.ipynb .pdf

Contents

5. Impacts

So far, we have focused on the characteristics of ENSO phases in the equatorial Pacific, but ENSO has far-reaching effects on global climate. To understand why, it is important to know that global atmospheric circulation is composed of an interconnected network of circulation cells. We have already discussed the Walker cell (Fig. 3.1), which describes how the equatorial atmosphere circulates in the east-west direction. This cell does not operate in isolation, however, and it intersects with circulation cells that move air in the north-south direction—namely the Hadley cell (Fig. 5.1).

The Hadley cell circulates air between the low tropical latitudes and mid-latitudes around 30ºN and is in turn connected to other cells, which are connected to yet other cells, and so on. Because all these cells are interconnected, the large ENSO-related changes to atmospheric circulation in the equatorial Pacific result in a reorganization of global atmospheric circulation and weather patterns. The distant impacts of ENSO are called teleconnections.

Fig. 5.1 \(~\)|\(~\) Global circulation of Earth’s atmosphere (source).

5.1. Impacts in North America

In this section, we will take North America as an example to see how ENSO teleconnections affect winter weather in locations far from the equatorial Pacific. Fig. 5.2 shows how weather patterns are reorganized between La Niña (left) and El Niño (right) winters. The chain of teleconnection begins over the North Pacific Ocean where the connection between atmospheric circulation cells produces atmospheric pressure anomalies. These pressure anomalies affect the path of the jet stream over North America, which affects the pattern of surface temperature and precipitation over the continent.

In the following questions, we will examine atmospheric pressure and precipitation reanalysis data too see the manifestation of these conditions in individual El Niño, La Niña, and neutral years.

Every ENSO event is unique

It is worth highlighting again that not all El Niño and La Niña events are the same. The features in Fig. 5.2 represent what we expect in a typical ENSO event by averaging over many El Niño and La Niña winters. Any one of these features may be different in pattern or intensity during any single event. But on the whole, these are the expected outcomes of ENSO variations based on our historical observations.

Fig. 5.2 \(~\)|\(~\) Impacts of La Niña (left) and El Niño (right) on North American climate (source.

Questions

1. Fig. 5.3 shows winter (December-February) sea level pressure and precipitation anomalies for three selected years. Based on the schematics of impacts in Fig. 5.2, characterize each year in Fig. 5.3 as either El Niño, La Niña, or neutral. There is one of each type.

2. The 2020 California wildfire season was a record-setting year during which 4% of the state’s roughly 100 million acres of land burned. While ENSO was not the only factor in producing the record-setting fire season, it did play a significant role. In fact, one of the winters represented in Fig. 5.3 corresponds to winter 2020, which directly preceded the catastrophic fire season. Which year in Fig. 5.3 do you think corresponds winter 2020? Based on your answers to the previous question, infer the impact of El Niño and La Niña events on the California fire season.

3. What is the relationship between rainfall in California and Hawaiʻi across the years in Fig. 5.3? Does ENSO have the same effect in both locations?

ENSO impacts in Hawaiʻi

If you would like to read more about the impacts of El niño in Hawaiʻi specifically, follow the link below to a short document from the National Weather Service that summarizes various impacts on the islands.

Impacts of El Niño in Hawaiʻi

Fig. 5.3 \(~\)|\(~\) (top) Winter (December–February) sea level pressure anomaly and (bottom) winter precipitation anomaly for three selected years from the ECMWF ERA5 reanalysis.

5.2. Impacts on ecosystems

From orangutans in Indonesia to monkeys in Central/South America to iguanas in the Galapagos and even the frequency of snakebites in Coast Rica, the impact of ENSO on biology and ecosystems is pervasive and global.

Ocean ecosystems are among the most profoundly impacted by ENSO. The following questions address two of these.

Fig. 5.4 \(~\)|\(~\) As water temperatures rise during an El Niño year, small pelagic (i.e., open-ocean) fish in the eastern equatorial Pacific scatter, either migrating south or descending to depths of 150 to 200 meters, where they are not accessible to traditional fishing vessels. (source).

Questions

1. Fig. 5.4 shows that as water temperatures rise during an El Niño year, small pelagic (i.e., open-ocean) fish in the eastern equatorial Pacific scatter, either migrating south or descending to depths of 150 to 200 meters. This stresses large fisheries and the global food supply, because important fish stocks—such as the Peruvian anchovy—cannot be reached by the traditional fishing vessels. It is not the warm temperature itself that drives away the fish, however, it is the lack of nutrients (and the plankton that need those nutrients) in the warmer surface waters. Note that nutrients tend to accumulate deep in the ocean, and think back to the Section 3 of this lab when we explored the effect of ENSO on the thermocline in the equatorial Pacific. Why do you think nutrients are less abundant in the warm surface waters of the eastern equatorial Pacific during an El Niño?

2. When ocean temperatures are too warm, the algae that coral depend on get stressed and leave the coral. This is called coral bleaching, and it can be devastating for coral reef ecosystems, including in Hawaiʻi. Fig. 5.5 shows elevated bleaching risk in tropical regions around the global ocean in the season following the 2016 El Niño winter. Given the response of corals to temporary warming from El Niño, what can you say about the potential impact of climate change on corals? Will corals be more likely or less likely to recover from major El Niño events in the future?

Fig. 5.5 \(~\)|\(~\) NOAA assessment of coral reef bleaching risk due to thermal stress for February–March, 2016 following the El Niño winter of 2016 (source).

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4. Onset of an El Niño