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 April 25, 2016
El Niño is Earth's rechargeable heat battery

 The recent El Niño has been in the news of late because the warm waters in the Pacific have helped lift Earth's temperatures to new records. Recent research is helping to track energy flows between the ocean waters and the atmosphere as the El Niño builds, then slowly decays and even changes to a La Niña. This new information is an important advancement of our understanding of the Earth's climate.

As a background, a part of the Pacific Ocean flips between cold (La Niña) and warm (El Niño) phases over a few-year-long period. Sometimes the oceans are in neither a cold or warm phase, and we call that neutral.

The flipping back and forth always occurs, but the duration and regularity can change. In general, the cycles occur over 3 to 7 years, sometimes with longer duration, other times shorter. But regardless, this El Niño/La Niña process is really important for the rest of the world. It affects the whole atmosphere through what are termed teleconnections.

Consider for instance a situation when the waters are warm (El Niño), resulting in more evaporation from the ocean waters into the atmosphere. Conversely when the ocean waters are cold, there is often less evaporation. Because evaporation requires a great deal of thermal energy -- it cools the ocean while moistening the atmosphere -- it's an engine that moves heat.

Simply put, the El Niño--Southern Oscillation (ENSO; the latter is in the atmosphere) cycle can supercharge this movement of energy, or it can temporarily sequester the heat. But regardless, once the energy gets into the atmosphere, it changes the atmospheric winds around the globe and affects weather elsewhere.

So, a new study, led by Dr. Michael Mayer from the University of Vienna, focused on the energy flows during the ENSO process. The study recalls prior work that has led to a view of ENSO that is a bit like a rechargeable battery. During La Niña, heat builds up in the Pacific and then during El Niño, the heat is dissipated to other regions. The dissipation occurs in both laterally via atmospheric energy transports and vertically via radiation to space. When the warm water moistens the air (through evaporation), it invigorates storms and flow of energy in the atmosphere. As a result of these teleconnections, a substantial fraction of the heat released from the Pacific during El Niño subsequently appears in the Atlantic and Indian Oceans.

The authors also looked at climate models, which are computerized representations of actual processes occurring in the real world. They wanted to know how well models reproduce the energy flows associated with ENSO. Traditionally, models have struggled to correctly simulate ENSO. The authors found that models tend to underestimate the ENSO processes in the upper 700 meters of the Pacific. While observations show substantial cooling of the tropical Pacific during El Niño as measured by ocean heat content, most models lack this typical signature.

This is related to biases in simulated low-level clouds, which in turn influence the amount of absorbed solar radiation. In addition, the authors found that models in general struggle to get the winds associated with ENSO correct, which hampers their ability to correctly simulate vertical redistribution of heat within the Pacific. Moreover, the models underestimate the strength of teleconnections to the Atlantic and Indian Oceans.

While models continue to get better, their current errors have to be considered when using them, and further improvements are necessary. But practice makes perfect and studies like this advance their capabilities. Such improvements are essential if we are to better understand the intersection of El Niño and climate change, and where and how extreme weather events occur.

I communicated with Dr. Mayer and he told me,

First, it is important to distinguish between the different oceans. The buildup and release of oceanic heat during La Niña and El Niño mainly occurs in the tropical Pacific. What is happening in the other basins during warm and cold ENSO events tends to be of opposite sign. This means that oceanic heat is released from the Pacific during El Niño while ocean heat content increases in the tropical Atlantic and Indian Ocean at the same time. The opposite tends to be the case during La Niña. This "seesawing" of oceanic energy between the different ocean basins is a result of the atmospheric teleconnections we describe. Besides this energy exchange between the different ocean basins there is also considerable radiative heat loss to space during El Niño. In the paper we checked whether the CMIP models replicate these pathways of anomalous energy flows.

We found that the models underestimate the ocean heat content variations in the Pacific associated with ENSO and they also underestimate the strength of teleconnections to the other basins as measured by ocean heat content variations in the Atlantic and Indian Oceans. One reason of the found deficiencies in modeled ENSO are biases in the climatological mean states of the models, which adversely affects simulated variability of energy flows.

Getting ENSO correct is important not only because it impacts society. It is superimposed on a long-term human-caused climate change. As we warm the planet from our emission of heat trapping gases, the ENSO fluctuations occur on top of that warming. Potentially, this can make the next ENSO cycle even more disruptive and costly.