¡EL NIÑO!

Objectives:

Nearly one-fifth of Earth’s surface area is within the oceanic area termed the tropical Pacific. This region stretches about one-third of the way around the globe. The vast expanse of tropical ocean and overlying atmosphere form a coupled system that makes its presence felt far beyond its boundaries. Its influence on world-wide weather and climate can have major ecological, societal, and economic consequences. Every 3 to 7 years this coupled ocean/atmosphere system takes on conditions termed El Niño, which typically persists for 12 to 18 months and may alternate with the less frequent La Niña. El Niño is one example of the variability of weather and climate on time and spatial scales that go beyond the basic weather map.

After completing this investigation, you should be able to:

• Describe the neutral (long-term average) conditions of the tropical Pacific Ocean and atmosphere.
• Compare El Niño and La Niña conditions to neutral conditions.
• Explain how atmospheric conditions during El Niño are transmitted beyond the tropical Pacific area.

Introduction:

Tropical Pacific during Neutral (Long-Term Average) Conditions

1. Examine Figure 1, the neutral (long-term average or normal) conditions in the tropical Pacific Ocean from about Borneo in the western Pacific Ocean to the west coast of South America (greatly exaggerated in the vertical scale). The scene depicts the ocean surface with atmosphere above and a cross-section of the ocean below. Fair weather appears in the eastern tropical Pacific (near 80 degrees W) while the cloud diagram implies that [(fair)(stormy)] weather prevails in the western Pacific (near 120 degrees E).

Figure 1. Atmospheric-oceanic block diagram of neutral (“normal”) conditions in the tropical Pacific Ocean.

2. The large-scale motions in the atmosphere show a convection cell (convective loop). The bold dark arrows show that air is rising in the stormy weather area of the western Pacific and [(rising)(sinking)] in the eastern tropical Pacific.

3. The bold black arrow along the ocean surface in the convective loop represents the trade winds and points in the direction toward which the prevailing winds are blowing in the equatorial region. As indicated by the arrows, winds during neutral (long-term average) conditions blow toward the [(east)(west)] along the equator.

4. The large white, open arrows provide surface ocean current information. The surface current arrows indicate that during neutral conditions, surface water flows towards the [(east)(west)] driven by the prevailing winds.

5. Colored areas on the top of the block diagram portion of the figure denote sea surface temperatures (SST) during neutral conditions. The red colored area in the western Pacific denotes the highest SST. These highest SST occur under [(considerable cloudiness)(clear skies)] in the tropical Pacific. This SST pattern is caused by relatively strong trade winds pushing sun-warmed surface water westward, as indicated by the direction of surface current arrows.

6. Strong trade winds also cause the warm surface waters to pile up in the western tropical Pacific so that the sea surface in the western Pacific is somewhat higher than in the eastern Pacific. Transport of surface waters to the west also causes the thermocline (the transition zone between warm surface water and cold deep water shown by the blue layer in the ocean side view) to be [(deeper)(shallower)] in the eastern tropical Pacific than in the western Pacific.

7. Warm surface water transported by the wind away from the South American coast is replaced by cold water rising from below in a process called upwelling. Upwelling of cold deep water results in relatively [(high)(low)] SST in the eastern Pacific compared to the western Pacific.

8. Cold surface water cools the air above it, which leads to increases in the surface air pressure. Warm surface water adds heat and water vapor to the atmosphere, lowering the surface air pressure. These air-sea interactions result in tropical surface air pressure being highest in the [(eastern)(western)] tropical Pacific.

9. Whenever air pressure changes over distance, a force will move air from where the pressure is relatively high to where pressure is relatively low. The trade winds blow from east to the west because from east to west the surface air pressure [(increases)(decreases)].

10. Rainfall in the tropical Pacific is also related to SST patterns. There are reasons for this relationship. The higher the SST, the greater the rate of evaporation of seawater and the more vigorous the atmospheric convection. Consequently, during neutral conditions, rainfall is greatest in the western tropical Pacific where SST are [(highest)(lowest)].

Tropical Pacific During El Niño

11. Figure 2 shows atmospheric and oceanic conditions during El Niño. Compared to Figure 1 (neutral or long-term average conditions), the area of stormy weather during El Niño has moved [(eastward)(westward)]. While no two El Niño episodes are exactly alike, all of them exhibit most of the characteristics shown in the El Niño schematic of Figure 2. With the onset of El Niño, tropical surface air pressure patterns change. Compare El Niño conditions in the western and central tropical Pacific with the neutral conditions of Figure 1. During neutral conditions, surface air pressure in the central Pacific is higher (accompanied by fair weather) than to the west. During El Niño, the surface air pressure to the west is higher than in the central Pacific. This reversal in the atmospheric pressure pattern, called the Southern Oscillation, was first studied in an attempt to explain monsoon failure and drought in India.

Figure 2. Diagram of El Niño conditions in the tropical Pacific.

