et al., 2009). Decreasing ventilation ages at intermediate depths in the western North Pacific Ocean suggest an increased level of mixing and water mass formation. Greater oxygen concentrations during H1 in a Bering Sea sediment core from a depth of about 2000 m further corroborate this conclusion (Okazaki et al., 2005). In addition, Kienast et al. (2006) showed that biological production increases in the eastern equatorial Pacific Ocean during Heinrich events, which is attributable to the upwelling of abundant nutrient-rich deep water.

Using an idealized earth system model by forcing a North Atlantic freshwater perturbation, Saenko et al. (2004) showed that a Pacific meridional overturning circulation (PMOC) can be established with AMOC weakening. They found AMOC weakening by creating an artificial PMOC through freshwater extraction in the North Pacific. The actual existence of such pan-oceanic interplay is an interesting topic that requires further investigation. A strong PMOC was also obtained in waterhosing experiments described in Timmermann et al. (2005). While these studies describe idealized modeling solutions, we attempt to further link the existing paleo-proxy evidence for major reorganizations of North Pacific flow with climate model sensitivity experiments conducted using two climate–carbon cycle models.

In our modern continental configuration, freshwater is exported from the Bering Sea into the Arctic Ocean. A major freshwater perturbation in the North Atlantic, often used in idealized waterhosing experiments, raises the sea-level in the Arctic and leads to the reversal of Bering Strait throughflow. The freshwater perturbation from the North Atlantic/Arctic Ocean then spills into the North Pacific, thereby preventing the formation of deep water. Under glacial conditions, however, the Bering Strait was closed (Dyke et al., 1996) and the dilution of North Pacific waters by additional North Atlantic freshwater discharge was prevented (Hu et al., 2007, Okumura et al., 2009). Such conditions may have preconditioned the North Pacific for intensified intermediate and deep water formation.

In this study, we describe the results of a North Atlantic freshwater perturbation experiment using two coupled climate models under glacial conditions (including a closed Bering Strait) to investigate the oceanic and biogeochemical response of the North Pacific to an AMOC shutdown. We describe the global climate response to an AMOC shutdown in both models, and then focus our research more specifically on climate and carbon cycle response in the North Pacific. Finally, we compare the results of both models with paleo proxies.

Section snippets

Two coupled models

In this study, we use a coupled atmosphere–ocean general circulation model (CGCM) MIROC version 3.2 (K-1 Model Developers, 2004), and an earth system model of intermediate complexity LOVECLIM (Goosse et al., 2010, Menviel et al., 2008). Because this CGCM predicts climate and ocean fields without marine carbon cycle, we additionally employ an off-line ocean biogeochemical model forced by the ocean circulation and diffusion fields obtained from MIROC. In LOVECLIM, the climate and carbon systems

Glacial climate and carbon cycle simulations

In the LGM control runs, the global mean surface air temperatures are 5.0 and 4.5 °C lower in MIROC and LOVECLIM, respectively. The maximum North Atlantic Deep Water (NADW) flow is 26.3 and 27.5 Sv, respectively. The NADW flow may have been partly affected by the Bering Strait closure, which prevents freshwater transport from the North Pacific to the North Atlantic (Hu et al., 2010). The resulting changes in the poleward heat transport affect the SST (Fig. 2) and sea ice concentration in the

Paleoproxy reconstruction of Heinrich 1 meltwater event

We compare the model results with multiple proxy data obtained for H1 (Table 1). In the hosing experiment, both models simulate a cooling effect in the North Atlantic and western North Pacific, which agrees well with proxy data based on foraminiferal δ18 O in the North Atlantic (Vidal et al., 1997) and Mg/Ca and alkenone temperatures in the western North Pacific and Okhotsk Sea (Harada et al., 2008, Harada et al., 2012, Sagawa and Ikehara, 2008). The South Atlantic and South Pacific warming in

