By Andy May
We’ve seen a lot of news stories about an upcoming El Niño, that may turn into a so-called “super” El Niño over the next year. This will affect our weather for a year or two, but what is the climatic effect of this weather feature, if any? Here we examine the history of warm ENSO events.
Los Niños warm Earth’s atmosphere for a few years because they cause excess thermal energy (heat) to be expelled from the topical Pacific Ocean and the heat is then circulated around the planet via atmospheric circulation, especially in the Northern Hemisphere where most of us live. But this is warm weather, not climate. Climate is normally defined as the average weather over a period of more than 30 years. Over 30 years, Los Niños are a cooling event since nearly all the heat they transfer to the atmosphere is eventually radiated to space. Very little of the heat released from the oceans during an El Niño is returned to the oceans because downwelling infrared radiation from the atmosphere cannot penetrate the ocean surface (Wong & Minnett, 2018). Only solar radiation can penetrate to the deeper ocean and significantly warm it.
Many Los Niños are very powerful weather features and can be traced back in time with lake sediment proxies in Ecuador as has been done by Christopher Moy and colleagues at Syracuse University (Moy et al., 2002). Figure 1 shows Moy’s El Niño proxy record and Rosenthal’s Makassar Strait proxy temperature record since 0AD. Moy’s sediment record from the Laguna Pallcacocha drainage basin is well located to record warm El Niño events since these events cause anomalous sea surface temperatures off the coast of Ecuador which initiate strong and widespread convection in the area.
The important point is that during the Medieval Warm Period Los Niños were rare and did not become common until the Little Ice Age began around 1200 AD and then declined as the Little Ice Age progressed and the world became colder. They have since become common again as the world has warmed, as shown in figure 2 which is a plot of the NOAA ERSST Niño 3.4 Index where Los Niños are positive and Las Niñas are negative values.
Los Niños were extremely rare during the Holocene Climatic Optimum, only increasing in number as the Neoglacial began as shown in figure 3. The paucity of Los Niños during the Holocene Climatic Optimum is confirmed by numerous geological proxies from around the Pacific basin as discussed in Moy et al. (Moy et al., 2002).
The paucity of Los Niños during the Holocene Climatic Optimum has been connected to Earth’s orbital cycles by Clement et al. (Clement et al., 2000). A discussion of the effects of orbital cycles on climate can be seen here. During the Holocene Climatic Optimum, Northern Hemisphere summer insolation was maximal. It appears that when this happens Los Niños are suppressed. Since the Neoglacial began, around 3800 BC, Northern Hemisphere summer insolation has declined significantly.
Figures 1 to 3 suggest that a warm stable climate is associated with very few Los Niños, but when Earth’s climate is beginning to cool, as at the beginning of the Neoglacial Period or the early cooling years of the Little Ice Age, there are more Los Niños. Los Niños were very common as we cooled into the depths of the Little Ice Age (~1750 or so) and then as we began to warm coming out of the deepest period of the Little Ice Age the number of Los Niños dropped off. We are currently at the end of Modern Solar Maximum or the Modern Warm Period, and we are seeing more Los Niños, suggesting the world is beginning to cool.
Works Cited
Clement, A., Seager, R., & Cane, M. (2000). Suppression of El Niño during the mid-Holocene by changes in Earth’s orbit. Paleooceanography, 15(6), 731-737. https://doi.org/10.1029/1999PA000466
Moy, C., Seltzer, G., & Rodbell, D. (2002). Variability of El Niño/Southern Oscillation activity at millennial timescales during the Holocene epoch. Nature, 420, 162-165. Retrieved from https://doi.org/10.1038/nature01194
Rosenthal, Y., Linsley, B., & Oppo, D. (2013, November 1). Pacific Ocean Heat Content During the Past 10,000 years. Science. Retrieved from http://science.sciencemag.org/content/342/6158/617
Vinther, B., Buchardt, S., Clausen, H., Dahl-Jensen, Johnsen, Fisher, . . . Svensson. (2009, September). Holocene thinning of the Greenland ice sheet. Nature, 461. Retrieved from https://www.nature.com/articles/nature08355
Wong, E. W., & Minnett, P. J. (2018). The Response of the Ocean Thermal Skin Layer to Variations in Incident Infrared Radiation. Journal of Geophysical Research: Oceans, 123(4). https://doi.org/10.1002/2017JC013351
From DeepSeek:
How Wind-Driven Overturning Destroys the Stratified “Warm Layer”
This is a subtler, but critically important, seasonal process at mid and high latitudes.
The Mechanism: The ocean is heated from above, creating a thin, warm surface layer sitting over much colder, denser water. This strong density stratification prevents vertical mixing.
The Overturning Connection: Strong wind stress—particularly from autumn and winter storms—forces turbulent mixing. This mechanically stirs the surface layer, deepening the mixed layer and entraining cold water from below into the surface. This is part of a vertical overturning circulation where cold water is mixed upward as warm water is mixed downward.
SST Reduction: The net effect at the sea surface is a rapid and widespread cooling, deepening the mixed layer from tens of meters in summer to hundreds of meters in winter. This is the primary reason for seasonal SST cooling outside the tropics.
If the wind stress field is such that the Ekman transport pulls surface waters apart (e.g., a cyclone in the Northern Hemisphere, or along the equator), a divergence zone is created. This literally “sucks” deep water up to the surface through a process called Ekman suction.
My own observation based on a study of change in SST at all latitudes by month of year is that the change in SST in the vicinity of the Galapagos islands is positively correlated with change in SST at all latitudes across the globe. This suggests that an El Nino event is due to a relaxation in wind stress globally. Walker noticed that it is associated with a relaxation of the trade winds and the Indian Monsoon.
The El Nino event is likely associated with the release of heat from the ocean to the atmosphere that would otherwise be associated with warmer water under the surface.
One should not underestimate the heat released via evaporative cooling of vegetation and the release of the latent heat due to condensation. Cloud cover is associated with the presence of vegetation. Clearing of vegetation associated with farming, animal husbandry and urbanisation must have reduced this mode of cooling over time
In the upshot the status of surface temperature is unlikely to be positively correlated with an increase in energy trapped within the Earth system as a whole.