Deep Sea Research Part II: Topical Studies in Oceanography
Variability in North Pacific intermediate and deep water ventilation during Heinrich events in two coupled climate models
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 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 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.
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