The normal condition of the tropical Pacific Ocean and atmosphere system is characterized by a warm pool of water in the west and a cold tongue of water in the east (off the coast of Peru), maintained by easterly trade winds that induce upwelling of cold subsurface water in the cold-tongue region and that push the warm-pool water towards the west. The resultant east-west difference (gradient) of sea surface temperature (SST) in turn helps maintain the easterly trade winds through the large-scale Walker Circulation of the atmosphere. Historically, every 2-7 years or so, the easterly trade winds relax or reverse abnormally, and the cold tongue is weakened or disappears. This abnormal warming in the cold tongue region (relative to the normal condition) is referred to as El Niño.
The relaxation of the easterly trade winds associated with El Niño is usually triggered by westerly wind bursts in the western tropical Pacific. These wind bursts have two effects. The first effect is the excitation of oceanic Kelvin waves that travel to the eastern equatorial Pacific, depressing the thermocline (the interface between the warm, surface water and colder, subsurface water), raising sea level, and suppressing the influence of the colder subsurface waters on the surface layer by reducing upwelling and vertical mixing. This causes an initial increase of SST in the eastern equatorial Pacific. The warmer SST reduces the east-west SST gradient and thus further weakens the trade winds, which in turn enhances the SST warming in the cold-tongue region. This process is referred to as the thermocline or Bjerknes feedback. The second effect of the westerly wind bursts is to push the edge of the warm pool towards the central and eastern equatorial Pacific, which also affects the large-scale east-west SST gradient and thus the trade winds. This process is referred to as the zonal advective feedback. These two processes together control the development of El Niño and its characteristics. When the Bjerknes feedback dominates, the El Niño usually has a maximum warming in the eastern equatorial Pacific (often referred to as the eastern-Pacific El Niño). When the zonal advective feedback dominates, the El Niño usually has a maximum warming in the central equatorial Pacific (often referred to as the central-Pacific El Niño or El Niño Modoki). A combination of these two processes can result in a diversity of El Niño characteristics. The precondition of heat content in the western Pacific is also important because a large upper-ocean heat content in the western tropical Pacific can be transmitted to the cold tongue region through oceanic Kelvin waves generated by the westerly wind bursts to trigger an El Niño.
Our knowledge of El Niño has increased significantly since the last major event in 1997-98, which was one of the largest ever recorded. The 1997-98 event was the first major El Niño that was observed extensively by satellites, including those that measured SST and sea surface height (SSH). These measurements are helpful to examine the evolution of an El Niño event and the consequence of the thermocline/Bjerknes feedback. Here we provide a brief overview of the current conditions associated with these parameters in relation to the conditions that preceded the 1997-98 El Niño.