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Historic El Nino events 125,000 years ago

Historic El Nino events to blame for heavy rainfalls in the Northern Atacama Desert 125 000 years ago.

Fingerprints of marine and terrestrial organisms preserved in marine sediments off the Peruvian Pacific coast helped scientists to reconstruct the climate of the Northern extension of the Atacama Desert ca. 125000 years ago. These fossil records were the tell-tale evidence for researchers from the Max Planck Institute for Marine Microbiology and their international colleagues to prove that long lasting El Nino-like conditions led to heavy rainfalls during the last interglacial.

Heavy rainfalls must have flooded the desert landscape and purged fresh water organisms like diatoms from rivers and flowers and plants from the continent into the Pacific Ocean. When analysing sediment cores from the Ocean Drilling Project (ODP) collected from the Peruvian continental shelf, the research team found fossil biomarkers of terrestrial plants and fresh water diatoms and defined the point in time when they were buried.

The normal weather and the El Nino weather phenomenon today

The Atacama Desert on the west coast of the South American continent is one of the most arid deserts of the world, virtually devoid of vegetation. On the South American coast, the Eastern Pacific cold waters are the result of the cold Humboldt Current heading towards the Equator, and the physical effect of coastal upwelling that drags cold and nutrient-rich waters from deeper levels to the surface in this area. The nutrients are the basis for the highly productive food chain. As a side effect the cold waters cool down the atmosphere, and rain clouds do not form on the western side of the Andes. Trade winds blowing towards the West across the tropical Pacific pile up warm surface water in the equatorial West Pacific (i.e. Indonesia). The strength of the trade winds is closely linked to the temperature difference between the warm Western and the cold Eastern Pacific surface waters. When warm surface waters from the equator extend to the west coast of South America, the temperature difference decreases and the trade winds slow down. As a consequence upwelling of nutrient rich deep seawater is reduced, plankton concentration drops and fish will stay out. This inter-annual climate phenomenon is observed every 2nd to 7th years with the most drastic consequences around Christmas. For centuries it has been called "El Nino" by the Peruvian fishermen referring to Christ child in Spanish (see also Figure 1).

Reconstruction of the weather situation 125 000 years ago

The reason for the heavy rainfalls during the last interglacial was due to El Nino conditions similar to the contemporary changes observed in this region. Data interpretation of sea surface temperature recorded in cores from the equatorial Pacific (East and West) support this scenario: the temperature gradient between East and West decreased during the last interglacial interpreted as weakening of the trade winds. This led to warm subtropical waters spreading along the Peruvian coast and turned off the upwelling of cold and typical nutrient rich waters. Sunlight warmed the surface of the ocean and the temperature rose more than 3 degree centigrade. Warm humid air started to ascend, clouds formed and rain showers watered the Atacama Desert. The desert started to blossom. In the Eastern Pacific less nutrients led to decreased primary production and hence to less food for the fish. The most surprising results found by Contreras and colleagues, is that this climate change can be best explained by most prevailing EN-like conditions during the LIG.

The global temperatures during the last interglacial were only slightly higher than today. This study from Contreras and colleagues suggests that a further increase in the present-day global temperature associated with El Nino-like conditions would have severe, but as-yet-neglected, implications on the Eastern tropical Pacific coast. Sergio Contreras, born in Chile, says: "When we found biomarkers from terrestrial plants and fresh water organisms like diatoms in this core from the bottom of the Pacific Ocean, I was convinced we are after something really special." Marcel Kuypers, director at the Max Planck Institute for Marine Microbiology adds:" El Nino effects have dramatic consequences on ecology and economy. To understand climate change we have to learn from history."

Manfred Schloesser and Anja Kamp

Further questions to be addressed to: Dr. Sergio Contreras Phone.:+49 421 2028 630 scontrermpi-bremen.de

Dr. Marcel Kuypers Phone +49 421 2028 647 mkuypersmpi-bremen.de

and Public relation officers: Dr. Manfred Schloesser, Phone: +49 421 2028 704; mschloesmpi-bremen.de Dr. Anja Kamp, Phone.: +49 421 2028 856; akampmpi-bremen.de

Max Planck Institute for Marine Microbiology Celsiusstr. 1, D-28359 Bremen, Germany

Original title: A rainy northern Atacama Desert during the last interglacial. Sergio Contreras, Carina B. Lange, Silvio Pantoja, Gaute Lavik, Daniel Rincón Martinez, and Marcel M. M. Kuypers. Geophysical Research Letters 2010, 37: LXXXXX, doi: 10.1029/2010GL045728

Contributing Research Institutes:

-Max Planck Institute for Marine Microbiology in Bremen -Department of Oceanography and Center for Oceanographic Research in the Eastern South Pacific, University of Concepción, Concepción, Chile -Alfred-Wegener-Institut für Polar- und Meeresforschung, Bremerhaven

Legend to Fig1

Figure 1: The El Nino Effect. Known to Peruvian fishermen for a long time, they call this weather anomaly El Nino. It occurs every 2nd to 7th year with the most drastic consequences near the end of December, and the name refers in Spanish to the new born Christ child. The consequences for the fishermen are dramatic, as shoals of fish stay out. Climate researchers in the 80s found that the reduced equatorial temperature gradients in the Pacific were the reason. The left panel A depicts the normal weather situation. The equatorial Western Pacific surface waters have average temperature around 28 C, which is four degrees higher than in the equatorial Eastern Pacific. Here the forces of the Humboldt Current coming from the South and the winds coming from the East interact and cause upwelling of cold deep water rich in nutrients. The temperature gradient establishes the Walker cell, a wind system across the Pacific Ocean. In the Western Pacific a low pressure (L) system builds up due to the rising of warm humid air. During the rise the air is cooled down and water condenses to form clouds which lead to rainfalls. On the other side of the South Pacific a high pressure system (H, anticyclone) forms. Dried air sinks down and strengthens the trade winds towards the low pressure system in the West.

Right panel B) During El Nino the situation is reversed. The stable south easterly winds have broken down, and the trade winds are so reduced in strength that they can even change direction close to the equator. Tropical and subtropical warm waters from the Western Pacific spread out eastbound until they reach the South American coast. The wind system of the Walker cell breaks down. It starts to rain on the West side of the Andes, as huge amounts of water evaporate and the water condenses when cooled down during the rise in the atmosphere. The nutricline is much deeper and as a consequence upwelling of nutrient rich deep seawater is reduced, plankton concentration drops and fish will stay out with disastrous decreases in Peruvian fisheries. Weather changes associated with strong El Nino events result in warmer and wetter years along northern South American coast, with torrential rainfalls and destructive flooding from Ecuador to Northern Peru, in coastal areas where precipitation is absent or normally scarce.

idw :: 22.12.2010