9.1: The Walker Circulation

El Niño affects California's weather as well, and is commonly talked about in the news and during weather reports. So we’ve all heard about it from somewhere, but how many of us know what El Niño really is? Answer the following question before moving ahead in the text.

1. What do you think El Niño is? 

Neutral Conditions

El Niño is a phase in the Walker Circulation. La Niña is another phase in the same circulation, and the state of the circulation between El Niño and La Niña is referred to as the Neutral phase. To understand what El Niño and La Niña are, we first need to understand 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 image.

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

2. Warmer sea surface temperatures in the western Pacific create warm, rising air, and thus ______ pressure conditions near East Asia.:
   1. high
   2. low 

3. Cooler sea surface temperatures in the eastern Pacific create cool, sinking air, and thus ______ pressure conditions near South America:
   1. high
   2. low

 Let's confirm our understanding using Figure \(\PageIndex{2}\), which shows typical surface air pressure and winds, centered over the equatorial Pacific. 

Figure \(\PageIndex{2}\): Surface Air Pressure and Wind centered over the Equator (black line). (CC BY 4.0; Pressure Contours via Windy.com) Alternative description of image.

WARNING: While it’s very tempting to focus on the longitude coordinates and believe that the left side of Figure \(\PageIndex{2}\) 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 is still on the left, and the east is still on the right. Thus, the eastern Pacific refers to the Pacific Ocean along North and South America, while the western Pacific refers to that along East Asia and Australia.

4. In general, there is ______, which is consistent with the Walker circulation during a Neutral Phase (hint: Blue indicates low pressure and Red indicates high pressure).
   1. Lower pressure in the eastern equatorial Pacific and higher pressure in the western equatorial Pacific
   2. Higher pressure in the eastern equatorial Pacific and lower pressure in the western equatorial Pacific 

5. This inequality in pressure between the eastern and western Pacific creates a pressure gradient force, which pushes additional air:
   1. from west to east
   2. from east to west

The surface air rises to higher altitudes due to the low pressure near Indonesia, makes its way back to Central and South America, and descends back to the surface due to the high pressure in the region. This circulation of air in the equatorial Pacific has significant implications for the local climate.

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

7. 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 \(\PageIndex{3}\).

Figure \(\PageIndex{3}\): Positions of TAO/Triton Array Buoys across the Equatorial Pacific. (Public Domain; Global Tropical Moored Buoy Array via NOAA/PMEL) Alternative description of image.

Sensors on the buoys measure various oceanographic and meteorological parameters, such as sea surface temperature, 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 \(\PageIndex{4}\), which shows 5-day mean temperatures and anomalies for the period ending July 2, 2018. These maps enable oceanographers and meteorologists to examine the behavior of the Walker Circulation closely.

Figure \(\PageIndex{4}\): TAO/Triton measured Sea Surface Temperature Observations (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 image.

The top panel in Figure \(\PageIndex{4}\) 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 the wind speed. 

8. According to Figure \(\PageIndex{4}\), the hottest sea surface temperatures (top panel) are located:
   1. In the western Pacific
   2. In the central Pacific
   3. In the eastern Pacific 

9. The coldest sea surface temperatures are located:
   1. In the western Pacific
   2. In the central Pacific
   3. In the eastern Pacific 

10. Except for the eastern third of the map, the winds in the top panel were generally blowing:
    1. From west to east
    2. From east to west

The bottom panel in Figure \(\PageIndex{4}\) indicates Anomalies in sea surface temperatures and winds, which are differences from the average. For example, if the class average on a test was 80%, and someone earned an 85%, that would be 5% greater than average, hence a 5% positive anomaly. Anomalies are shaded similarly to mean sea surface temperatures. Colder waters are represented by dark blue and purple, near-average temperatures are shaded in green, while Hot anomalies are depicted in orange and red. There are also isolines (lines of constant value) to help detect the pattern of anomalies.

11. The anomalies in Figure \(\PageIndex{4}\), 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

12. Trade wind anomalies, indicated by the wind arrows, are:
    1. Very strong and directed eastward
    2. Light and varying in direction 

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