PerspectivePALEOCLIMATE

The Glacial Tropical Pacific--Not Just a West Side Story

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Science  12 Jul 2002:
Vol. 297, Issue 5579, pp. 202-203
DOI: 10.1126/science.1073841

The tropical oceans—especially the tropical Pacific—serve as a heat engine for Earth's climate [HN1] and as a vapor source for its hydrological cycle [HN2]. The impact of the tropical Pacific on global climate is well known during El Niño [HN3] events. By implication, tropical climate must have played a major role in ice age cycles [HN4]. But until recently, researchers could not resolve the relatively subtle changes in tropical sea surface temperature (SST) [HN5] that occurred during glacial episodes. The impact of tropical climate cycles has therefore remained elusive.

Two papers on pages 222 and 226 in this issue (1, 2) now report the first high-resolution records of past climate for two key regions of the tropical Pacific: the cool waters of the Galápagos region in the east, and the warm waters south of Mindanao in the Philippines in the west [HN6]. The records reveal several new aspects of tropical Pacific climate during the last glaciation.

The authors combine two recent advances to derive their records. First, two new methods, alkenone unsaturation ratios [HN7] (3) and foraminiferal Mg/Ca ratios [HN8] (4), enable second-hand (proxy) information on past SSTs to be obtained. Second, usable, rapidly accumulating sediments have been recovered from the tropical Pacific—a difficult endeavor because of the great depth and extensive carbonate dissolution of much of the central basin [HN9].

The Galápagos data of Koutavas et al. [HN10] (2) suggest that the cold tongue of the eastern Pacific [HN11] cooled less than other regions (see the figure). According to sediment geochemical data, most of the warm (>26°C) tropical Pacific cooled by about 3°C (35), and this inference is corroborated by the new Mindanao data (1). In contrast, glacial cooling may have been only 1°C in the coldest part of the eastern tropical Pacific (2). This finding is important because the temperature of the eastern Pacific cold tongue is a direct diagnostic of trade wind [HN12] strength, thermocline depth, and the upwelling [HN13] of cold subsurface waters.

Mean annual sea surface temperature and salinity in the tropical Pacific.

Modern climate is characterized by strong temperature gradients in the east and dominance of salinity gradients in the west. Symbol size and color indicate estimated changes during cold glacial periods from sediment records. Salinity changes are relative to the oceanic mean and do not include the effect of lower sea levels. The absolute magnitude of glacial salinity changes is uncertain because it depends on the oxygen-isotope composition of glacial precipitation, but the sense of change is likely to be correct. Diamonds indicate sites where there is no apparent change. [Modern climatology adapted from (14)]

This result challenges the paradigm of stronger trade winds, a steeper east-west thermocline tilt, and more intensive eastern boundary cooling during glacial episodes (6). It further suggests that cold (La Niña) [HN14] episodes, which are characterized by such conditions, are not a suitable analog for glacial conditions (7).

The Mindanao record of Stott et al. [HN15] (1), on the other hand, mostly demonstrates the rapid response of the hydrological cycle of the western Pacific on millennial time scales. The authors exploit extremely high accumulation rates (∼65 cm/1000 years) and the combination of Mg paleothermometry and oxygen isotopes [HN16] to pinpoint the timing and magnitude of these hydrological shifts.

Today, gradients in the western tropical Pacific are dominated by salinity rather than temperature (see the figure). The salinity gradients reflect the interplay of sites of convection and the Asian monsoon [HN17]. Spatial and temporal salinity patterns respond strongly to the effects of El Niño-Southern Oscillation (ENSO) [HN18] on precipitation over the modern western Pacific (8).

During glacial sea-level lows, the enlargement of the western maritime continent likely led to changes in patterns of precipitation and runoff. Earlier studies used patterns of oxygen-isotope change between the last glacial maximum and modern climate to suggest large-scale shifts in western Pacific salinity (9).

Stott et al. show that the rapid millennial-scale climate events (10) that characterize the last glaciation are marked by large shifts in surface water hydrology. They find warmer and wetter conditions during brief warmings known as interstadials, which they interpret as an indication that oscillation between cool stadials and warm interstadials (10) was akin to extreme ENSO cycles. During stadials the center of atmospheric convection appears to have moved east of the Mindanao site, as it does today during El Niño episodes. The hypothesis of drier El Niño-like conditions during glacial episodes is in line with the Galápagos data of Koutavas et al. (2).

