Relative importance of meridional and zonal sea surface temperature gradients for the onset of the ice ages and Pliocene-Pleistocene climate evolution

Abstract

[1] During the early Pliocene (roughly 4 Myr ago), the ocean warm water pool extended over most of the tropics. Subsequently, the warm pool gradually contracted toward the equator, while midlatitudes and subpolar regions cooled, establishing a meridional sea surface temperature (SST) gradient comparable to the modern about 2 Myr ago (as estimated on the eastern side of the Pacific). The zonal SST gradient along the equator, virtually nonexistent in the early Pliocene, reached modern values between 1 and 2 Myr ago. Here, we use an atmospheric general circulation model to investigate the relative roles of the changes in the meridional and zonal temperature gradients for the onset of glacial cycles and for Pliocene-Pleistocene climate evolution in general. We show that the increase in the meridional SST gradient reduces air temperature and increases snowfall over most of North America, both factors favorable to ice sheet inception. The impacts of changes in the zonal gradient, while also important over North America, are somewhat weaker than those caused by meridional temperature variations. The establishment of the modern meridional and zonal SST distributions leads to roughly 3.2°C and 0.6°C decreases in global mean temperature, respectively. Changes in the two gradients also have large regional consequences, including aridification of Africa (both gradients) and strengthening of the Indian monsoon (zonal gradient). Ultimately, this study suggests that the growth of Northern Hemisphere ice sheets is a result of the global cooling of Earth's climate since 4 Myr rather than its initial cause. Thus, reproducing the correct changes in the SST distribution is critical for a model to simulate the transition from the warm early Pliocene to a colder Pleistocene climate.Citation: Brierley, C. M., and A. V. Fedorov (2010), Relative importance of meridional and zonal sea surface temperature gradients for the onset of the ice ages and Pliocene-Pleistocene climate evolution, Paleoceanography, 25, PA2214,

Late Pliocene changes in the North Atlantic Current

“…This suggests that changes in the NAC and subsequent decrease in northward heat transport were a cause for the intensification of NHG rather than a result. This agrees with recent modeling results (Brierley and Fedorov, 2010;Lunt et al, 2008), which show that cooling of the higher latitudes was a necessity for the development of large continental ice sheets in the Northern Hemisphere. Although closing of the CAS might have led to a more intense northward heat transport in the early Pliocene (e.g., Haug and Tiedemann, 1998), our results show that during the late Pliocene northward heat transport diminished as continental ice volume increased.…”

“…So far, none of these have given a complete satisfactory explanation and the ultimate cause remains an enigma. Nevertheless, recent studies suggest that cooling of the higher latitudes and increase in meridional SST gradient were crucial for the intensification of NHG (Berger and Wefer, 1996;Brierley and Fedorov, 2010;De Schepper et al, 2009;Lunt et al, 2008). This means that the NAC, by which the excess in heat from the tropics was transported northwards during the Pliocene, had to weaken and/or change its path during the intensification of the NHG in order to allow the higher latitudes to cool and the meridional SST gradient to increase.…”

Effects of meridional sea surface temperature changes on stratospheric temperature and circulation

“…Hoerling et al (2001) demonstrated that the progressive warming of the equatorial oceans is associated with climate changes over the North Atlantic, and warmer tropical SSTs force a positive North Atlantic Oscillation phase; and this was further confirmed by Magnusdottir et al (2004). Brierley and Fedorov (2010) revealed that the impacts over North America of changes in the meridional SST gradient are somewhat stronger than those from zonal SST variations. Some earlier studies (e.g., Rind et al, 1990;Olsen et al, 2007) provided evidence that an enhanced SST meridional gradient results in a greater meridional temperature gradient in the subtropical troposphere.…”

Pleistocene sea-surface temperature evolution: Early cooling, delayed glacial intensification, and implications for the mid-Pleistocene climate transition

“…Ma and 3.5 Ma, respectively (Brierley and Fedorov, 2010). However, the stability of the Coral Sea SSTs (MD06-3018) through the Pleistocene suggests that the southern margin of the WPWP did not contract equatorward, and suggests that Hadley Circulation in the southern hemisphere was probably not strengthened in association with the MPT (Russon et al, 2010).…”

“…Both snowpack formation and survival are enhanced by a strengthening of the meridional SST gradient by reducing the number of positive degree days and increasing snowfall below 70°N (Brierley and Fedorov, 2010). Increased zonal SST gradients in the equatorial Pacific appear to precipitate a weaker response in ice volume: a reduction in positive degree days occurs in the likely source regions of the Laurentide ice-sheet and increased snowfall ensues in Alaska and Greenland (Brierley and Fedorov, 2010).…”