Pliocene El Nino
Closing of the Indonesian Seaway and El Niño teleconnections and tropical anomalies as a blueprint for pre-Ice Age climates
Work of Bill Chaisson, Christina Ravelo, Kira Lawrence, Tim Herbert, and their collaborators has shown that before continental ice sheets formed in Canada and Fennoscandia, sea-surface temperatures in the Pacific showed little east- west variability, as they do during El Niño events (Fig. 1). In addition differences between pre-Ice Age and present-day climates in a number of regions resemble those El Niño teleconnections, as we might expect if the atmosphere responded to a permanent El Niño state [Molnar and Cane, 2002]. Further analysis (extends this synthesis to suggest that the El Niño of 1997-98 represents the pre-Ice Age equatorial Pacific sea-surface temperature distribution better than that of other El Niño events [Molnar and Cane, 2007].
As much of Canada warms, at least in winter, during El Niño events, we have explored the possibility that a permanently warm eastern equatorial Pacific would, via its teleconnections to Canada, prevent ice sheets from growing there.
As El Niño writes its signature largely in winter, but summer climate dictates whether winter snow survives and can accumulate into an ice sheet, Huybers and Molnar  examined the correlation of El Niño occurrence with positive- degree-days in Canada and found that indeed summers during El Niño events tended last longer and offer greater capability of melting winter snow. When scaled to eastern equatorial Pacific sea-surface temperatures over the past 4 million years, the predicted greater number of positive-degree-days than today should have sufficed to prevent ice sheets from growing.
The apparent permanent El Niño state in pre-Ice Age times poses the question of what geologic process destroyed that state. We are pursuing two variants on the theme that the closing of the Indonesian Seaway and the conversion of a largely maritime region into the Maritime Continent both blocked warm water to help form the warm pool in the western equatorial pacific [Cane and Molnar, 2001] and increased precipitation over the expanding Maritime Continent that strengthened the Walker Circulation, whose easterly winds along the equator sustain the warm pool [Dayem et al., 2007]. For the former, Jochum et al.  exploited a coupled ocean-atmosphere general circulation model, and found that although the closing of the Indonesian Seaway should affect tropical Pacific sea-surface temperatures, the calculated change is too small to be important. We have not abandoned the possibility that a closing of that seaway was important, but its effect does not seem to be as simple as we had thought.
For the latter, geologic evidence shows that Indonesian islands were both fewer and lower in elevation 3-5 million years ago. Insofar as these islands act as attractors for precipitation and enhance ascent over them, they offer a qualitative argument consistent with the idea that the growth of these islands enhanced precipitation, which strengthened the Walker Circulation over the equatorial Pacific and created the cold tongue in its eastern part.
Finally, Molnar  synthesized evidence that the closing of the Central American Seaway in late Cenozoic time altered ocean circulation and led to a state in which ice ages were possible. He concluded that although one could refute that suggestions, the evidence for it shares much with a house of cards.
Sea surface temperatures and ODP sites from the equatorial Pacific. The top map shows mean winter (January-March) sea surface temperature for the period 1972-2000, and the bottom map shows the mean sea surface temperature (in °C) for January through March of the El Niño year 1998. In the El Niño year high temperatures extend into the eastern Pacific, eliminating the climatological cold tongue. The image was obtained using the NCEP data [Kalnay et al., 1996] provided by NOAA-CIRES, Boulder Colorado on their Web site at http://www.cdc.noaa.gov/. Site numbers beside the white circles indicate the locations of ODP sites used.
Cane, M. A., and P. Molnar (2001), Closing of the Indonesian seaway as a precursor to east African aridification around 3–4 million years ago, Nature, 411, 157-162.
Huybers, P; Molnar, P (2007), Tropical cooling and the onset of North American glaciation. Clim. Past., 3 (3) 549-557
Molnar, P (2008), Closing of the Central American Seaway and the ice age: A critical review. Paleoceanography, 23 (1) , Art. No. PA2201, doi: 10.1029/2007PA001574
Molnar, P; Cane, MA (2007), Early pliocene (pre-Ice Age) El Nino-like global climate: Which El Nino?. Geosphere, 3 (5) 337-365, doi: 10.1130/GES00103.1
Molnar, P., and M. A. Cane (2002), El Niño's tropical climate and teleconnections as a blueprint for pre-Ice Age climates, Paleoceanography, 17 (2), 10.1029/2001PA000663.