The pattern of deglaciation in the two records, and in other records from this region, is another important diagnostic. Results from low-resolution equatorial Pacific records suggest that tropical SSTs warmed before ice volume shrank (4). No such lead is found in records from the South China Sea (3, 5), where the SST response to millennial-scale climate cycles appears to follow the Northern Hemisphere pattern (11). In contrast, the new Mindanao and Galápagos records clearly indicate the SST lead on deglaciation, as does a record from the Sulu Sea west of Mindanao (12).

The temperature lead thus appears to be a robust feature of tropical Pacific deglaciation, whereas the South China Sea responds to Northern Hemisphere forcing by way of the Asian monsoon (5, 13). This analysis suggests that the Northern Hemisphere pattern of deglaciation did not extend to the open tropical Pacific. The Mindanao results (1) imply, however, that the tropical water cycle in the western Pacific did respond to millennial-scale climate change, either through the postulated link to ENSO or, as suggested for the South China Sea, by way of the Asian monsoon (13).

The new Mindanao record (1) should help researchers to piece together a picture of the structure and climatology of the glacial western Pacific. Available data (see the figure) suggest that glacial cooling was fairly uniform, but that the pattern of hydrological change was complex. The latter may reflect the combined effects of southward migration of atmospheric convection, changes in the strength of the Asian monsoon, and shifts in ENSO.

But the Galápagos record (2) indicates that tropical Pacific climate evolution is not just a West Side Story. Much of the climate variability in the tropical Pacific reflects trade wind strength and the atmospheric Walker circulation [HN19]. The weaker winds implied by the minimal glacial cooling in the cold tongue must therefore be linked to changes in atmospheric convergence and convection in the western Pacific. As the temperature and salinity structure of the glacial tropical Pacific becomes more certain, researchers should be able to establish how such critical linkages changed during glaciation.

HyperNotes Related Resources on the World Wide Web

General Hypernotes

Dictionaries and Glossaries

A Glossary of Oceanography and the Related Geosciences with References is provided by S. Baum, Department of Oceanography, Texas A&M University.

A glossary of carbon dioxide and climate is provided by the Carbon Dioxide Information Analysis Center (CDIAC).

A climate glossary is provided by NASA's Goddard Institute for Space Studies (GISS).

Web Collections, References, and Resource Lists

WINDandSEA is an oceanic and atmospheric sciences Internet locator provided by the Central Library of the National Oceanic and Atmospheric Administration (NOAA).

The Open Directory Project provides Internet links to resources on oceanography and global change.

Academic Info provides links to Internet resources on oceanography and climate and atmospheric sciences.

The WWW Virtual Library of Paleoclimatology and Paleoceanography is maintained by P. Farrar.

R. H. Cummins, School of Interdisciplinary Studies, Western College Program, Miami University, OH, offers a collection of Internet resources on paleoclimate, greenhouse warming, El Niño, and climate change.

NOAA's Office of Global Programs provides links to Internet resources on climate and climate change.

The NOAA Paleoclimatology Program at the National Geophysical Data Center is a central location for paleoclimate data, research, and education. A collection of Internet links and an introduction to paleoclimatology science are provided.

Online Texts and Lecture Notes

Resources in Atmospheric Sciences, provided by B. Geerts, Department of Atmospheric Sciences, University of Wyoming, includes a section of articles on oceans and climate.

Fundamentals of Physical Geography, a Web textbook by M. Pidwirny, Department of Geography, Okanagan University, Kelowna, BC, Canada, includes chapters on climatology and meteorology and hydrology.

The Department of Earth and Environmental Sciences, Columbia University, provides lecture notes for a course on the climate system. A presentation by A. Gordon on general circulation and climate zones is included.

L. Stott, Department of Earth Sciences, University of Southern California, offers lecture notes for a course on climate change. A presentation on Late Pleistocene millennial scale climate change is included.

R. Myneni, Department of Geography, Boston University, provides lecture notes and Internet links for a course on global climate change and environmental impacts. A section on observed climate variability and change is included.

S. Fitzsimons, Department of Geography, University of Otago, New Zealand, provides lecture notes for a course on climate change of the past.

