Thursday, September 3, 2015

Currently, a major El Niño ocean-atmosphere event is developing in the Equatorial Pacific Ocean. This article describes current conditions and compares them to previous major El Niño events.

The term El Niño was first used by Peruvian fisherman to describe weak annual ocean warming off the coast of Peru. The word itself refers to “the Child” in Spanish, aptly named given the December timeframe (i.e., Christmas season) when the ocean warming was observed.  Historically, this term has taken on new meaning. Research since the 1970s (Wyrtki et al., 1976) has shown that the annual (December) warming off Peru can intensify every 3-5 years.  At times the extreme warming can extend westward over much of the Equatorial Pacific and affect weather and climate patterns around the world. Thus, today, the word El Niño has become associated with unusual ocean warming events that occur every few years in the Eastern Pacific.

The advent of Earth satellite observing platforms in the early 1980s provide the opportunity to observe the global ocean in a span of a few days, including the 1982-1983 and 1997-1998 El Niño events and their global impacts. Current anomalous conditions in both sea level and sea surface temperature (SST) in the Equatorial Pacific can be compared with previous El Niño events to gain a potential sense about future development and related impacts of the currently developing El Niño.

Sea level measurements from NASA’s TOPEX/Poseidon (launched August 1992) and Jason-2 (launched June 2008) satellite missions, along with SST data from the National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR), allow one to compare the current anomalous conditions in the Equatorial Pacific with measurements from the 1982-1983 and 1997-1998 El Niño events. Changes in sea level can be used as an indicator of the changes in the heat content while changes in SST are more indicative of the coupling between the ocean and the atmosphere from wind forcing.  Such comparisons give us insight into the diversity and similarity of El Niño characteristics and related impacts.

Figure 1. Sea level anomalies for January 1997 and 2015.
Figure 2.  Comparison of sea level anomalies from January through July for the 1997 and 2015 El Niño events.

Figure 1 shows the sea level anomaly for January 1997 derived from TOPEX/Poseidon and January 2015 derived from Jason-2. Sea level anomaly is defined as the departure of sea level from normal conditions observed during a specific time of year, in this case January. Sea level variation is linked to the change of heat stored in the upper ocean (i.e., upper-ocean heat content) that affects El Niño development. Therefore, sea level measurements enable the study of the redistribution of upper ocean heat content. Red colors indicate sea level that is above normal by more than 20 cm. Comparison of sea level anomalies indicates that significant differences existed between January 1997 and January 2015 in the Equatorial Pacific. In January 1997 sea level anomalies in the Tropical Pacific were still close to normal, while in 2015 large anomalies had already appeared.  Figure 2 shows the progression of sea level anomalies for 1997 and 2015 from January to July. During the development of the 1997 El Niño large anomalies were not visible in the Eastern Equatorial Pacific until July.  This year (2015) has shown large anomalies of over 20 cm dominating the Equatorial Pacific since January.  

Figures 3-5 shows monthly SST anomalies derived from the AVHRR Optimal Interpolation (OI) SST dataset (Reynolds and Smith, 1994; Reynolds et al., 2002; Reynolds et al., 2007). SST anomalies during January, March, May, July, and December of the 1982-1983 El Niño and 1997-1998 El Niño events are shown in Figures 3-4. Large positive SST anomalies greater than 3°C were observed in December 1982 for the 1982-1983 El Niño event, whereas they were observed as early as July 1997 for the 1997-1998 El Niño event. SST anomalies during January, March, May and July 2015 are shown in Figure 5, providing a comparison of present conditions with past El Niño events.  Differences in the pattern of SST anomalies are clearly evident (Figures 3-5), with large positive anomalies observed off the West Coast of North America throughout 2015 and large positive SST anomalies as early as May 2015 in the Equatorial Pacific in comparison with the 1982-1983 and 1997-1998 El Niño events.

It is clear from these comparisons that not all El Niños are alike. Two of the largest recorded El Niño events (i.e., 1982-1983 and 1997-1998) show differences in both the spatial patterns of peak warming and also the time of year when the maximum anomalies occur.  The 2015 event clearly shows large anomalies in both sea level (upper ocean heat content) and SST.  Additional factors that determine the intensity of El Niño events are changes in the strength and the direction of the Trade Winds. How these anomalies will develop, along with changes in the Trade Winds over the next few months, will determine the impacts to different parts of the globe. This includes local fisheries such as those off California (Jacox et al., 2015) as well as the drought affecting the West Coast of North America.  

Figure 3. Monthly SST anomalies during January, March, May, July, and December 1982. 
Figure 4. Monthly SST anomalies during January, March, May, July, and December 1997. 
Figure 5. Monthly SST anomalies during January, March, May, and July 2015. 


PO.DAAC Science Team