Conclusion

The responses of North Pacific intermediate and deep water ventilation and biogeochemical fields to North Atlantic freshwater perturbations were evaluated using the MIROC and LOVECLIM models. In our study, the North Pacific Ocean circulation and corresponding water properties were sensitive to the AMOC shutdown. Both models succeeded in reproducing the deeper penetration of high-oxygen and low-nutrient younger water at intermediate depths in the North Pacific Ocean. The existence of oxygenated

Acknowledgments

We would like to thank two anonymous external reviewers and the editor, K. Takahashi, for helpful comments. The authors are also grateful to H. Tatebe for his stimulating discussion. This research was conducted by JAMSTEC-IPRC Initiative (JII) project. The numerical simulations were preformed on the Earth Simulator at JAMSTEC and HITACHI SR11000 at University of Tokyo. AT was supported through the National Science Foundation grant AGS 1010869 and JAMSTEC through its co-sponsorship of the IPRC.

References (68)

  • A. Rosell-Mele et al.

    Biomarker evidence for “Heinrich” events

    Geochim. Cosmochim. Acta

    (1997)
  • K. Tachikawa et al.

    Glacial/interglacial sea surface temperature changes in the Southwest Pacific ocean over the past 360 ka

    Quatern. Sci. Rev.

    (2009)
  • L. Vidal et al.

    Evidence for changes in the North Atlantic Deep Water linked to meltwater surges during the Heinrich events

    Earth Planet. Sci. Lett.

    (1997)
  • N. Ahagon et al.

    Mid-depth circulation in the northwest Pacific during the last deglaciation: evidence from foraminifera radiocarbon ages

    Geophys. Res. Lett.

    (2003)
  • M.A. Altabet et al.

    The effect of millennial-scale changes in Arabian Sea denitrification on atmospheric CO2

    Nature

    (2002)
  • A.P. Ballantyne et al.

    Meta-analysis of tropical surface temperatures during the Last Glacial Maximum

    Geophys. Res. Lett.

    (2005)
  • E. Bard et al.

    Interhemispheric synchrony of the last deglaciation inferred from alkenone paleothermometry

    Nature

    (1997)
  • E. Bard et al.

    Hydrological impact of Heinrich events in the subtropical northeast Atlantic

    Science

    (2000)
  • R.J. Behl et al.

    Brief interstadial events in the Santa Barbara basin, NE Pacific, during the past 60 kyr

    Nature

    (1996)
  • G. Bond et al.

    Iceberg discharges into the North Atlantic on millennial time scales during the last glaciation

    Science

    (1995)
  • P. Braconnot et al.

    Results of PMIP2 coupled simulations of the Mid-Holocene and Last Glacial Maximum. Part 1. Experiments and large-scale features

    Clim. Past

    (2007)
  • B.G. Brunelle et al.

    Evidence from diatom-bound nitrogen isotopes for subarctic Pacific stratification during the last ice age and a link to North Pacific denitrification changes

    Paleoceanography

    (2007)
  • M.O. Chikamoto et al.

    Response of deep-sea CaCO3 sedimentation to Atlantic meridional overturning circulation shutdown

    J. Geophys. Res.

    (2008)
  • M.O. Chikamoto et al.

    Glacial marine carbon cycle sensitivities to Atlantic ocean circulation reorganization by coupled climate model simulations

    Clim. Past Discuss.

    (2011)
  • W. Dansgaard et al.

    Evidence for general instability of past climate from a 250-kyr ice-core record

    Nature

    (1993)
  • S.A. Dyke et al.

    Marine molluscs as indicators of environmental change in glaciated North America and Greenland during the last 18 000 years

    Geogr. Phys. Quatern.

    (1996)
  • H. Goosse et al.

    Description of the Earth system model of intermediate complexity LOVECLIM version 1.2

    Geosci. Model Dev.

    (2010)
  • N. Harada et al.

