Overturning circulation, nutrient limitation, and warming in the Glacial North Pacific

Abstract

Although the Pacific Ocean is a major reservoir of heat and CO2, and thus an important component of the global climate system, its circulation under different climatic conditions is poorly understood. Here, we present evidence that during the Last Glacial Maximum (LGM), the North Pacific was better ventilated at intermediate depths and had surface waters with lower nutrients, higher salinity, and warmer temperatures compared to today. Modeling shows that this pattern is well explained by enhanced Pacific meridional overturning circulation (PMOC), which brings warm, salty, and nutrient-poor subtropical waters to high latitudes. Enhanced PMOC at the LGM would have lowered atmospheric CO2—in part through synergy with the Southern Ocean—and supported an equable regional climate, which may have aided human habitability in Beringia, and migration from Asia to North America.

Deep‐Water Formation in the North Pacific During the Late Miocene Global Cooling

“…Previous studies suggested that North Pacific deep-water could form during the last deglacial abrupt cold events due to increased surface salinity driven by subdued local freshwater flux (Okazaki et al, 2010;Rae et al, 2014Rae et al, , 2020Yu et al, 2020). Precipitation in the North Pacific mid-latitudes is mainly shaped by the storm tracks, the position of which can be affected by the equator-to-pole temperature gradient (Brayshaw et al, 2008;Shaw et al, 2016).…”

“…More saline and oxygen-rich surface water would act directly to erode the halocline and facilitate deep-water formation. Higher salinity in the North Pacific may also be driven by increased delivery of more saline subtropical waters into the North Pacific due to an intensification of the westerly winds in the presence of pronounced cooling in the North Hemisphere (Gray et al, 2018;Rae et al, 2020). The late Miocene global cooling may also have caused enhanced Ekman suction in the North Pacific, which could bring up salt from subsurface waters and lead to increased salinity in surface layer (Gray et al, 2018;Okazaki et al, 2010).…”

“…. It has been proposed that deep-water, formed due to increased surface salinity driven by subdued freshwater flux during the last deglacial cold events, may have played a key role in global heat distribution and carbon cycles (Okazaki et al, 2010;Rae et al, 2014Rae et al, , 2020Yu et al, 2020). However, some studies argue that during the deglacial cold events, only enhanced intermediate-water ventilation occurred above a depth of ∼2,000 m in the North Pacific driven by the active polynya formation and brine rejection during sea ice formation in the Okhotsk Sea and/or western Bering Sea, while the deep ocean remained isolated (Gong et al, 2019;Jaccard & Galbraith, 2013;Max et al, 2014;Ohkushi et al, 2003 Kawabe & Fujio, 2010); also shown are the locations of IODP Sites U1425 and U1430 in the Japan Sea and other sites referred in this study.…”

Delayed Collapse of the North Pacific Intermediate Water After the Glacial Termination

“…Recently this synchronization was ascribed to the formation of NPIW, which regulated the intermediate‐to‐deep ocean stratification and thus North Pacific deep water upwelling, and further modulated the delivery of nutrient and CO 2 ‐rich deep water to the surface ocean (Gong et al., 2019; Worne et al., 2019). Besides, the NPIW formation could also regulate the vertical convection and preformed nutrient content and the associated net efficiency of the biological pump between surface and subsurface North Pacific, which further controlled the CO 2 outgassing (Gray et al., 2018; Rae et al., 2020).…”

The atmospheric bridge communicated the <i>δ</i><sup>13</sup>C decline during the last deglaciation to the global upper ocean

“…A manual detailing code installation, basic model configuration, and tutorials covering various aspects of model configuration and experimental design, plus results output and processing, is assigned a DOI: https://doi.org/10.5281/zenodo.4903426 (Ridgwell et al, 2021a).…”