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        Gazette IERS Gazette IERS Gazette IERS Gazette IERS Gazette
                                    _______________________________________
        No 21, 30 September 1997   /
__________________________________/                 Contact: iers@obspm.fr
                                     ftp:  hpiers.obspm.fr (145.238.100.28)
                              WWW:  http://hpiers.obspm.fr
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        Title: El Nino/Southern Oscillation and Earth rotation variations     
        Author: D. Salstein

                                  Call for

Investigations of interactions between El Nino/Southern Oscillation and 
Earth rotation variations


The El Nino/Southern Oscillation (ENSO) phenomena is a large-scale process 
in the ocean-atmosphere system, primarily  in the tropical Pacific. During 
ENSO, the normal tropical atmospheric and oceanic states are disturbed. 
Each El Nino event includes increases of sea surface temperature (SST) 
along the west coast of South America and elsewhere. This oceanic warming 
is part of one phase of the so-called Southern Oscillation (SO), a shifting 
of atmospheric mass across the breadth of the Pacific Ocean. In the El Nino 
phase, the oscillation typically produces anomalously high pressures over 
the West Pacific-Australian-Indonesian region and low pressures further to 
the east, over the central Pacific. Modifications of zonal winds during an 
El Nino episode are observed over the entire equatorial Pacific. Easterly 
winds collapse and westerlies usually increase. The influence of ENSO 
phenomena on atmospheric processes go beyond the tropics to connect to 
middle latitude regions.

Motion and redistribution of masses in the atmosphere and ocean may be
accompanied by noted variations of Earth rotation. Strong positive anomalies 
of length of day occur during ENSO events; such signals have been 
investigated by many past studies.  The impact on polar motion can be 
important as well.  The global ENSO signal in Earth rotation may be related 
to other climate phenomena.  Additionally, the correlation between geodetic 
and atmospheric excitation Earth rotation functions may be particularly 
high during times of active El Nino activity.

It is clear that a new strong El Nino is presently unfolding with important 
climatic consequences.  Given past results, detailed investigation of ENSO 
influences on Earth rotation variations is a timely pursuit.  Because each 
El Nino event has separate  characteristics, different analyses might be done 
for individual cases. Study of recent cases, as well as the current 1997 event, 
are particularly important because their occurrence during a period of high 
accuracy of Earth rotation parameters (EOP). An understanding of the 
relationships between lengthy records of ENSO and EOP is also important.

Weekly updates for relevant climate parameters, including SST, 850-hPa wind, 
and outgoing longwave radiation (OLR), a measure related to SO phase, are 
available on the Climate Prediction Center homepage at:

         http://nic.fb4.noaa.gov (ENSO Update)
     
We ask that observers of geophysical fluids (atmosphere and ocean) and of the 
various techniques for Earth rotation/polar motion consider the details of 
their products keeping in mind the analysis and possible forecast of the 
geodetic impacts of El Nino, especially for the recent and current El Nino 
events.  We will coordinate the results of such investigations on this topic. 
Discussions of this topic will be included at the 1998 IERS Workshop at other 
relevant science conferences.