Pacific warm pool subsurface heat sequestration modulated Walker circulation and ENSO activity during the Holocene

Abstract

Dynamics driving the El Niño–Southern Oscillation (ENSO) over longer-than-interannual time scales are poorly understood. Here, we compile thermocline temperature records of the Indo-Pacific warm pool over the past 25,000 years, which reveal a major warming in the Early Holocene and a secondary warming in the Middle Holocene. We suggest that the first thermocline warming corresponds to heat transport of southern Pacific shallow overturning circulation driven by June (austral winter) insolation maximum. The second thermocline warming follows equatorial September insolation maximum, which may have caused a steeper west-east upper-ocean thermal gradient and an intensified Walker circulation in the equatorial Pacific. We propose that the warm pool thermocline warming ultimately reduced the interannual ENSO activity in the Early to Middle Holocene. Thus, a substantially increased oceanic heat content of the warm pool, acting as a negative feedback for ENSO in the past, may play its role in the ongoing global warming.

Central Equatorial Pacific Cooling During the Last Glacial Maximum

“…Our results do not agree with CLIMAP and MARGO, which suggest little to no SST change near the Line Islands and throughout most of the central Pacific. In contrast, our data are consistent with both model simulations that show ∼2.0°C-2.5°C cooling near the Line Islands (Brady et al, 2013;DiNezio et al, 2011) as well as proxy records from the eastern and western equatorial Pacific (Benway et al, 2006;Bolliet et al, 2011;Dang et al, 2020;de Garidel-Thoron et al, 2007Hertzberg et al, 2016;Hollstein et al, 2018;Koutavas & Joanides, 2012;Lea et al, 2000Lea et al, , 2006Leduc et al, 2007;Rosenthal et al, 2003;Sagawa et al, 2012;Steinke et al, 2006;Stott et al, 2007Stott et al, , 2002Xu et al, 2010). The recent LGM data assimilation study of Tierney et al (2020) shows cooling at our core sites, but of a higher magnitude (−3.9°C) than we find in this study.…”

“…However, it should be noted that there were very few geochemical measurements in the MARGO compilation from the central tropical Pacific and the MARGO project had to rely on foraminiferal assemblage data, much of it generated for the early CLIMAP study. In contrast, geochemical records from the eastern and western equatorial Pacific from Mg/Ca (Benway et al., 2006; Bolliet et al., 2011; Dang et al., 2020; de Garidel‐Thoron et al., 2005, 2007; Hertzberg et al., 2016; Hollstein et al., 2018; Koutavas & Joanides, 2012; Koutavas et al., 2002; Lea et al., 2000, 2006; Leduc et al., 2007; Rosenthal et al., 2003; Sagawa et al., 2012; Steinke et al., 2006; Stott et al., 2002, 2007), alkenones (Kienast et al., 2001; Koutavas & Sachs, 2008; Leduc et al., 2007), TEX 86 (Hertzberg et al., 2016), and clumped isotopes (Tripati et al., 2014) indicate 1°C–4°C cooling. Several proxy‐based analyses indicate that the magnitude of overall tropical ocean cooling was likely 2.0°C–3.0°C (Ballantyne et al., 2005; Crowley, 2000), much larger than the moderate cooling suggested by CLIMAP and MARGO.…”

Precession cycles of the El Niño/Southern oscillation-like system controlled by Pacific upper-ocean stratification

“…In turn, subsurface waters of the western equatorial Pacific (WEP) originate from the subduction of southern Pacific subtropical waters [3][4][5][6][7][8] , presenting extratropical control mainly through oceanic tunnels 9 . Therefore, historical records of subsurface waters could provide clues on the past ENSO-like process and the linkage of extratropical climate change to the tropical Pacific 10 .…”

“…A subT record off Papua New Guinea demonstrated clear obliquity cycles since 110 ka, with a warming subT caused by weakened subtropical gyre circulation due to high obliquity 14 . Overall, most existing records of subsurface waters of the WPWP have focused on either the last glacial period 10 or regions far from the center of the WPWP 12,14 and have shown nonuniform information.…”

Gradually Cooling of the Yellow Sea Warm Current Driven by Tropical Pacific Subsurface Water Temperature Changes Over the Past 5 kyr

“…Temperature reconstructions of intermediate waters (500–600 m water depth) in the WPWP have revealed a cooling of 2.1 ± 0.4°C during the mid‐to‐late Holocene (Rosenthal et al., 2013). The cooling of the intermediate waters is accompanied by a cooling of the WPWP thermocline (Dang et al., 2020) and surface water temperatures (Linsley et al., 2010), a contraction of the WPWP (Moffa‐Sanchez et al., 2019) and a southward shift of Kuroshio extension (Isono et al., 2009). The concordant decrease of the BWTs in the southern YS and intermediate/thermocline water temperatures in the WPWP might imply that the temperature signal of the low‐latitude source waters was transmitted by the Kuroshio Current, which flows northward along the western side of the Okinawa Trough and carries large amounts of heat (and temperature signal) from the tropics to mid‐latitudes (Wu et al., 2012 and references therein), into the YS (Figures 2a–2d) via its intermediate/thermocline waters.…”