Wind‐Driven Evolution of the North Pacific Subpolar Gyre Over the Last Deglaciation

• Published in: Geophysical research letters Vol. 47; no. 6
• Main Authors: Gray, William R., Wills, Robert C. J., Rae, James W. B., Burke, Andrea, Ivanovic, Ruza F., Roberts, William H. G., Ferreira, David, Valdes, Paul J.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28.03.2020; American Geophysical Union
• Subjects: Albedo; Albedo (solar); Atmospheric circulation; Atmospheric circulation changes; Atmospheric models; Biogeochemistry; Carbon dioxide; Carbonates; Circulation; Climate change; Climate models; Climate system; Cold surfaces; Computer simulation; Continental interfaces, environment; Cores; Deglaciation; Foraminifera; Fossils; Glaciation; Global climate; gyre circulation; Gyres; Heat transport; Hydroclimate; Ice ages; Ice sheets; Isotopes; Last Glacial Maximum; Meltwater; Nonlinear response; North Pacific; Numerical simulations; Ocean circulation; Ocean currents; Ocean, Atmosphere; Oceans; Oxygen; Oxygen isotopes; Plankton; Sciences of the Universe; Sea surface; Sea surface temperature; Surface temperature; Surface water; Water circulation; Westerlies; Wind; Wind patterns; Wind stress; Winds
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2019GL086328

North Pacific atmospheric and oceanic circulations are key missing pieces in our understanding of the reorganization of the global climate system since the Last Glacial Maximum. Here, using a basin‐wide compilation of planktic foraminiferal δ18O, we show that the North Pacific subpolar gyre extended ~3° further south during the Last Glacial Maximum, consistent with sea surface temperature and productivity proxy data. Climate models indicate that the expansion of the subpolar gyre was associated with a substantial gyre strengthening, and that these gyre circulation changes were driven by a southward shift of the midlatitude westerlies and increased wind stress from the polar easterlies. Using single‐forcing model runs, we show that these atmospheric circulation changes are a nonlinear response to ice sheet topography/albedo and CO2. Our reconstruction indicates that the gyre boundary (and thus westerly winds) began to migrate northward at ~16.5 ka, driving changes in ocean heat transport, biogeochemistry, and North American hydroclimate. Plain language summary Despite the North Pacific's importance in the global climate system, changes in the circulation of this region since the last ice age are poorly understood. Today, the North Pacific Ocean has distinct properties north and south of ~40°N: To the south, the warm surface waters form a circulation cell that moves clockwise (the subtropical gyre); to the north, the cold surface waters form a circulation cell that moves anticlockwise (the subpolar gyre). This difference in surface ocean circulation north and south of ~40°N is determined by the wind patterns. Here, using a compilation of oxygen isotopes measured in the carbonate shells of fossil plankton from sediment cores across the basin, which tracks changes in the spatial pattern of temperature, we reconstruct how the position of the boundary between the gyres changed since the last ice age. Our results show that the boundary between the gyres was shifted southward by ~3° during the last ice age; this indicates that the westerly winds were also shifted southward at this time. Using numerical simulations of the climate, we find that this ice age shift in the westerly winds is primarily due to the presence of a large ice sheet over North America. Key Points Planktic foraminiferal δ18O data indicate that the North Pacific subpolar gyre expanded southward by ~3° during the Last Glacial Maximum Climate models show that changes in gyre extent/strength are driven by a nonlinear response of the westerlies to ice sheet albedo/topography and CO2 Proxy data and model simulations indicate that the gyre boundary and westerlies began to migrate northward at ~16.5 ka, during Heinrich Stadial 1