8.4: El Niño-Southern Oscillation

The Walker Circulation

The Walker Circulation occurs in the equatorial eastern Pacific and begins with the trade winds, which commonly blow from East to West. As the winds blow along the ocean surface, they push warm surface waters to the west as well. As a result, warm surface waters accumulate in the Western Pacific, near Asia, Indonesia, and Australia, while cooler surface waters upwell (rise from below) in the Eastern Pacific, along the coast of Peru. Figure \(\PageIndex{1}\) shows the Walker Circulation during such neutral conditions.

Figure \(\PageIndex{1}\): The Walker Circulation during Neutral Conditions. (Public Domain; Schematic Diagrams via NOAA/PMEL). Alternative description of the image.

Using this information, let's understand the atmospheric dynamics of the Neutral Phase of the Walker Circulation.

17. Warmer sea surface temperatures in the western Pacific create warm, rising air, and thus ______ pressure conditions near east Asia.
    1. high
    2. low
18. Meanwhile, cooler sea-surface temperatures in the eastern Pacific produce cooler, sinking air, and thus ______ pressure conditions near South America.
    1. high
    2. low

While it’s very tempting to focus on the longitude coordinates and believe that the left side of Figure \(\PageIndex{1}\) is the “eastern” Pacific, while the right side is the “western” Pacific. This is not true. The reason the coordinates switch from west to east while traveling west on the map is that you cross the International Date Line and enter the Eastern Hemisphere. The west remains on the left, and the east remains on the right. Thus, the eastern Pacific refers to the Pacific Ocean along the coasts of North and South America, whereas the western Pacific refers to the Pacific Ocean along the coasts of East Asia and Australia.

The surface air rises to higher altitudes over Indonesia, moves back toward Central and South America, and descends to the surface due to the region's high pressure. This air circulation in the equatorial Pacific has significant implications for the local climate.

19. The western Pacific has warm, rising air and more moisture, which allows for the formation of:
    1. Stormy weather
    2. Calm clear weather

20. On the other hand, the eastern Pacific has cooler sinking air and less moisture, which makes it:
    1. Stormier
    2. Drier

Following the 1982-1983 El Niño event, which led to numerous extreme weather events worldwide, a series of buoys jointly operated by the U.S. National Oceanic and Atmospheric Administration (NOAA) and the Japanese Meteorological Agency (JMA) were deployed along the Equatorial Pacific. This array is called the TAO/Triton Array, and the locations of the buoys are shown in Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): Positions of the Global Tropical Moored (TAO/Triton) Buoy Array across the Equatorial Pacific (Public Domain; Global Tropical Moored Buoy Array via NOAA/PMEL). Alternative description of the image.

Sensors on the buoys measure various oceanographic and meteorological parameters, such as sea surface temperature (SST), wind speed and direction, air temperature, and humidity. The data is then transmitted in near-real time via NOAA's polar-orbiting satellites to NOAA's Pacific Marine Environmental Laboratory (PMEL). The data is then plotted on maps, such as Figure \(\PageIndex{3}\), which shows 5-day SST and wind speed means as well as anomalies for the period ending July 2, 2018. These maps enable oceanographers and meteorologists to closely examine the behavior of the Walker Circulation.

Figure \(\PageIndex{3}\): TAO/Triton measured Sea Surface Temperature (SST) and wind speed mean values (top) and anomalies (bottom) for the 5-day period ending July 2, 2018. (Public Domain; Monthly Mean SST and Winds via NOAA/PMEL). Alternative description of the image.

There are two panels on Figure \(\PageIndex{3}\). The top panel shows 5-day mean observed sea surface temperatures and wind vectors. Here’s how to read these:
• Temperature: Blues are cold, greens are mild, yellows are warm, reds are hot.
• Wind Vectors: The arrow points in the direction the wind is blowing towards, and the length of the arrow indicates how fast the wind is moving.

21. According to Figure \(\PageIndex{3}\), the hottest sea surface temperatures (top panel) are located ______, while the coldest sea surface temperatures are located _______.
    1. In the western Pacific; in the eastern Pacific
    2. In the central Pacific; in the eastern Pacific
    3. In the eastern Pacific; in the western Pacific

The bottom panel in Figure \(\PageIndex{3}\) indicates anomalies in SST and winds, which are differences from the average. For example, if the class average on a test was 80% and someone earned 85%, that would be 5% above average, hence a 5% positive anomaly. Anomalies are shaded in the same manner as the mean SST. Colder waters are represented by dark blue and purple; near-average temperatures are shaded in green; hot anomalies are depicted in orange and red. There are also isolines (lines of constant value) to help detect the pattern of anomalies.

