9.3: Alternative Text Descriptions for Investigation 9

Figure 9.1.1 The Walker Circulation during Neutral Conditions

This figure illustrates normal atmospheric and oceanic circulation patterns across the tropical Pacific Ocean under non-El Niño conditions.

Layout:

• This is a 3D cross-section of the equatorial Pacific Ocean, oriented with:
  • Australia and Southeast Asia (west) on the left side (labeled ~120°E longitude)
  • South America (east) on the right side (labeled ~80°W longitude)
  • The Equator runs horizontally across the center.
• Ocean and Atmosphere Features:
  • Warm ocean surface waters are piled up in the western Pacific due to easterly trade winds.
  • Cooler ocean water dominates the eastern Pacific, near the coast of South America.
  • A sloping thermocline—the boundary between warm surface water and cold deep water—is shown dipping deeper in the west and rising toward the surface in the east.
  • Upwelling is occurring along the South American coast, where cold water is rising to the surface.
  • White arrows near the ocean surface indicate the direction of surface winds (easterlies)—from east to west.
  • Convective clouds and rising air are located above the warm waters in the western Pacific, signifying strong convection and thunderstorm activity.
  • Dashed arrows show the atmospheric return flow aloft, moving from west to east, completing the Walker circulation loop.

Annotations:

• Labeled “Normal Conditions” at the top, this configuration shows typical Walker Cell circulation:
  • Surface winds blow from east to west (trade winds).
  • Warm water and rising air in the western Pacific support deep convection and rainfall.
  • Cooler water and sinking air in the eastern Pacific result in dry conditions.  

Figure 9.1.2 Surface Air Pressure and Wind over the Equator.

This is a global weather visualization map from Windy.com, displaying atmospheric surface pressure patterns over a large portion of Earth, including North and South America, the Pacific Ocean, Australia, and parts of Asia. The image is divided into a Northern Hemisphere (top) and Southern Hemisphere (bottom) view.

Key Features:

• Color Shading and Isobars:
  • Areas of high pressure (H) are labeled with values (e.g., 1030 hPa, 1026 hPa).
  • Areas of low pressure (L) are labeled with values (e.g., 999 hPa, 985 hPa).
  • White contour lines (isobars) connect areas of equal pressure, illustrating the shape and gradient of pressure systems.
• Notable Pressure Systems:
  • Strong high-pressure systems exist:
  • Over the eastern Pacific Ocean (~1030 hPa),
  • North of Hawaii and east of Japan,
  • Over Australia and parts of South America.
•  Low-pressure systems appear:
  • Off the west coast of the U.S. (~999 hPa),
  • Near the Philippines, South Pacific, and eastern South America.
• Wind Flow Visualization:
  • Particle animation (visible as fine streaks) illustrates the direction of surface winds, which generally flow:
  • Clockwise around high-pressure systems in the Southern Hemisphere,
  • Counterclockwise around lows in the Northern Hemisphere.
• Wind patterns are tightly packed near strong pressure gradients, such as around tropical lows.   

Figure 9.1.3 Positions of TAO/Triton Array Buoys

This map displays the Global Tropical Moored Buoy Array, which consists of oceanographic buoys distributed across the tropical Pacific, Indian, and Atlantic Oceans as of July 2025. These buoys are part of three regional networks used for long-term climate and ocean monitoring:

• RAMA-2.0 (Indian Ocean, left side of map)
• TAO (Pacific Ocean, center of map)
• PIRATA (Atlantic Ocean, right side of map)

Map Layout:

• The map spans latitudes from 30°N to 30°S and longitudes from 60°E to 0°E, wrapping around the globe from the Indian Ocean to the Atlantic.
• Landmasses include Africa, South America, Southeast Asia, and Australia.
• Buoy positions are marked across the ocean basins with various colored squares, each color representing a different type of mooring instrumentation.

Buoy Types and Symbols:

Each square represents a buoy site. Two fill types are used:

• Solid squares = currently operating
• Open squares = planned deployment

Figure 9.1.4 Sea Surface Temperatures for July 2018

This figure contains two horizontal panels representing TAO/TRITON ocean monitoring data for the equatorial Pacific Ocean, covering the five-day period ending July 2, 2018. The panels display sea surface temperature (SST) and surface wind vectors, with the lower panel also showing anomalies relative to climatological averages.

Top Panel – “Means” (Sea Surface Temperature and Wind):  

• The top panel shows SST in degrees Celsius (°C) across a horizontal section of the equatorial Pacific, from 140°E to 100°W longitude, and 10°S to 10°N latitude.
• A gradient ranges from:
  • 30°C in the western Pacific near 140°E,
  • 20–26°C as it moves eastward toward South America.
  • Contour lines labeled (e.g., 30, 29, 28…) indicate lines of equal SST (isotherms).
  • Wind vectors (arrows) represent average surface winds, generally showing easterly trade winds blowing from east to west across the central Pacific.

Bottom Panel – “Anomalies” (SST and Wind Deviations from Normal):

• This panel shows SST anomalies (differences from long-term averages) for the same region and time period.
• The color gradient ranges from:
• +2 to +4°C for positive anomalies (warmer than normal),
• -2 to -4°C for negative anomalies (cooler than normal).
• Overall, the anomalies are mostly near zero, indicating neutral ENSO conditions (neither El Niño nor La Niña).
• Contour lines labeled with numbers (e.g., 0, +1, -1) delineate SST anomaly values.
• Wind anomaly vectors (arrows) show small deviations in direction and magnitude, indicating slight variations from typical trade wind patterns.  

