In 2003, paleoceanographers and paleoclimatologists pressed forward with numerous innovative studies into past climate changes, many of which coalesced in two closely related themes: (a) past variations in tropical rainfall patterns, and (b) the behavior of the El Niño-Southern Oscillation (ENSO) in the near and distant past.
Past tropical rainfall patterns
The Indian Ocean monsoon has a profound influence on the climate of the Indian subcontinent and the Arabian peninsula. Three new studies add to growing evidence of persistent links between the monsoon and the climate of the North Atlantic over a range of timescales. Anil Gupta and colleagues (Nature, v. 421, p. 354-357) used the abundance of G. bulloides, a planktonic foraminifer which thrives in upwelled waters, to reconstruct the Holocene history of monsoon-driven upwelling offshore Oman. Their work revealed stronger monsoonal upwelling in the early Holocene, as expected from the summer insolation maximum at that time, but also showed higher frequency reductions in monsoon strength, which correlated with known ice-rafting events in the North Atlantic.
In a related study, Dominik Fleitmann and coworkers (Science, v. 300, p. 1737-1739) analyzed the oxygen isotopic composition (delta18O) of a Holocene stalagmite from a southern Oman cave. Depleted isotopic values during the early Holocene attest to enhanced rainfall (due to the isotopic "amount effect") and confirm the presence of a strengthened monsoon and its close covariance with North Atlantic climate.
Stalagmite delta18O was also used by Steve Burns and colleagues (Science, v. 301, p. 1365-1367) to demonstrate a close match between Greenland temperatures and monsoonal precipitation on Socotra Island in the Indian Ocean between 42,000 and 55,000 years ago, an interval marked by several abrupt Dansgaard-Oeschger cycles.
Rainfall in Central and South America is governed by the seasonal migration of the Intertropical Convergence Zone (ITCZ), a belt of perpetual thunderstorm activity and cumulous convection. Sediments from the anoxic Cariaco Basin offshore Venezuela reveal much about past ITCZ variability. Pushing the temporal resolution of this archive to new limits, Gerald Haug and coworkers (Science, v. 299, p. 1731-1735) carried out seasonally resolved measurements of titanium (a proxy for rainfall intensity) in laminated sediments dated between A.D. 730-930 to investigate the influence of climate on the decline and collapse of the Mayan civilization in nearby Yucatan. Their results showed evidence of repeated multiyear droughts, likely caused by southward dislocations of the ITCZ, whose timing matched the documented dates of Mayan city abandonment. Their finding strengthens previous assertions that climate was an important factor in the Mayan downfall.
Meridional shifts of the ITCZ were also implicated in rapid temperature switches in Cariaco surface waters during the last deglaciation, revealed by foraminiferal magnesium/calcium ratios measured by David Lea and colleagues (Science, v. 301, p. 1361-1364). Abrupt cooling and warming of Cariaco surface temperatures occurred in step with the onset and termination of the high-latitude Younger Dryas event, and require varying strength of wind-driven upwelling caused by shifts of the ITCZ.
ENSO in the recent and distant
How might ENSO change in the future? Knowledge of the past range of ENSO variability is key to addressing this question. Kim Cobb and colleagues (Nature, v. 424, p. 271-276) offer new perspective into this system's behavior in recent centuries. Cobb and coworkers analyzed the oxygen isotopic composition of fossil corals from Palmyra Island, located in an ENSO-sensitive area of the central Pacific, splicing together individual coral records into extended time windows, which added up to an impressive 450-year aggregate spread over the last 11 centuries. Their results document changes in both mean ocean conditions and in the frequency and amplitude of ENSO, without, however, a clear relationship between the two. The data contain evidence of ENSO "regime shifts," sudden changes in behavior accomplished within a decade, which defy explanation but may arise from internal ENSO dynamics.
In a related study, Matthew Huber and Rodrigo Caballero (Science, v. 299, p. 877-881) examined the character of ENSO during the extreme global warmth of the Eocene, 55 million to 35 million years ago (Geotimes, April 2003). Their analysis of Eocene ENSO, simulated by a coupled ocean- atmosphere model, indicated ENSO variance as strong or stronger than at the present time. Their result is at variance with a hypothesis proposed by George Philander and Alexey Fedorov (Paleoceanography, v. 18, 10.1029/2002PA000837), invoking a "perennial-El Niño" state in the tropical Pacific throughout the warmer climates of the Cenozoic, up until 3 million years ago. This state is theorized to have been a consequence of warmer deep-ocean temperatures (by up to 12 degrees Celsius) and a depressed tropical thermocline, which inhibited the formation of an equatorial cold tongue in the eastern Pacific. These studies underscore our still incomplete understanding of ENSO dynamics over both interannual and longer timescales.
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