12. In response to changes in the air pressure pattern across the tropical Pacific, the trade winds weaken (and wind directions can reverse, especially in the western Pacific as shown by the bold dark arrows). No longer being pushed toward and piled up in the western Pacific, the warm surface water reverses flow direction. As shown by the surface currents arrows, the surface water during El Niño flows toward the east. As evident in the appropriate sea surface temperature shading, this causes SST in the eastern tropical Pacific to be [(higher)(lower)] than during neutral conditions.

13. In response to changes in surface currents, sea surface heights in the eastern tropical Pacific are higher than during neutral conditions. At the same time, the arrival of the warmer water in the east causes the surface warm-water layer to thicken. Evidence of this is the [(shallower)(deeper)] depth of the thermocline to the east compared with neutral conditions.

Tropical Pacific During La Niña

14. Figure 3 shows atmospheric and oceanic conditions during La Niña. At times the tropical Pacific experiences trade winds stronger than neutral conditions with SST lower than usual in the eastern tropical Pacific and higher than usual in the western tropical Pacific. Because stronger trade winds produce stronger surface currents during La Niña, the warm water is pushed westward and colder water wells up to cause below-average SST in the eastern tropical Pacific. It also follows that SST in the western tropical Pacific must be [(higher)(lower)] than during a typical El Niño episode.

Figure 3. Diagram of La Niña conditions in the tropical Pacific.

15. Changes in surface air pressure, areas of large-scale convection, and upper air flow patterns as shown in Figures 2 and 3 alter the planetary wind circulation and affect the weather elsewhere in the world. Figure 4 shows some weather patterns that have been statistically associated with El Niño conditions. This figure shows that during our Northern Hemisphere winter when El Niño is taking place, the southeastern states are usually [(drier and warmer)(wetter and cooler)] than normal. Figure 5 shows some weather patterns linked to La Niña conditions.

The planetary-scale circulation of the atmosphere along the Intertropical Convergence Zone (ITCZ) includes the northeasterly trade winds of the Northern Hemisphere converging with the southeasterly trades of the Southern Hemisphere. But this generalized picture does not describe all the fluctuations of the dynamic Earth-atmosphere system. Changing temperatures in the upper layers of the Pacific Ocean and the overlying atmosphere along the equator lead to the Southern Oscillation and El Niño/La Niña episodes. In much of 1997 and early 1998, for example, the tropical Pacific Ocean experienced an unusually strong El Niño. The effects of these tropical ocean-atmosphere conditions extended well beyond the tropics and may well have set the stage for the extensive storminess along the West Coast, relatively warm and dry weather in the Southeast, the mild winter in the northern states, and weather extremes elsewhere.

Figure 4. Weather patterns statistically associated with El Niño conditions.

Figure 5. Weather patterns statistically associated with La Niña conditions. http://www.cpc.ncep.noaa.gov/products/precip/CWlink/ENSO/ENSO-Global-Impacts/ ()

Following the intense El Niño episode of 1982-83 with its worldwide weather impacts, an instrumented array of buoys (Tropical Atmosphere Ocean (TAO) or TAO/TRITON array) was deployed across the tropical Pacific from ten degrees North latitude to ten degrees South latitude. Figure 6 is a map showing the buoy locations. This array, along with satellite observations, has allowed real-time monitoring of tropical Pacific ocean and atmosphere conditions and provided input for models used to predict future episodes.

Figure 6. The locations of TAO-Triton instrumented buoys in the tropical Pacific Ocean. [http://www.pmel.noaa.gov/tao/proj_over/map_array.html ()]

16. The reporting of TAO/TRITON surface data for November 2013 is presented in Figure 7. The upper panel of Figure 7 depicts the mean tropical Pacific SST and wind conditions for that month. The SST are shaded with isotherms drawn at one-half Celsius degree intervals. Wind directions are shown by arrows originating at the buoy site with the length of the arrow depicting the relative wind speed. The shading and isotherms indicate that the warmest waters across the tropical Pacific are located near [(160° E)(180°)(140° W)] longitude. [Note, the Pacific east of 180° longitude (the International Dateline) has W(est) numbered longitudes while the Pacific west of 180° has E(ast) numbers.]

Figure 7. November 2013 oceanic means and anomaly SST and wind conditions from TAO array.

17. Across the tropical Pacific, winds were generally from [(west to east)(east to west)] and stronger in the eastern half of the region.

18. The lower panel of Figure 7 displays Anomalies, that is, departures from the long-term average. Positive temperature anomaly isotherms are drawn as thin solid lines and negative anomaly isotherms are be presented as dashed lines. The anomaly interval between lines is also one-half degree Celsius. The bold solid line denotes the 0-degree departure (i.e. average). The broad pattern of current SST anomalies over the tropical Pacific region along the equator, in general, shows values that were: [(negative everywhere across the entire equatorial Pacific)(positive everywhere across the entire equatorial Pacific)(negative in all but the southeast area)(positive in all but the southeast area)].

