***What is an El Nino?*
El Nino is a disruption of the ocean-atmosphere system in the  tropical Pacific having importantconsequences for weather around  the globe. Among these consequences are increased rainfall across the southern tier of the U.S. and in Peru, which has caused  destructive flooding, and drought in thewest Pacific, sometimes  associated with devastating brush fires in Australia.  Observations  ofconditions in the tropical Pacific are considered essential for  predicting short-term (a few monthsto one year) climate variations.   To provide necessary data, NOAA operates a network of buoys that measure temperature, currents, and winds in the equatorial band.  These buoys daily transmit data thatare available in real time to  researchers and forecasters around the world. 
In normal, non-El Nino conditions (top panel of schematic diagram),  the trade winds blow towards thewest across the tropical Pacific.   These winds pile up warm surface water in the west Pacific, making sea surface temperature about 8 degrees Celsius higher than the  east off South America.  This coldwater is nutrient rich, supporting  high levels of primary productivity, diverse marine ecosystems,and  major fisheries. 
During El Nino (bottom panel of the schematic diagram), the trade  winds relax in the central andwestern Pacific leading to a  deepening of the layer of warm surface water in the eastern Pacific, and an elevation of this layer in the west.  This leads to a reduction  in the upwelling and cuts offthe supply of nutrient rich deep water  to the upper ocean layers.  The result is a rise in seasurface  temperature and a drastic decline in primary productivity, the latter  of which adverselyaffects higher trophic levels of the food chain,  including commercial fisheries in this region. Rainfall follows the  warm water eastward, with associated flooding in Peru and drought  in Indonesiaand Australia.  The eastward displacement of the  atmospheric heat source overlaying the warmest waterresults in  large changes in the global atmospheric circulation, which in turn  force weather changesin regions far removed from the tropical  Pacific. 
El Nino can be seen in measurements of the sea surface temperature,  such as those shown above, whichwere made from the Tropical  Atmosphere-Ocean Array of moored buoys.  In January 1991, the sea surface temperatures and the winds were near normal, with warm  water in the Western Pacific Ocean (inred on the top panel of  January 1991 plot),  and cool water, called the "cold tongue" in the  EasternPacific Ocean (in green on the top panel of the January 1991  plot).  The winds in the Western Pacificare very weak (see the  arrows pointing in the direction the wind is blowing towards), and  the windsin the Eastern Pacific are blowing towards the west  (towards Indonesia).  The bottom panel of theJanuary 1991 plot  shows anomalies, the way the sea surface temperature and wind  differs from anormal January.  In this plot, the anomalies are very  small (yellow/green), indicating a normalJanuary. 
January 1992 was the peak of an El Nino year.  In January 1992, the  warm water (red in the top panelof the January 1992 plot) has  spread from the western Pacific Ocean towards the east (in the direction of South America), the "cold tongue" (green color in the top  panel of the January 1992plot) has weakened, and the winds in the  western Pacific, usually weak, are blowing strongly towardsthe  east, pushing the warm water eastward.  The anomalies show clearly  that the water in the centerof the Pacific Ocean is much warmer  (red) than in a normal January. 
An animation of the 1991 El Nino is on the PMEL server at:
http://www.pmel.noaa.gov/toga-tao/el-nino-story.html**