S. Lund, Department of Earth Sciences, University of Southern California, offers lecture notes for an oceanography course. A presentation on paleoclimate and paleoceanography is included.

R. Stewart, Department of Oceanography, Texas A&M University, offers textbook notes for a course on physical oceanography. A section on equatorial processes is included. R. Stewart's Introduction to Physical Oceanography is available in PDF format.

D. Wright, Department of Geosciences, Department of Oceanography, Oregon State University, provides lecture notes for a course on the geography of the oceans.

M. Tomczak, School of Chemistry, Physics and Earth Sciences, Flinders University, Adelaide, Australia, offers lecture notes for a course on physical oceanography. Regional Oceanography: An Introduction by M. Tomczak and J. S. Godfrey is made available in PDF format.

E. Takle, Department of Geological and Atmospheric Sciences, Iowa State University, provides lecture notes and Internet links for a course on global change.

General Reports and Articles

R. Muller, Physics Department, University of California, Berkeley, makes available introductions to the history of climate and ice age theories from his book Ice Ages and Astronomical Causes.

Climate Change 2001: The Scientific Basis is made available on the Web by the Intergovernmental Panel on Climate Change. A chapter on observed climate variability and change is included.

The 27 April 2001 issue of Science had a special section on paleoclimate. Included was a review by J. Zachos et al. titled “Trends, rhythms, and aberrations in global climate 65 Ma to present.”

The 16 June 2000 issue was a special issue on understanding Earth's dynamics. A Web supplement provides a selection of previously published Science articles about El Niño and ocean-atmosphere coupling and modeling.

The December 2001 issue of Geotimes had an article by M. Huber titled “Climate change: A glance in the rear view mirror.”

Numbered Hypernotes

1. Tropical oceans and climate. M. Tomczak offers lecture notes on the oceans and climate for a physical oceanography course. NASA's Earth Observatory Web site includes a presentation by D. Herring on oceans and climate. The 10 November 2000 issue of Science had an Enhanced Perspective by M. Cane and M. Evans titled “Do the tropics rule?” The 2 October 1998 issue had a Perspective by M. Cane titled “A role for the tropical Pacific.” The 27 April 2001 issue of Science had a news article by R. Kerr titled “The tropics return to the climate system.” The 15 February 2000 issue (which includes a collection of articles on abrupt climate change) of the Proceedings of the National Academy of Sciences had a perspective article by R. Pierrehumbert titled “Climate change and the tropical Pacific: The sleeping dragon wakes.”

2. Earth's hydrological cycle. Weather World 2010 Project (WW2010, presented by the Department of Atmospheric Sciences, University of Illinois, offers an introduction to the hydrologic cycle. The Remarkable Ocean World, provided by W. S. Chamberlin, Division of Natural Sciences, Fullerton College, CA, offers an introduction to the hydrologic cycle. E. Takle provides lecture notes on the global hydrological cycle for a course on global change. NOAA's Climate Diagnostics Center provides a section on hydrologic cycle climate research in its 1997 Science Program Review.

3. El Niño. WW2010 offers a presentation on El Niño. The International Research Institute for Climate Prediction at Columbia University provides a presentation on the influence of El Niño and La Niña on seasonal climate. NASA's Earth Observatory Web site offers a feature by J. Weiber titled “El Niño's extended family: An introduction to the cyclic patterns that determine global weather.” The Norwegian Meteorological Institute offers a presentation on El Niño. “El Niño and La Niña: Tracing the dance of ocean and atmosphere” is a presentation of the National Academies of Science. The NOAA El Niño Page provides links to Internet resources about El Niño. Other NOAA El Niño resource pages are provided by the Pacific Marine Environmental Laboratory, the Climate Diagnostics Center, and the Climate Prediction Center. The Australian Bureau of Meteorology provides an information page on climate variability and El Niño. The Center for Ocean-Atmospheric Prediction Studies, Florida State University, provides an El Niño and La Niña resource page including links to movies and animations on El Niño and La Niña. The 23 February 2001 issue of Science had an Enhanced Perspective by J. Cole titled “A slow dance for El Niño” about a report in that issue by A. Tudhope et al. titled “Variability in the El Niño-Southern Oscillation through a glacial-interglacial cycle.” The 16 Jun 2000 issue had a review by A. Fedorov and S. G. Philander titled “Is El Niño changing?”

