Cooperative Institute for Research in Environmental Sciences

Abstracts: 20

Closing of the Indonesian Seaway as a Possible Key to the Ice Ages

Peter Molnar (CIRES) and Mark A. Cane (Lamont-Doherty Earth Observatory)

Several facts point toward the closing of the Indonesia Seaway, which links the Pacific and Indian Oceans, as a key to the change in global climate some 3 to 5 million years ago, including the onset of the Ice Age (as well as the aridification of East Africa when and where bipedal hominids evolved). At present, cold water from the North Pacific passes into the Indian Ocean, but warm surface water occupies much of the western Pacific south of the equator. The mean state of the equatorial Pacific Ocean includes a cold east Pacific and strong easterly winds that maintain the warm pool in the west. This configuration gives way every few years to El Niño events, in which the eastern Pacific becomes warm, easterly trade winds weaken, and heat is exported from the tropics to high latitudes. Buried beneath this current surface climatological configuration, however, is a marked gradient in temperature (and salinity) of water in the mixed layer (to depths of ~150 m) across the equator. Water north of the equator is colder and fresher than that south of it.

New Guinea, the northern part of the Australian continent, moves steadily and inexorably northward at 60-70 kilometers per million years. Thus, 3 to 5 Million years ago (Ma), its northern edge, which reaches the equator today, lay 2-3° south of the equator. Both simple calculations and runs using a General Circulation Model of the Ocean show that with a northern edge of the New Guinea-Australia continent south of the equator, most of the Indonesian Throughflow would come from the Pacific south of the equator, not north of it as currently occurs. Thus, at 3-5 Ma warm water from the southern Pacific should have passed into the Indian Ocean. By analogy with the present climate, such warmer water would have induced a rainy atmospheric circulation over East Africa, consistent with the evidence that that region warm humid until 3-5 Ma. Moreover, the blockage of the warmer South Pacific mixed layer may have played a key in the formation of the current warm pool over the western Pacific. Thus, the blockage of that warm mixed layer by New Guinea would have transformed an equatorial Pacific Ocean from a permanent El Niño state to one with only occasional El Niño events. In a permanent El Niño state, ought not the enhanced atmospheric heat transport prevent the formation of ice over Canada?

One test of a perpetual El Niño state to the present is a comparison between differences between pre-Ice Age climates and present climates with El Niño teleconnections and anomalies. We show that in early Pliocene time (3-5 Ma), the eastern Pacific was warmer and the thermocline deeper than today, consistent with a permanent El Niño state. Similarly, many difference between pre-Ice Age climates and present climates resemble El Niño teleconnections. Thus, although by no means a proof that the closing of the Indonesian Seaway triggered the Ice Ages, the idea passes one test that can be made.