|Fig. 1: Comparison of sea-surface temperature anomalies in the Pacific Ocean during a strong La Niña (b, December 1988) and El Niño (a, December 1997).  (Courtesy of McMillan Publishers Ltd.)|
One of the most startling impressions from taking a course titled the Physics of Energy is being confronted with the sheer range of energy scales surrounding us and trying to interpret them in quantitatively and qualitatively insightful ways. This year, 2015, presents one of the most staggering and relatable examples of a large-scale energy system on Earth, the El Niño climate event. In this article, we view this climate event in terms of a large-scale battery and estimate its energy content as compared to the amount of total electricity consumed in the United States for the month of August 2015 (400,000 Megawatt Hours or 1.4 × 106 Gigajoules (GJ)). 
El Niño and La Niña are together part of the periodic El Niño Southern Oscillation (ENSO), a massive semi-periodic climate event characterized by anomalies in sea surface temperature (SST) and sea surface height (SSH) over vast expanses of the equatorial Pacific Ocean. [2,3] During an El Niño event, a change in trade winds that typically blow from east to west over the Pacific weakens and the eastern equatorial Pacific begins absorbing more heat from the atmosphere than usual. La Niña is the cold swing of this oscillation. Remote sensing observations by NASA satellites  and in-situ measurements have documented this phenomenon for over five decades, but 2015 is remarkable because it is currently predicted to be the strongest El Niño event on record, characterized by large deviations in SST. Fig. 1 compares the difference from average SST from December 1988, a La Niña event, and December 1997, an El Niño event.
|Fig. 2: Heat energy in the top 700 meters of the ocean in 2014 compared to the average from 1993-2014. Values displayed with colormap in Gigajoules per square meter. (Adapted from Johnson and Parsons, Fig. 3.5a. )|
Eventually, the Pacific Ocean releases this stored residual heat in fall and winter, driving dramatic warm weather systems in the western United States and severe drought in eastern Australia, among other effects.  What is interesting from an energy standpoint is just how effective a salty body of water like the Pacific is at storing vast quantities of energy and releasing it over extended periods of time. If we consider the heat energy in the top 700 meters of the ocean in 2014 compared to the average from 1993-2014, we see the spatial extent and density of stored energy in gigajoules per square meter in Fig. 2. Integrating these average heat content deviations over the entire ocean surface of Earth yields an energy surplus of 5.8 × 1013 GJ as compared to the 1993-2014 average. 
Based on the total electricity consumed in the United States for the month of August 2015, this quantity of energy is sufficient to power the United States for 3.45 million years [(5.8 × 1013)/(12 × (1.4x106) years]. Assuming this energy is completely dissipated by ocean circulation, rain and water vapor transport by the end of the El Niño climate cycle, approximately four months in duration, the average power output over this period is 5.8 × 1013 GJ/(122 days × 24 hrs × 3600 seconds) = 5.5 × 1015 Watts. This total energy and power estimate makes El Niño one of nature's most formidable naturally occurring battery systems, capable of producing markedly warm winters for the western United States. [2,3]
© Ved Chirayath. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 "Annual Energy Outlook 2015," U.S. Energy Information Administration, DOE/EIA-0383(2015), April 2015.
 N. C. Stenseth et al., "Studying Climate Effects on Ecology Through the Use of Climate Indices: the North Atlantic Oscillation, El Niño Southern Oscillation and Beyond." Proc. R. Soc. Lond. B 270, 2087 (2003).
 C. F. Ropelewski and M. S. Halpert, "Global and Regional Scale Precipitation Patterns Associated with the El Niño/Southern Oscillation," Mon. Weather Rev. 115, 1606 (1987).
 G. C. Johnson and A. R. Parsons, Eds., "Global Oceans," in State of the Climate in 2014, Bull. Am. Meteorol. Soc. 96, No. 7, S59 (2015).
 T. Felis et al., "Pronounced Interannual Variability in Tropical South Pacific Temperatures During Heinrich Stadial 1," Nat. Commun. 3, 965 (2012).