4. Ice ages. M. Pidwirny's Fundamentals of Physical Geography includes a section on Earth's climatic history. The University of Michigan's Global Change Curriculum makes available lecture notes by P. Samson on climate patterns past and present and the paleoclimate record and climate models. Ices Ages is a Web exhibit presented by the Illinois State Museum. J. Aber, Earth Science Department, Emporia State University, Kansas, provides an introduction to ice ages for a course on ice age environments. E. Thomas, Department of Earth and Environmental Sciences, Wesleyan University, provides lecture notes on Pleistocene ice ages in a collection of notes on paleoceanography. A graphic depiction of ice age climate is presented by the Deutsche Klimarechenzentrum. B. Geerts's Resources in Atmospheric Sciences makes available a presentation by E. Linacre on the climate of the last ice age. C. Lithgow-Bertelloni, Department of Geological Sciences, University of Michigan, provides illustrated lecture notes on the cryosphere and ice ages for a geology course. The 22 February 2002 issue of Science had an Enhanced Perspective by T. Crowley titled “Cycles, cycles everywhere.” The 14 May 1999 issue had a Perspective by E. Bard titled “Ice age temperatures and geochemistry.”

5. Sea surface temperature is defined in the CDIAC glossary. Climate Change 2001: The Scientific Basis includes a section on sea surface temperature (SST). NOAA's Climate Diagnostics Center provides SST information and animations. NOAA's Office of Satellite Data Processing and Distribution provides SST images. NASA's Visual Earth offers a collection of SST images.

6. The Pacific Ocean and the Galápagos and Mindanao regions. Geography.com provides an introduction to the Pacific Ocean. The National Geographic Society provides a map of the western Pacific Ocean, as well as a map of the Galápagos Islands. The Holt, Rinehart, and Winston World Atlas Web page provides a map of the South Pacific Ocean as well as maps of the Philippines and Indonesia. The Perry-Castañeda Library Map Collection provides a map of Oceania showing the region around Mindanao. The Galapagos Conservation Trust provides an introduction to the islands and a map. World Sites Atlas Web site provides a physical map and a political map of the Galápagos Islands and a map of the Mindanao region.

7. Alkenone unsaturation ratios. F. Prahl, College of Oceanic and Atmospheric Sciences, Oregon State University, provides lecture notes on biomarker clues for past oceanographic conditions for a course on ocean research frontiers. The Paleoceanography Group of the Department of Geological Sciences, Brown University offers a research presentation on alkenone paleotemperature measurements. The Joint Oceanographic Institutions/U.S. Science Support Program makes available (in PDF format) an article by B. Simoneit titled “Organic compounds as thermal sensors for sediment alteration” that discusses alkenone unsaturation ratios. Volume 165 (May 2000) of the Scientific Results of the Ocean Drilling Program includes a contribution by T. Herbert and J. Schuffer titled “Alkenone unsaturation estimates of sea-surface temperatures at Site 1002 over a full glacial cycle” with an introduction to the method. The 1999 Goldschmidt conference Web site makes available (in PDF format) an extended abstract by S. C. Brassell, titled “Evolution of temperature control on alkenone biosynthesis.”

8. Foraminiferal Mg/Ca ratios. An introduction to foraminifera is provided by the Department of Paleobiology of the Smithsonian National Museum of Natural History. The University of California Museum of Paleontology offers a presentation on foraminifera. The 14 January 2000 issue of Science had a Perspective by G. Dwyer titled “Unraveling the signals of global climate change” about a Research article in that issue by C. H. Lear, H. Elderfield, and P. A. Wilson titled “Cenozoic deep-sea temperatures and global ice volumes from Mg/Ca in benthic foraminiferal calcite.” The 8 September 2000 issue had a Perspective by D. Nürnberg titled “Taking the temperature of past ocean surfaces” about a Research article in that issue by D. Lea, D. Pak, and H. Spero titled “Climate impact of late Quaternary equatorial Pacific sea surface temperature variations” (4). The 30 March 1999 issue of the Proceedings of the National Academy of Sciences had an article by M. Kastner titled “Oceanic minerals: Their origin, nature of their environment, and significance.”

