Discovery News reports on P10 research [http://dsc.discovery.com/news/2009/04/21/warming-day-length.html] related to a recent EGU presentation by Elfrun Lehmann, Felicitas Hansen and co-authors (session G3 „Observing and understanding Earth rotation variability and its geophysical excitation“). The authors reported on an increase in length-of-day variations under climate warming. They link longer days to a more frequent emerge of strong El Niño events and increasing stratospheric jets towards the end of the 21st century.
In their presentation the authors compare observed zonal wind anomalies (1961-2000) to results from a coupled atmosphere-ocean climate simulation (IPCC A1B scenario) that projects a warming climate towards the end of the 21th century. Comparing model results for time periods of 1971-2000 (IPCC 20C scenario) and 2071-2100 (IPCC A1B scenario) suggests a mean increase of 21% in length-of-day variations assuming that carbondioxid levels will have increased from about 380 ppm of pre-industrial concentrations to 700 ppm at the end of the 21th century. The climate simulation projects the strongest warming of climate up from year 2050. From this time period up the model also projects a more frequent emerge of strong El Niño events in the tropical Pacific ocean and an extensive increase in zonal winds, in particular within the stratosphere (200-10 hPa). Those jets move within zonal bands in atmospheric heights between 10 to 15 km around the Earth. They reach wind speeds of about 100 km to more than 400 km per hour. Comparing climate means of 1971-2000 (IPCC 20C scenario) and 2071-2100 (IPCC A1B scenario) indicates a strengthening of zonal stratospheric winds that lead to a mean increase in length-of-day variations of 31%.
Three strong EL Niños have been observed between 1982 and 2000. Compared to this time period the model scenario for climate warming(IPCC A1B) projects nine strong El Niño events between 2082-2100. Since the Earth acts like a closed system, an increse of jets leads to a proportional slowdown of the rotation rate of the solid Earth and therefore, to a lengthing of day. During an El Niño event Earth rotation slows down for some milliseconds per day. However, this effect declines when sea surface temperatures in the tropical Pacific region are cooling again.
However, not every El Niño event affects length-of-day variations with the same magnitude. In their study, the authors analyzed physical feedback processes associated with the varying effect of El Niños on length-of-day variations. An analysis of observed data indicates that in relation to increased ocean surface temperatures also an increase in length-of-day variations is observed that vary between 15% and 73%. The authors conclude that the varying effect of El Niños on length-of-day variations can be associated with varying anomalies of zonal wind patterns in the upper troposphere and stratosphere.
The study extends its analysis on feed back mechanism between ocean and zonal wind anomalies including results from a coupled atmosphere-ocean model simulation. Results from a model scenario for climate warming indicate that sea surface temperatures are steadily increasing leading to a more frequent emerge of El Niño events towards the end of the 21th century. In the tropical Pacific region ocean surface temperatures increase in average up to 2.9°C (IPCC A1B scenario). The climate warming model scenario projects zonal wind anomalies similar to those observations when model projected El Niño events have a strong effect on length-of-day variations. Under these conditions 75% of model projected strong El Niño events can be associated with a periodic increase in length-of-day variations of 50% and more.