The term El Nino means 'Christ Child' and was first used by Peruvian fishermen in the late 1800's to describe the warm current appearing off the western coast of Ecuador and Peru around Christmas time. Today El Nino describes the warm phase of a naturally ccurring sea surface temperature oscillation in the tropical Pacific Ocean. When this warming occurs the usual upwelling of cold, nutrient rich deep ocean water is significantly reduced. Sometimes an extremely warm event can develop that lasts for much longer time periods. In the 1990s, strong El Ninos developed in 1991 and lasted until 1995, and from fall 1997 to spring 1998. This oscillation is associated with the atmosphere, and thus the term ENSO - which incorporates the southern oscillation phenomenon - is commonly used.
So how can you determine if an El Nino event is occurring today? Certain changes typically occur to both the atmosphere and ocean during periods of El Nino and La Nina, and thus if factors such as wind velocities, sea surface temperatures, surface air pressure and sea level are monitored these changes can be detected. Here are some things you can focus on in order to determine whether an El Nino is currently occurring.
Sea Surface Temperatures
During non-El Nino and non-La Nina conditions sea surface temperatures are approximately 6-8 degrees Celsius warmer in the western tropical Pacific than in the eastern tropical Pacific. These temperature disparities typically occur because the easterly trade winds that blow across the tropical Pacific move the warm surface water with them from east to west. Thus, you could look at SST data to determine whether an El Nino event is occurring at present, or not.
During non-El Nino and non-La Nina conditions trade winds typically blow to the west across the tropical Pacific, but during an El Nino event the trade winds typically slacken or reverse. Thus, you could look at wind velocity and direction to determine whether an El Nino event is occurring at present, or not.
Other factors to consider when determining whether an El Nino event is occurring: during non-El Nino and non-La Nina conditions the sea level is typically 0.5m higher in the western tropical Pacific because the trade winds move water with them from east to west. This movement of water also causes the thermocline in the west to be deeper than in the east, and as result upwelling typically occurs in the eastern tropical Pacific. During an El Nino event the trade winds typically slacken or reverse as shown above and thus less water is moved from east to west, so SST, sea level and the thermocline all increase in the east.
The Southern Oscillation Index (SOI) can also be monitored and recorded, indicating when El Nino events are taking place. During an El Nino event the surface air pressure is typically higher in the western tropical Pacific, than in the eastern tropical Pacific. The (SOI) measures the monthly/seasonal fluctuations in surface air pressure differences at Tahiti and Darwin (Equation = Tahiti - Darwin), and thus the SOI usually has a negative value during an El Nino event.
Source: NOAA-OGP Climate Information project