01.08.: What drives the monsoon?

What drives the monsoon?

MARUM Researchers get new insights into the tropical climate system.

The Australian-Indonesian monsoon is one of the most important climate-systems in the world, affecting the meteorological processes in a wide spread area reaching from the Eastern Indian Ocean to the Indonesian and Philippines islands and to the Western Pacific. The monsoon can cause cyclones and heavy rain-fall as well as periods of drought. Still, it is unclear what influences the intensity of the monsoon. An international team of researchers now presents a preview on what might be a first explanation for the phenomena. The surprising result: Climatic phenomena in the northern hemisphere seem to control the intensity of the Australian-Indonesian monsoon in the southern hemisphere. The researchers analysed the past 20,000 years and found out how and why the monsoon has changed its behaviour during this period. Their results will be published in NATURE GEOSCIENCE (p.540 ff.)

From July to September it is winter in the southern hemisphere and the Australian-Indonesian monsoon comes from the southeast. From January to March the direction turns about 180 degrees and with wind coming from northwest, heavy rainfalls occur in Eastern Indonesia and Northeastern Australia. Research which covers the long-term changes shows that the intensity of the monsoon has varied over the millenniums: The southern winter can be more or sometimes less dry and frigid and the southern summer can be either very humid or less.

Dr. Mahyar Mohtadi from MARUM and his research team have found the reason for these changes: “Our data shows that the difference in the intensity is caused by phenomena in the northern hemisphere.” Furthermore, the first-author of the NATURE GEOSCIENCE study also points out: “This is valid for the summer monsoon as well for the winter monsoon.”

Working together with researchers from the Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover and also with colleagues from the USA and Chile, the MARUM-Team has examined oceanic sediments from the shore of Java. These sediments are a precise archive on the climate of the past 20,000 years and the seasonal variation of the monsoon. “After extensive research, we could show that the heaviest rainfall was caused by the summer-monsoon when the climate in the northern hemisphere was mild. That has been the case for the last 2500 years but also in the period from 13,000 to 14,500 years”, states Co-Editor Dr. Stephan Steinke, researcher at MARUM.

In order to trace for example the changes in the winter monsoon during the past 20,000 years, the scientists examined the abundance of calcite shells of planktonic foraminifera in various sediment layers. In order to proliferate, those tiny planktonic creatures need nutritive substances which is supplied by water from greater depths. The winter monsoon acts in this cycle like a water-pump, as it forces water on the surface away from the shores of Java. In return, water with more nutritive substances is pumped from the deeper layers of the ocean to the surface. In the case of the examined sediment layers, the researchers found just a small amount of shells in sediment-layers of periods when the insolation on the northern hemisphere was very low. This indicates that the insolation in the northern hemisphere affects the intensity of the “water pump” and the winter monsoon in the southern hemisphere.

“We want to proceed our work and we are already planning further research cruises to Southeastern Asia”, says Dr.Mahyar Mohtadi. “It is true that we examine the climate of the past, but our work will also contribute to understanding the climate in the future. On a longterm basis it will help us to estimate the risk of extreme incidents caused by climate.”


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Albert Gerdes
MARUM – Public Relations –MARUM
Phone: +49 - 421 – 218-65541
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MARUM aims at understanding the role of the oceans in the Earth’s system by employing state-of-the-art methods. It examines the significance of the oceans within the framework of global change, quantifies interactions between the marine geosphere and biosphere, and provides information for sustainable use of the ocean.
MARUM comprises the DFG research centre and the cluster of excellence "The Ocean in the Earth System".