A hemispheric circulation asymmetry during Late Tertiar

Abstract

Recent data obtained within the Deep Sea Drilling Project (Legs 28, 29, 35, 36) in the southern oceans revealed that the formation of Antarctic sea-ice started 38 m. y. b. p. ago at the begin of the Oligocene. The Antarctic ice-cap reached nearly its present volume during the Middle Miocene (14–12 m. y. b. p.) and a volume greater than the present during the Messinian (6–5 m. y. b. p.). Apart from local mountain glaciations, the first large-scale glaciation of the northern continents was developed about 3 m. y. b. p. followed by the formation of the Arctic sea-ice, which has probably survived more than 20 fully developed interglacials, certainly during the last 0.7 m. y. This evolution indicates a period of about 10 m. y. during which East Antarctica was highly glaciated, while the Arctic Ocean was essentially ice-free. This asymmetric pattern is compared with the present asymmetry (unipolar versus bipolar glaciation); the annually averaged position of the “meteorological equator” had then displaced from about Lat. 6 °N to-day to about 10 °N. Some estimates of the heat budget terms in polar latitudes at a glaciated continent and at an ice-covered ocean are given. A preliminary review of paleoclimatic data reveals significant shifts of the position of the climatic belts at both hemispheres. These shifts coincide rather well with an extrapolation of the actual seasonal variations which confirm a simple equation for the latitude of the subtropical anticyclonic belts as the boundary between Ferrel and Hadley regimes. Finally, the role of equatorial upwelling — as correlated with an asymmetric circulation pattern — for the water balance is outlined. These asymmetries may become of major importance, if an expected rise of CO2 level above an estimated threshold should restore an ice-free Arctic Ocean.