Mechanisms for millennial‐scale global synchronization during the last glacial period

Abstract

Global climate during the last glacial period was punctuated by abrupt warmings and occasional pulses of freshwater into the North Atlantic that disrupted deepwater production. These massive freshwater pulses known as Heinrich events arose, in part, from instabilities within the Laurentide ice sheet. Paleoevidence from the North Atlantic suggests that these events altered the production of deep water and changed downstream climate throughout the Northern Hemisphere. In the tropical western Pacific sea, surface temperatures and salinity varied together with ocean and climate changes at high latitudes. Here we present results from coupled modeling experiments that shed light on a possible dynamical link between the North Atlantic Ocean and the western tropical Pacific. This link involves a global oceanic standing wave pattern brought about by millennial‐scale glacial density variations in the North Atlantic, atmospheric teleconnections triggered by meridional sea surface temperature gradients, and local air‐sea interactions. Furthermore, our modeling results are compared with hydrological records from the Cariaco basin, the Indian Ocean, the Sulu Sea, and northern Australia.

Eastern tropical Pacific hydrologic changes during the past 27,000 years from D/H ratios in alkenones

“…However, it is in agreement with both terrestrial and marine paleorecords from the tropical Atlantic and South America [e.g., Arz et al, 1998;Baker et al, 2001b;Bush and Colinvaux, 1990;Hooghiemstra and van der Hammen, 1993;Hughen et al, 1996;Leyden, 1985;Mora and Pratt, 2001;Peterson et al, 2000;Schmidt et al, 2004;Wang et al, 2004Wang et al, , 2006, as well as several model studies, that consistently suggest a southward shift of the ITCZ at times of high Northern Hemisphere ice volume and at times of reduced North Atlantic Deep Water (NADW) production [Chiang and Bitz, 2005;Dahl et al, 2005;Timmermann et al, 2005Timmermann et al, , 2007b. The disparity with salinity records from $8°N in the eastern tropical Pacific are likely due to the complexity of precipitation sources in this area.…”

“…[3] Climate modeling evidence [e.g., Stouffer et al, 2007;Timmermann et al, 2005Timmermann et al, , 2007bZhang and Delworth, 2005] supports the notion that meltwater pulses in the northern North Atlantic can disrupt the large-scale Atlantic meridional overturning circulation (AMOC) by reducing surface density and deepwater formation. A weaker AMOC leads to a decrease of the poleward heat transport in the North Atlantic [Stocker and Johnsen, 2003] and a cooling of surface waters.…”

“…The resulting pressure gradient intensifies the northern trade wind circulation, which results in an enhancement of the oceanic subtropical cells, increased upwelling and colder sea surface conditions in the tropical and North Pacific [Timmermann et al, 2004]. Furthermore, the seasonal cycle of clouds generates a meridional asymmetry of net annual mean shortwave radiation on precessional timescales [Timmermann et al, 2007a]. The resulting heating asymmetry also affects the meridional position of the ITCZ considerably.…”

“…[4] Cooling in the Caribbean generates an anticyclonic surface circulation in the atmosphere, which spreads into the tropical North Pacific across Central America [Timmermann et al, 2005[Timmermann et al, , 2007bZhang and Delworth, 2005]. Over the tropical Pacific, enhanced northeasterly trade winds lead to enhanced Ekman pumping north of the equator, evaporative cooling, and a southward shift of the ITCZ.…”

Hindcasting the continuum of Dansgaard–Oeschger variability: mechanisms, patterns and timing

“…7h, changes in the barotropic transport across the Indonesian archipelago occur almost in unison with the AMOC. This surprisingly fast adjustment can be attained by two processes: (i) wind changes in the Pacific (Timmermann et al, 2005b), and (ii) fast oceanic adjustment processes involving wave propagation from the Atlantic into the Indian and Pacific oceans, as discussed in Timmermann et al (2005a). The former can modulate the Indonesian Throughflow via the island rule (Godfrey, 1989), whereas the latter would have to change the joint effect of baroclinicity and relief (JEBAR) term in the barotropic transport equation (Sarkisyan and Ivanov, 1971;Cane et al, 1998).…”