9. Ocean sediment recovery. An introduction to deep-sea cores is presented by the British Ocean Sediment Core Repository. The Lamont-Doherty Earth Observatory Deep-Sea Sample Repository offers a presentation on coring the ocean floor. IMAGES (International Marine Past Global Changes Study) organizes cruises to collect marine sediment; an introduction to methodologies is included.

10. A. Koutavas, J. Lynch-Stieglitz, and T. M. Marchitto Jr., are at the Lamont-Doherty Earth Observatory and in the Department of Earth and Environmental Sciences, Columbia University. J. P. Sachs is in the Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology.

11. Eastern Pacific cold tongue. Cold tongue is defined by the Tracking El Niño Web site of NOVA Online. An introduction to the eastern Pacific cold tongue is provided by the Eastern Equatorial Pacific Sea Surface Temperature Anomalies Web page provided by the Joint Institute for the Study of the Atmosphere and Ocean, University of Washington. G. Johnson, Pacific Marine Environmental Laboratory, makes available a preprint (in PDF format) by B. Sloyan, G. Johnson, and W. Kessler titled “The Pacific cold tongue: An indicator of interhemispheric exchange.” The 4 October 1996 issue of Science had a report by F.-F. Jin titled “Tropical ocean-atmosphere interaction, the Pacific cold tongue, and the El Niño-Southern Oscillation.”

12. Trade winds. Trade winds are defined in the climate glossary provided by the Australian Bureau of Meteorology. An introduction to the trade winds is provided by PBS Online's Savage Seas Web site. N. Atkins, Department of Meteorology, Lyndon State College, Lyndonville, VT, includes a presentation on trade winds in lecture notes for a meteorology course. B. Geerts's Resources in Atmospheric Sciences offers an animated presentation on trade winds and El Niño.

13. Thermocline and upwelling are defined in the CDIAC glossary. Thermocline is defined in S. Baum's glossary. Upwelling current is defined in the GISS climate glossary. Introductions to upwelling and thermocline are provided by the WW2010 Web site.

14. La Niña. The Climate Information Project offers an introduction to La Niña. The International Research Institute for Climate Prediction offers a presentation on understanding La Niña. A La Niña Web page is provided by NOAA's Pacific Marine Environmental Laboratory.

15. L. Stott, C. Poulsen, and S. Lund are in the Department of Earth Sciences, University of Southern California. R. Thunell is in the Department of Geological Sciences, University of South Carolina.

16. Oxygen isotope analysis. Oxygen isotope analysis is defined in S. Baum's glossary. The NASA Goddard Institute for Space Studies makes available an article by G. Schmidt titled “Cold climates, warm climates: How can we tell past temperatures?” about oxygen isotope analysis. E. Thomas provides lecture notes titled “Oxygen isotopes: The thermometer of the Earth” in a collection of notes on paleoceanography.

17. Monsoon is defined in S. Baum's glossary and in the CDIAC glossary. An introduction to monsoons is provided in the Encyclopedia of the Atmospheric Environment. B. Geerts's Resources in Atmospheric Sciences offers an introduction to monsoons. Monsoon On Line is a Web resource provided by D. Stephenson of the Climate Analysis Group, Department of Meteorology, University of Reading, UK, and K. R. Kumar, Indian Institute of Tropical Meteorology, Pune. The Paleoceanography Group of the Department of Geological Sciences, Brown University, offers a research presentation on monsoon variability and evolution. A section on the variability of the Asian-Australian monsoon system is included in the Initial Implementation Plan of CLIVAR, an international research program investigating climate variability and predictability. The GEOS-1 Multiyear Assimilation Web site offers a presentation titled “The Asian monsoon from the GEOS-1 multiyear assimilation.”

18. ENSO (El Niño Southern Oscillation). A glossary of ENSO terms is provided by NOAA's Climate Diagnostics Center. The Climatic Research Unit, University of East Anglia, Norwich, UK, offers an information sheet on ENSO and climate. NOAA's Climate Prediction Center offers a tutorial on the ENSO cycle. K. Carslow, School of the Environment, University of Leeds, UK, provides lecture notes on ENSO and short-term climate variability for a course on the scientific issues of climate change.

19. Walker circulation is defined in S. Baum's glossary. The Australian Bureau of Meteorology includes a section on Walker circulation in its presentation on El Niño.

20. D. W. Lea is in the Department of Geological Sciences, University of California, Santa Barbara.

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