Millennial meridional dynamics of Indo-Pacific Warm Pool during the last deglaciation
2. Results and Discussion
2.4 Meridional IPWP SST gradient and the southward-shifted ITCZ precipitation boundary
A rapid warming in S-IPWP during the H1 and YD periods, shown in the new compiled SST records [Figure 3-3A], may cause an altered Hadley Cell (HC) circulation, reorientation of the cross equatorial current, and a consequent precipitation reduction in the East Asian Monsoon region. Modern observatory data over the past six decades [Figure 12 of Feng et al., 2013] expresses that an equator-ward shifting of the NH convection branch of HC can be induced by an oceanic warming located at ~10oS This equator-ward shifting of northern branch of HC can cause a southward shift of ITCZ for about 10o [Feng et al., 2013]. This oceanic
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process, combined with fast atmospheric bridging [Chiang and Bitz, 2005; Broccoli et al., 2006] and confirmed by the marine and cave proxies data [Wang et al., 2001; Lea et al., 2003; Wang et al., 2007], may connect the NH and SH climate systems through a coupled low latitude ocean-atmosphere pathway.
Distinct different precipitation conditions between 10oS in the IPWP during H1 and YD events are illustrated in Figure 3-S3. Shiau et al. [2011] proposed an enhanced precipitation during the events in the Coral Sea by using marine sediment thorium isotopic proxy record, which shows a good correlation to the Australian summer monsoon reported by the Lynch’s crater records [Muller et al., 2008].
Stalagmite δ18O records from Flores Island are also featured with intense precipitation during YD [Griffiths et al., 2009]. Marine evidences reveal a reduced precipitation/
increased salinity in the northern IPWP region north of 10oS, including South China Sea [Stenike et al., 2006], Sulu Sea [Rosenthal et al., 2003], Philippines Sea [Stott et al., 2002], Java Island [Mohtadi et al., 2012], and Solomon Sea [This study] [Figure 3-S3].
On the basis of previous terrestrial and marine hydrological records and our new data, we speculate a sharp precipitation boundary between maritime continents and Australia at about 10oS from Solomon Sea, Arafura Sea, Timor Sea, to the eastern Indian Ocean during the last deglacial period [Figure 3-S3]. The IPWP meridional SST gradient variations and the altered HC circulation may further enhance the precipitation reduction in the Asian monsoon region during the H1 and YD periods.
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3. Conclusions
On the basis of our records and previous reports, we found that the meridional IPWP thermal conditions strongly link to interhemispheric high-latitude climates during the last deglaciation: N-IPWP paced by NH and S-IPWP governed by SH. Ice volume corrected δ18OSW stacked records show an increasing salinity gradient between N- and S-IPWP over the last Termination. We proposed a hypothetical precipitation boundary at about 10oS. Advanced high-resolution model simulations are required to clarify the role of IPWP meridional thermal/hydrological gradient to the altered HC and its relationship with regional and global climate systems over the last deglaciation.
Acknowledgements
MD05-2925 site location was selected by Meng-Yang Lee and collected during the IMAGES PECTEN Cruise, which was conducted by Luc Beaufort and Min-Te Chen. Sediment samples are provided from Taiwan TORI. Chien-Ju Chou, Wan-Lin Hu, and Yu-Ting Hsiao helped to pick foraminifera samples. Yang-Hui Hsu helped to operate climatological database and plotted figures. Thank Delia W. Oppo and Braddock K. Linsley for their generous offering the non-overlapping method MatLab code. This research were funded by Taiwan ROC NSC (98-2811-M-002-129, 99-2611-M-002-005, 100-2116-M-002-009 to CCS, and 95-2611-M-002-019, 96-2611-M-002-019 to KYW), and National Taiwan University (101R7625 to CCS)
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Figure 3-1. Climatological map of the Indo-Pacific Warm Pool (IPWP) sea surface temperature (SST, left) and precipitation (right) during 1950-2004 AD [Reynolds et al., 2002]. Upper panels are from the June-July-August (JJA), and lower panels are from December-January-February (DJF) averages of (A, C) SSTs and (B, D) precipitation distribution maps. SST and precipitation are at 0.5oC and 2 mm/day intervals. Green star is the study site MD05-2925.
Orange and green dots are the previous study sites in IPWP region for reconstruction of meridional thermal and precipitation variations during the glacial/interglacial change.
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Figure 3-2. Geochemical proxies from the Site MD05-2925. (A) δ18OSW-IVC (blue line) and (B) SST (red dots and line) were reconstructed with G. ruber Mg/Ca ratios and δ18OC [Anand et al., 2003]. Gray line is the Greenland ice core NGRIP [Northern Greenland Ice Core Project Members, 2004] oxygen isotope record. Dark gray line denotes the Antarctica EPICA deuterium isotope record [Stenni et al., 2003]. The superimposed dark gray lines are the 200-yr smoothed records. Black triangles are AMS 14C dates [Table 2-1].
Vertical bars denote the H1and YD periods.
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Figure 3-3. Four-hundred-year non-overlapping binned (A) SST and (C) δ18OSW-IVC
of N- (orange solid lines) and S-IPWP (green solid lines). Lower panel are the differences of (B) SST and (D) δ18OSW-IVC between N- and S-IPWP, respectively. The compilations of N- and S-IPWP surface water thermal and hydrological records [see Chapter 2, Table 2-3] were calculated with a non-overlapping binned method [Linsley et al., 2010, see Chapter 2]. All dashed lines represent 1-sigma uncertainty range. Gray bars represent the H1 and YD events.
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Figure 3-S1. G. ruber geochemical proxy records of site MD05-2925, including (A) oxygen isotope (δ18OC), (B) Mg/Ca ratio, and (C) temperature corrected-only seawater oxygen isotope (δ18OSW). Triangle symbols are corrected radiocarbon dates.
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Figure 3-S2. EOF analysis on SST [Dataset from the Reynolds et al., 2002] and selected sites [Table 2-3] used for stacked N- and S-IPWP records. (A) EOF1 explains 83.4% of total variance, which mainly represent the intra-annual seasonality. (B) EOF2 shows a clear zonal change pattern. Red stars represent the selected sites for the N-IPWP group and blue ones for the S-IPWP group.
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Figure 3-S3. Hypothetical proxy-inferred precipitation boundary during the H1 and/or the YD events (modified from the Linsley et al. [2010]). Blue dots represent a relatively wet condition, and brown ones a dry condition. The segment between Java and Flores Islands of this sharp boundary (red dotted line) was proposed by Mohtadi et al. [2011], and the one between the Solomon and Coral Seas by this study.
Figure S5
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