22. The anomalies in Figure \(\PageIndex{3}\), indicate that the sea surface temperatures were:
    1. Much hotter (more than +3°C) than normal
    2. Much colder (less than -3°C) than normal
    3. Almost completely normal

These conditions represent the “neutral condition" of the Walker circulation.

El Niño

However, every few years, key components of the Walker Circulation shift. The trade winds, which usually carry warm water to the Western Pacific, either slow or reverse, allowing warm water to accumulate over the eastern Pacific. This allows for the collection of warmer, rising air and lower surface pressure to build in the eastern Pacific, near Peru. On the other hand, as the trade winds slacken, there is less warm water over the western Pacific, leading to less warm air and drier conditions near East Asia. Because this always happens around Christmas, Peruvians began calling it "El Niño". Officially, it is called the El Niño-Southern Oscillation(ENSO). Figure \(\PageIndex{4}\) shows the Walker Circulation during the El Niño phase. You'll notice that the Walker circulation from Figure \(\PageIndex{1}\) has reversed.

Figure \(\PageIndex{4}\): The Walker Circulation in an El Niño phase. (Public Domain; Schematic Diagrams via NOAA/PMEL) Alternative description of the image.

La Niña

La Niña is the opposite of El Niño. During this phase of the Walker Circulation, rather than weakening or reversing from the Neutral Phase, the trade winds intensify. This intensification of the east-to-west Trade winds leads to more warm water accumulating over the western Pacific, near East Asia, which in turn fuels more warm, rising air. On the other hand, the eastern Pacific, near Peru, experiences even cooler-than-average sea surface temperatures and sinking air. Figure \(\PageIndex{5}\) shows Walker Circulation during the La Niña Phase.

Figure \(\PageIndex{5}\): The Walker Circulation during La Niña. (Public Domain; Schematic Diagrams via NOAA/PMEL) Alternative description of the image.

Let us assess whether we can identify these changing sea-surface temperature patterns and wind directions using the TAO/Triton buoy array in the equatorial Pacific. Figure \(\PageIndex{6}\) shows the TAO/Triton measured mean sea surface temperature, winds, and anomalies for December 2015.

Figure \(\PageIndex{6}\): Sea-surface temperatures and anomalies for December 2015. (Public Domain; Monthly mean SST and Winds via NOAA/PMEL) Alternative description of the image.

23. According to the lower panel in Figure \(\PageIndex{6}\), in the central and eastern Pacific, the most significant temperature anomalies were:
    1. Very cold (colder than -2°C below normal)
    2. Closer to average
    3. Very hot (hotter than +2°C above normal)

24. These conditions indicate that the Walker Circulation was in a(n) ______ phase.
    1. La Niña
    2. El Niño
    3. Neutral

Figure \(\PageIndex{7}\) shows data from the TAO/Triton array during December 2010. You'll notice that the sea-surface temperatures and wind speed anomalies are different from Figure \(\PageIndex{6}\). Sea-surface temperatures are much cooler in the eastern Pacific, and the trade winds are blowing stronger from east to west.

Figure \(\PageIndex{7}\): TAO/Triton sea-surface temperatures and wind anomalies for December 2010. (Public Domain; Monthly mean SST and Winds via NOAA/PMEL) Alternative description of the image.

25. Considering mean temperatures, trade winds and anomalies in Figure \(\PageIndex{7}\), we can confidently say that during December 2010, the Walker Circulation was in a(n) ______ phase.
    1. Neutral
    2. El Niño
    3. La Niña

Additional Reading

In this investigation, we didn't cover the impacts of El Niño and La Niña because the focus was on identifying the signatures and detecting their presence. If you want to learn more about the effects of El Niño and La Niña on our weather, check out the following links.

• How El Niño and La Niña Affect California Weather - El Niño often brings wetter, cooler conditions and increased rainfall to Central and Southern California. At the same time, La Niña typically leads to drier, warmer conditions, increasing drought and wildfire risks in the south.
• El Niño and La Niña Affect Weather in the U.S. and Worldwide: El Niño suppresses Atlantic hurricane activity by increasing vertical wind shear. La Niña, by contrast, reduces vertical wind shear, leading to stronger and more frequent Atlantic hurricanes.