    Freshwater impacts recorded in tetraunsaturated alkenones and alkenone sea surface temperatures from the Okhotsk Sea across millennial-scale cycles

    Paleoceanography

    (2008)
  • A. Hu et al.

    Role of the Bering Strait in the thermohaline circulation and abrupt climate change

    Geophys. Res. Lett.

    (2007)
  • A. Hu et al.

    Influence of Bering Strait flow and North Atlantic circulation on glacial sea-level changes

    Nat. Geosci.

    (2010)
  • K-1 Model Developers 2004. In: Hasumi, H., Emori, S. (Ed.). K-1 Coupled GCM (MIROC) Description. K-1 Technical Report...
  • J.P. Kennett et al.

    Latest quaternary paleoclimatic and radiocarbon chronology, Hole 1017E, southern California margin

    Proc. Ocean Drilling Program Sci. Results

    (2000)
  • M. Kienast et al.

    Eastern Pacific cooling and Atlantic overturning circulation during the last deglaciation

    Nature

    (2006)
  • S.S. Kienast et al.

    Export production in the subarctic North Pacific over the last 800 kyrs: no evidence for iron fertilization?

    J. Oceanogr.

    (2004)
  • Cited by (55)

    • Late quaternary sea-ice and sedimentary redox conditions in the eastern Bering Sea – Implications for ventilation of the mid-depth North Pacific and an Atlantic-Pacific seesaw mechanism

      2020, Quaternary Science Reviews
      Citation Excerpt :

      The reasons for a stronger PMOC, however, are still under debate. Several numerical simulations suggest prominent changes in the atmospheric circulation over the subtropical and subarctic North Pacific in response to reduced northward heat transport in the Atlantic during an AMOC-off mode (Chikamoto et al., 2012; Gong et al., 2019; Menviel et al., 2012; Okazaki et al., 2010; Okumura et al., 2009; Wu et al., 2008). These changes include a southward shift in the Intertropical Convergence Zone (ITCZ) (Chikamoto et al., 2012; Okumura et al., 2009; Wu et al., 2008), stronger midlatitude westerlies (Gong et al., 2019; Okumura et al., 2009), and a strengthened Aleutian Low over the subarctic North Pacific (Chikamoto et al., 2012; Gong et al., 2019; Okumura et al., 2009).

    • Glacial discharge into the subarctic Northeast Pacific Ocean during the last glacial

      2020, Global and Planetary Change
      Citation Excerpt :

      These events could have been triggered by the onset of North Pacific deep water formation and Pacific meridional overturning circulation (PMOC), which has previously been observed during Heinrich event 1. However, debate remains about the prevalence of PMOC during the Younger Dryas and whether ventilation occurred to immediate or lower depths (Okazaki et al., 2010; Chikamoto et al., 2012; Hu et al., 2012; Rae et al., 2014; Liu and Hu, 2015; Gong et al., 2019). There is also a lack of studies examining the presence/absence of PMOC during other Heinrich events as well as other abrupt events through the last glacial.

    • Millennial scale cycles in the Bering Sea during penultimate and last glacials; their similarities and differences

      2019, Quaternary International
      Citation Excerpt :

      The revised age model does not contradict the previously published one (Riethdorf et al., 2013), but is based on more key time points (Fig. 3). So the approach was based on the synchronicity of millennial-scale climate changes between the North Atlantic and the North Pacific (Nürnberg and Tiedemann, 2004; Caissie et al., 2010; Chikamoto et al., 2012; Max et al., 2012; Rella et al., 2012). Compatibility of age model constructed by Riethdorf et al. (2013) and new approach over MIS 1-5 allow us to use this suit of proxies (color b*, Ca/Ti, chlorin and PM) to modify age model of the studied core during MIS 6 based on their correlation with absolutely dated Chinese interstadials recorded in δ18O curves of Sanbao and Hulu cave stalagmites (Wang et al., 2008) (Fig. 1).

    View all citing articles on Scopus
    View full text