Trade wind forcing of upwelling, seasonally, and Heinrich events as a response to sub‐Milankovitch climate variability

Abstract

Planktonic foraminifera recovered from two cores in the northern Benguela upwelling system reveal a history of rapid events with a variability at sub‐Milankovitch frequencies during the last 140 kyr. The “cold‐water” planktonic foraminifer, left coiling Neogloboquadrina pachyderma (Ehrenberg), shows rapid fluctuations in relative abundance, indicating changes in upwelling intensity. The periods of high abundance in left coiling N. pachyderma are referred to as “PS events” (pachyderma sinistral) and indicate increased intensity and zonality of the South Atlantic trade winds controlling the Benguela upwelling system. The good correlation between PS events, the North Atlantic Heinrich events, and the Dansgaard‐Oeschger cycles from the Greenland Ice Sheet Program (GISP2) ice core suggests large‐scale global oceanographic or climatic teleconnections between the South and North Atlantic via the trade wind system. The radiocarbon constrained timing of PS events younger than 45 kyr indicates that the South Atlantic leads the North Atlantic's response to trade wind changes, particularly during isotope stages 4–2 when the Earth was dominated by large ice sheets. At times of increased trade wind strength, tropical and subtropical waters are forced across the equator enhancing the pool of warm water to be transferred to the high latitudes of the North Atlantic via the Gulf Stream and North Atlantic Drift, increasing the pull of the thermohaline convective conveyor. The increased supply of warm water to the polar regions of the northern hemisphere increases the ice‐ocean moisture gradient and accelerates ice sheet growth, leading to eventual instability and collapse.