Figure 9.2.1 The Walker Circulation during El Niño

This is a 3D schematic cross-section of the equatorial Pacific Ocean, illustrating El Niño conditions as part of the El Niño–Southern Oscillation (ENSO) cycle.

Layout:

• The diagram spans from 120°E (left) to 80°W (right) along the equator, depicting ocean and atmospheric patterns across the tropical Pacific.
• The left side represents the western Pacific (near Indonesia and Australia).
• The right side represents the eastern Pacific (near South America).

Key Features:

Ocean Structure:

• Warm surface water stretches uniformly across the Pacific, from west to east.
• Cooler water is confined to deeper layers and pushed downward throughout the entire basin.
• The thermocline, the boundary between warm surface water and cooler deep water, is shown as flat and deep, unlike in normal conditions when it is tilted.
• Upwelling of cold water is suppressed near South America.

Surface Winds:

• The usual trade winds (from east to west) are weakened or reversed.
• Black arrows along the ocean surface indicate weakened or reversed wind direction, blowing from west to east across the equator.

Atmosphere:

• Rising air and deep convection (clouds and upward arrows) are now centered in the central and eastern Pacific, rather than over the western Pacific.
• Dashed white arrows show upper-level atmospheric return flow, completing a disrupted Walker circulation loop.
• Rainfall and thunderstorm activity are displaced eastward, toward the central Pacific.

Figure 9.2.2 The Walker Circulation during La Niña.

This is a 3D schematic cross-section of the equatorial Pacific Ocean, illustrating La Niña conditions, a component of the El Niño–Southern Oscillation (ENSO) climate pattern.

Layout:

• The cross-section extends from 120°E (left) to 80°W (right) along the equator.
• Western Pacific (left side) represents the region near Australia and Southeast Asia.
• Eastern Pacific (right side) represents the region near South America.

Ocean Features:

• Warm surface waters are pushed farther west than normal, accumulating in the western Pacific.
• The eastern Pacific is dominated by cooler waters.
• The thermocline, the boundary between warm and cold water, is steeper than normal, being much shallower near South America and deeper in the western Pacific.
• Strong upwelling of cold, nutrient-rich water is shown off the coast of South America.

Atmospheric Circulation:

• Surface trade winds (arrows along the surface) are stronger than normal, blowing east to west across the Pacific.
• These winds drive warm surface water westward and enhance upwelling in the east.
• Rising air and deep convection (indicated by black upward arrows and clouds) are concentrated over the western Pacific, where the water is warmest.
• A dashed white arrow above the surface indicates upper-level atmospheric return flow moving west to east, completing the Walker circulation.
• Sinking air and dry conditions are shown over the eastern Pacific, near South America.

Figure 9.2.3 Sea Surface Temperatures and Anomalies for December 2015

This figure shows sea surface temperature (SST) and wind data across the equatorial Pacific Ocean during December 2015, based on measurements from the TAO/TRITON buoy array. The data is presented in two horizontal panels:

Top Panel – “Means” (Sea Surface Temperature and Wind):  

• This panel shows average SSTs (in °C) and surface winds between 10°N and 10°S latitude, from 140°E to 100°W longitude.
• The SSTs are displayed using a color gradient from:
  • 30°C in the western and central Pacific,
  • 26–28°C in the east,
  • 18–22°C near South America.
  • Isotherms (contour lines) mark temperatures at 1°C intervals (e.g., 30°C, 29°C, 28°C).
  • Wind vectors (arrows) show the direction and magnitude of surface winds:
• Arrows mostly point from west to east across the central and eastern Pacific,
• Indicating weakened or reversed trade winds, which are typical of El Niño events.

Bottom Panel – “Anomalies” (SST and Wind):

• This panel shows deviations from the average (climatological) conditions.
• Color shading indicates SST anomalies (in °C), with:
  • Positive anomalies (+2 to +3.5°C) in the central and eastern equatorial Pacific.
  • The strongest warming occurs between 160°W and 100°W, peaking at about +3.5°C.
  • Areas with cooler-than-normal SSTs (very limited in this case).
• Contour lines trace SST anomaly values (e.g., 1.5, 2.0, 3.0).
• Wind anomaly vectors display eastward-pointing arrows, indicating weakened or reversed easterly winds, consistent with El Niño.

Figure 9.2.4 Sea Surface Observations and Anomalies for December 2010

This figure presents monthly sea surface temperature (SST) and wind data from the TAO/TRITON buoy array for the equatorial Pacific Ocean during December 2010. It includes two horizontal panels:

Top Panel – “Means” (Sea Surface Temperature and Wind):  

• The panel displays average SSTs and wind vectors between 10°N and 10°S latitude, from 140°E to 100°W longitude.
• A gradient represents SSTs from:
  • 30°C in the western Pacific,
  • 18–24°C as it moves eastward toward South America.
  • Contour lines (isotherms) labeled at 1°C intervals (e.g., 29, 28, 26°C) trace regions of equal temperature.
  • Arrows show surface wind direction and speed:
• Arrows point strongly from east to west, indicating intensified easterly trade winds typical of La Niña.

Bottom Panel – “Anomalies” (SST and Wind):

• This panel shows anomalies (differences from the long-term average).
• SST anomalies from -1°C to -3°C dominate the central and eastern Pacific, showing cooler-than-average water.
• Contour lines labeled -1.5, -2.0, etc., trace the extent of cold anomalies.
• Wind anomaly vectors show stronger-than-normal easterlies, reinforcing the cooling effect.