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
past
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.
Back to index
 |
Geotimes Home | AGI Home | Information Services | Geoscience Education | Public Policy | Programs | Publications | Careers  |