19. The magnitudes in the broad area of the most positive SST anomalies over the region were generally between [(0.5 and 1.0)(1.0 and 1.5)(1.5 and 2.0)] Celsius degrees.

Anomalous winds are departures from the average of both speed and direction. For example, at 110° W longitude the two actual winds (upper panel) were from the southeast while the anomalous winds (lower panel) were very small, meaning essentially than average.

20. The anomalous winds are generally [(all strongly from the east)(all strongly from the west)(light and variable except near 160 °E where they were greater easterly than average)] over the tropical Pacific.

For contrast and comparison, we will look at TAO data acquired during significant El Niño and La Niña events. In much of 1997 and early 1998, the tropical Pacific Ocean experienced a strong El Niño. Figure 8 is a depiction of the average ocean surface temperatures and atmospheric surface winds in the tropical Pacific for the month of November 1997 as measured by the TAO array, near the peak of the 1997-98 El Niño episode.

Figure 8. November 1997 oceanic means and anomaly SST and wind conditions from TAO array.

21. The top view (November 1997 Means) is the average sea surface temperatures and surface winds for the month of November 1997. The sea surface temperatures (SST) across the region ranged from about 26 °C as the “coolest” in the southeast corner to about 30 °C as the “warmest” just south of the Equator, west of center. These highest SST were located at about [(170° W)(120° W)] Longitude in the tropical Pacific.

22. The wind directions in the eastern Pacific were generally from the southeast. In the western Pacific, along the Equator (from about 140° E to 150° W), winds were generally light with most blowing from the west. Compare these observed winds and SST with the depiction of the Figure 2 schematic for an El Niño where surface winds are the horizontal arrows and the SST are color coded. The observations and the schematic model generally [(were)(were not)] consistent. (For larger views of these schematics, see http://www.pmel.noaa.gov/tao/elnino/nino_normal.html ().)

The bottom view of Figure 8 (November 1997 Anomalies) is a depiction of SST and wind anomalies, departures of the observed values shown in the top view from the long-term average. (Recall: Positive temperature anomalies are solid lines in intervals of one-half degree Celsius. A heavy line labeled 0 shows where no temperature anomaly exists, i.e. conditions are average.)

23. The SST anomalies in the eastern Pacific were positive, with the greatest values being more than [(1.5)(4.5)(7.5)] C°. SST anomalies along the equator were virtually all positive or zero. The location and degree of the warm SST anomalies is what defines the El Niño situation.

24. Now examine Figure 9. These are the tropical Pacific SST and wind conditions for November 1998, one year after Figure 8, showing that La Niña conditions had replaced El Niño. For November 1998, the sea-surface temperatures along the Equator in the eastern Pacific were near 22 °C, several degrees [(warmer)(cooler)] than those of the same area during the El Niño in November 1997. The winds across the entire Pacific area of the depiction were generally blowing from the east at that time. The warmest waters were found in the extreme western Pacific.

Figure 9. November 1998 oceanic means and anomaly SST and wind conditions from TAO array.

25. These observed winds and SST in November 1998 generally [(were)(were not)] consistent with the depiction of those of the Figure 3 schematic for a La Niña.

26. The lower panel of November 1998 Anomalies shows the Pacific SST anomalies along the Equator being almost all negative, denoted by the dashed lines, with negative values dropping below [(–2)(–3)] C°. This relatively cool (compared to the Neutral (“Normal”) Conditions) water is characteristic of La Niña.

27. Compare Figure 7 with Figures 1, 2, and 3 as well as with Figures 8 and 9. That comparison shows that Figure 7’s oceanic means and anomalies indicated [(strong La Niña)(neutral)(strong El Niño)] conditions existed in November 2013. This was confirmed by the NOAA Climate Prediction Center (CPC) ENSO description of weak warm anomalies that had covered most of the equatorial Pacific. The CPC’s forecast suggested that ENSO-neutral conditions would continue through the Northern Hemisphere summer in 2014.

For additional displays of current Pacific information related to El Niño and La Niña conditions, including SST, anomalies, depth cross-sections, winds and some animations, go to: http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml ().

Suggestions for further activities: You might investigate the El Niño/La Niña websites given above to determine the instrumentation used to obtain these in situ oceanic buoy measurements. Also, the sites display the Southern Oscillation Index (SOI). You can explore the discovery and meaning of this indicator of tropical Pacific conditions.

The El Niño theme page, http://www.pmel.noaa.gov/tao/elnino/nino-home.html (), links to a three-dimensional animation of the tropical ocean conditions as El Niño evolves. Global impacts of El Niño are shown at: http://www.cpc.noaa.gov/products/analysis_monitoring/ensocycle/elninosfc.shtml (), while global La Niña impacts are shown at: http://www.cpc.noaa.gov/products/analysis_monitoring/ensocycle/laninasfc.shtml ().