Wednesday, May 14, 2014

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.


Figures 1 and 2 show a comparison of SST and sea level anomalies for similar periods of time in 1997 (preceding the large 1997-98 El Niño event), 2013 (considered a normal year), and 2014 (preceding a possible 2014-15 El Niño event).  Anomalies are deviations from normal, wherein normal is a user-defined time average. SST anomalies are herein defined as the difference between total SST on April 23 of a respective year (1997, 2013, or 2014) to an average of SST on April 23 from 1985-2010. Sea level anomalies are sea surface heights computed with respect to a twenty-year mean. There are clear similarities between current conditions of the tropical Pacific and those preceding the 1997-98 El Niño. Similar to 1997 conditions, SST anomalies greater than 2°C are seen off the coast of South America in 2014, although the warming had not yet reached a maximum and did not extend as far west within the tropical Pacific (Figure 1). Similar patterns were also seen in sea level anomalies (Figure 2). Such similarities indicate that the observed 2014 conditions could be a precursor for a 2014-15 El Niño. If conditions continue towards an El Niño state it will peak near the end of the year, such as December 1997 for the 1997-98 El Niño event.

El Niño is also affected by the longer-period change in the background state. A major question concerning the development of an El Niño event is the potential effect of longer-period climate modes such as the Pacific Decadal Oscillation (PDO).  For example, the 1997-98 event occurred during a shift from a warm to cold phase of the PDO, but the recent warming in the Pacific is occurring during a cold phase of the PDO. How the difference in the state of the PDO between 1997 and 2014 will affect El Niño remains to be investigated.

Questions remain about the chance of an El Niño developing this year and its structure (spatial pattern and magnitude) because different structures may result in different impacts. The U.S. Climate Prediction Center has predicted a 65% chance for an El Niño to develop after August 2014.  This could be welcome news for a drought stricken California, but perhaps not for Australia or Peru. The answers to these questions will become clear in the coming months.


El Niño originates in the tropical Pacific Ocean, but has global implications. In an eastern-Pacific El Niño, abnormal warming reaches the coast of South America and weakens upwelling.  Weakened upwelling can lead to a collapse of  fisheries, which rely upon an influx of nutrient-rich subsurface waters, impacting local economies (e.g., Peru) that rely upon the fisheries industry. Other implications include variations in precipitation. During El Niño, the southwestern and southeastern United States often receive above average rainfall, whereas Australia experiences severe drought. However, there is large variability in rainfall associated with these areas, even during El Niño years. As an example, Figure 3 shows rainfall as measured at the Los Angeles Civic Center from 1878 to 2014, with El Niño years indicated. Although many El Niño events are associated with above average rainfall, some are not. Thus the variability is quite large.  The diversity of El Niño characteristics results in different teleconnections (climatic anomalies occurring over large distances) and impacts, much of which remains to be fully understood.

Figure 3. Average annual rainfall as measured at the Los Angeles Civic Center.

Other impacts of an El Niño include an increase in global temperature (by 0.1°C to 0.2°C), alterations in U.S. continental climate and patterns including Atlantic hurricanes, and a shift in marine ecosystems. Major questions remain. Will a warming planet bring more El Niño events? If so, will they be an eastern-Pacific, central-Pacific El Niño or a mix of both? How strong will the current warming be? What will be the impacts of this El Niño?  Will it bring relief to a drought stricken California?  The satellite data in the coming months will provide some of the answers to these questions.